Image forming apparatus, control method for controlling the same and temperature control apparatus

An image forming apparatus includes a fixing unit having a heat source, a temperature detecting unit for detecting a temperature of the fixing unit, an initial operation necessity detecting unit for determining whether an initial operation should be performed in the image forming apparatus, an initial operation selecting control unit for selecting an initial operation from among a plurality of predetermined initial operations based on the temperature detected by the temperature detecting unit when the initial operation necessity detecting unit detects that the initial operation should be performed, and a control unit for controlling the fixing unit so that the fixing unit performs the initial operation selected by the initial operation selecting control unit. Furthermore, a temperature control apparatus includes a determination unit for determining, based on the detected temperature of the body, whether the temperature of the body should be increased or decreased, a temperature increasing control unit for applying an AC voltage from the AC power supply to the heater so that the temperature of the body is increased when the determination unit determines that the temperature of the body should be increased, and a temperature decreasing control unit for applying to the heater a pulse-shaped AC voltage which is repeatedly turned on and off at a frequency so that the temperature of the body is decreased when the determination unit determined that the temperature of the body should be decreased, the frequency being generally not perceived by people.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention generally relates to an image forming apparatus, such 
as a copy machine, a facsimile machine and a printer, for forming images 
in accordance with an electrophotographic process, and particularly to an 
image forming apparatus having a heat source used for fixing of developer 
in the electrophotographic process and a control method for controlling 
the image forming apparatus. 
The preset invention further relates to a temperature control unit for 
controlling a temperature of a body, such as a fixing unit used in a laser 
printer, which is to be heated by a heater connected to an AC power supply 
line so that the temperature of the body is maintained in a predetermined 
temperature range, and an image forming apparatus using the temperature 
control unit. 
(2) Description of the Related Art 
FIG. 1 shows a first example of a conventional image forming apparatus. 
Referring to FIG. 1, a fixing unit 191, a temperature detecting device 
195, an initial operation necessity detecting unit 197, a maximum 
temperature driving instruction unit 198, a temperature control unit 196 
and a driving control unit 199. The fixing unit 191 is used for fixing of 
developer on a recording medium (a recording sheet) in a laser printer 
(the image forming apparatus for forming images in accordance with the 
electrophotographic process). The fixing unit 191 has a heat roller 192 
including a heat source 193 such as a halogen lamp and a pressure roller 
194 for pressing a recording medium against the heat roller 192. The 
temperature detecting device 195, which is made, for example, of a 
thermistor, detects a surface temperature of the heat roller. After a 
power of the image forming apparatus is turned on, after open-and-close 
operations of covers of the image forming apparatus are performed to 
remove jammed papers, to change consumables or do other things, and when a 
reset signal is received from an external unit, such as a computer unit, 
for supplying printing data, predetermined initial operations must be 
performed. The initial operation necessity detecting unit 198 determines 
whether or not the initial operations should be performed. In a case where 
the initial operations are required, the fixing unit 191 operates in a 
maximum temperature mode. In the maximum temperature mode, the maximum 
temperature of a plurality of temperatures is set as a target temperature 
of the heat roller 192, and immediately after the initial operations 
starts, the heat source 193 is turned on and the heat roller 192 is 
rotated to heat the pressure roller 194. The maximum temperature driving 
instruction unit 198 supplies instructions for the maximum temperature 
mode to the temperature control unit 196 and the driving control unit 199. 
The temperature control unit 196 controls the fixing unit 191 so that 
detected temperature from the temperature detecting device 195 is equal to 
the target temperature set in the apparatus. The driving control unit 199 
carries out driving control of the fixing unit 191. 
The initial operation are operations carried out to provide for thermal 
fixing of toner on a recording medium (a recording sheet). By the initial 
operation, the heat source 193 is activated while the heat roller 192 is 
being rotated so that the fixing unit 191 is heated. That is, the pressure 
roller 194 is heated at a temperature so that the minimum fixing factor is 
obtained. The fixing factor indicates a degree of whether the developer 
(the toner) is fixed on the recording sheet. 
In the initial operations, the maximum temperature of a plurality of 
temperatures is set as the target temperature, so that a waiting time 
required for starting of the printing operation can be decreased. 
As shown in FIG. 2, in a case where a detected temperature is less than 
100.degree. C. (step S53), the maximum temperature 180.degree. C. is 
selected as the target temperature. The target temperature is further 
controlled based on whether the detected temperature is less than 
40.degree. C. On the other hand, when the detected temperature is equal to 
or greater than 100.degree. C., a temperature of 170.degree. C. which is 
to be set in a continuous printing operation is set as the target 
temperature. 
After the initial operations have been performed, the target temperature is 
selected from among the plurality of temperatures based on conditions of 
the recording medium (the recording sheet). When a thick recording sheet 
(over 70 kg/m.sup.2) is used as the recording medium, a high temperature 
is set as the target temperature of the fixing unit 191. When a thin 
recording sheet (less than 70 kg/m.sup.2) is used as the recording medium, 
a low temperature is set as the target temperature of the fixing unit 191. 
After the recording medium has passed between the heat roller 192 and the 
pressure roller 194, the heat roller 192 is being maintained, for a 
predetermined time, at a temperature which is to be set in the printing 
operation in order to provide for the fixing of toner on the next 
recording sheet. A mode in which the heat roller 192 is being rotated for 
the predetermined time at the temperature to be set in the printing 
operation is referred as a wait mode. Due to the waiting mode, the 
temperature of the fixing unit 191 is not decreased, so that a time for 
reactivation of the fixing unit 191 can be eliminated and a number of 
times which the printing operation is performed in a unit time can be 
prevented from being decreased. 
In a case where there is no printing instruction for a predetermined time, 
the fixing unit 191 is controlled in a stand-by mode in which a 
predetermined temperature than the temperature to be set in the printing 
operation is set as the target temperature. In general, in a state where 
the fixing unit 191 is being controlled in the stand-by mode, an printing 
instruction is supplied from the external unit. 
When the printer receives the printing instruction, the following print 
starting process is activated to provide for the printing operation. 
In the print starting process, the temperature control of the fixing unit 
191 starts, an optical system is activated and initial operations of a 
processing unit (for performing the electrophotographic process) which are 
performed while a photosensitive drum is being rotated starts. After a 
time required for the initial operations of the processing unit elapses, 
it is confirmed that the optical system is activated. After this, when the 
detected temperature of the fixing unit 191 reaches the predetermined 
temperature, the printing operation starts. 
A description will now be given, with reference to FIGS. 8A, 8B, 9 and 10, 
of a second example of the conventional image forming apparatus. 
FIG. 8A shows a printer according to the second example of the conventional 
image forming apparatus. 
Referring to FIG. 8A, a printer 291 has a temperature detecting device 83, 
a heater controller 84, a power supply unit 85, an interface control unit 
86, a mechanism controller 87 and mechanisms 88. The printer 291 further 
has a heater 81 and a heater driving circuit 82. The heater 81 is included 
in a fixing unit used for fixing of toner on a recording sheet. When a 
detected temperature output from the temperature detecting device 83 is 
less than a minimum setting temperature, the heater controller 84 supplies 
an on-signal to the heater driving circuit 82 via a control line 7. When 
the detected temperature output from the temperature detecting device 83 
is equal to or greater than a maximum setting temperature, the heater 
controller 84 supplies an off-signal to the heater driving circuit 82 via 
the control line 7. The heater driving circuit 82 which receives the 
on-signal supplies an AC voltage to the heater 81 via a power line 6. As 
shown in FIG. 8B, the heater driving circuit 82 has a driving element 92 
and a power thermistor 93. 
FIG. 9 shows a timing chart illustrating control of the heater 81. The 
dissipation power of the heater 81 is 1100 watts, and a total dissipation 
power of other devices in the printer is 100 watts. The heater 81 is 
controlled at 150.degree. C..+-.2.degree. C. When the heater control 
signal has a low level "L", the heater driving circuit 82 turns on the 
heater 81. When the heater control signal has a high level "H", the heater 
driving circuit 82 turns off the heater 81. In this control of the heater 
81, when the detected temperature is equal to or greater than 152.degree. 
C., the heater 81 is turned off. In addition, when the detected 
temperature is less than 148.degree. C., the heater 81 is turned on. In 
general, an on-state of the heater 81 is continued for a few seconds and 
an off-state of the heater 81 is continued for a time falling within a 
range from a few seconds to a few tens seconds. A detailed state where the 
heater 81 is activated (see a point A in FIG. 9) is shown in FIG. 10. 
In general, although the fixing unit should be controlled at a high 
temperature in order to obtain a high fixing factor, a low temperature 
should be set as a target temperature of the fixing unit in order to 
prevent recording sheets from curling and being wrinkled. 
The first example of the conventional image forming apparatus has the 
following disadvantages. 
(a) With increasing of the number of times which the printing operation is 
repeated, the degree of positive curl of the recording sheet is enlarged. 
The positive curl defines a state where the recording sheet is curled 
around the heat roller 48 so that a printed side of the recording sheet is 
inside, as shown in FIG. 36B. When the temperature of the pressure roller 
47 is increased, the degree of the positive curl of the recording sheet is 
enlarged, as shown in FIG. 36A. 
(b) In a case where the fixing unit is not sufficiently heated immediately 
after the power is turned on, the degree of reverse curl of the recording 
sheet having moisture is large. The reverse curl defines a state where the 
recording sheet is curled around the pressure roller 47 so that the 
printed side of the recording sheet outside. When the temperature of the 
pressure roller 47 is decreased, the degree of the reverse curl of the 
recording sheet is enlarged, as shown in FIG. 36A. 
In a two-sided printing apparatus, when an images are printed on a second 
side of the recording sheet having a large reverse curl, it is difficult 
to separate the recording sheet from the photosensitive drum, so that a 
sheet jam occurs. 
(c) Even if the fixing unit is controlled at a required temperature, the 
predetermined initial operations are performed. Thus, a warm-up time is 
increased. 
(d) A temperature at which the fixing unit is controlled may be too high 
and a time for which the initial operations are performed may be too long. 
Thus, the temperature in the printer is increased. 
(e) The photosensitive drum is vacantly rotated every time the initial 
operations are performed and the print starting process is performed. 
Thus, characteristics of the photosensitive drum may deteriorates in 
processes and operations other than the printing operation. 
(f) Since the photosensitive drum is vacantly rotated every time the print 
starting process is performed, the first-print time can not be decreased. 
The recording sheet is curled (the positive curl and the reverse curl) as 
follows. 
The positive and reverse curls of the recording sheet depend on the 
temperatures of the heat roller and pressure roller, a type of the 
recording sheet (a thickness, fibers included therein, material and the 
like) and a state (the moisture content) of the recording sheet. 
FIG. 3 shows an example of temperature transition of the pressure roller in 
a case where the printing operation is repeated starting from turning-on 
of the power supply (a cold start operation). In this example, the initial 
operations are executed after the power supply for the printer is turning 
on. After this, the print instruction is not received for a predetermined 
time, so that the fixing unit is brought into the standby mode. In this 
example, a set of five printing instructions is repeatedly received five 
times at intervals of five minutes (in the standby mode). In accordance 
with each set of the five printing instructions, the printing operation in 
which characters are printed on a recording sheet having a size of A4 is 
repeatedly executed at intervals of 30 seconds. The printing operation is 
repeatedly executed page by page. The print starting process is performed 
every time the printing operation for a page is received. 
As shown in FIG. 3, after the initial operation is performed for a short 
time in a state where the fixing unit is maintained at a relatively low 
temperature, the fixing unit is controlled in the standby mode. Thus, the 
temperature of the pressure roller is not almost increased. After the 
fixing unit is controlled in the standby mode for a predetermined time, 
the printing operation starts in accordance with the printing instruction. 
A time for which the print starting process is performed is relatively 
long. Thus, after starting the printing operation, the temperature of the 
pressure roller is rapidly increased. The printing operation is repeatedly 
performed at the intervals, and the temperature of the pressure roller is 
increased so as to head for a saturation temperature. The saturation 
temperature depends on the recording sheet which passes between the heat 
roller and the pressure roller. In this case where the recording sheet 
having the size of A4 laterally passes between the heat roller and the 
pressure roller, the saturation temperature is about 130.degree. C. 
The positive curl of the recording sheet occurs as follows. 
In a general case where the printing operation is repeatedly executed at 
intervals, the temperature of the pressure roller is increased as shown in 
FIG. 3. The fixing factor of the fixing unit depends on the temperatures 
of the heat roller and the pressure roller. The temperature of the heat 
roller is decided based on a condition in which the required fixing factor 
is obtained even if the temperature of the pressure roller is low. Thus, 
the heat roller may be controlled at the predetermined temperature under a 
condition in which the pressure roller is in a heated-up state. In this 
case, when the printing operation is repeatedly executed under the 
condition in which the heat roller is controlled at the predetermined 
temperature without regard to the temperature of the pressure roller, the 
fixing factor is increased with creasing of the temperature of the 
pressure roller. Under a condition in which the temperature of the 
pressure roller is in a saturated temperature range (e.g., 115.degree. 
C.), the fixing unit has the necessary and sufficient fixing factor. 
However, the positive curl of the recording sheet may exceed an allowable 
level and/or the recording sheet may be easily wrinkled. 
The positive curl of the recording sheet is based on the following cause 
of: 
(a) excessive increasing of the temperature of the pressure roller; and/or 
(b) overshoot of the temperature of the heat roller in the print starting 
process. 
The temperature of the pressure roller is excessively increased as follows 
(the above item (a)). 
In a general printing operation, only the recording sheet removes heat from 
the pressure roller and the heat roller is freely heated by the heat 
roller. 
A time for which the recording sheet passes through the fixing unit is less 
than a total time required for the print starting process and the printing 
operation. As shown in FIG. 4, after the recording sheet passes through 
the fixing unit, the heat roller controlled at the predetermined 
temperature is rotated. Thus, in this case, the pressure roller is freely 
heated by the heat roller. 
FIG. 4 shows a timing chart of on-and-off operation of the heater of the 
fixing unit. 
In a case where the printer having the fixing unit controlled in the 
standby mode receives a print instruction 1, the photosensitive drum and 
the optical system are activated to provide for the print operation. After 
a time t.sub.1 elapses, it is confirmed that the photosensitive drum and 
the optical system have been activated. The printer then returns a VR 
signal to a host unit. The host transmits printing data to the printer 
after a time t.sub.2 elapses from when the host unit receives the VR 
signal. After the printer receives the printing instruction 1, a 
temperature at which the fixing unit should be controlled in the printing 
operation is set as a target temperature for the fixing unit to provide 
for the printing operation. When supply of the printing data to the 
printer is suspended, the printer waits for the next printing instruction 
for the predetermined time t.sub.4 under a condition in which the fixing 
unit is controlled at the temperature to be set in the printing operation. 
When the printer receives the next printing instruction 2, the printer 
transmits the VR signal to the host unit. When the host unit receives the 
VR signal, the host unit starts to transmit printing data to the printer 
from a time t.sub.2. In the conventional printer, as has described above, 
when supply of the printing data to the printer is suspended, the fixing 
unit is controlled at the temperature to be set in the printing operation 
for the time t.sub.4. In a case where there is no printing instruction for 
the predetermined time t.sub.4, the photosensitive drum, the optical 
system and the fixing unit are controlled in the standby mode. 
In the conventional control system, even if the printing operation is 
terminated, the fixing unit is controlled at the temperature to be set in 
the printing operation. As a result, the pressure roller is excessively 
heated. 
Further, after the power supply of the printer is turned on, a regular 
warm-up operation is always performed without regard to whether or not the 
fixing unit has been warmed up. In a case where the fixing unit is cold 
(the cold start operation), the fixing unit is not easily warmed up to the 
predetermined temperature. After the power supply is reset for some 
reason, after the open-and-close operations of the covers of the printer 
with the power supply in the on state are performed to remove jammed 
papers and/or to change the consumables and after the printer receives the 
reset signal from the host unit, such as a computer unit, the 
predetermined warm-up operation of the fixing unit is executed. In these 
cases, the fixing unit is excessively warmed up. 
The overshoot of the temperature of the heat roller occurs as follows (the 
above item (b)). 
On the assumption that the printing operation is continuously repeated (a 
continuous printing condition), a target temperature is set for the fixing 
unit so that the temperature of the fixing unit is maintained at the 
predetermined temperature to be controlled in the printing operation. In 
addition, when the printing instruction is received by the printer, the 
target temperature is changed to the predetermined temperature. Thus, as 
shown in FIGS. 5, 6 and 7, when a recording sheet enters the fixing unit 
after the printing operation starts, the overshoot of the temperature of 
the heat roller occurs, so that the controlled temperature of the heat 
roller exceeds the target temperature set in the continuous printing 
condition. Thus, developer (toner) on the first recording sheet is fixed 
at the overshot temperature by the fixing unit. In a case where the 
printing operation is continuously repeated, the intervals at which 
recording sheets are supplied to the fixing unit is short, so that 
developer (toner) on each recording sheet is not excessively fixed 
(fused). 
In a case where the printing operation is intermittently performed, the 
temperature of the heat roller is overshot at each print starting time. 
Examples of the overshoot of the temperature of the heat rollers is shown 
in FIGS. 5, 6 and 7. 
Further, in the case where the printing operation is intermittently 
performed, the intervals at which a recording sheet passes through the 
fixing unit is long. As a result, due to the overshot temperature of the 
heat roller, the pressure roller is excessively heated. Thus, in the case 
where the printing operation is intermittently performed, the developer on 
the recording sheet is excessively fixed (fused), so that a large positive 
curl of the recording sheet occurs. 
The above phenomena are summarized in a table shown in FIG. 29. 
The reverse curl of the recording sheet occurs as follows. 
Immediately after the power supply of the printer is turned on, the initial 
operations for the fixing unit are performed so that the heat roller and 
the pressure roller are warmed up. 
After a predetermined waiting time elapses, the fixing unit is controlled 
in the standby mode to provide for start of the printing mode. 
Referring to the temperature transition of the pressure roller as shown in 
FIG. 3, in the initial operations, the pressure roller is not almost 
heated. After the printing operation starts, although the temperature of 
the pressure roller is increased by 30.degree. C., the pressure roller is 
not sufficiently warmed up. At this time, the developer on the recording 
sheet fixed mainly due to the temperature of the heat roller. 
In general, after recording sheets are set in a sheet cassette, the 
recording sheets may be left alone in the sheet cassette for few hours or 
more. In this condition, the recording sheets may absorb moisture. In the 
printing operation using a recording sheet which absorbs moisture 
immediately after the power supply is turned on, the recording sheet is 
curled such that a side opposite to a printed side is inside. That is, the 
reverse curl of the recording sheet occurs. Due to the reverse curl of the 
recording sheet, the print quality deteriorates. 
Handling of one-sided prints which are reverse curled is troublesome. In a 
case where two-sided prints are formed, when printing is performed on the 
second side of the recording sheet which has been reverse curled in 
printing on the first side, it is difficult to separate the recording 
sheet from the photosensitive drum. As a result, the recording sheet may 
be jammed. 
As the result of investigation, it was ascertained that when a recording 
sheet with moisture absorption passes through the pressure roller having a 
low temperature, a large amount of reverse curl of the recording sheet 
occurred. That is, in a case where although the pressure roller is 
controlled at a temperature by which the predetermined fixing factor can 
be obtained, the controlled temperature is lower than the predetermined 
temperature, the reverse curl of the recording sheet occurs. 
Thus, to avoid the reverse curl, the pressure roller must be warmed-up to a 
temperature higher than the temperature by which the predetermined fixing 
factor can be obtained. Although the pressure roller is freely heated by 
the heat roller as has been described above, immediately after the power 
supply is turned on, the temperature of the pressure roller does not reach 
a temperature sufficient to prevent the reverse curl of the recording 
sheet. 
The above phenomena is summarized on a table shown in FIG. 30. 
The above disadvantages (c) and (d) are based on the following causes. 
The initial operations is performed to warm up the fixing unit which is 
cold. In the case where the open/close operations of the covers of the 
printer are performed and in the case where the reset signal is received 
from the host unit, such as the computer unit, the initial operations are 
performed. In these cases, since the fixing unit is warm, a time for the 
warm-up operation is too long. 
In the above cases, the target temperature of the heat roller in the 
warm-up operation is too high. That is, the fixing unit is heated at the 
high temperature for the long time. As a result, the temperature on the 
inside of the printer is increased. Further, the overshoot of the 
temperature of the heat roller causes the temperature of the inside of the 
printer to increase. Thus, thermal deterioration of parts of the printer 
and thermal stress in toner stored in a process cartridge may occur. In 
addition, the toner is fused and fixed on parts of the printer, so that 
various operation troubles may occur. To avoid operation trouble, a 
cooling fan may be provided in the printer. However, in this case, the 
production cost is increased. 
In addition, after the recording sheet completely pass though the fixing 
unit, the heater roller controlled at the predetermined temperature is 
rotated for a predetermined time. This control of the fixing unit is one 
of causes of increasing of the temperature in the printer. 
The above disadvantages (e) and (f) are based on the following causes. 
In a printer in which the heat roller of the fixing unit is interlocked 
with the photosensitive roller, the initial operations which are continued 
for a long time causes the photosensitive drum to deteriorate. 
Further, as shown as a conventional case in FIG. 33A, every time the 
printing operation starts, the heat roller is rotated to warm up the 
pressure roller. In a case where the pressure roller has been warmed up, 
it is not necessary to perform this warm-up operation. In this case, the 
first-print-time is increased. 
In addition, every time the print operation starts, the photosensitive drum 
interlocked with heat roller is rotated for a purpose other than the 
printing purpose. Thus, the lifetime of the photosensitive drum is 
decreased. 
The following disadvantages occur in the second example of the conventional 
image forming apparatus. 
In a page printer, the dissipation power in a case where the heater is in 
the off state is 1/20 to 1/10 as large as the dissipation power in a case 
where the heater is in the on state. That is, the dissipation power of the 
printer almost depends on the operation of the heater. In such a case, the 
on and off operations of the heater cause rapid variation of a current 
through power lines in the page printer. Due to the rapid variation of the 
current, a voltage of the power supply is varied. There may be a case 
where the standard regarding the voltage variation of the power supply is 
not satisfied. 
Printing operation in the page printer is accelerated year by year. Thus, 
it is required that the heater is rapidly controlled to a target 
temperature. The current which flows in an on-operation and an 
off-operation of the power supply tends to be increased. It is further 
difficult, due to this tendency, to satisfy the standard regarding the 
voltage variation. 
The standard regarding the voltage variation specifies allowable voltage 
variation of the power supply line. When the voltage of the power supply 
line is varied, lighting fixtures connected to the power supply line may 
be flickered and characteristics of the other electronic units connected 
to the power supply line may be affected thereby. 
As to the voltage variation of the power supply line, the second example of 
the conventional image forming apparatus has the following disadvantages. 
In the on-operation of the heater and the off-operation thereof, the 
dissipation power is greatly varied so that the current flowing through an 
AC cable is greatly varied. That is, the output voltage of the AC power 
line is varied so that there is a case where the standard regarding the 
voltage variation is not satisfied. 
To satisfy the standard regarding the voltage variation, the difference 
between currents in periods N and F shown in FIG. 9 should be minimized. 
As shown in FIGS. 8A and 8B, the power line 6 for the heater driving 
circuit 82 is serially provided with the power thermistor 93. The power 
thermistor 93 has a characteristic by which the higher the temperature the 
lower the resistance. Due to this characteristic, when the heater 81 has a 
low temperature, a large current is not supplied from the heater driving 
circuit 82 to the heater 81. The heater 81 which is a resistor has a low 
resistance at a low temperature and a high resistance at a high 
temperature. Thus, at a low temperature, the power thermistor 93 having 
the high resistance limits the current supplied to the heater 81. At a 
high temperature, although the power thermistor 93 has a low resistance, 
the current flowing through the heater 81 is limited by the high 
resistance of the heater 81 itself. As a result, almost a constant current 
flows through the heater 81 without regard to the temperature of the 
heater 81. In addition, a large amount of current is not momentarily 
supplied to the heater 81. In the printing operation of the page printer, 
every time a recording sheet passes through the fixing unit, the recording 
sheet removes the heat from the heater 81. Although a central portion of 
the heater 81 is warm, the temperature detecting unit detects that the 
surface temperature of the heater 81 has been decreased. The power 
thermistor 93 is hardly cooled in contrast to the heater 81. Thus, when 
the heater 81 is turned on again after turning off, the current is 
supplied to the heater 81 under a condition in which the resistance of the 
power thermistor 93 is not sufficiently increased. However, since 
temperature detecting unit detects decreasing of the temperature of the 
heater 81 under the condition in which the central portion of the heater 
81, the current starts to flow through the heater 81 having a large 
resistance. As a result, a large amount of current does not flow through 
the heater 81. 
On the other hand, in a case where the page printer is in the idle state, 
when the heater 81 is turned off, the temperature of the central portion 
of the heater 81 and the temperature of the surface of the heater 81 are 
simultaneously decreased. Thus, when the heater is turned on again, an 
phenomenon in which the heater 81 is sufficiently cooled but the power 
thermistor 93 is not sufficiently cooled may occur. In this case, a large 
amount of current flows through the heater 81. 
In addition, in order that the standard regarding the voltage variation is 
satisfied, a current rapidly flowing through the heater 81 when heater 81 
is changed from the off state to the on state should be restrained. 
The period F, shown in FIG. 9, in the printing operation greatly differs 
from that in the idling state. Thus, there may be a case where the power 
thermistor 92 is sufficiently cooled in the period F and a case where the 
power thermistor 92 is not sufficiently cooled. That is, the standard 
regarding the voltage variation cannot be satisfied simultaneously both in 
the printing operation and in the idle state. 
If the heater 81 is repeatedly turned on and off at short intervals so that 
the current is intermittently supplied to the heater 81 in the period N 
shown in FIG. 9, the difference between the amounts of currents in the 
periods N and F shown in FIG. 9 may be decreased. In a case where the 
period N is merely shortened, a long time is needed to control the heater 
81 which has been cooled to a target temperature. As a result, the start 
of the printing operation is delayed. 
The environmental temperature and the operation sate of the printer may be 
varied. Thus, the heater 81 cannot be effectively controlled by a single 
manner. Due to the rapid temperature variation, variation of the thickness 
of the recording sheet which passes through the heater 81 and removes heat 
from the heater 81, variation of timing at which the recording sheet 
passes though the heater 81 and the like, a speed at which the heater 81 
is warmed up and a speed at which the heater 81 is cooled down may be 
varied. 
In a case where the heater 81 is warm up and cooled down at a constant 
speed, after the actual temperature of the heater 81 exceeds the maximum 
target temperature and is below the minimum target temperature, the heater 
81 may be turned off and turned on. In this case, the recording sheet is 
excessively heated and cooled. As a result, the recording sheet may be 
curled and the toner may not be completely fixed on the recording sheet. 
SUMMARY OF THE INVENTION 
Accordingly, a general object of the present invention is to provide novel 
and useful image forming apparatus and temperature control apparatus in 
which the disadvantages of the aforementioned prior art are eliminated. 
A first specific object of the present invention is to provide an image 
forming apparatus in which the positive curl of a recording sheet is 
small. 
A second specific object of the present invention is to provide an image 
forming apparatus in which the reverse curl of a recording sheet having 
moisture absorption is small, so that the recording sheet is prevented 
from being jammed. 
A third specific object of the present invention is to provide an image 
forming apparatus in which the warm-up time is short, so that the printing 
operation can rapidly start. 
A fourth specific object of the present invention is to provide an image 
forming apparatus in which the temperature therein is not excessively 
increased, so that the dissipation power can be saved. 
A fifth specific object of the present invention is to provide an image 
forming apparatus in which the lifetime of the photosensitive drum can be 
improved. 
A sixth specific object of the present invention is to provide an image 
forming apparatus in which a first-print-time is short, so that a first 
print can be rapidly formed. 
A seventh specific object of the present invention is to provide an image 
forming apparatus in which a voltage variation of the power supply line 
does not cause the lighting fixtures connected the power supply line to be 
flickered and does not affect other electric product connected to the 
power supply line, so that a standard regarding the voltage variation is 
satisfied. 
A eighth specific object of the present invention is to provide an image 
forming apparatus in which a standard regarding the voltage variation is 
satisfied both in the printing operation and in the idle state. 
A ninth specific object of the present invention is to provide an image 
forming apparatus in which a standard regarding the voltage variation is 
satisfied without affecting initial operations. 
A tenth specific object of the present invention is to provide an image 
forming apparatus in which a fixing unit is not excessively warmed up and 
cooled down so that the recording sheet is prevented from being curled and 
the toner is can be completely fixed on the recording sheet. 
The above objects of the present invention are achieved by the following 
image forming apparatuses in which: 
An image forming apparatus comprising a fixing unit having a heat source, 
temperature detecting means for detecting a temperature of said fixing 
unit, initial operation necessity detecting means for determining whether 
an initial operation should be performed in said image forming apparatus, 
initial operation selecting control means for selecting an initial 
operation from among a plurality of predetermined initial operations based 
on the temperature detected by said temperature detecting means when said 
initial operation necessity detecting means detects that the initial 
operation should be performed; and control means for controlling said 
fixing unit so that said fixing unit performs the initial operation 
selected by said initial operation selecting control means; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said heat roller, temperature detecting means for detecting 
a surface temperature of said heat roller, initial operation necessity 
detecting means for determining whether an initial operation should be 
performed, printing instruction detecting means for detecting a printing 
instruction supplied from an external unit, means for setting, as a target 
temperature, a temperature (A) higher than a temperature which should be 
set in a continuous print operation and outputting an instruction for 
activating said heat source immediately after the initial operation starts 
and rotating said heat roller to warm up said pressure roller after the 
detected temperature reaches a predetermined temperature (C), in a case 
where a temperature detected by said temperature detecting means when said 
initial operation necessity detecting means determines that the initial 
operation should be performed is lower than a first temperature, means for 
setting a target temperature corresponding to an idle state and outputting 
an instruction for driving said fixing unit after the initial operation is 
completed, in a case where said printing instruction detecting means has 
not yet detected the printing instruction before the initial operation is 
completed, means for outputting an instruction for a print operation after 
the initial operation is completed, in a case where said printing 
instruction detecting means has detected the printing instruction before 
the initial operation is completed, temperature control means for 
controlling fixing unit so that the detected temperature agrees with the 
target temperature, and driving means for driving said fixing unit in 
accordance with the instruction; 
An image forming apparatus comprising, a fixing unit having a heat roller 
in which a heat source is mounted and a pressure roller pressing a 
recording sheet against said heat roller, temperature detecting means for 
detecting a surface temperature of said heat roller, initial operation 
necessity detecting means for determining whether an initial operation 
should be performed, printing instruction detecting means for detecting a 
printing instruction supplied from an external unit, means for setting, as 
a target temperature, a temperature (B) lower than a temperature (A) 
higher than a temperature which should be set in a continuous print 
operation and outputting an instruction for activating said heat source 
immediately after the initial operation starts and rotating said heat 
roller to warm up said pressure roller, in a case where a temperature 
which is detected by said temperature detecting means when said initial 
operation necessity detecting means determines that the initial operation 
should be performed falls within a range between a first temperature and a 
second temperature, means for setting a target temperature corresponding 
to an idle state and outputting an instruction for driving said fixing 
unit after the initial operation is completed, in a case where said 
printing instruction detecting means has not yet detected a printing 
instruction before the initial operation is completed, means for 
outputting an instruction for the print operation in a case where said 
printing instruction detecting means have detected before the initial 
operation is completed, temperature control means for controlling fixing 
unit so that the detected temperature agrees with the target temperature, 
and driving means for driving said fixing unit in accordance with the 
instruction; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said heat roller, temperature detecting means for detecting 
a surface temperature of said heat roller, initial operation necessity 
detecting means for determining whether an initial operation should be 
performed, printing instruction detecting means for detecting a printing 
instruction supplied from an external unit, means for setting, as a target 
temperature, a stand-by temperature which should be set when supply of the 
printing instruction pauses and said image forming apparatus is on 
stand-by and outputting an instruction for activating said heat source 
immediately after the initial operation starts and rotating said heat 
roller to warm up said pressure roller, in a case where a temperature 
which is detected by said temperature detecting means when said initial 
operation necessity detecting means detects that the initial operation 
should be performed is higher than a second temperature, means for setting 
a target temperature corresponding to an idle state and outputting an 
instruction for driving said fixing unit, in a case where said printing 
instruction detecting means has not yet detected a printing instruction 
before the initial operation is completed, means for outputting an 
instruction for a print operation in a case where said printing 
instruction detecting means have detected a printing instruction before 
the initial operation is completed, temperature control means for 
controlling fixing unit so that the detected temperature agrees with the 
target temperature, and driving means for driving said fixing unit in 
accordance with the instruction; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said heat roller, temperature detecting means for detecting 
a surface temperature of said heat roller, temperature control means for 
controlling fixing unit so that a temperature detected by said temperature 
detecting means agrees with a target temperature, printing instruction 
detecting means for detecting a printing instruction supplied from an 
external unit, sheet passing detecting means for detecting whether a 
recording sheet passes through said fixing unit, and idle state setting 
means for informing said temperature control means that said fixing unit 
should be controlled in an idle state, when said sheet passing detecting 
means detects that the recording sheet passes through said fixing unit, in 
a case where said print instruction detecting means have not yet a next 
printing instruction during a print operation; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said pressure roller, temperature detecting means for 
detecting a surface temperature of said heat roller, print start detecting 
means for detecting a print start time at which a print operation based on 
printing instruction received in an idle state after an initial operation 
is completed or after a previous print operation is completed should 
start, target temperature setting means for setting, as a target 
temperature, a temperature lower than a temperature which should be set in 
a continuous print operation when said print start detecting means detects 
the print start time, and temperature control means for controlling the 
fixing unit so that a temperature detected by said temperature detecting 
means agrees with the target temperature set by said target temperature 
setting means; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said heat roller, temperature detecting means for detecting 
a surface temperature of said heat roller, print start detecting means for 
detecting a print start time at which a print operation based on printing 
instruction received in an idle state after an initial operation is 
completed or after a previous print operation is completed should start, 
target temperature setting means for setting a target temperature so that 
a overshot temperature of the fixing unit is equal to a temperature which 
should be set in the continuous print operation, and temperature control 
means for controlling the fixing unit so that a temperature detected by 
said temperature detecting means agrees with the target temperature set by 
said target temperature setting means; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said heat roller temperature detecting means for detecting a 
surface temperature of said heat roller, print start detecting means for 
detecting a print start time at which a print operation based on printing 
instruction received in an idle state after an initial operation is 
completed or after a previous print operation is completed should start, 
printing instruction detecting means for detecting a print instruction 
supplied from an external unit, target temperature setting means for 
setting, as a target temperature, a temperature lower than a temperature 
which should be set in a continuous print operation when said print start 
detecting means detects the print start time, and immediately setting, as 
the target temperature, the temperature which should be set in the 
continuous print operation when said printing instruction detecting means 
detects the next printing instruction during a print operation, and 
temperature control means for controlling the fixing unit so that a 
temperature detected by said temperature detecting means agrees with the 
target temperature set by said target temperature setting means; 
An image forming apparatus comprising a fixing unit having a heat roller in 
which a heat source is mounted and a pressure roller pressing a recording 
sheet against said heat roller, temperature detecting means for detecting 
a surface temperature of said heat roller, initial operation necessity 
detecting means for determining whether an initial operation should be 
performed, printing instruction detecting means for detecting a printing 
instruction from an external unit, target temperature setting means for 
setting, as a target temperature, a temperature (A) higher than a 
temperature which should be set in a continuous print operation when said 
printing instruction detecting means detects the next printing instruction 
during a print operation based on an printing instruction detected by said 
printing instruction detecting means before the initial operation is 
completed, in a case where a temperature which is detected by said 
temperature detecting means when said initial operation necessity 
detecting means detects that the initial operation should be performed is 
less than a first temperature, and temperature control means for 
controlling the fixing unit so that a temperature detected by said 
temperature detecting means agrees with the target temperature set by said 
target temperature setting means; and 
An image forming apparatus comprising a fixing unit having a heat source, 
temperature detecting means for detecting a temperature of said fixing 
unit, power supply control means for supplying a voltage to said heat 
source of said fixing unit in an on-period and for shutting off the 
voltage to said heat source in an off-period alternately so that the 
temperature detected by said temperature detecting means agrees with a 
target temperature, and on-and-off control means for repeatedly turning on 
and off the voltage to said heat source at a cycle less than a minimum 
cycle of a range which is not perceived by people, in the on-period. 
Some of the above objects of the present invention are achieved by the 
following temperature control apparatuses in which: 
A temperature control apparatus for controlling a temperature of a body 
which is heated by a heater connected to an AC power supply based on a 
detected temperature of said body so that the temperature of said body 
falls in a predetermined temperature range, said temperature control 
apparatus comprising determination means for determining, based on the 
detected temperature of said body, whether the temperature of said body 
should be increased or decreased, temperature increasing control means for 
applying an AC voltage from said AC power supply to said heater so that 
the temperature of said body is increased when said determination means 
determines that the temperature of said body should be increased, and 
temperature decreasing control means for applying to said heater a 
pulse-shaped AC voltage which is repeatedly turned on and off at a 
frequency so that the temperature of said body is decreased when said 
determination means determined that the temperature of said body should be 
decreased, the frequency being generally not perceived by people; 
A temperature control apparatus for controlling a temperature of a body 
which is heated by a heater connected to an AC power supply based on a 
detected temperature of said body so that the temperature of said body 
falls in a predetermined temperature range, said temperature control 
apparatus comprising a plurality of heater elements into which said heater 
is divided, determination means for determining, based on the detected 
temperature of said body, whether the temperature of said body should be 
increased or decreased, and connection switching means for connecting said 
plurality of heater elements so that said heater radiates an amount of 
heat which makes the temperature of said body be increased when said 
determination means determines that the temperature of said body should be 
increased and for connecting said plurality of heater elements so that 
said heater radiates an amount of heat which makes the temperature of said 
body be decreased when said determination means determines that the 
temperature of said body should be decreased; and 
A temperature control apparatus for controlling a temperature of a body 
which is heated by a heater connected to an AC power supply based on a 
detected temperature of said body so that the temperature of said body 
falls in a predetermined temperature range, said temperature control 
apparatus comprising a power dissipation element connected to said AC 
power supply so as to be connected to said heater in parallel, current 
by-pass means for supplying the AC current to said heater by-passing said 
power dissipation element when a predetermined control signal is supplied 
to said current by-pass means, determination means for determining, based 
on the detected temperature of said body, whether the temperature of said 
body should be increased or decreased, temperature increasing control 
means for supplying the predetermined control signal to said current 
by-pass means when said determination means determines that the 
temperature of said body should be increased, and temperature decreasing 
control means for supplying the AC current to said heater via said power 
dissipation element when said determination means determines that the 
temperature of said body should be decreased.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description will now be given of aspects of the present invention. 
FIG. 11 shows a first aspect of the present invention. Referring to FIG. 
11, an image forming apparatus includes a fixing unit 10 having a heat 
source, a temperature detecting unit 5, an initial operation necessity 
detecting unit 17, an initial operation selecting control unit 18 and a 
printing operation control unit 16. The temperature detecting unit 5 
detects the temperature of the fixing unit. The initial operation 
necessity detecting unit 17 determines whether or not initial operations 
should be performed. For example, after power supply of the image forming 
apparatus is turned on, after the open-and-close operations of covers of 
the image forming apparatus with the power supply in the on state are 
performed to remove jammed sheet and/or to change consumables and after a 
reset signal is received from a host unit, such as computer unit, for 
supplying printing data, the initial operations should be performed. When 
the initial operation necessity detecting unit 17 determines that the 
initial operations should be performed, the initial operation selecting 
control unit 18 selects, based on a detected temperature from the 
temperature detecting unit 5, an initial operations from among a plurality 
of initial operations and outputs an instruction for executing the 
selected initial operation. The printing operation control unit 16 
controls the fixing unit 10 and other units (not shown) in accordance with 
the instruction from the initial operation selecting control unit 18 so 
that the selected initial operation is performed. 
The initial operations is an operation for warming up the fixing unit 10 to 
provide for thermal fixing of developer (toner) in a printing operation. 
For example, a heat roller in the fixing unit 10 is rotated with a heater 
in an active state, as the initial operation. 
The fixing unit 10 has, for example, a heat roller including a heat source 
and a pressure roller pressing a recording sheet against the heater 
roller. 
The image forming apparatus according the first aspect of the present 
invention is operated in accordance with a procedure shown in FIG. 17. 
Referring to FIG. 17, in step S1, the initial operation necessity detecting 
unit determines that the initial operation should be performed. In step 
S2, then, the temperature detecting unit 5 detects the temperature of the 
fixing unit 10. After this, the initial operation selecting control unit 
18 selects, based on the detected temperature, an initial operation from 
among a plurality of initial operations, and outputs an instruction for 
executing the selected initial operation. In step S3, the printing 
operation control unit 16 controls the fixing unit 10 in accordance with 
the instruction from the initial operation selecting control unit 18. 
The initial operation selecting control unit 18 may select, for example, a 
target temperature for the fixing unit 10 among from a plurality of target 
temperature, as an selecting operation for the initial operation. In this 
case, if the temperature of the fixing unit 10 is low when the initial 
operation should be performed, the initial operation selecting control 
unit 16 select a high target temperature. On the other hand, if the 
temperature of the fixing unit 10 is high when the initial operation 
should be performed, a low target temperature is selected. As a result, a 
time for a warm-up operation of the fixing unit 10 can be decreased and 
the fixing unit 10 can be effectively warmed up. 
In addition, since the warm-up time is decreased, in the apparatus having a 
photosensitive drum interlocked with the fixing unit 10, a time for which 
the photosensitive drum is rotated in the initial operation is decreased. 
As a result, the photosensitive drum is not wastefully rotated in the 
initial operation. Thus, the lifetime of the photosensitive drum can be 
improved. 
FIG. 12 shows a second aspect of the present invention. In FIG. 12, those 
parts which are the same as those shown in FIG. 11 are given the same 
reference numbers. 
Referring to FIG. 12, the initial operation selecting unit 18 has a target 
temperature setting unit 20, and the printing operation control unit 16 
has a temperature control unit 19. When the initial operation necessity 
detecting unit 17 determines that the initial operation should be 
performed, the target temperature setting unit 20 sets a target 
temperature at which the fixing unit 10 should be controlled, based on the 
detected temperature from the temperature detecting unit 5. The 
temperature control unit 20 controls the temperature of the fixing unit 10 
so that the detected temperature reaches the target temperature set by the 
target temperature setting unit 20. 
When the detected temperature is low, the reverse curl of the recording 
sheet may occur. In this case, the target temperature set by the target 
temperature setting unit 20 is higher than a temperature which should be 
normally set in a continuous printing operation. As a result, the actual 
temperature rapidly reaches the target temperature which should be 
normally set in the continuous printing operation. On the other hand, when 
the detected temperature is high, the positive curl of the recording sheet 
may occur. In this case, the target temperature is lower than the 
temperature which should be normally set in the continuous printing 
operation. As a result, the temperature of the fixing unit 10 can be 
prevented from being excessively increased by the initial operation. 
Thus, the pressure roller can be effectively warmed up by the initial 
operation. It is not necessary to rotate the heat roller to warm up the 
pressure roller every time the printing operation starts. As a result, a 
first-print-time which is a time required for forming of a first print can 
be shortened. 
FIG. 13 shows a third aspect of the present invention. Referring to FIG. 
13, the image forming apparatus includes a fixing unit 11, a temperature 
detecting unit 15, the initial operation necessity detecting unit 17, a 
print instruction detecting unit 22, a target temperature/driving 
instruction unit 23 and the printing operation control unit 16. The 
printing operation control unit 16 has a temperature control unit 21 and a 
driving control unit 24. The fixing unit 11 has a heat roller 12 including 
a heat source 13 and a pressure roller 14 for pressing a recording sheet 
against the heat roller 12. The temperature detecting unit 15 detects a 
surface temperature of the heat roller 12. The print instruction detecting 
unit 22 detects a print instruction from an external unit, such as a 
computer unit. When the initial operation necessity detecting unit 17 
determines that the initial operation should be performed and when the 
detected temperature is lower than a predetermined temperature 1, the 
target temperature/driving instruction unit 23 sets, as a target 
temperature, a temperature A higher than a temperature which should be set 
in the continuous printing operation. The target temperature/driving 
instruction unit 23 outputs instructions to activate the heat source 13 
immediately after the initial operation starts and to rotate the heat 
roller 12 so that the pressure roller is warmed up after the detected 
temperature reaches a temperature C. In a case where a printing 
instruction is not received before the initial operation is completed, the 
target temperature/driving instruction unit 23 outputs a target 
temperature at which the fixing unit 11 should be controlled in an idle 
state after the initial operation is completed and outputs an instruction 
to drive the heat roller 12 of the fixing unit 11. Further, in a case 
where a printing instruction is received before the initial operation is 
completed, the target temperature/driving instruction unit 23 outputs an 
instruction to start a printing operation after the initial operation is 
completed. The temperature control unit 21 controls the heat source 13 of 
the fixing unit 11 so that the detected temperature is maintained at the 
target temperature. The driving control unit 24 performs driving control 
for the fixing unit 11. 
The temperature C is less than the temperature A. The predetermined 
temperature 1 and the temperatures A and C depend on characteristics and 
performance of the fixing unit 11. These temperatures are experimentally 
decided. 
The image forming apparatus according the third aspect of the present 
invention is operated in accordance with a procedure shown in FIG. 18. 
Referring to FIG. 18, in step S11, the initial operation necessity 
detecting unit 17 determines that the initial operation should be 
performed. In step S12, then, the temperature detecting unit 15 detects 
the surface temperature of the heat roller 12 of the fixing unit 11. The 
target temperature/driving instruction unit 23 carries out the following 
steps based on the detected temperature from the temperature detecting 
unit 15. 
In step S13, the target temperature/driving instruction unit 23 determines 
whether the detected temperature is less than the temperature 1. The 
temperature 1 is near a room temperature which is sufficiently less than a 
temperature at which the fixing unit 10 should be controlled in a standby 
mode. The temperature 1 is decided based on performance of the image 
forming apparatus, and is set, for example, at 40.degree. C. 
When the detected temperature is less than the temperature 1, the process 
proceeds to step S14. In step S14, the target temperature/driving 
instruction unit 23 supplies to the driving control unit an instruction to 
carry out the initial operation. At this time, the temperature A which is 
higher than the temperature which should be set in the continuous printing 
operation is set as a target temperature. 
The target temperature/driving instruction unit 23 further supplies to the 
driving control unit 24 instructions to activate the heat source 13 and to 
rotate the heat roller 12 of the fixing unit 11 after the detected 
temperature reaches the temperature C so that the pressure roller 14 is 
warmed up. 
Since the detected temperature is less than the temperature 1, the heat 
roller 12 is rotated after the detected temperature reaches the 
temperature C. As a result, the pressure roller 14 can be efficiently 
warmed up. The temperature is less than the temperature A, and is set, for 
example, at 120.degree. C. 
In step S15, it is determined that the surface temperature (the detected 
temperature) reaches the temperature 1, the process proceeds to step S16 
and the initial operation is then completed. As a result, the image 
forming apparatus is brought into a state where the printing operation can 
be executed. 
In a case where, in step S17, the printing instruction detecting unit 22 
detects an printing instruction supplied from the host unit before the 
initial operation is completed, an instruction to start the printing 
operation is supplied from the target temperature/driving unit 23 to the 
driving control unit 24, in step S19. On the other hand, in a case where, 
in step S17, the printing instruction detecting unit 22 does not detect an 
printing instruction before the initial operation is completed, an 
instruction to control the fixing unit 11 in the idle state, in step S18. 
According to the third aspect of the present invention, the temperature at 
which the pressure roller is controlled is not too low, so that the 
reverse curl of the recording sheet can be prevented. 
When the printing instruction detecting unit 22 does not detect the 
printing instruction before the initial operation is completed, the target 
temperature/driving unit 23 sets a target temperature corresponding to the 
idling state and outputs a driving instruction. When a first printing 
instruction is received, the target temperature/driving unit 23 sets the 
temperature A as the target temperature and outputs an instruction for the 
printing operation. 
When the initial operation necessity detecting unit 17 determines that the 
initial operation should be performed and when the detected temperature is 
lower than a predetermined temperature 2 which is higher than the 
predetermined temperature 1, the target temperature/driving instruction 
unit 23 sets, as the target temperature, a temperature B lower than the 
above temperature A. The target temperature/driving instruction unit 23 
outputs instructions to activate the heat source 13 immediately after the 
initial operation starts and to rotate the heat roller 12 so that the 
pressure roller is warmed up. In a case where a printing instruction is 
received before the initial operation is completed, the target 
temperature/driving instruction unit 23 outputs an instruction to start a 
printing operation after the initial operation is completed. 
The detailed operation in this case shown in FIG. 19. 
Referring to FIG. 19, when the initial operation necessity detecting unit 
17 determines, in step S31, that the initial operation should be 
performed, the target temperature/driving instruction unit 23 operates as 
follows based on the surface temperature of the heat roller 12 of the 
fixing unit 11 which is detected by the temperature detecting unit 15. 
The target temperature/driving instruction unit 23 determines, in step S33, 
whether or not the detected temperature falls within a range between the 
predetermined temperatures 1 and 2. 
The predetermined temperature 2 is sufficiently higher than a room 
temperature, but less than the standby temperature. The predetermined 
temperature 2 depends on characteristics of the image forming apparatus, 
and is set, for example, at 100.degree. C. 
When it is determined that the detected temperature falls within the range, 
the target temperature/driving instruction unit 23 sets the temperature B 
as the target temperature after the initial operation starts, in step S34. 
The temperature B is a temperature at which the heat roller 12 should be 
controlled in the continuous printing operation. Since the fixing unit 11 
has been sufficiently warmed up, the heat roller 12 starts to be rotated 
immediately after the heat source 13 is activated. 
When the detected temperature reaches the predetermined temperature (in 
step S35), it is determined, in step S36, the initial operation is 
completed and the image forming apparatus is in a state where the printing 
operation can be performed. 
In step S37, when a print instruction is detected by the print instruction 
detecting unit 22 before the initial operation is completed, the process 
proceeds to step S39. In step S39, the printing operation is performed. On 
the other hand, when a print instruction is not detected, the image 
forming apparatus is controlled in the idle state. 
According to the above operations of the target temperature/driving unit 
23, the heat roller 12 is not excessively increased, so that the recording 
sheet is prevented from being curled (the positive curl) and the image 
forming apparatus is prevented from being wasteful with the dissipation 
power. 
Further, when the initial operation necessity detecting unit 17 determines 
that the initial operation should be performed and when the detected 
temperature is higher than the predetermined temperature 2, the target 
temperature/driving instruction unit 23 sets, as the target temperature, a 
standby temperature which should be controlled in the standby state. The 
target temperature/driving instruction unit 23 outputs instructions to 
activate the heat source 13 immediately after the initial operation starts 
and to rotate the heat roller 12 so the pressure roller is warmed up. In a 
case where a printing instruction is not received before the initial 
operation is completed, the target temperature/driving instruction unit 23 
sets a target temperature corresponding to the idle state and outputs a 
driving instruction. On the other hand, in a case where a printing 
instruction is received before the initial operation is completed, the 
target temperature/driving instruction unit 23 outputs an instruction to 
start a printing operation after the initial operation is completed. 
The detailed operation in this case shown in FIG. 20. 
Referring to FIG. 20, after it is determined, in step S41, that the initial 
operation should be performed and the temperature of the fixing unit 11 is 
detected in step S42, it is determined, in step S43, whether or not the 
detected temperature obtained in step S42 is higher than the predetermined 
temperature 2. 
When the detected temperature is higher than the predetermined temperature 
2, the process proceeds to step S44. In step S44, the target 
temperature/driving instruction unit 23 sets the standby temperature which 
should be set in the idle state and supplies to the driving control unit 
24 an instruction to rotate the heat roller 12 to warm up the pressure 
roller 14. When it is determined, in step S45, that the detected 
temperature reaches the predetermined temperature, the initial operation 
completed and the image forming apparatus is in a state when the printing 
operation can be performed. 
Due to the above process, the temperature of the pressure roller 14 is not 
excessively increased, so that the recording sheet is prevented from being 
curled (the positive curl) and the image forming apparatus can be 
prevented from being wasteful with the dissipation power. 
FIG. 14 shows a fourth aspects of the present invention. Referring to FIG. 
14, the image forming apparatus includes the fixing unit 11, the 
temperature control unit 21, the print instruction detecting unit 22, a 
sheet detecting unit 25 and an idle state setting unit 2. The fixing unit 
11, the temperature control unit 21 and the print instruction detecting 
unit 22 have the same structures as those shown in FIG. 13. The sheet 
detecting unit 25 detects whether a recording sheet passes through the 
fixing unit 11. The idle state setting unit 27 outputs to the temperature 
control unit 21 an instruction to set the idle state when the sheet 
detecting unit 25 detects that the recording sheet has passed through the 
fixing unit 11 in a case where the next printing instruction is not 
received. 
According to the fourth aspect of the present invention, in a case where 
the next print instruction is not received during the printing operation, 
when the recording sheet has passed through the fixing unit 11, the fixing 
unit 11 is set in the idle state. Thus, the pressure roller is not 
excessively heated, so that the recording sheet is prevented from being 
curled (the positive curl) and the image forming apparatus is prevented 
from being wasteful with the dissipation power. 
FIG. 15 shows a fifth aspect of the present invention. 
Referring to FIG. 15, the image forming apparatus includes the fixing unit 
11, the temperature detecting unit 15, a temperature control unit 28, a 
print start detecting unit 29 and a target temperature setting unit 30. 
The fixing unit 11 and the temperature detecting unit have the same 
structures as those shown in FIG. 13. When a print instruction is received 
in the idle state after the initial operation is completed or the previous 
print operation is completed, the print start detecting unit 29 determines 
that the print operation should start. When the printing operation starts, 
the target temperature setting unit 30 sets, as the target temperature, a 
temperature lower than a temperature which should be set in the continuous 
printing operation. The temperature control unit 28 controls the heat 
source 13 so that the detected temperature is maintained at the target 
temperature set by the target temperature setting unit 30. 
In the image forming apparatus according to the fifth aspect of the present 
invention, when the printing operation starts, the temperature lower than 
the temperature (the temperature B) which should be set in the continuous 
printing operation is set at the target temperature. Thus, even if the 
overshoot of the temperature of the heat roller 12 occurs, the maximum 
temperature in the overshoot is relatively low. Thus, the recording sheet 
is prevented from being curled (the positive curl). 
The target temperature setting unit 30 may set a temperature as the target 
temperature so that an overshot temperature is equal to a temperature to 
be controlled in the continuous printing operation. 
In this case, the fixing factor is prevented from deteriorating and being 
excess, so that the optimum fixing factor can be obtained. 
FIG. 16 shows a sixth aspect of the present invention. 
Referring to FIG. 16, the image forming apparatus includes the fixing unit 
11, the temperature detecting unit 15, the print instruction detecting 
unit 22, the print start detecting unit 29 and a target temperature 
setting unit 31. The fixing unit 11, the temperature detecting unit 15, 
the print instruction detecting unit 22 and the print start detecting unit 
29 have the same structures as those shown in FIGS. 13 and 15. When the 
printing operation starts, the target temperature setting unit 31 sets, as 
the target temperature, a temperature lower than a temperature which 
should be set in the continuous printing operation. When the next print 
instruction is received during the printing operation, the target 
temperature setting unit 31 immediately sets, as the target temperature, 
the temperature which should be set in the continuous printing operation. 
The temperature control unit 28 controls the heat source 13 so that the 
detected temperature is maintained at the target temperature set by the 
target temperature setting unit 31. 
In the image forming apparatus according to the sixth aspect of the present 
invention, when the printing operation starts, the temperature lower than 
the temperature which should be set in the continuous printing operation 
is set as the target temperature. In addition, when the next print 
instruction is received during the printing operation, the temperature 
which should be set in the continuous operation is immediately set as the 
target temperature. Thus, the maximum temperature in the overshoot which 
occurs at start of the printing operation is relatively low. As a result, 
the recording sheet is prevented from being curled (the positive curl) at 
the start of the printing operation. In addition, in the continuous 
printing operation, a relative high temperature is set as the target 
temperature. Thus, the reverse curl of the recording sheet and a fixing 
error can be prevented. 
Further, there is a case where the surface temperature of the heat roller 
12 (the detected temperature) is lower than predetermined temperature 1 
when it is determined that the initial operation should be performed. In 
this case, when the next print instruction is received during the printing 
operation based on the previous print instruction received before the 
initial operation is completed, the target temperature setting unit 31 
sets, as the target temperature, the temperature A higher than the 
temperature which should be set as the target temperature in the 
continuous printing operation. 
In this case, even if the temperature of the pressure roller 14 detected 
when it is determined that the initial operation should be performed is 
low, the pressure roller 14 can be sufficiently warmed up by setting a 
high temperature as the target temperature. Thus, the reverse curl of the 
recording sheet can be prevented. 
FIG. 21 shows a seventh aspect of the present invention. 
Referring to FIG. 21, the image forming apparatus includes the fixing unit 
10, the temperature detecting unit 5, a voltage supply control unit 35 and 
a first cycle control unit 36. The fixing unit 5 has a heat source. The 
temperature detecting unit 5 detects the temperature of the fixing unit 
10. The voltage supply control unit 35 controls, based on a temperature 
detected by the temperature detecting unit 5, whether or not the voltage 
is supplied to the heat source of the fixing unit 10 so that the detected 
temperature is maintained at a target temperature. While the voltage is 
being applied to the heat source in accordance with the control of the 
voltage supply control unit 35, the first cycle control unit 36 repeatedly 
turns on and off the voltage applied to the heat source at a predetermined 
cycle (referred to as a first on-and-off cycle). The first on-and-off 
cycle is less than the minimum cycle in a range which can be perceived by 
people. The minimum cycle in the range which can be perceived by people 
is, for example, 40 milliseconds (msec). When light is flickered at an 
on-and-off cycle of about 110 milliseconds (corresponding to about 8.8 
Hz), people generally have the most uncomfortable feeling. In accordance 
with increasing and decreasing of the on-and-off cycle from about 110 
milliseconds, a degree of the unconformable feeling is decreased. 
In the image forming apparatus according to the seventh aspect of the 
present invention, the voltage applied to the heat source of the fixing 
unit is repeatedly turned on and off at the first on-and-off cycle which 
is not perceived by people. Thus, even if lighting devices sharing the 
power supply with the heat source of the fixing unit are flickered at the 
first on-and-off cycle, the people do not feel uncomfortable. Further, 
characteristics of the electric devices sharing the power supply for the 
heat source of the fixing unit do not deteriorate. 
The voltage supplied by the voltage supply control unit 35 may be not only 
AC (alternating current) voltage but also DC (direct current) voltage. 
FIG. 22 shows an eighth aspect of the present invention. 
Referring to FIG. 22, the image forming apparatus includes the fixing unit 
10, the temperature detecting unit 5 and the voltage supply control unit 
35 in the same manner as that shown in FIG. 21. The image forming 
apparatus according to the eighth aspect of the invention further includes 
a second cycle control unit 37. In a period in which the voltage supply 
control unit 35 supplies a voltage to the heat source of the fixing unit 
10, the second cycle control unit 37 repeatedly turns on and off the 
voltage at a predetermined cycle (referred to as a second on-and-off 
cycle) so that the temperature of the fixing unit 10 reaches the target 
temperature. The second on-and-off cycle falls within a range to which 
people generally have slightly uncomfortable feeling, which range is 
included in the range which can be perceived by people. 
People generally can perceive flicker of a lighting device at cycle longer 
than 40 milliseconds. When the lighting device is flickered at cycle 
sufficiently longer than 110 milliseconds, people feels slightly 
uncomfortable. Thus, in the case where the voltage applied to the heat 
source of the fixing unit 10 is repeatedly turned on and off at the second 
on-and-off cycle, even if lighting devices sharing the power supply with 
the heat source of the fixing unit 10 are flickered at the second 
on-and-off cycle, the uncomfortable feeling of people is generally slight. 
In addition, characteristics of electric devices sharing the power supply 
with the heat source of the fixing unit does not deteriorate. 
FIG. 23 shows a ninth aspect of the present invention. 
Referring to FIG. 23, the image forming apparatus includes the fixing unit 
10, the temperature detecting unit 5 and the voltage supply control unit 
in the same manner as in the cases shown in FIGS. 21 and 22. The image 
forming apparatus according to the ninth aspect of the present invention 
further includes the first cycle control unit 36, the second cycle control 
unit 37, a selecting unit 38 and an operation state detecting unit 39. The 
operation state detecting unit 39 detects whether the image forming 
apparatus is in a state where the printing operation is performed 
(referred to as a printing state) or in the idle state. When the operation 
state detecting unit 39 detects that the image forming apparatus is in the 
printing state, the selecting unit 38 selects the first cycle control unit 
36. In this case, in a period in which the voltage supply control unit 35 
supplies the voltage to the heat source of the fixing unit 10, the first 
cycle control unit 36 repeatedly turns on and off the voltage applied to 
the heat source at the first on-and-off cycle. On the other hand, when the 
operation sate detecting unit 39 detects that the image forming apparatus 
is in the idle state, the selecting unit 38 selects the second cycle 
control unit 37. In this case, in a period in which the voltage supply 
control unit 35 supplies the voltage to the heat source of the fixing unit 
10, the second cycle control unit 37 repeatedly turns on and off the 
voltage applied to the heat source at the second on-and-off cycle. 
When the image forming apparatus is in the printing state, the voltage 
applied to the heat source of the fixing unit 10 is repeatedly turned on 
and off at the first on-and-off cycle less than the minimum cycle in the 
range which can be generally perceived by people. That is, in the printing 
state, the voltage applied to the heat source of the fixing unit 10 is 
repeatedly turned on and of at a relatively short cycle. 
When the image forming apparatus is in the idle state, the voltage applied 
to the heat source of the fixing unit 10 is repeatedly turned on and off 
at the second on-and-off cycle longer than the cycle for which people 
generally have the most uncomfortable feeling. That is, in the idle state, 
the voltage applied to the heat source of the fixing unit 10 is repeatedly 
turned on and off at a relatively long cycle. 
FIG. 24 shows a tenth aspect of the present invention. 
Referring to FIG. 24, the image forming apparatus includes the fixing unit 
10, the temperature detecting unit 5, the voltage supply control unit 35 
and the first cycle control unit 36 in the same manner as that shown in 
FIG. 21. The image forming apparatus according the tenth aspect of the 
present invention further includes the initial operation necessity 
detecting unit 17 and a switching unit 40. When the initial operation 
necessity detecting unit 17 determines that the initial operation should 
be performed, the switching unit 40 performs a switching operation so as 
to couple the voltage supply control unit 35 to the heat source of the 
fixing unit 10. In this case, the voltage supply control unit controls, 
based on the detected temperature, whether or not the voltage is applied 
to the heat source of the fixing unit 10. As a result, the voltage is 
applied to the heat source at intervals. In a period in which the voltage 
is applied to the heat source, the temperature of the fixing unit 10 can 
rapidly reach the target temperature. 
On the other hand, when the detected temperature reaches a predetermined 
temperature, the switching unit 40 performs the switching operation so as 
to couple the voltage supply control unit 35 to the first cycle control 
unit 36. In this case, in a period in which the voltage supply control 
unit 35 supplies the voltage to the heat source of the fixing unit 10, the 
voltage is repeatedly turned on and off at the first on-and-off cycle. 
FIG. 25 shows an eleventh aspect of the present invention. 
Referring to FIG. 25, the image forming apparatus includes the fixing unit 
10, the temperature detecting unit 5 and the voltage supply control unit 
35. The image forming apparatus according to the eleventh aspect of the 
present invention further includes an off-control unit 41. In a case where 
the temperature of the fixing unit 10 (the detected temperature) exceeds a 
predetermined temperature, in a period in which voltage supply control 
unit 35 should not supply the voltage to the heat source of the fixing 
unit 10, the voltage applied to the heat source is completely shut off 
after the off-control unit 41 repeatedly turns on and off once or a few 
times. 
According to the above control of the voltage supply to the heat source of 
the fixing unit 10, the temperature of the fixing unit is slowly decreased 
from the predetermined temperature. 
FIG. 26 shows a twelfth aspect of the present invention. 
Referring to FIG. 26, the image forming apparatus includes the fixing unit 
10, the temperature detecting unit 5 and the voltage supply control unit 
35. The image forming apparatus according to the twelfth aspect of the 
present invention further includes a temperature variation detecting unit 
42 and a voltage supply rate control unit 43. When the temperature 
variation detecting unit 42 determines that the detected temperature is 
increased at a speed which exceeds a predetermined speed, the voltage 
supply rate control unit 43 decreases a rate at which the voltage is 
applied to the heat source of the fixing unit. On the other hand, when the 
temperature variation detecting unit 42 determines that the detected 
temperature is increased at a speed which is less than a predetermined 
speed, the voltage supply rate control unit 43 increases the rate at which 
the voltage is applied to the heat source of the fixing unit 10. The rate 
at which the voltage is applied to the heat source is a rate of a time for 
which the voltage is actually applied to the heat source to one cycle at 
which the voltage applied to the heat source is turned on and off. 
According to the above control of the voltage supply to the heat source of 
the fixing unit 10, in a case where the detected temperature is slowly 
increased, the rate at which the voltage is applied to the heat source is 
increased. Thus, the recording sheet is prevented from being curled (the 
reverse curl) due to a temperature which is too low. In addition, 
developer (toner) is prevented from being incompletely fixed on the 
recording sheet. 
A description will now be given of embodiments of the present invention. 
FIG. 27 shows a printer according to an embodiment of the present 
invention. The printer prints images on a recording sheet in accordance 
with the electrophotographic process. 
Referring to FIG. 27, the printer has a sheet cassette 141, a picking 
roller 142, a registering roller 143, a transfer roller 44, a 
photosensitive drum 45, a fixing unit 46, an ejecting roller 49, a process 
cartridge 50, an optical system 51 and a power supply unit 52. Recording 
sheets are housed in the sheet cassette 141. The picking roller 142 picks 
out recording sheets from the sheet cassette 141 one by one. The 
registering roller 143 registers each recording sheet fed by the piking 
roller 142 and feeds it toward the process cartridge 50. Toner images are 
formed on and retained by the photosensitive drum 45. The transfer roller 
44 is used to transfer toner images formed on the photosensitive drum 45 
to a recording sheet. The fixing unit 46 has a heat roller 48 and a 
pressure roller 47 and thermally fixes the toner image on the recording 
sheet. The ejecting roller 49 ejects the recording sheet on which the 
toner image is fixed. The process cartridge 50 includes the photosensitive 
drum 45 and the transfer roller 46 and performs the electrophotographic 
process so that a toner image is formed on a recording sheet. The optical 
system 51 projects a light beam which is modulated in accordance with 
image data on the photosensitive drum 45 so that an electrostatic latent 
image is formed on the photosensitive drum 45 (an exposure process is 
executed). 
When a printing instruction is supplied to the printer, the optical system 
51, the process cartridge 50 and the fixing unit 46 are activated. When 
predetermined preparations are made for the print operation, a recording 
sheet is picked out from the sheet cassette 141 by the picking roller 141. 
The recording sheet picked out from the sheet cassette 141 is fed through 
a path 140. That is, the recording sheet is fed to a transfer position 
between the photosensitive drum 45 and the transfer roller 44 and a toner 
image is transferred from the photosensitive drum 45 to the recording 
sheet. The recording sheet is then fed to the ejecting roller 49 via the 
fixing unit and is ejected from a housing of the printer. 
FIG. 28 shows a structure of a printer engine 53. Referring to FIG. 28, the 
printer engine 53 has a mechanical system 54 and a mechanical system 
controller 55. The mechanical system 54 includes the fixing unit 46, a 
transfer system 73 for transferring a toner image from the photosensitive 
drum 45 to a recording sheet, the optical system 51 and a sheet feeding 
system 75 for feeding recording sheets. 
The mechanical system controller 55 includes various sensors 57, an optical 
system control unit 58 for controlling the optical unit 51, a motor 
control unit 59 for controlling motors so that the photosensitive drum 45 
and various rollers are rotated and a fixing temperature control unit 60 
for controlling the temperature of the fixing unit 46. 
The mechanical system controller 55 receives print control signals 
including a print instruction and video signals which are print data. The 
mechanical system controller 55 controls the mechanical system 54 based on 
the print control signals and the video signals so that preparations for 
the printing operation and the printing operation are repeatedly 
performed. 
FIG. 31 shows a functional structure of a control system of the printer 
according to a first embodiment of the present invention. 
Referring to FIG. 31, the fixing unit 46 has the heat roller 48 in which a 
halogen lamp 70 is mounted, the pressure roller 47 pressing a recording 
sheet against the heat roller 48 and an AC driver 62 for driving the 
halogen lamp 70. The fixing unit 46 is provided with a temperature sensor 
71 (e.g., a thermistor) for detecting the surface temperature of the heat 
roller 48. The control system includes a control unit 60 formed of a CPU 
and programs. The output signal of the temperature sensor 71 which is 
analog data is converted into digital data by an analog-to-digital 
converter 61. An operation/display unit 63 is used by a user to enter 
information and display information to be supplied to the user. An 
interface 64 controls connection of the control system to a host unit 65. 
The heat roller 48 is a cylinder made of aluminum. The pressure roller 47 
is made of rubber. 
The control unit 60 (the CPU and the programs) includes, as functional 
units, a print sequence control unit 65, a temperature comparing unit 68, 
an initial operation necessity detecting unit 69, a target 
temperature/driving instruction unit 66, a heater on/off control unit 67, 
a print start detecting unit 76, a printing instruction detecting unit 77, 
a sheet passing detecting unit 78 and an idle state setting unit 79. The 
print sequence control unit 65 controls printing operations. The 
temperature comparing unit 68 compares a temperature detected by the 
temperature sensor 71 with a target temperature. The initial operation 
necessity detecting unit 66 determines whether or not the initial 
operation should be performed. Immediately after the power supply unit 52 
is turned on, immediately after open/close operations of covers of the 
printer are performed to remove jammed papers and/or to change consumables 
under a condition in which the power supply 52 is in the on state and 
immediately after the printer receives a reset signal from the host unit 
56, the initial operations should be performed. The target 
temperature/driving instruction unit 66 selects a target temperature based 
on instructions and a comparison result obtained by the temperature 
comparing unit 68. The target temperature/driving instruction unit 66 
further outputs driving instructions. The heater on/off control unit 67 
turns on and off an AC voltage supplied to the AC driver 62. The AC driver 
62 drives the halogen lamp 70 which is a heat source of the heat roller 48 
based on the AC voltage controlled by the heater on/off control unit 67. 
When a print instruction is received in the idle state after the initial 
operations are completed or after the print operation is completed, the 
print operation starts. The print start detecting unit 76 determines that 
the print operation should start. The printing instruction detecting unit 
77 detects whether the printing instruction is received. The sheet passing 
detecting unit 78 detects whether a recording sheet has passed through the 
fixing unit 46. When the sheet passing detecting unit 78 detects that a 
recording sheet has passed through the fixing unit 46 in a case where the 
next printing instruction has not yet received during the printing 
operation, the idle setting unit 79 sets a target temperature in the idle 
state and supplies it to the target temperature/driving instruction unit 
66. 
A description will now be given, with reference to FIGS. 32 through 40, of 
temperature control of the fixing unit 46. 
FIG. 32 shows procedures of the temperature control. Referring to FIG. 32, 
the initial operation necessity detecting unit 69 detects, in step S50, 
that the open/close operation of the covers of the printer, detects, in 
step S51, that the power supply is turned on, or detects in step S52, that 
the reset signal is received from the host unit 56, such as a computer 
unit or a word-processing unit. After this, in step S53, the temperature 
comparing unit 68 compares the surface temperature of the heat roller 48 
detected by the temperature sensor 71 with a reference temperature of 
100.degree. C. and determines whether or not the detected temperature is 
equal to or less than the reference temperature (100.degree. C.). The 
reference temperature of 100.degree. C. corresponds to the predetermined 
temperature 2 described above. 
If the detected temperature is equal to or less than the reference 
temperature (100.degree. C.), the temperature comparing unit 68 compares 
the surface temperature of the heat roller 48 with a reference temperature 
of 40.degree. C., in step S54. The reference temperature of 40.degree. C. 
corresponds to the predetermined temperature 1 described above. When it is 
determined, in step S54, that the surface temperature of the heat roller 
48 is equal to or greater than the reference temperature of 48.degree. C., 
the process proceeds to step S55. 
In step S55, the target temperature/driving instruction unit 66 sets, as 
the target temperature, the temperature B (e.g., 170.degree. C.) which 
should be set in the continuous print operation. The halogen lamp 71 is 
then driven by the target temperature/driving instruction unit 66. 
In step S56, to warm up the pressure roller 47 made of rubber, the heat 
roller 48 is rotated. Due to the rotation of the heat roller 48, the 
pressure roller is rotated. 
After the heat roller 48 is rotated for 11 seconds (step S57), the heater 
on/of control unit 67 controls the AC driver 62 so that the AC voltage 
applied to the halogen lamp 70 is repeatedly turned on and off. As a 
result, the surface temperature of the heat roller 48 reaches the target 
temperature of 170.degree. C. 
When it is determined, in step S58, that the surface temperature of the 
heat roller 48 reaches the target temperature of 170.degree. C., the 
control system of the printer outputs a print ready signal in step S59. 
The temperature control manner in accordance with steps S55 through S59 
described above is shown in FIG. 33B (2). After a time of about 40 seconds 
elapses from when the power supply is turned on (a time at which the 
initial operation should be performed), the print ready signal is output 
from the control system. 
Returning to FIG. 32, when it is determined, in step S60, that the printing 
instruction and print data is received before the initial operations are 
completed, the process proceeds to step S61. On the other hand, it is 
determined, in step S60, that there is no printing instruction, the 
process proceeds to step S81. 
The printing instruction and the print data are received at times in the 
timing chart shown in FIG. 34. after a time t.sub.1 elapses from when the 
first print instruction is received, the control system of the printer 
returns the VR (Video Synchronized request) signal to the host unit 56 in 
response to the printing instruction. In addition, after a time t.sub.2 
elapses from when the VR signal is supplied to the host unit 56, the print 
data is output from the host unit 56. The print operation starts to be 
executed in synchronism with rising of the VR signal. 
Returning to FIG. 32, when the printing instruction detecting unit 
determines, in step S60, that the printing instruction is received, a 
state where the target temperature of 170.degree. C. is set is maintained, 
in step S61. The print sequence control unit 65 executes the print 
operation in step S62. 
When it is then determined, in step S63, that the next printing instruction 
is received during the print operation, the process returns to step S61. 
The printing operation is the executed in the state where the temperature 
(170.degree. C.) to be set in the continuous print operation is set as the 
target temperature. 
On the other hand, the temperature comparing unit 68 determines, in step 
S54, that the surface temperature of the heat roller 48 is equal to or 
less than the reference temperature of 40.degree. C. (the predetermined 
temperature 1), the target temperature/driving instruction unit 66 sets a 
temperature of 180.degree. C. as the target temperature, in step S64. The 
heater on/off control unit 67 then controls the AC driver 62 so that the 
halogen lamp 70 is turned on. 
The temperature of 180.degree. C. corresponds to the temperature A which is 
to be set in the continuous print operation and is higher than the 
temperature B, as described above. 
When the temperature comparing unit 68 determined, in step S65, that the 
temperature of the fixing unit 46 is equal to or higher than a reference 
temperature of 120.degree. C., the process proceeds to step S66. In step 
S66, the heat roller 48 of the fixing unit 46 is rotated. The reference 
temperature of 120.degree. C. corresponds to the temperature C described 
above. 
In this case, after the surface temperature of the heat roller 48 reaches 
120.degree. C., the heat roller 48 starts to be rotated. Thus, the 
pressure roller 47 can be effectively warmed up by the heat roller 48. 
If it is determined, in step S67, that the heat roller 48 is rotated for 80 
seconds, it is determined whether or not the surface temperature of the 
heat roller 48 has reached the target temperature of 180.degree. C. When 
the surface temperature of the heat roller 48 has reached the target 
temperature of 180.degree. C., the control system of the printer outputs 
the print ready signal, in step S69. 
The temperature control manner in accordance with steps S65 through S69 is 
shown in FIG. 33B (1). That is, the procedure from turning on the power 
supply to outputting of the print ready signal takes about 120 seconds. 
Returning to FIG. 32, when it is determined, in step S70, that the printing 
instruction and the print data are received from the host unit 56 before 
the initial operation is completed, the process proceeds to step S71. In 
step S71, the temperature 180.degree. C. is set as the target temperature 
is maintained, and the print operation is performed in step S72. 
On the other hand, when it is determined, in step S70, that the printing 
instruction is not received before the initial operation is completed, the 
process proceeds to steps 70-2 and 74. As a result, after the initial 
operation is completed, a temperature of 140.degree. C. is set as the 
target temperature so that the printer is in the idle state. 
After this, if the printing instruction and the print data are received in 
step S75, the process returns to step S71. In step S71, the target 
temperature is changed from 140.degree. C. to 180.degree. C. higher than 
the temperature which should be set in the continuous print operation. 
In addition, it is determined, in step 73, that the next printing 
instruction and the print data are received during the printing operation 
is being performed, the process returns to step S71. In step S72, the 
temperature of 180.degree. C. is set as the target temperature. 
In this case, since the surface temperature is low (equal to or less than 
40.degree. C.), the temperature of 180.degree. C. which is higher than the 
temperatures (170.degree. C. and 160.degree. C.) to be set in the printing 
operation is set as the target temperature. As a result, the pressure 
roller 47 can be sufficiently warmed up by the heat roller 48. Thus, the 
recording sheet is prevent from being curled (the reverse curl). 
On the other hand, it is determined, in step S53, that the surface 
temperature of the heat roller 48 is equal to or greater than 100.degree. 
C., the process proceeds to step S76. In step S76, the target 
temperature/driving instruction unit 66 sets, as the target temperature, a 
temperature of 140.degree. C. which should be set in the idle state. The 
heater on/off control unit 67 then controls the AC driver 62 so that the 
halogen lamp 70 is turned on. 
In step S77, the heat roller 48 of the fixing unit 46 is rotated so that 
the pressure roller 47 is warmed up. 
When it is determined, in step S78, that the heat roller 48 has been 
rotated for 11 seconds, and when it is determined, in step S79, that the 
surface temperature of the heat roller 48 reaches the target temperature 
(140.degree. C.), the control system of the printer outputs the print 
ready signal. 
The temperature control manner in accordance with steps S76 through S80 is 
shown in FIG. 33B (3). In this case, the procedure from turning on the 
power supply to outputting of the print ready signal takes about 15 
seconds. 
The printing instruction detecting unit 77 may detects, in step S60, that 
the printing instruction is not received before the initial operation is 
completed. Further, the printing instruction detecting unit 77 may 
detects, in steps S63 and S73, that the next printing instruction is not 
detected during the print operation. In these cases, when the sheet 
detecting unit 78 detects that the recording sheet has passed through the 
fixing unit 46, the process proceeds to step S81. In step S81, the idle 
setting unit 79 sets the printer in the idle state. 
As shown in FIG. 34, in a conventional case, after t.sub.4 seconds elapses 
from a time at which the print data completely transmitted, the printer is 
set in the idle state. In the present embodiment of the present invention, 
after t.sub.3 seconds elapses from a time at which the print data 
completely transmitted, the printer is set in the idle state. In the idle 
state, the target temperature is changed to a temperature of 140.degree. 
C. which should be set in the standby state (the idle state) and the 
halogen lamp 70 (the heat source) is turned off. 
That is, a period from a time at which the VR signal is output after the 
printing instruction is received to a time at which t.sub.3 elapses from 
completion of receiving of the print data is defined as a print operation 
period. 
In step S82, the target temperature/driving instruction unit 66 sets the 
temperature of 140.degree. C. as the target temperature. 
After the printer outputs the print ready signal in step S80, or after the 
temperature of 140.degree. C. is set as the target temperature in step 
S82, it is determined, in step S83, whether the printing instruction and 
the print data are received. When the printing instruction and the print 
data are received, the print start detecting unit 76 detects that the 
print operation starts. 
After this, the target temperature/driving instruction unit 66 sets a 
temperature of 160.degree. C. as the target temperature in step S84. The 
maximum value of the overshot temperature of the heat roller 48 which is 
heated based on the target temperature set at 160.degree. C. may be about 
170.degree. C. which should be set in the continuous print operation. 
In step S85, the print operation starts. 
The printing operation at the start is shown in FIG. 35. Referring to FIG. 
35, when the printing instruction is received in the idle state, the 
photosensitive drum 45 and the heat roller 48 of the fixing unit 46 start 
to be rotated. 
At the start of the print operation, the rotation of the photosensitive 
drum 45 and the heat roller 48 is continued for about 8.5 seconds (not 11 
seconds). In first about 5 seconds out of 8.5 seconds, the process unit 50 
is initialized and the optical system is activated. In second about 3.5 
seconds out of 8.5 seconds, the pressure roller 47 is warmed up to a 
predetermined temperature. 
In the conventional case, after 8.5 seconds elapses from receiving of the 
printing instruction, a recording sheet is picked out from the sheet 
cassette. On the other hand, in this embodiment of the present invention, 
immediately after the process unit 50 is initialized and the optical 
system is activated (about 5 seconds), a recording sheet is picked out 
from the sheet cassette. Thus, in this embodiment, the first print time is 
shorter than that in the conventional case. 
In the embodiment of the present invention, in the initial operation, the 
pressure roller is sufficiently warmed up in accordance with, for example, 
steps S64 through S75. Thus, at the start of the print operation, a time 
for the idle rotation of the heat roller 48 to warm up the pressure roller 
47 can be decreased. 
When the printing instruction detecting unit 77 detects, in step S86, that 
the next printing instruction is received during the printing operation, 
the control system determines that the continuous print operation is 
required. As a result, the process proceeds to step S61. The target 
temperature is thus set at 170.degree. C., and the steps after step S61 
are successively executed. 
On the other hand, when it is determined, in step S86, that the next 
printing instruction is not received during the printing operation, the 
process exceeds to step S81 and the printer is maintained in the idle 
state. 
The temperature control manner in accordance with steps S78 through S80 is 
shown in FIG. 33B (3). 
As has been described above, in the case where the print data is received 
during the initial operation, the target temperature (e.g., 170.degree. C. 
or 180.degree. C.) set for the initial operation is maintained in the 
continuous print operation. As a result, the toner image can be fixed on 
the recording sheet at a high fixing factor. 
A description will now be given of experiment results in the case of the 
printer according to the above embodiment of the present invention. 
FIG. 36A shows relationships between the temperature of the pressure roller 
47 and fixing ability, and FIGS. 36B and 36C show states in which 
recording sheets are curled (the positive curl and the reverse curl). 
Referring to FIG. 36A, when 5 minutes elapsed from turning on the power 
supply, the printing operation started. Recording sheets having A4 size 
passed through the fixing unit 46 at intervals of 30 seconds. Each 
recording sheet was led to the fixing unit 46 from a longitudinal edge 
thereof. Under the above conditions, the temperature of the pressure 
roller 47 varied as shown in FIG. 36A. 
In FIG. 36A, a thin line indicates temperature variation in the 
conventional case and a thick line indicates temperature variation in the 
case of the embodiment of the present invention. 
The fixing error (1), a large amount of reverse curl of the recording sheet 
(2) and a large amount of positive curl of the recording sheet (3) occur 
based on the temperature of the pressure roller 47. In a case where the 
pressure roller 47 has a low temperature immediately after the power 
supply is turned on, the fixing error (1) and the large amount of reverse 
curl of the recording sheet (2) may occur. In a case where the pressure 
roller 47 has a high temperature in the printing operation which is 
repeatedly performed, the large amount of positive curl of the recording 
sheet may occur. Thus, to avoid these problems, it is necessary to control 
the temperature of the pressure roller 47 in a stable region shown in FIG. 
36A in a period in which the printing operation starts and is repeatedly 
performed. 
In the conventional case, at the start of the printing operation, the 
pressure roller is not sufficiently warmed up. In the first and second 
print operations, the heat roller 48 is rotated for a long time so that 
the pressure roller 47 is rapidly warmed up. However, the pressure roller 
47 can not be warmed up at a temperature sufficient to avoid the reverse 
curl of the recording sheet. As a result, the quality of the recording 
sheet may deteriorate and the recording sheet may be jammed in the 
printing operation. 
Further, while the printing operation is repeatedly performed, the 
temperature of the pressure roller 48 is increased. Thus, the large amount 
of positive curl of the recording sheet may occur. 
In the embodiment of the present invention, the pressure roller is 
sufficiently warmed up in the initial operation. Although the temperature 
of the pressure roller 47 is slightly decreased until the print operation 
starts, at the start of the print operation, the pressure roller 47 is 
warmed up for a short time. As a result, the amount of reverse curl of the 
recording sheet can be decreased. 
Variations of the temperature detected by the temperature sensor 71 (the 
surface temperature of the heat roller 46) in the case of the embodiment 
of the present invention are shown in FIGS. 38, 39 and 40. Corresponding 
variations of the detected temperature in the conventional case are shown 
in FIGS. 5, 6 and 7. 
In the cases of the embodiment of the present invention, a low-temperature 
is selected as the target temperature at the start of the print operation. 
Thus, the overshoot of the temperature is suppressed. As a result, the 
large amount of positive curl of the recording sheet is prevented from 
occurring. 
According to the above embodiment of the invention, the following effects 
can be obtained. 
In the initial operation, if the surface temperature of the heat roller 48 
is low, a relatively high temperature is set as the target temperature so 
that the heat roller 48 is sufficiently heated. In the initial operation, 
on the other hand, if the surface temperature of the heat roller 48 is 
high, a relatively low temperature is set as the target temperature so 
that the heat roller 48 is not excessively heated. 
Further, in the case where the surface temperature of the heat roller 48 is 
low, after the surface temperature of the heat roller 48 reaches a 
predetermined temperature, the heat roller 48 starts to be rotated (see 
steps 65 and 66 in FIG. 32). That is, after the heat roller 48 is 
sufficiently heated, the heat roller is rotated 48, so that the pressure 
roller 47 is effectively warmed up. In the case where the surface 
temperature of the heat roller 48 is higher than a predetermined 
temperature, a relatively low temperature is set as the target temperature 
and the heat roller 48 immediately starts to be rotated. After the heat 
roller 48 is rotated for a predetermined time, the initial operation is 
completed. 
In the case where the next printing instruction is not received during the 
print operation, when the recording sheet has passed through the fixing 
unit 46, the target temperature for the fixing unit 41 is changed to a 
relatively low temperature. As a result, the pressure roller 47 is not 
excessively warmed up. 
At the start of the print operation, the temperature lower than the 
temperature which should be set in the continuous print operation is set 
as the target temperature. Even if the temperature of the heat roller 48 
is overshot, the maximum temperature is less than that in the conventional 
case. As a result, the amount of positive curl of the recording sheet can 
be reduced. 
In this case, the target temperature is set so that the overshot 
temperature is in a range suitable for the heat roller 48 in the 
continuous print operation. As a result, the toner image can be fixed on 
the recording sheet with the optimum fixing factor. 
For example, as shown in FIG. 33B (1), (2) and (3), an initial operation 
type is selected from a plurality of initial operation types based on the 
detected temperature of the fixing unit 46. In one type of the initial 
operation, a relatively high temperature is set as the target temperature 
and the heat roller is rotated for a long time. In another type of the 
initial operation, a relative low temperature is set as the target 
temperature and the heat roller is rotated for a short time. As a result, 
the initial operation is prevented from excessively continuing. 
In addition, since the initial operation does not excessively continue, a 
time for which the photosensitive drum is idled is reduced. Further, it is 
not necessary to rotate the pressure roller 48 with the photosensitive 
drum every time the print operation starts. The life time of the 
photosensitive drum can be improved and the fast print time can be 
reduced. 
A description will now be given, with reference to FIGS. 41A through 43. 
A printer 91 according to the second embodiment is shown in FIG. 41A. 
Referring to FIG. 41A, the printer 91 has a fixing unit 80, a temperature 
sensor 83, a heater controller 84, a waveform controller 89. The printer 
91 further has a power supply unit 85, an IF controller 86, a mechanism 
controller 87. 
The fixing unit 80 has a heater 81 (the heat source) and a heater driving 
circuit 82. The temperature sensor 83 detects the temperature of the 
fixing unit 80. The heater controller 84 performs an on-and-off control of 
an AC voltage used to drive the heater 81, based on the detected 
temperature, so that the detected temperature controlled at the target 
temperature. In the on-and-off control, an on-period in which the AC 
voltage is to be applied to the heater 81 and an off-period in which the 
AC voltage is not applied to the heater 81 are determined by the heater 
controller 84. In the on-period, the waveform controller 84 supplies to 
the heater driving circuit 82 a control signal to repeatedly turn on and 
off the AC voltage applied to the heater 81 at an on-and-off cycle less 
than a minimum value in a range which can be perceived by people. The 
power supply unit 85 supplies the AC voltage to the heater 81 via the 
heater driving circuit 82 and supplies a DC voltage to the IF controller 
86, the mechanism controller 87 and the heater controller 84. 
The waveform controller 89 is formed, for example, of an OR gate 90, as 
shown in FIG. 41B. The OR gate 90 outputs a logical sum signal of the 
control signal 7 from the heater controller 84 and a clock signal C as 
shown in FIG. 41C. 
The minimum cycle in the range which can be perceived by people is about 40 
milliseconds. An on-and-off cycle in a range which cannot be perceived by 
people is less than 40 milliseconds. 
The waveform of the AC voltage output from the power supply unit 85 is 
shown in FIG. 42 (a). The on-and-off cycle of the AC voltage is 20 
milliseconds which is not perceived by people. The waveform of the control 
signal 7 from the heater controller 84 is shown in FIG. 42 (b). A period 
in which the control signal has a high level (H) corresponds to the 
off-period described above. A period in which the control signal has a low 
level (L) corresponds to the on-period described above. The clock signal C 
has, as shown in FIG. 42 (c), a cycle (B) of 40 milliseconds and a low 
period (A) of 20 milliseconds. Thus, the output signal 1 (the logical sum 
of the control signal 7 and the clock signal C) of the waveform controller 
89 has the waveform as shown in FIG. 42 (d). The heater driving circuit 82 
generates a driving voltage, as shown in FIG. 42 (e), based on the output 
signal 1 of the waveform controller 86 and the AC voltage supplied from 
the power supply unit 85. The driving voltage is applied to the heater 81. 
In the on-period N of the driving voltage, an AC voltage is repeatedly 
turned on and off for one cycle. The AC voltage may be in the on state for 
a half cycle and in the off state for one cycle alternately. The 
on-and-off cycle at which the AC voltage is repeatedly turned on and off 
in the on-period is equal to or less than 40 milliseconds. 
The printer according to the second embodiment of the present invention 
operates as follows. 
When the temperature detected by the temperature sensor 83 is less than the 
minimum target temperature, the heater controller 84 supplies to the 
waveform controller 89 the control signal having the low level which 
represents the on-period. When the temperature detected by the temperature 
sensor 83 is equal to or greater than the maximum target temperature, the 
heater controller 84 outputs the control signal having the high level 
which represents the off-period. 
The waveform controller 89 outputs to the heater driving circuit 82 the 
logical sum signal of the control signal 7 and the clock signal C. When 
the signal 1 output from the waveform controller 89 has the low level (L), 
the AC voltage is supplied to the heater 81 as the driving voltage 6. On 
the other hand, when the signal 1 output from the waveform controller 89 
has the high level (H), the AC voltage is not supplied to the heater 81. 
The low-period (A) and the cycle (B) of the clock signal C are variable. In 
FIG. 42, the low-period (A) of the clock signal C is set at 20 
milliseconds and the cycle (B) of the clock signal C is set at 40 
milliseconds (see FIG. 42 (c)). According to the clock signal C, the AC 
voltage applied to the heater 81 is repeatedly turned on and off for one 
cycle. 
In FIG. 43, the low-period (A) of the clock signal C is set at 10 
milliseconds, and the cycle (B) of the clock signal C is set at 30 
milliseconds. As a result, the AC voltage is in the on state for the half 
cycle and in the off state for one cycle alternately. 
In the second embodiment, the heater 81 is not applied with the AC voltage 
at a rate (a duty-cycle) of 100%. Since the rate at which the AC voltage 
is actually applied to the heater 81 is less than 100%, the difference 
between amounts of current which pass through the heater 81 in the 
on-period (N) and the off-period (F) can be reduced. 
In the example shown in FIG. 42, the rate at which the AC voltage is 
actually applied to the heater 81 is 50%. In the example shown in FIG. 43, 
the rate at which the AC voltage is actually applied to the heater 81 is 
33.3%. In both the cases, the on-and-off cycle at which the AC voltages is 
repeatedly turned on and off is less than 40 milliseconds. Thus, the 
standard regarding the voltage variation of the power supply can be 
satisfied. 
A description will now be given, with reference to FIG. 44, of a third 
embodiment of the present invention. 
The printer according to the third embodiment has the same structure as the 
printer shown in FIG. 41A. However, a function of the waveform controller 
in the third embodiment differs from that of the waveform controller 89 in 
the second embodiment. 
The waveform controller 89 in the third embodiment of the present invention 
controls the heater driving circuit 82 so that the AC voltage is 
repeatedly turned on and off at an on-and-off cycle in the on-period. The 
on-and-off cycle, in this case, is in a range to which people generally 
have slightly uncomfortable feeling. This range is included in the range 
which can be perceived by people. 
An on-and-off cycle in the range to which people generally have slightly 
uncomfortable feeling is greater than 110 milliseconds (corresponding to 
8.8 Hz) to which the people generally have the most uncomfortable feeling. 
Thus, it is preferable that the on-and-off cycle is set as large as 
possible. In this embodiment, the on-and-off cycle at which the AC voltage 
is repeatedly turned on and off is set at 2 seconds (2000 milliseconds). 
However, if the on-and-off cycle is too large, the temperature control of 
the fixing unit cannot be normally performed. Thus, the on-and-off cycle 
should be decided so that the temperature control of the fixing unit can 
be normally performed. 
In the third embodiment, when the printer is in a predetermined sate such 
as the idle state, the AC voltage is repeatedly turned on and off at a 
cycle of 2 seconds in the on-period N shown in FIG. 9. For example, the AC 
voltage is in the on state for 1800 milliseconds (corresponding to 90 
cycles) and in the off state for 200 milliseconds (corresponding to 10 
cycles) alternately, as shown in FIG. 44. 
According to the above control of the AC voltage applied to the heater 81, 
the on-period N and the off-period F in the AC voltage supply control can 
be made longer. This matter is favorable to satisfy the standard regarding 
the voltage variation of the power supply. 
A description will now be given, with reference to FIG. 45, of a fourth 
embodiment of the present invention. In FIG. 45, those parts which are the 
same as those shown in FIG. 41A are given the same reference numbers. 
Referring to FIG. 45, a page printer 95 has the fixing unit 80, the 
temperature sensor 83, the heater controller 84, the power supply unit 85, 
the IF controller 86, the mechanism controller 87 and the mechanisms 88 in 
the same manner as that shown in FIG. 41A. The page printer 95 according 
to the fourth embodiment further has a waveform controller 96 different 
from that shown in FIG. 41A and an operation state determining unit 97. 
The operation state determining unit 97 determines, based on information 
from the mechanism controller 87, whether the page printer 95 is in the 
print operation state or in the idle state. The operation state 
determination unit 97 outputs an operation state signal 2 which indicates 
either the print operation state or the idle state. 
The waveform controller 96 controls the heater driving circuit 82 so that 
the on-period and the off-period in the AC voltage supply control are 
varied in accordance with the operation state signal 2. As a result, in 
both the idle state and the print operation state, the standard regarding 
the voltage variation of the power supply can be satisfied. 
When the operation state determining unit 97 determines that the page 
printer 95 is in the print operation state, the waveform controller 96 
functions in the same manner as the waveform controller 89 shown in FIG. 
41A in the second embodiment. That is, in this case, the AC voltage 
applied to the heater 81 is in the on state for one or half cycle and in 
the off state for one or half cycle alternately in the on-period. On the 
other hand, when the operation state determining unit 97 determines that 
the page printer is in the idle state, the waveform controller 96 
functions in the same manner as that in the third embodiment as shown in 
FIG. 44. That is, the AC voltage supplied to the heater 81 is repeatedly 
turned on and off at a cycle equal to or greater than 2 seconds in the 
on-period. A period of the on state corresponds to a few cycles of the AC 
voltage, and a period of the off state corresponds to a number of cycles 
of the AC voltage expressed in tens. 
A description will now be given, with reference to FIG. 46, of a fifth 
embodiment of the present invention. In FIG. 46, those parts which are the 
same as those shown in FIG. 41A are given the same reference numbers. 
Referring to FIG. 46, a page printer 98 has the fixing unit 80, the 
temperature sensor 83, the heater controller 84, the power supply unit 85, 
the IF controller 86, the mechanism controller 87 and the mechanisms 88 in 
the same manner as that in the second embodiment shown in FIG. 41A. The 
page printer 98 further has a waveform controller 99 having a function 
different from the function of the waveform controller 89 shown in FIG. 
41A and a power-on detecting unit 100. 
When the power of the page printer 98 is turned on, the waveform controller 
99 starts to control the heater driving circuit 82 so that the AC voltage 
is continuously supplied from the power supply unit 85 to the heater 81. 
When the detected temperature reaches a predetermined temperature, the 
waveform controller 99 changes the function so as to control the heater 
driving circuit 82 in the same manner as in the second embodiment. That 
is, the AC voltage applied to the heater 81 is repeatedly turned on and 
off at a cycle in the on-period. 
When the power-on detecting unit 100 detects that the power of the page 
printer 98 is turned on, a power-on detecting signal 4 is output from the 
power-on detecting unit 100. When receiving the power-on detecting signal, 
the waveform controller 99 outputs the heater control signal 1 in which 
the low-period (A) (see FIGS. 42 and 43) is set to 0 millisecond so that 
the AC voltage is continuously supplied to the heater 81. 
After this, when the detected temperature reaches a predetermined 
temperature, the waveform controller 99 outputs the heater control signal 
1 in which the low-period (A) and the cycle (B) are set in the same manner 
as in the second embodiment. That is, the AC voltage is in the on state 
for one or half cycle and in the off state for one or half cycle 
alternately as shown in FIG. 42 or 43. 
According to the fifth embodiment, the fixing unit 80 can be rapidly warmed 
up from when the power of the page printer 98 is turned on (the cold 
start). Further, after the temperature of the fixing unit 80 reaches the 
predetermined temperature, the voltage variation of the power line to 
which the power supply unit 85 is connected can be reduced. 
A description will now be given, with reference to FIG. 47, of a sixth 
embodiment of the present invention. 
A page printer according to the sixth embodiment has a waveform controller 
101 having a function different from the function of the waveform 
controller 89 in the second embodiment shown in FIG. 41A. 
The waveform controller 101 of the page printer according to the sixth 
embodiment is formed as shown in FIG. 47 (f). That is, the waveform 
controller 101 has an AND gate 102 to which the heater control signal 7, 
as shown in FIG. 47 (b), supplied from the heater controller 84 and the 
clock signal C as shown in FIG. 47 (c) are input. The waveform controller 
101 outputs the heater control signal 1 as shown in FIG. 47 (d). The 
heater driving circuit 82 is controlled using the heater control signal 1, 
so that the AC voltage which varies as shown in FIG. 47 (e) is applied to 
the heater 81. 
The waveform controller 101 further has a function for determining whether 
or not the detected temperature from the temperature sensor 83 reaches a 
predetermined temperature. 
When the detected temperature reaches the predetermined temperature, the 
power supply to the heater 81 is turned on. The off-period then starts. In 
the off-period, the AC voltage is not completely in the off state. That 
is, in the off-state, the AC voltage is repeatedly turned on and off at 
short intervals one or a few times. As a result, a time for which the 
fixing unit 80 is cooled to a predetermined temperature is made longer. 
Thus, in the off-period, when the temperature of the fixing unit 80 
reaches the predetermined temperature, the power thermistor of the heater 
driving circuit 82 is sufficiently cooled so as to have a large 
resistance. When the AC voltage starts to be applied to the heater element 
at the next time (the next on-period starts), a large amount of current is 
prevented from passing through the heater 81. In addition, the total 
period of the on-period (N) and the off-period (F) is made longer. 
Thus, the voltage variation of the power line can be reduced. 
A description will now be given, with reference to FIGS. 48A, 48B and 48C, 
of a seventh embodiment of the present invention. 
In the seventh embodiment, the temperature variation of the fixing unit 80 
is detected at intervals. If it is determined, based on the detected 
temperature variation, that the temperature of the fixing unit 80 is 
excessively increased, a density at which the AC voltage applied to the 
heater 81 is in the on state is reduced. On the other hand, if it is 
determined, based on the detected temperature variation, that the 
temperature of the fixing unit 80 increases at too a low rate or 
decreases, the density at which the AC voltage applied to the heater 81 is 
in the on state is increased. 
In this embodiment, the waveform controller has a function for monitoring 
the detected temperature from the temperature sensor and detecting the 
temperature variation at predetermined intervals, and a function for 
selecting a on-and-off cycle and a period of the on state. 
In the on-period (N) shown in FIG. 9, if it is determined that the 
temperature of the fixing unit 80 excessively increased, a on-and-off 
cycle of the AC voltage and a period of the on state thereof are selected 
so that the density at which the AC voltage applied to the heater 81 is in 
the on state is decreased. On the other hand, if it is determined that the 
temperature of the fixing unit 80 increases at too a low rate or 
decreases, another on-and-off cycle of the AC voltage and another period 
of the on state are selected so that the density at which the AC voltage 
applied to the heater is in the on state is increased. 
In addition, in the off-period (F) shown in FIG. 9, the power supply 
control as described above may be performed in accordance with a rate at 
which the temperature of the fixing unit decreases. 
According to the seventh embodiment, the total period of the on-period (N) 
and the off-period (F) can be made longer. Thus, the level of the AC 
voltage applied to the heater 81 can be slowly varied. 
In the seventh embodiment, the temperature variation of the heater 81 is 
detected at intervals of 200 milliseconds as shown in FIG. 48A. A clock 
signal corresponding to the detected temperature variation (t.degree.C.) 
is selected from among four clock signals in accordance with a rule shown 
by a table in FIG. 48B. 
The density at which the AC voltage applied to the AC voltage applied to 
the heater 81 is in the on state is defined by a duty ratio A/B of the 
clock signal C, where A is the period of on state (the low-period) and B 
is the on-and-off cycle as shown in FIG. 48C. 
In addition, according to the above control of the power supply to the 
heater 81, the fixing unit is not excessively heated and cooled. Thus, the 
recording sheet is prevented from being curled and toner image is 
prevented from being fixed on the recording sheet. 
As has been described above, in the image forming apparatus, such as a 
laser printer and a copy machine, in which an image is formed on a 
recording sheet in accordance with the electrophotographic method, the AC 
voltage applied to the heater of the fixing unit is repeatedly turned on 
and off in accordance with the detected temperature of the heater. In this 
case, the voltage variation occurs in the power line to which the power 
supply unit of the image forming apparatus is connected. If the voltage 
variation is large, lighting devices which share the power line may be 
flickered and the performance of other electronic units which share the 
power line may deteriorate. 
In a case where the voltage in the power line to which the lighting devices 
are connected is varied in rectangular shape, it is experimentally known 
that people generally have uncomfortable feeling in accordance with a 
characteristic as shown in FIG. 49. In FIG. 49, the axis of ordinate 
represents the amplitude of voltage variations .DELTA.V (corresponding to 
the difference between light and darkness of a lighting device), and the 
axis of abscissas represents a number of times of voltage variation for 
one minute (corresponding to a number of times of flicker of a lighting 
device). In a case where an amplitude of the voltage variation and a 
number of times of the voltage variation are included in an area under a 
curve Q shown in FIG. 49, people generally do not have the uncomfortable 
feeling to the flicker of the lighting device. On the other hand, in a 
case where an amplitude of the voltage variation and a number of times of 
the voltage variation are included in an area over the curve Q shown in 
FIG. 49, people generally have the uncomfortable feeling to the flicker of 
the lighting device. In this characteristic shown in FIG. 49, if the 
number of times of the voltage variation exceeds "C", the voltage 
variation cannot be perceived by people. In this case, the flicker of the 
lighting device based on the voltage variation cannot be perceived by 
people. Thus, the people do not have the uncomfortable feeling to any 
amplitude of the voltage variation. The number "C" of times of the voltage 
variation corresponds to about 33 Hz (the cycle of about 30 milliseconds). 
When the number of times of the voltage variation is "D", the people 
generally has the most uncomfortable feeling. The number "D" of times of 
the voltage variation corresponds to about 8.8 Hz. 
According to the above characteristic, in a case where the number of times 
of the voltage variation is "B" and the amplitude of the voltage variation 
is ".DELTA.B", if the amplitude of the voltage variation is reduced from 
".DELTA.B" to ".DELTA.A", the uncomfortable feeling of the people can be 
eliminated. In addition, in a case where the number of time of the voltage 
variation is "A" and the amplitude of the voltage variation is ".DELTA.A", 
if the number of times of the voltage variation is reduced from the "A" to 
"B", the uncomfortable feeling of the people can be eliminated. 
The temperature control of the fixing unit in the printer according to the 
second embodiment through the seventh embodiment is performed based on the 
above characteristic shown in FIG. 49. Hereinafter, furthermore, other 
examples of temperature controller which is applied to the fixing unit (a 
heated body) will be described below. 
A first example of a temperature controller for the fixing unit is formed 
as shown in FIG. 50. This temperature controller corresponds to a portion 
formed of the heater controller 84, the waveform controller 89 and the 
heater driving circuit 82 which has been described above. 
Referring to FIG. 50, the temperature controller has a switching circuit 
501, a temperature sensor (e.g., a thermistor), a first temperature 
detecting circuit 510, a second temperature detecting circuit 520, a high 
frequency oscillator circuit 530, an OR circuit 540 and an AND circuit 
550. A DC power E is supplied to the first temperature detecting circuit 
510, the second temperature detecting circuit 520 and the high frequency 
oscillator circuit 530. A capacitor C is connected between AC power lines. 
The switching circuit 501 is provided in one of the power lines to which a 
heater 500 of the fixing unit. The switching circuit 501 is formed of four 
diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 and a transistor Tr.sub.1. 
When the transistor Tr.sub.1 is turned on and off, the AC voltage supplied 
from the AC power line to the heater 500 is turned on and off. 
The first temperature detecting circuit 510 has a comparator 511, an 
inverter circuit 512 and resistors R.sub.1 through R.sub.4. A combined 
resistance value of the temperature sensor 502 and the resistor R.sub.1 
depends on the temperature of the fixing unit. A detecting signal 
corresponding to the combined resistance value is supplied to an inverting 
input terminal (-) of the comparator 511. A reference voltage depending on 
the resistors R.sub.2, R.sub.3 and R.sub.4 is supplied to a non-inverting 
input terminal (+) of the comparator 511. The reference voltage 
corresponding to a lower limit of a controlled temperature range depends 
on the resistors R.sub.2 and R.sub.3 connected between the DC power lines 
(E). the reference voltage corresponding to an upper limit of the 
controlled temperature range depends on the resistor R.sub.4 connected 
between the non-inverting input terminal (+) of the comparator 511 and an 
output terminal thereof. As shown in FIG. 51, when the temperature of the 
fixing increases and the detected temperature reaches the upper limit of 
the controlled temperature range, the output of the comparator 511 (M1) 
rises to a high level (H). After this, even if the detected temperature is 
less than the upper limit of the controlled temperature, the output of the 
comparator 511 (M1) is maintained at the high level. When the detected 
temperature reaches the lower limit of the controlled temperature range, 
the output of the comparator 511 (M1) falls to a low level (L). The output 
signal of the comparator 511 is supplied to the OR circuit 540 via the 
inverter circuit 512. 
The second temperature detecting circuit 520 has a comparator 521, an 
inverter circuit 522 and resistors R.sub.5 through R.sub.8. A combined 
resistance of the temperature sensor 502 and the resistor R.sub.5 depends 
on the temperature of the fixing unit. A detecting signal corresponding to 
the combined resistance is supplied to an inverting input terminal (-) of 
the comparator 521. A reference voltage depending on the resistors 
R.sub.6, R.sub.7 and R.sub.8 is supplied to a non-inverting input terminal 
of the comparator 521. The lower limit of the controlled temperature range 
depends on the resistors R.sub.6 and R.sub.7 connected between the DC 
power lines (E). A reference voltage corresponding to a maximum 
temperature (MAX) greater than the upper limit depends on the resistor 
R.sub.8 connected between the non-inverting input terminal of the 
comparator 521 and an output terminal thereof. As shown in FIG. 51, when 
the temperature of the fixing unit increases and the detected temperature 
reaches the maximum temperature (MAX), the output of the comparator 521 
(M2) rises to a high level (H). After this, even if the detected 
temperature is less than the maximum temperature (MAX), the output of the 
comparator 521 (M2) is maintained at the high level (H). When the detected 
temperature reaches the lower limit of the controlled temperature range, 
the output of the comparator 521 falls to a low level (L). The output 
signal of the comparator 521 is supplied to the AND gate 520 via the 
inverter circuit 522. 
The high frequency oscillator circuit 530 has a comparator 531 and 
resistors R.sub.9 and R.sub.10. A reference voltage depending on the 
resistors R.sub.9 and R.sub.10 is supplied to an inverting input terminal 
(-) of the comparator 531. A non-inverting input terminal (+) of the 
comparator 531 is provided with a triangular signal supplied from an 
external unit. As a result, the triangular signal input to the 
non-inverting input terminal is sliced by the reference voltage level 
input to the inverting input terminal, so that the comparator 531 outputs 
a rectangular pulse signal. The output signal (the rectangular pulse 
signal) is supplied to the OR circuit 540. 
The temperature controller having the above structure controls the 
temperature of the fixing unit (the heater 500) as follows. 
In a print operation, until the detected temperature (a fixing temperature) 
reaches the upper limit of the controlled temperature range, both the 
first temperature detecting circuit 510 and the second temperature 
detecting circuit 520 output the detecting signals having the high level 
(H). Thus, a signal having the high level (H) is supplied through the AND 
circuit 550 to the transistor Tr.sub.1 of the switching circuit 501 (an 
portion A), so that the transistor Tr.sub.1 is turned on. The heater 500 
is thus continuously provided with the AC voltage. As a result, the fixing 
temperature (the temperature of the fixing unit) increases in each 
"on-period" shown in FIG. 52. 
When the fixing temperature increases and the detected temperature reaches 
the upper limit, although the output signal of the second temperature 
detecting circuit 520 is maintained at the high level (H), the output 
signal of the first temperature detecting circuit 510 falls to the low 
level (L). Thus, the pulse signal from the high frequency oscillator 
circuit 530 is supplied to the transistor Tr.sub.1 of the switching 
circuit 501 via the OR circuit 540 and the AND circuit 550. The transistor 
Tr.sub.1 is repeatedly turned on and off in synchronism with the pulse 
signal. As a result, the AC voltage is intermittently applied to the 
heater 500 in synchronism with the pulse signal in each "off-period" shown 
in FIG. 52. Energy supplied to the heater 500 in the case where the AC 
voltage is intermittently applied to the heater 500 is less than energy 
supplied to the heater 500 in the case where the AC voltage is 
continuously applied to the heater. Thus, in this case, the fixing 
temperature is gradually decreased. Then, when the fixing temperature 
reaches the lower limit again, the output signal of the first temperature 
detecting circuit 510 rises to the high level (H), so that the signal 
having the high level (H) is supplied to the transistor Tr.sub.1 of the 
switching circuit 501. As a result, the AC voltage is continuously applied 
to the heater 500, so that the fixing temperature is increased again. 
During the print operation, control of the AC voltage applied to the heater 
500 in the "on-period" and in the "off-period" as described above is 
repeatedly performed. As a result, the fixing temperature is controlled so 
as to be maintained within a range between the lower limit and the upper 
limit. 
The frequency of the pulse signal output from the high frequency oscillator 
circuit 530 is greater than 33 Hz corresponding to the number "C" of times 
of the voltage variation shown in FIG. 49. Thus, even if the AC voltage 
applied to the heater 500 is repeatedly turned on and off in synchronism 
with the pulse signal in the "off-state", the flicker of a lighting device 
which shares the power lines connected to the heater 500 is not perceived 
by people. 
The "off-period" depends on the amount of energy emitted from the heater 
500, that is, the duty ratio of the above pulse signal. The duty ratio is 
a ratio of a on-period (the high level) to one cycle of the pulse signal. 
The duty ratio of the pulse signal is set so that the fixing temperature 
is decreased in accordance with an allowable characteristic. The duty 
ratio of the pulse signal is, for example, less than 50%. Further, the 
duty ratio of the pulse signal is set so that a number of times which the 
"on-period" and the "off-period" are repeated for a predetermined time 
(e.g., 1 minute) and the amplitude of the voltage variation are located in 
the area under the curve Q show in FIG. 49. That is, in a case where the 
number of times which the "on-period" and the "off-period" are repeated 
for the predetermined time is increased, the duty ratio is decreased so 
that the "off-period" is shortened. In addition, in a case where the 
number of times which the "on-period" and the "off-period" are repeated 
for the predetermined time is decreased, the duty ratio is increased so 
that the "off-period" is made longer. 
The duty ratio of the pulse signal is controlled by control of the 
reference voltage level supplied to the comparator 531 of the high 
frequency oscillator circuit 530. That is, the ratio of the resistances of 
the resistors R.sub.9 and R.sub.10 is controlled, so that the duty ratio 
of the pulse signal is controlled. 
In a case where the print operation is not performed, recording sheets do 
not pass through the fixing unit. In this case, the heat of the fixing 
unit is not radiated via the recording sheets. Thus, in the "off-period", 
the fixing temperature may not be decreased (see FIG. 52). When the fixing 
temperature increases and reaches the maximum temperature (MAX), the 
output signal of the second temperature detecting circuit 520 falls to the 
low level (L), so that the transistor Tr.sub.1 of the switching circuit 
501 is turned off. Thus, the power supply to the heater 500 is forced to 
be shut off. The fixing temperature is decreased. When the fixing 
temperature reaches the lower limit, the power supply control in the 
"on-state" and in the "off-state" described above is alternately performed 
again. 
According to the above temperature controller, the duty ratio of the pulse 
signal from the high frequency oscillator circuit 530 is controlled which 
duty ratio affects the "off-period". As a result, lighting devices which 
share the power lines can be prevented from being flickered based on 
repeatedly turning on and off the power supply to the heater 500. 
A second example of the temperature controller for the fixing unit is shown 
in FIG. 53. In the first example described above, when the fixing 
temperature reaches the maximum temperature (MAX), the power supply to the 
heater 500 is forced to be shut off. Alternatively, in the second example, 
when the print operation is completed, the power supply to the heater 500 
is forced to be shut off. If the fixing temperature decreases to the lower 
limit, the normal power supply control resumed. 
Referring to FIG. 53, the temperature controller has the switching circuit 
501, the temperature sensor 502, the first temperature detecting circuit 
510 and the high frequency oscillator circuit 530 in the same manner as 
that in the first example. The output signal of the first temperature 
detecting circuit 510 is supplied to the OR circuit 540 and is supplied to 
an AND circuit 551 via an inverter circuit 560. The AND circuit 551 is 
further provided with a print signal (a portion B). When the print 
operation is performed, the print signal has the high level (H). When the 
print operation is not performed, the print signal has the low level (L). 
The output signal of the AND circuit 551 and the pulse signal from the 
high frequency oscillator circuit 530 are respectively supplied to an AND 
circuit 552. The output signal of the AND circuit 552 is supplied to an OR 
circuit 541. 
In the print operation (the print signal having the high level (H)), the 
detecting signal having the high level (H) is supplied from the first 
temperature detecting circuit 510 to the transistor Tr.sub.1 of the 
switching circuit 501 via the OR circuit 541 until the fixing temperature 
reaches the upper limit. As a result, the AC voltage is continuously 
applied to the heater 500 (the "on-period"). In addition, when the fixing 
temperature reaches the upper limit, the detecting signal from the first 
temperature detecting circuit 510 falls to the low level (L). As a result, 
the pulse signal from the high frequency oscillator circuit 530 is 
supplied to the transistor Tr.sub.1 of the switching circuit 501 via the 
AND circuit 552 and the OR circuit 541. The AC voltage is intermittently 
applied to the heater 500 in synchronism with the pulse signal (the 
"off-period"). 
On the other hand, in the case where the print operation is not performed 
(the print signal having the low level (L)), when the fixing temperature 
reaches the upper limit, the detecting signal from the first temperature 
detecting circuit 510 falls to the low level in a state where the print 
signal has the low level (L), so that the transistor Tr.sub.1 of the 
switching circuit 501 is turned on. Thus, in the "off-period", the power 
supply to the heater 500 is forced to be shut off. After this, when the 
fixing temperature reaches the lower limit, the detecting signal from the 
first temperature detecting circuit 510 rises to the high level (H), and 
the AC voltage is continuously applied to the heater 500. 
According to the second example of the temperature controller, in a state 
where the print operation is completed and there is not recording sheet 
supplied to the fixing unit, the power supply to the heater 500 is forced 
to be shut off in the "off-period". Thus, the temperature of the fixing 
unit to which no recording sheet is supplied can be prevented from being 
excessively increased. 
A third example of the temperature controller for the fixing unit is shown 
in FIG. 54. The temperature controller controls the power supply to the 
heater 500 so that the AC voltage is slightly applied to the heater 500 in 
the "off-period". 
Referring to the FIG. 54, the temperature controller has the switching 
circuit 501, the temperature sensor 502, the first temperature detecting 
circuit 510 and the second temperature detecting circuit 520 in the same 
manner as that of the first example. The temperature controller further 
has a switching control circuit 570. The switching control circuit 570 
performs a driving control for the transistor Tr.sub.1 of the switching 
circuit 501. 
The switching control circuit 570 has an amplifier 571, transistors 
Tr.sub.2 through Tr.sub.6 and resistors R.sub.11 through R.sub.19. An 
input terminal of the amplifier 571 is provided, from an external unit, 
with a full-wave rectification signal synchronized with the commercial 
frequency (the frequency of the AC voltage in the power supply). The DC 
voltage (E) is applied to the switching control circuit 570. The 
transistor Tr.sub.3 is controlled by the output of the amplifier 571. 
The detecting signals from the first temperature detecting circuit 510 and 
the second temperature detecting circuit 520 are supplied to an AND 
circuit 583. In addition, the detecting signal from the first temperature 
detecting circuit 510 is supplied to an AND circuit 582 via an inverter 
circuit 581, and the detecting signal from the second temperature 
detecting circuit 520 is directly supplied to the AND circuit 582. The 
output signal of the AND circuit 582 is supplied to the transistor 
Tr.sub.6 of the switching control circuit 570 via the resistor R.sub.14 so 
that the transistor Tr.sub.6 is turned on and off by the output signal of 
the AND circuit 582. The output signal of the AND circuit 583 is supplied 
to the transistor Tr.sub.5 of the switching control circuit 570 via the 
resistor R.sub.16 so that the transistor Tr.sub.5 is turned on and off by 
the output signal of the AND circuit 583. 
Control currents are supplied from the DC power line (E) to the transistor 
Tr.sub.1 of the switching circuit 501 through two paths in the switching 
control circuit 57. The first path extends from the DC power line (E) to 
the transistor Tr.sub.1 via the transistor Tr.sub.4 and the resistor 
R.sub.18. The second path extends from the DC power line (E) to the 
transistor Tr.sub.1 via the transistors Tr.sub.2 and Tr.sub.3 and the 
resistor R.sub.19. 
In a case where the fixing temperature increases from the lower limit to 
the upper limit, both the detecting signals of the first temperature 
detecting circuit 510 and the second temperature detecting circuit 520 are 
maintained at the high level (H). Due to the output signal of the AND 
circuit 583, the transistor Tr.sub.5 is in the on state. As a result, the 
transistor Tr.sub.4 is in the on state, and the control current is 
supplied from the DC power line (E) to the transistor Tr.sub.1 of the 
switching circuit 501 via the transistor Tr.sub.4 and the resistor 
R.sub.18. In this case, the resistance of the resistor R.sub.18 and other 
circuit constants are decided so that an amount of the control current is 
sufficient to completely turn on the transistor Tr.sub.1. Thus, the AC 
voltage is continuously supplied to the heater 500 via the transistor 
Tr.sub.1, so that the fixing temperature is increased in the "on-period" 
shown in FIG. 55. 
When the fixing temperature is increased and reaches the upper limit, the 
detecting signal from the first temperature detecting circuit 510 falls to 
the low level (L). At this time, the output signal of the AND circuit 583 
falls to the low level (L) and the output signal of the AND circuit 582 
rise to the high level (H). Due to the output signal of the AND circuit 
582, the transistor Tr.sub.6 is in the on state. As a result, the 
transistor Tr.sub.2 is in the on state, the control current is supplied 
from the DC power line (E) to the transistor Tr.sub.1 of the switching 
circuit 501 via the transistors TR.sub.2 and Tr.sub.3 and the resistor 
R.sub.19. In this case, the amount of control current is adjusted using 
the resistance of the resistor R.sub.19 and other circuit constants so 
that the amplitude of the AC voltage applied to the heater 500 is reduced 
as shown in FIG. 56. The amplitude of the AC voltage applied to the heater 
500 is reduced, so that the fixing temperature is decreased in the 
"off-period" shown in FIG. 55. 
In a case where there is not recording sheet supplied to the fixing unit 
(after termination of the print operation), although the amplitude of the 
AC voltage applied to the heater 500 is reduced, the fixing temperature 
may be increased. In this case, when the fixing temperature reaches the 
maximum temperature (MAX), the detecting signal from the second 
temperature detecting circuit 520 falls to the low level (L). As a result, 
the output signals of the AND circuits 582 and 583 falls to the low level 
(L), and the control current is not supplied to the transistor Tr.sub.1 of 
the switching circuit 501. In this case, the AC voltage applied to the 
heater 500 is forced to be turned off, so that the fixing temperature is 
decreased from the maximum temperature (MAX) to the lower limit as shown 
in FIG. 55. 
According to the power supply control as has been described above, the 
variation of the AC voltage applied to the heater 500 in the cases where 
the "on-period" is changed to the "off-period" and vise versa is reduced. 
Thus, the flicker of the lighting devices which share the AC power line 
for the heater 500 is improved. Further, in the case where the AC voltage 
applied to the heater 500 is forced to be shut off in the "off-period", 
the "off-period" is made longer. As a result, the number of repeat of the 
"on-period" and the "off-period" for a predetermined time is reduced. The 
uncomfortable feeling with respect to the flicker of the lighting devices 
which share the AC power line for the heater 500 is improved (see the 
characteristic shown in FIG. 49). 
A fourth example of the temperature controller of the fixing unit is shown 
in FIG. 57. In this temperature controller, in the "on-period" and the 
"off-period", the AC voltage to be applied to the heater 500 is modulated 
using pulse signals having different duty ratios. 
Referring to FIG. 57, the temperature controller has the switching circuit 
501, the temperature sensor 502, the first temperature detecting circuit 
520 and the second temperature detecting circuit 530 in the same manner as 
that of the first example. The temperature controller further has a first 
high frequency oscillator circuit 530A and a second high frequency 
oscillator circuit 530B. 
The first high frequency oscillator circuit 530A has a first comparator 
531A and resistors R.sub.20 and R.sub.21. The first high frequency 
oscillator circuit 530A outputs a first pulse signal used to modulate the 
AC voltage in the "on-period". The duty ratio of the first pulse signal is 
set, in the same manner as in the first example, based on a reference 
voltage (an on-reference level) and a triangular signal both of which are 
supplied to the first comparator 531A. The on-reference level depends on 
the resistance ratio of the resistors R.sub.20 and R.sub.21. The first 
pulse signal has, for example, a frequency equal to or greater than 20 KHz 
as shown in FIG. 59. The duty ratio of the first pulse is greater than 50% 
(a period of the high level is greater than a period of the low level). 
The second high frequency oscillator circuit 530B has a second comparator 
531B and resistors R.sub.22 and R.sub.23. The second high frequency 
oscillator circuit 530B outputs a second pulse signal used to modulate the 
AC voltage in the "off-period". The duty ratio of the second pulse signal 
is set, in the same manner as that of the first high frequency oscillator 
circuit 530A described above, based on a reference voltage (an 
off-reference level) and the triangle signal both of which are supplied to 
the second comparator 531B. The off-reference level depends on the 
resistance ratio of the resistors R.sub.22 and R.sub.23. The off-reference 
level is greater than the on-reference level so that the duty ratio of the 
second pulse signal is less than the duty ratio of the first pulse. The 
second pulse signal has, for example, a frequency equal to or greater than 
20 KHz as shown in FIG. 60. The duty ratio of the second pulse signal is 
less than 50% (a period of the high level is less than a period of the low 
level). 
The detecting signal from the first temperature detecting circuit 510 is 
input to an AND circuit 587 and is input to an AND circuit 586 via an 
inverter circuit 585. The first pulse signal from the first high frequency 
oscillator circuit 530A is supplied to the AND circuit 587, and the second 
pulse signal from the second high frequency oscillator circuit 530B is 
supplied to the AND circuit 586. The output signals of the AND circuits 
586 and 587 are input to the OR circuit 540, and the output signal of the 
OR circuit 540 is input to the AND circuit 550. The detecting signal from 
the second temperature detecting circuit 520 is supplied to the AND 
circuit 550, the output signal of the AND circuit 550 is supplied, as the 
control signal, to the transistor Tr.sub.1 of the switching circuit 501. 
When the fixing temperature decreases and reaches the lower limit, the 
detecting signal from the first temperature detecting circuit 510 rises 
the high level (H) under a condition in which the detecting signal from 
the second temperature detecting circuit 520 is maintained at the high 
level (H). As a result, the first pulse signal from the first high 
frequency oscillator circuit 530A is supplied, as the control signal, to 
the transistor Tr.sub.1 of the switching circuit 501 via the AND circuit 
587, the OR circuit 540 and the AND circuit 550. The transistor Tr.sub.1 
is repeatedly turned on and off in synchronism with the first pulse 
signal, so that the AC voltage modulated using the first pulse signal is 
applied to the heater 500. In this case, since the first pulse signal has 
a duty ratio greater than 50% (see FIG. 59), the fixing temperature is 
increased in the "on-period" shown in FIG. 58. 
When the fixing temperature reaches the upper limit, the detecting signal 
from the first temperature detecting circuit 510 falls to the low level 
(L). As a result, the second pulse signal from the second high frequency 
oscillator circuit 530B is supplied, as the control signal, to the 
transistor Tr.sub.1 of the switching circuit 501 via the OR circuit 540 
and the AND circuit 550. The transistor Tr.sub.1 is repeatedly turned on 
and off in synchronism with the second pulse signal, so that the AC 
voltage modulated using the second pulse signal is applied to the heater 
500. In this case, since the duty ratio of the second pulse signal is less 
than 50% (see FIG. 60), the fixing temperature is decreased from the upper 
limit in the "off-state" shown in FIG. 58. 
In a case where no recording sheet is supplied to the fixing unit (after 
termination of the print operation), the fixing temperature may increase 
from the upper limit in the "off-period". In this case, when the fixing 
temperature reaches the maximum temperature (MAX), the detecting signal 
from the second temperature detecting circuit 520 falls to the low level 
(L), so that the control signal supplied to the transistor Tr.sub.1 of the 
switching circuit 501 falls to low level (L). As a result, the transistor 
Tr.sub.1 is turned off, and the power supply to the heater 500 is shut 
off. Thus, the fixing temperature is decreased from the maximum 
temperature (MAX) as shown in FIG. 58. 
In the temperature controller as described above, the "on-period" and the 
"off-period" are adjusted by adjusting of the duty ratios of the first 
pulse signal and the second pulse signal so that the number of times which 
the "on-period" and the "off-period" are repeated for the predetermined 
time is located in the area under the curve Q shown in FIG. 49. 
A fifth example of the temperature controller is shown in FIG. 61. In this 
example, the current supplied to the heater 500 is continuously controlled 
based on the detected temperature. 
Referring to FIG. 61, the AC power line to which the heater 500 of the 
fixing unit is connected is provided with the capacitor C and the 
switching circuit 501 in the same manner as that in the first example. The 
detected temperature signal is supplied to a non-inverting input terminal 
(+) of a differential amplifier 590. The detected temperature signal has a 
level corresponding to a combined resistance of the temperature sensor 502 
(the thermistor) and a resistor R.sub.24. A reference voltage supply 
circuit is connected to the DC power line via a resistor R.sub.25. A 
reference voltage V.sub.ref from the reference voltage supply circuit is 
supplied to an inverting input terminal of the differential amplifier 590. 
The differential amplifier 590 outputs a control signal having a level 
corresponding to the difference between the reference voltage V.sub.ref 
and the level of the detected temperature signal. The control signal is 
supplied to the transistor Tr.sub.1 of the switching circuit 501. 
The reference voltage V.sub.ref is set at a value based on a temperature at 
which the fixing unit should be controlled. 
According to the temperature controller described above, when the detected 
temperature increases, the resistance of the temperature sensor 502 
decreases so that the level of the detected temperature signal decreases. 
In this case, the level of the control signal which is supplied from the 
differential amplifier 590 to the transistor Tr.sub.1 of the switching 
circuit 501 decreases. Thus, the amount of current supplied to the heater 
500 is controlled by the transistor Tr.sub.1 so as to be decreased. As a 
result, the fixing temperature is decreased. On the other hand, when the 
detected temperature decreases, the resistance of the temperature sensor 
502 increases so that the level of the detected temperature signal 
increases. In this case, the level of the control signal which is supplied 
from the differential amplifier 590 to the transistor Tr.sub.1 of the 
switching circuit 51 increases. Thus, the amount of current supplied to 
the heater is controlled by the transistor Tr.sub.1 so as to be increased. 
As a result, the fixing temperature is increased. 
According to the power supply control for the heater 500, the fixing 
temperature gently varies within a temperature range including a target 
temperature corresponding to the reference voltage V.sub.ref as shown in 
FIG. 62. 
Since the current supplied to the heater 500 of the fixing unit gently 
varies, the flicker of the lighting devices which share the power lines 
for the heater 500 is improved. 
A sixth example of the temperature controller is shown in FIG. 63. In this 
example, pulse-width modulation in accordance with the detected 
temperature is applied to the AC voltage supplied to the heater 500 so 
that the AC voltage supplied to the heater 500 is continuously controlled. 
Referring to FIG. 63, the temperature controller has the switching circuit 
501, the temperature sensor 502, the differential amplifier 590, the 
reference voltage supply circuit outputting the reference voltage 
V.sub.ref and the resistors R.sub.24 and R.sub.25, in the same manner as 
that of the fifth example. The temperature controller further has a 
comparator 591. 
The differential amplifier 590 outputs the signal having the level 
corresponding to the detected temperature in the same manner as that in 
the fifth example. The output signal from the differential amplifier 590 
is supplied, as a reference, to an inverting input terminal (-) of the 
comparator 591. A non-inverting input terminal (+) of the comparator 591 
is provided with a triangular signal from an external unit. The comparator 
591 outputs a pulse signal having a frequency and a duty ratio both of 
which depend on the reference and the triangular signal. The pulse signal 
from the comparator 591 is supplied, as the control signal, to the 
transistor Tr.sub.1 of the switching circuit 501. The transistor Tr.sub.1 
is repeatedly turned on and off in synchronism with the control signal 
(the pulse signal). 
When the fixing temperature decreases, the reference level (the level of 
the output signal from the differential amplifier 590) for the comparator 
591 is decreased. As a result, the duty ratio of the pulse signal output 
from the comparator 591 is increased. The amount of heat from the heater 
500 to which the AC voltage controlled using the pulse signal is applied 
is increased. Thus, the fixing temperature is increased. 
When the fixing temperature increases, the reference level for the 
comparator 591 is increased. As a result, the duty ratio of the pulse 
signal output from the comparator 591 is decreased. The amount of heat 
from the heater 500 to which the AC voltage controlled using the pulse 
signal is applied is decreased. Thus, the fixing temperature is decreased. 
According to the sixth example, in the same manner as the fifth example, 
the fixing temperature is controlled so as to gently vary within a 
temperature range including a target temperature corresponding to the 
reference voltage V.sub.ref, as shown in FIG. 64. Thus, the flicker of the 
lighting devices which share the power line for the heater 500 is 
improved. 
A seventh example of the temperature controller is shown in FIG. 65. 
In the sixth example, since the heater 500 is at a room temperature (a 
minimum temperature) when the power supply of the printer (the image 
forming apparatus) is turned on (a cold start). Thus, at the cold start of 
the printer, the control signal (the output signal of the comparator 591) 
used to control the AC voltage applied to the heater 500 has a maximum 
pulse width. When the power supply to the heater 500 is controlled using 
the control signal having the maximum pulse width at the cold start of the 
printer, the current rapidly flows through the heater 500. In order to 
prevent the current from rapidly flowing through the heater 500, in the 
seventh example, the duty ratio of the control signal used to control the 
AC voltage applied to the heater 500 is gradually increased from a minimum 
value when the power supply of the printer is turned on. 
Referring to FIG. 65, the temperature controller has the switching circuit 
501, the temperature sensor 502, the differential amplifier 590, the 
comparator 591, the reference voltage supply circuit (V.sub.ref) and the 
resistors R.sub.24 and R.sub.25 in the same manner as that of the sixth 
example. The temperature controller further has a reference level control 
circuit 600. 
The reference level control circuit 600 has resistors R.sub.26, R.sub.27 
and R.sub.28, a capacitor C.sub.2 and a diode D.sub.5. At the cold start 
of the printer, the reference level control circuit 600 controls the 
reference level supplied to the inverting input terminal (-) of the 
comparator 591 so that the reference level is gradually decreased from a 
maximum level. 
When the DC power supply is turned on at the cold start of the printer, the 
DC voltage (E) (the maximum voltage) is supplied, as the reference 
voltage, to the inverting input terminal (-) of the comparator 591 via the 
capacitor C.sub.2 and the diode D.sub.5. After this, while the capacitor 
C.sub.2 is being charged, the voltage level applied to the inverting input 
terminal (-) of the comparator 591 is gradually decreased. As a result, 
the duty ratio of the pulse signal which is output from the comparator 591 
is gradually increased from the minimum value. When the capacitor C.sub.2 
is fully charged, the duty ratio of the control signal reaches a value 
which is decided based on the relationship between the reference voltage 
depending on the detected temperature and the triangular signal. After 
this, the duty ratio of the control signal supplied from the comparator 
591 to the transistor Tr.sub.1 of the switching circuit 501 varies in 
accordance with the detected temperature. As a result, the fixing 
temperature gently varies within a temperature range including a target 
temperature as shown in FIG. 64. 
According to the seventh example of the temperature controller, at the cold 
start of the printer, the amount of current which flows through the heater 
500 is not rapidly varied. Thus, the voltage of the AC power line is 
prevented from being rapidly varied when the power supply of the printer 
is turned on. 
A eighth example of the temperature controller is shown in FIG. 66. 
In the case of the fifth example (see FIG. 61), since the heater 500 is at 
a room temperature (a minimum temperature) when the power supply of the 
printer is turned on (at the cold start of the printer), the maximum 
amount of current flows through the heater 500. In order to prevent the 
maximum amount of current from flowing through the heater 500 at the cold 
start, in the eighth example, the amount of current flowing through the 
heater 500 is controlled so as to be gradually increased from a minimum 
value. 
Referring to FIG. 66, the temperature controller has the switching circuit 
501, the temperature sensor 502, the differential amplifier 590, the 
reference voltage supply circuit (V.sub.ref) and the resistors R.sub.24 
and R.sub.25 in the same manner as that of the fifth example (see FIG. 
61). The temperature controller further has a reference level control 
circuit 600. The reference level control circuit 600 has the resistors 
R.sub.26, R.sub.27 and R.sub.28, the capacitor C.sub.2 and the diode 
D.sub.5 in the same manner as that in the seventh example (see FIG. 65). 
The reference level control circuit 600 controls the level of the 
reference voltage supplied to the inverting input terminal (-) of the 
differential amplifier 590 so that the level of the reference voltage is 
gradually decreased from a maximum value at the cold start of the printer. 
When the power supply of the printer is turned on (the cold start) and the 
DC power supply (E) is turned on, the DC voltage having the maximum level 
is supplied, as the reference voltage, to the inverting input terminal (-) 
of the differential amplifier 590 via the capacitor C.sub.2 and diode 
D.sub.5. After this, while the capacitor C.sub.2 is being charged, the 
level of the reference voltage is gradually deceased. Finally, the level 
of the reference voltage reaches a level corresponding to the voltage 
V.sub.ref from the reference voltage supply circuit. In this case, the 
level of the control signal which is supplied from the differential 
amplifier 590 to the transistor Tr.sub.1 of the switching circuit 501 is 
gradually increased. As a result, the amount of current which is 
controlled by the transistor T.sub.1 and flows through the heater 500 at 
the cold start is gradually increased from a minimum value. 
After the capacitor C.sub.2 is completely charged, the inverting input 
terminal (-) of the differential amplifier 590 is regularly provided with 
the voltage corresponding to the reference voltage V.sub.ref. Thus, the 
amount of current corresponding to the detected temperature flows through 
the heater 500. As a result, the fixing temperature is controlled so as to 
gently vary within a temperature range including a target temperature as 
shown in FIG. 62. 
According to the eighth example, the amount of current which flows through 
the heater 500 at the cold start is gradually increased, so that the 
amount of current which flows through the heater 500 is not rapidly 
increased. Thus, the voltage of the AC power line is prevented from being 
varied at the cold start. 
A ninth example of the temperature controller is shown in FIG. 67. In this 
example, the AC voltage applied to the heater 500 in the "on-period" is 
controlled so that the AC current flowing through the heater 500 is 
repeatedly turned on and off one cycle by one cycle. 
Referring to FIG. 67, the temperature controller has the temperature 
detecting circuit 510 having the same structure as the first temperature 
detecting circuit 510 in the first example (see FIG. 50). The temperature 
detecting circuit 510 outputs a detecting signal. While the detected 
temperature is increasing from the lower limit to the upper limit, the 
detecting signal from the temperature detecting circuit 510 has the high 
level (H). On the other hand, while the detected temperature is decreasing 
from the upper limit to the lower limit, the detecting signal has the low 
level (L). 
An inverter circuit 611, a flip flop 612 and an inverter circuit 612 form a 
dividing circuit. The dividing circuit is provided with a clock signal 
(the portion A) in which the level thereof is repeatedly inverted in 
synchronism with the AC voltage to be supplied to the heater 500, as shown 
in FIG. 68. The dividing circuit makes a clock signal (the portion B) in 
which the level is inverted every cycle of the AC voltage (see FIG. 68). 
The clock signal from the dividing circuit and the detecting signal from 
the temperature detecting circuit 510 are supplied to an AND circuit 614. 
The output signal of the AND circuit 614 is supplied, as a control signal, 
to a control terminal of a triac 503 provided in the AC power line 
connected to the heater 500. The triac 503 is a type which is turned on 
and off in accordance with a zero-crossing method. 
When the detected temperature reaches the lower limit, the detecting signal 
from the temperature detecting circuit 510 rises to the high level (H) 
(the "on-period"). In this state, the clock signal (the portion B) in 
which the level is inverted every one cycle of the AC voltage is supplied, 
as the control signal, from the flip flop 612 to the control terminal of 
the triac 503 via the AND circuit 614. As a result, the triac 503 is 
repeatedly turned on and off by one cycle of the AC voltage. Thus, the AC 
voltage which is repeatedly turned on and off by one cycle as shown in 
FIG. 70 is applied to the heater 500. 
While the AC voltage as controlled above is being applied to the heater 
500, the fixing temperature is increased from the lower limit to the upper 
limit in the "on-period" shown in FIG. 69. In this case, the frequency at 
which the AC voltage applied to the heater 500 is turned on and off is 
close to the maximum value (corresponding to "C" shown in FIG. 49) which 
can be perceived by people. Thus, even if the voltage variation occurs in 
the AC power line due to the above temperature control for the fixing 
unit, people do not have uncomfortable feeling with respect to the flicker 
of the lighting devices which share the AC power line. 
When the fixing temperature is increased and reaches the upper limit, the 
detecting signal from the temperature detecting circuit 510 falls to the 
low level (L) (the "off-period"). As a result, the control terminal of the 
triac 503 is maintained at the low level (L), so that the triac 503 is in 
the off state. Thus, the AC voltage to the heater 500 is shut off, and the 
fixing temperature is decreased from the upper limit to the lower limit in 
the "off-period" shown in FIG. 69. 
According to the ninth example, the fixing temperature is repeatedly 
increased and decreased in a range between the lower limit and the upper 
limit as shown in FIG. 69. 
The AC power control described above can be realized by a control circuit 
as shown in FIG. 71. 
In this control circuit, a flip flop 615 and an inverter circuit 616 are 
added to the control circuit shown in FIG. 67. As shown in FIG. 72, the 
control clock signal (the portion B) to be supplied to the triac 503 is 
generated based on a clock signal (the portion A) which is made from the 
full-wave rectification waveform of the AC voltage having the commercial 
frequency. The control clock signal has the same waveform as that shown in 
FIG. 68. 
A tenth example of the temperature controller is shown in FIG. 73. In this 
example, the power supply to the heater 500 is controlled so that a period 
in which the AC power supply to the heater 500 is shut off is longer than 
a period in which the AC power is supplied to the heater 500 in the 
"off-state". 
Referring to FIG. 73, the temperature controller has the first temperature 
detecting circuit 510 and the second temperature detecting circuit 520 in 
the same manner as that of the first example. In addition, the triac 503 
is provided in the AC power line connected to the heater 500 so that the 
AC power supply to the heater 500 is controlled due to the on-and-off 
operation of the triac 503. In this example, also, the triac 503 is a type 
which is turned on and off in accordance with the zero-crossing method. 
The temperature controller further has an inverter circuit 621, flip flops 
622 and 623, inverter circuits 624 and 625, an AND circuit 626, an OR 
circuit 627 and an AND circuit 628. These circuits form a unit for 
generating a control signal for the triac 503. A clock signal (the portion 
A) having the high level (H) at each zero-crossing point of the AC voltage 
(an input voltage) as shown in FIG. 74 is supplied to a clock terminal of 
the flip flops 622 and 623 via the AND circuit 621. The flip flop 622 
outputs a pulse signal (the portion B) which is in the on state for a half 
cycle of the AC voltage and in the off-state for one cycle of the AC 
voltage alternately. This pulse signal (the portion B) is supplied, as the 
control signal, to the control terminal of the triac 503 via the OR 
circuit 627 and the AND circuit 628. The OR circuit 627 is controlled by 
the detecting signal from the first temperature detecting circuit 510 and 
the AND circuit 628 is controlled by the detecting signal from the second 
temperature detecting circuit 520. 
When the fixing temperature is decreased from the upper limit and reaches 
the lower limit, the detecting signal from the first temperature detecting 
circuit 510 rises to the high level (H) and the detecting signal from the 
second temperature detecting circuit 520 is maintained at the high level 
(H) (the "on-period"). As a result, the control terminal of the triac 503 
is maintained at the high level (H), so that the triac 503 is in the on 
state. Thus, the AC voltage is continuously applied to the heater 500. 
The AC voltage is continuously applied to the heater 500 as described 
above, so that the fixing temperature is increased from the lower limit to 
the upper limit in the "on-period" as shown in FIG. 75. When the fixing 
temperature is increased and reaches the upper limit, the detecting signal 
from the second temperature detecting circuit 520 is maintained at the 
high level (H) and the detecting signal from the first temperature 
detecting circuit 510 falls to the low level (L) (the "off-period"). As a 
result, the pulse signal which is in the on-state for a half cycle of the 
AC voltage and in the off-state for one cycle alternately as described 
above is supplied, as the control signal, from the flip flop 622 to the 
control terminal of the triac 503 via the OR circuit 627 and the AND 
circuit 628. The triac 503 is repeatedly turned on and off by the control 
signal, so that the AC voltage which is in the on-state for a half cycle 
and in the off-state for one cycle alternately is applied to the heater 
500. 
In this case, in the "off-period", a period in which the AC voltage is not 
supplied to the heater 500 is longer than a period in which the AC voltage 
is supplied to the heater 500. Thus, the fixing temperature is gradually 
decreased from the upper limit in the "off-period" as shown in FIG. F75. 
When the fixing temperature reaches the lower limit, the power supply 
control in the "on-period" as has been described above starts. After this, 
every time the fixing temperature reaches the upper limit and lower limit, 
the power supply control to the heater 500 is switched from the manner in 
the "on-period" to the manner in the "off-period" and vice versa. 
Further, for example, in a case where the recording sheet is not supplied 
to the fixing unit (termination of the print operation), when the fixing 
temperature is further increased from the upper limit to the maximum 
temperature (MAX) although the power supply to the heater 500 is 
controlled in the "off-period", the detecting signal from the second 
temperature detecting circuit 520 rises to the high level (H). As a 
result, the control terminal of the triac 503 is maintained at the low 
level (L), so that the triac 503 is in the off state. Thus, the AC voltage 
applied to the heater 503 is shut off, so that the fixing temperature is 
rapidly decreased. 
A eleventh example of the temperature controller is shown in FIG. 77. In 
this example, the heater of the fixing unit is divided into two heater 
elements and a connecting manner of the heater elements (coils) is changed 
in accordance with the detected temperature. 
Referring to FIG. 77, the heater 500 is divided into a first coil 500A and 
a second coil 500B. The first coil 500A and the second coil 500B are 
provided in the AC power line. A first triac 504 is provided in a path 
through which the AC voltage is serially supplied to the first coil 500A 
and the second coil 500B. A second triac 505 is provided in a path through 
which the AC voltage is supplied to the second coil 500B. A third triac 
506 is provided in a path through which the AC voltage is supplied to the 
first coil 500A. 
The temperature controller which performs the power supply control to the 
first coil 500A and the second coil 500B has the temperature sensor 502, 
the first temperature detecting circuit 510 and the second temperature 
detecting circuit 520 in the same manner as that of the first example. 
This temperature controller further has an inverter circuit 631 and AND 
circuits 632 and 633. The respective AND circuits 632 and 633 are 
controlled by the detecting signal from the second temperature detecting 
circuit 520. The detecting signal from the first temperature detecting 
circuit 510 is supplied, as the control signal, to a control terminal of 
the first triac 504 via the inverter circuit 631 and the AND circuit 632. 
The detecting signal from the first temperature detecting circuit 510 is 
supplied also, as the control signal, to control terminals of the second 
triac 505 and the third triac 506 via the AND circuit 633. 
When the fixing temperature is decreased from the upper limit and reaches 
the lower limit, the detecting signal from the second temperature 
detecting circuit 520 is maintained at the high level (H) and the 
detecting signal from the first temperature detecting circuit 510 rises to 
the high level (H). As a result, the second triac 505 and the third triac 
506 are controlled so as to be in the on state. Thus, the first coil 500A 
and the second coil 500B are provided with the AC voltage in parallel. In 
this case, the heater 500 radiates a rated amount of heat, so that the 
fixing temperature is increased from the lower limit. 
When the fixing temperature is increased and reaches the upper limit, the 
detecting signal from the second temperature detecting circuit 520 is 
maintained at the high level (H) and the detecting signal from the first 
temperature detecting circuit 510 falls to the low level (L). As a result, 
the second triac 505 and the third triac 506 are turned off and the first 
triac 504 is turned on. Thus, the AC voltage is supplied to the first coil 
500A and the second coil 500B which are serially connected. In this case, 
the heater 500 radiates only an amount of heat which is one fourth as much 
as the rated amount of heat, so that the fixing temperature is gradually 
decreased from the upper limit. 
According to the power supply control to the heater 500, the fixing 
temperature is maintained within a range between the upper limit and lower 
limit. In this case, since the fixing temperature is decreased under the 
condition in which the heater 500 radiates the amount of heat one fourth 
as much as the rated amount of heat, a temperature gradient at which the 
fixing unit is decreased is less than the temperature gradient in a case 
where the AC voltage is completely shut off. Thus, a period which needed 
to decrease the temperature from the upper limit to the low limit is made 
longer, so that the respective triacs are turned on and off at long 
intervals. As a result, even if the AC voltage supplied to the heater 500 
is repeatedly turned on and off by the triacs, people do not have 
uncomfortable feeling with respect to the flicker of the lighting device 
which share the AC power line (see FIG. 49). 
The intervals at which the AC voltage is turned on and off are adjusted, 
based on the number of coils into which the heater is divided, capacities 
of the respective coils and the like, within a range (see FIG. 49) in 
which people do not have uncomfortable feeling with respect to the flicker 
of the lighting devices which share the AC power line. 
If the fixing temperature is further increased from the upper limit to the 
maximum temperature (MAX) although the heater 50 radiates the amount of 
heat which is one fourth as much as the rated amount of heat, the 
detecting signal from the second temperature detecting circuit 520 falls 
to the low level (L). As a result, all the triacs 504, 505 and 506 are 
controlled to be in the off state, so that the AC voltage supplied to the 
heater 500 is shut off. Thus, the fixing temperature is rapidly decreased. 
A twelfth example of the temperature controller is shown in FIG. 78. In 
this example, a resistor is serially connected to the heater, so that the 
voltage applied to the heater is reduced. 
Referring to FIG. 78, the heater 500, the first triac 503 and a resistor 
R.sub.3 are provided in the AC line. A second triac 507 is connected to 
the resistor R.sub.30 in parallel. When the second triac 507 is in the on 
state, a current from the AC power supply by-passes the resistor R.sub.30 
and is supplied to the heater 500. The temperature controller has the 
temperature sensor 502, the first temperature detecting circuit 510 and 
the second temperature detecting circuit 520 in the same manner as that of 
the first example (see FIG. 50). The detecting signal from the first 
temperature detecting circuit 510 is supplied, as the control signal, to 
the control terminal of the second triac 507. The detecting signal from 
the second temperature detecting circuit 520 is supplied, as the control 
signal, to the control terminal of the first triac 503. 
When the fixing temperature is decreased from the upper limit and reaches 
the lower limit, the detecting signal from the second temperature 
detecting circuit 520 is maintained in the high level (H) and the 
detecting signal from the first temperature detecting circuit 510 rises to 
the high level (H). As a result, the first triac 503 is maintained in the 
on state and the second triac 507 is turned on. In this state, the current 
from the AC power supply by-passes the resistor R.sub.30 and is supplied 
to the heater 500. Thus, the heater radiates the rated amount of heat, so 
that the fixing temperature is increased from the lower limit. 
When the fixing temperature is increased and reaches the upper limit, the 
detecting signal from the second temperature detecting circuit 520 is 
maintained at the high level (H) and the detecting signal from the first 
temperature detecting circuit 510 falls to the low level (L). As a result, 
the first triac 503 is maintained in the on state and the second triac 507 
is controlled so as to be in the off state. The current from the AC power 
supply is supplied to the heater via the resistor R.sub.30. In this case, 
a part of power from the AC power supply is dissipated by the resistor 
R.sub.30 (radiated as heat), so that the heater 500 radiates only an 
amount of heat less than the rated amount of heat. Thus, the fixing 
temperature is gradually decreased from the upper limit. 
According to the power supply control to the heater 500 as described above, 
the fixing temperature can be controlled within a range between the upper 
limit and the lower limit. In this case, since the fixing temperature is 
decreased under the condition in which the heater 500 radiates the amount 
of heat less than the rated amount of heat, the temperature gradient at 
which the fixing temperature is decreased is less than that in a case 
where the AC voltage is completely shut off. Thus, a period in which the 
fixing temperature is decreased from the upper limit to the lower limit is 
made longer, so that the intervals at which the second triac 507 is 
repeatedly turned on and off is made longer. As a result, even if the AC 
voltage applied to the heater 500 is controlled by the second triac 507 
which is turned on and off as described above, people do not have 
uncomfortable feeling with respect to the flicker of the lighting devices 
which share the AC power line (see FIG. 49). 
The intervals at which the second triac 507 is turned on and off is 
adjusted, based on the resistance of the resistor R.sub.30, within a range 
in which people do not have the uncomfortable feeling with respect to the 
flicker of the lighting devices which share the AC power line (see FIG. 
49). 
In addition, if the fixing temperature is further increased from the upper 
limit and reaches the maximum temperature (MAX) although the current is 
supplied to the heater via the resistor R.sub.30, the detecting signal 
from the second temperature detecting circuit 520 falls to the low level 
(L). As a result, the first triac 503 is controlled so as to be off state, 
and the AC voltage applied to the heater 500 is shut off. Thus, the fixing 
temperature is rapidly decreased. 
A thirteenth example of the temperature controller is shown in FIG. 79. In 
this example, the heater of the fixing unit to which the AC voltage is 
continuously applied is cooled by a cooling fan in accordance with the 
detected temperature. 
Referring to FIG. 79, the AC current is supplied to the heater 500 via the 
triac 503. The temperature controller has the temperature sensor 502, the 
first temperature detecting circuit 510 and the second temperature 
detecting circuit 520 in the same manner as that of the first embodiment 
(see FIG. 50). The temperature controller further has a cooling fan 700 
used to cool the heater 500 and a driving circuit. The driving circuit has 
an inverter circuit 63, a transistor Tr.sub.10 and a DC power supply. The 
driving circuit performs a driving control for the cooling fan 700 based 
on the detecting signal from the first temperature detecting circuit 510. 
In addition, the detecting signal from the second temperature detecting 
circuit 520 is supplied, as the control signal, to the control terminal of 
the triac 503. 
When the fixing temperature is decreased and reaches the lower limit, the 
detecting signal from the second temperature detecting circuit 520 is 
maintained at the high level (H) and the detecting signal from the first 
temperature detecting circuit 510 rises to the high level (H). As a 
result, the transistor Tr.sub.10 is controlled so as to be off state, so 
that the cooling fan 700 is stopped. In this state, the heater 500 is 
provided with the AC current through the triac 503, so that the fixing 
temperature is increased from the lower limit to the upper limit in a 
"stop-period" as shown in FIG. 80. 
When the fixing temperature is increased and reaches the upper limit, the 
detecting signal from the first temperature detecting circuit 510 falls to 
the low level (L). As a result, the transistor Tr.sub.10 is controlled so 
as to be in the on state, so that the cooling fan 700 is driven. The 
heater 500 of the fixing unit is cooled by wind from the cooling fan 700, 
and the fixing temperature is gradually decreased in an "operation-period" 
as shown in FIG. 80. 
As has been described above, the heater 500 is continuously provided with 
the AC current and the cooling fun 700 is turned on and off in accordance 
with the detected temperature, so that the fixing temperature is 
controlled within a range between the upper limit and the lower limit. 
In addition, if the fixing temperature is further increased from the upper 
limit and reaches the maximum temperature (MAX) although the cooling fun 
700 is driven, the detecting signal from the second temperature detecting 
circuit 520 falls to the low level. The triac 503 is thus controlled so as 
to be in the off state. As a result, the current supplied to the heater 
500 is shut off, so that the fixing temperature is rapidly decreased from 
the maximum temperature. 
According to the thirteenth example, since the AC current is continuously 
supplied to the heater 500. Thus, the light devices which share the AC 
power line connected to the heater 500 is not affected. 
A fourteenth example of the temperature controller is shown in FIG. 81. In 
this example, the rotation of the cooling fan for cooling the heater is 
continuously controlled in accordance with the detected temperature. 
Referring to FIG. 81, in a state where the power supply of the printer is 
in the on state, the AC current is supplied to the heater 500. The 
temperature controller has the cooling fan 700 for cooling the heater 500 
and the driving circuit. The driving circuit performs the driving control 
for the cooling fan 700 in accordance with the detecting signal from the 
temperature sensor 502. The driving control circuit is formed of an 
amplifier 650, a reference power supply V.sub.ref, the DC power supply E, 
a transistor Tr.sub.20 and resistors R.sub.41, R.sub.42 and R.sub.43. The 
driving control circuit supplies to the cooling fan 700 a driving signal 
V.sub.o having a level depending of the level of the detecting signal from 
the temperature sensor 502. 
When the level of the detecting signal input to the amplifier 650 is 
decreased in accordance with increasing of the detected temperature, the 
level of the driving signal V.sub.o supplied to the cooling fan 700 is 
increased. When the level of the detecting signal input to the amplifier 
650 is increased in accordance with decreasing of the detected 
temperature, the level of the driving signal V.sub.o is decreased. 
According to such the characteristic of the driving circuit, the fixing 
temperature is controlled so as to be maintained within a range including 
a target temperature corresponding to the reference voltage V.sub.ref, as 
shown in FIG. 82. 
A fifteenth example of the temperature controller is shown in FIG. 73. In 
this example, there is a mechanism for controlling an amount of wind 
supplied from the cooling fun to the heater. 
Referring to FIG. 73, the AC current is supplied from the AC power line to 
the heater 500 of the fixing unit via the triac 503. The temperature 
controller has the temperature sensor 502, the first temperature detecting 
circuit 510 and the second temperature detecting circuit 520 in the same 
manner as that of the first example (see FIG. 50). The temperature 
controller further has the cooling fan 700 for cooling the heater 500 and 
a wind control mechanism for controlling amount of wind supplied to the 
heater 500. 
The wind control mechanism has a fixed screening plate 741 and a movable 
screening plate 742. These screening plates 741 and 742 are located 
between the cooling fan 700 and the heater 500. Each of these screening 
plates 741 and 742 has a plurality of holes formed thereon so as to be 
arranged in a matrix as shown in FIG. 84. The fixed screening plate 741 
faces the heater 500 and is fixed. The movable screening plate 742 faces 
the cooling fan 700 and is located so as to be parallel to the fixed 
screening plate 741. A motor driving circuit 720 performs a driving 
control for a motor 730 in accordance with the detected temperature. 
The motor driving circuit 720 has, as shown in FIG. 85, transistors 
Q.sub.1, Q.sub.2 and Q.sub.3, a capacitor, a resistor R.sub.50 and the DC 
power supply E. The motor driving circuit 720 drives the motor 730 so that 
the motor 730 is rotated in a direction corresponding to a level (the high 
level or the low level) of a control signal (the portion B signal) 
supplied to the transistor Q.sub.3. The detecting signal from the first 
temperature detecting circuit 510 is supplied, as the control signal, to 
the transistor Q.sub.3. The motor 73 is rotated until the capacity is 
completely charged. 
When the fixing temperature is decreased and reaches the lower limit, the 
detecting signal from the first temperature detecting circuit 510 rises 
the high level (H). The motor driving circuit 720 drives the motor 730 so 
that the motor 730 is rotated in a direction corresponding to the high 
level (H) of the detecting signal. As a result, the movable screening 
plate 742 is moved in a direction parallel to the fixed screening plate 
741. The movable screening plate 742 is stopped at a position so that the 
holes of the fixed screening plate 741 and the movable screening plate 742 
do not overlap with each other. As a result, the heater 500 is screened 
from the wind from the cooling fan 700 by these screening plates 741 and 
741. Thus, the heater 500 to which the AC current is continuously supplied 
radiates heat, and the fixing temperature is increased in a 
"closing-holes-period" as shown in FIG. 86. 
When the fixing temperature is increased and reaches the upper limit, the 
detecting signal from the first temperature detecting circuit 510 falls to 
the low level (L). The motor driving circuit 729 reversely drives the 
motor 730 so that the movable screening plate 742 is moved in a reverse 
direction. The movable screening plate 742 is stopped at a position so 
that the holes of the fixed screening plate 741 and the movable screening 
plate 742 overlap with each other. As a result, the wind from the cooling 
fun 700 passes through the holes of the respective screening plates 741 
and 742 and is supplied to the heater 500. Thus, the fixing temperature is 
gradually decreased in an "opening-holes-period" as shown in FIG. 86. 
If the fixing temperature is further increased from the upper limit and 
reaches the maximum temperature (MAX) although the wind from the cooling 
fan 700 is supplied to the heater 500 through the holes of the respective 
screening plates 741 and 742, the detecting signal from the second 
temperature detecting circuit 520 falls to the low level (L). The triac 
503 is thus turned off. As a result, the AC current to the heater 500 is 
shut off, so that the fixing temperature is rapidly decreased from the 
maximum temperature (MAX). 
In the above fourteenth and fifteenth examples, since the AC current is 
continuously supplied from the AC power line to the heater 500, the light 
devices which share the AC power line is not affected in the same manner 
as in the thirteenth example. 
The temperature controller according to the first example through the 
fifteenth example is applied to the temperature control of the fixing unit 
in the electrophotographic type printer and copy machine. Further, the 
temperature controller may be also applicable to a machine in which the 
on-and-off operation of the AC power supply is performed for the 
temperature control. 
The present invention is not limited to the aforementioned embodiments, and 
variations and modifications may be made without departing from the scope 
of the claimed invention.