Heater control apparatus with variable input voltage rectification

A heater control apparatus wherein a voltage level of the power supply is discriminated by an input voltage discrimination circuit, and in case that an input voltage applied by an AC power supply is the lowest level, the heater is excited by a current corresponding to a full-wave of the input voltage. In case that the input voltage applied by an AC power supply is a level other than the lowest level, the heater is excited by a current substantially equal to that in the case of the input voltage of the lowest level. The exciting current control circuit is composed of a bridge diode, a current limiting resistor for limiting a current flow from the AC power supply, a Zener diode for setting an input voltage discrimination standard value for discriminating a level of the input voltage, a photo coupler for generating an input voltage discrimination signal, and an exciting current signal generating circuit for generating an exciting current signal. In case that a voltage of an AC power supply is the lowest level, the heater is excited by a current corresponding to a full-wave of the input voltage, and in case that the input voltage is a high level, the heater is excited by a current corresponding to a 1/4 wave of the input voltage. An exciting current control circuit is provided so that an exciting current signal is generated to make the temperature control properties in the both cases are the same with each other.

BACKGROUND OF THE INVENTION
 1. Technical Field
 This invention relates to a heater control apparatus for use in laminaters
 or the like.
 2. Description of the Prior Art
 FIG. 13 shows a conventional heater control apparatus, wherein reference
 numeral 1 denotes a temperature control circuit, 2 denotes a power element
 consisting of a both-way conducting element, such as a triac, 2A denotes
 an AC power supply, 3 denotes a heater heated by a current applied from
 the AC power supply 2A through the power element 2, and 4 denotes a
 temperature detection element, such as a thermister for detecting the
 temperature of the heater 3 or an object to be heated by the heater 3.
 In such conventional heater control apparatus, the temperature control
 circuit 1 receives a detected temperature signal a from the temperature
 detection element 4 and outputs an exciting current control signal b for
 controlling the power element 2. The power element 2 is controlled by the
 exciting current control signal b and the heater 3 is heated.
 The temperature control circuit 1 comprises a detected temperature
 processing circuit 5 which receives the detected temperature signal a from
 the temperature detection element 4 and outputs a detected temperature
 processing signal c of a predetermined level, a temperature setting device
 6 for setting a temperature of the heater 3 or the object heated by the
 heater 3, a comparator 7 for comparing a temperature setting signal d
 outputted from the temperature setting device 6 with the detected
 temperature processing signal c outputted from the detected temperature
 processing circuit 5, and a zero crossing type power element driver 8b for
 receiving a control signal e from the comparator 7 and generating the
 exciting current control signal b, wherein the temperature of the heater 3
 or the object heated by the heater 3 is maintained at the temperature set
 by the temperature setting device 6.
 The zero crossing type power element driver 8b comprises a photo triac
 coupler and resistors as shown in FIG. 13, for example.
 R1 and R2 shown in FIG. 13 denote resistors.
 FIG. 14 shows a timing chart of an operation of the conventional heater
 control apparatus, wherein hatched wave form portions in FIG. 14(a) show a
 heater current and solid line portions show an input voltage. In FIG.
 14(b), a reference symbol c designates the detected temperature processing
 signal, and d designates the temperature setting signal. In FIG. 14(c), a
 reference symbol e designates the control signal. In FIG. 14(d), a
 reference symbol b designates the exciting current control signal.
 The function of the conventional heater control apparatus will now be
 explained with reference to FIG. 14.
 When the detected temperature processing signal c is lower in level than
 the temperature setting signal d, as shown in FIG. 14(b), the control
 signal e outputted from the comparator 7 is low level as shown in FIG.
 14(c), so that positive pulses and negative pulses of the exciting current
 control signal b are generated at such a timing that the input voltage
 (sine wave) applied by the AC power supply 2A becomes zero, as shown in
 FIG. 14(d).
 The power element 2 receives the positive and negative pulses of the
 exciting current control signal b and supplies the maximum heater current
 indicated by the hatched portions in FIG. 14(a) to the heater 3 for a
 period of time from t1 to t2 and a period of time from t3 to t4.
 In a period of time from t2 to t3, the detected temperature processing
 signal c is higher in level than the temperature setting signal d as shown
 in FIG. 14(b), and the control signal e outputted from the comparator 7
 becomes high level (H) as shown in FIG. 14(c), so that the zero crossing
 type power element driver 8b maintains the exciting current control signal
 b at zero volt.
 The power element 2 receives the-exciting current control signal b
 maintained at zero volt, and stops the current supply to the heater for
 the period of time from t2 to t3 as shown in FIG. 14(d).
 By repeating the above operations, the temperature of the heater 3 or the
 object heated by the heater 3 can be controlled to a temperature
 corresponding to the temperature setting signal d set by the temperature
 setting device 6.
 However, the detected temperature signal a detected by the temperature
 detection element 4 fluctuates in a range due to the time lag of the
 thermal transmission between the heating portion of the heater 3 and the
 temperature detection element 4.
 The range of fluctuation is varied according to the input voltage and
 becomes wide when the input voltage becomes high, so that the average
 temperature becomes high.
 Accordingly, it is necessary to adjust the temperature setting signal d set
 by the temperature setting device 6 so that the mean value of the detected
 temperature signal a in case that an input voltage of the lowest level is
 applied is equal to the mean value of the detected temperature signal a in
 case that an input voltage of a level other than the lowest level is
 applied, in the conventional heater control apparatus to which at least
 two levels of input voltage can be applied.
 Further, in the prior art, the range of the fluctuation of the detected
 temperature signal a cannot be adjusted.
 SUMMARY OF THE INVENTION
 An object of the present invention is to provide a heater control apparatus
 which solves the above tasks and problems.
 Another object of the present invention is to provide a heater control
 apparatus to which at least two levels of input voltage such as AC 100V
 and AC 200V are applied selectively, wherein the temperature control
 properties thereof in case that one level of input voltage is applied
 thereto is equal to that in case that the other level of input voltage is
 applied, and wherein no adjustment of the temperature control properties
 thereof is required.
 A further object of the present invention is to provide a heater control
 apparatus comprising a power element for exciting a heater, a temperature
 detection element for detecting a temperature of the heater or an object
 heated by the heater, and a temperature control circuit for generating an
 exciting current control signal for the power element, wherein in case
 that an input voltage applied by an AC power supply is the lowest level,
 the heater is excited by a current corresponding to a full-wave of the
 input voltage, and in case that the input voltage applied by an AC power
 supply is a level other than the lowest level, the heater is excited by a
 current substantially equal to that in the case of the input voltage of
 the lowest level, the current in case of the level other than the lowest
 level being obtained by setting a range of an applying time of the input
 voltage to the heater from a zero crossing point of the input voltage
 according to the level of the AC power source and the frequency thereof.
 The heater control apparatus further comprises an exciting current control
 circuit having a zero crossing detecting circuit for detecting the zero
 crossing point of the input voltage, an input voltage discrimination
 circuit for discriminating levels of the input voltage, a frequency
 setting switch which is set according to a zero crossing detecting signal,
 an input voltage discrimination signal and the frequency of the AC power
 supply, an exciting current signal generating circuit for detecting the
 state of the frequency setting switch and generating a first exciting
 current signal, and a transistor for receiving the first exciting current
 signal and generating a second exciting current signal.
 Yet further object of the present invention is to provide a heater control
 apparatus comprising a power element for exciting a heater, a temperature
 detection element for detecting a temperature of the heater or an object
 heated by the heater, and a temperature control circuit for generating an
 exciting current control signal for the power element, wherein in case
 that a voltage of an AC power supply is the lowest level, the heater is
 excited by a current corresponding to a full-wave of the input voltage,
 and in case that the input voltage is a high level, the heater is excited
 by a current corresponding to a 1/4 wave of the input voltage.
 The heater control apparatus further comprises an exciting current control
 circuit having a full-wave rectifier for rectifying a full-wave of the
 input voltage, a current limiting resistor for limiting a current flow
 from the AC power supply, a Zener diode for setting an input voltage
 discrimination standard value for discriminating a level of the input
 voltage, a photo coupler for generating an input voltage discrimination
 signal, an exciting current signal generating circuit for receiving the
 input voltage discrimination signal and generating a first exciting
 current signal, and a transistor for receiving the first exciting current
 signal and generating a second exciting current signal.
 The exciting current generating circuit comprises an inverter gate and
 three D-flip-flops, wherein a control signal is generated for applying a
 half cycle among the two cycles of the input voltage to the heater and a
 voltage of 1/4 of the full-wave of the input voltage is applied to the
 heater.
 The exciting current signal generating circuit comprises an inverter gate
 and five D-flip-flops, wherein a control signal is generated for applying
 one cycle among the four cycles of the input voltage to the heater and a
 voltage of 1/4 of the full-wave of the input voltage is applied to the
 heater.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 An embodiment of this invention will be explained with reference to FIGS. 1
 to FIG. 5.
 In FIG. 1, parts of the apparatus shown in FIG. 1 which are similar to
 corresponding parts of the apparatus shown in FIG. 13 have been given
 corresponding reference numerals and need not be further redescribed.
 However, kindly note that the power element driver 8b in the conventional
 heater control apparatus is of zero crossing type, whereas a power element
 driver 8a of the present invention is of the non-zero crossing type.
 The heater control apparatus of the present invention comprises an exciting
 current control circuit 9. The exciting current control circuit 9 having,
 as shown in FIG. 1, a zero crossing detecting circuit 10 which detects a
 zero crossing point of an input voltage applied by a AC power supply 2A
 and outputs a zero crossing detection signal f, an input voltage
 discrimination circuit 11 which discriminates a level of an input voltage
 applied by the AC power supply 2A and outputs an input voltage
 discrimination signal g, a frequency setting switch 12 which receives the
 zero crossing detection signal f and the input voltage discriminating
 signal g, and sets a frequency corresponding to a frequency of the AC
 power supply 2A, an exciting current signal generating circuit 13 which
 detects the setting state of the frequency setting switch 12 and generates
 an exciting current signal h1, and a transistor 14 as an inversion
 switching element, which receives the exciting current signal h1 and
 outputs an exciting current signal h2.
 The heater control apparatus of the present invention is characterized in
 that, the non-zero crossing type power element driver 8a is used instead
 of the conventional zero crossing type power element drover 8b in order to
 generates an exciting current control signal b when the non-zero crossing
 type power element driver 8a receives the exciting current signal h2
 outputed from the exciting current control circuit 9 and a control signal
 e outputted from a comparator 7. In FIG. 1, R3 denotes a resistor.
 FIG. 2, shows an example of the zero crossing detecting circuit 10 wherein
 a photo coupler 17a is turned ON when a current is flowed in the (+)
 direction from the AC power supply 2A to the photo coupler 17a passing
 through a current limiting resistor 15a, so that the zero crossing
 detecting signal f becomes low level "L". Further, the photo coupler 17a
 is turned OFF when a current is flowed in the (-) direction from the AC
 power supply 2A to the current limiting resistor 15a passing through the
 photo coupler 17a, so that the zero crossing detecting signal f becomes
 high level "H". The zero crossing detecting signal f is changed to "L"
 level from "H" level or to "H" level from "L" level at substantially zero
 volt of the AC power supply 2A.
 In FIG. 2, a diode 16 prevents a large counter voltage from being applied
 to the input terminal of the photo coupler 17a when the current is flowed
 in the (-) direction from the AC power supply 2A, and the current limiting
 resistor 15a limits the current passing through the photo coupler 17a and
 the diode 16. R4 denotes a resistor.
 FIG. 3 shows an example of the input voltage discrimination circuit 11
 which is applicable to three levels of AC power supply 2A. As shown in
 FIG. 3, an input voltage applied by the AC power supply is full-wave
 rectified by a bridge diode 18 and smoothed by a capacitor 19a to obtain a
 DC voltage. For example, AC 100V is converted into DC 141V, AC 120V is
 converted into DC 170V or AC 200V is converted into DC 283V. A first DC
 voltage level at which a photo coupler 17b is turned ON is set by a
 current limiting resistor 15b and a Zener diode 20a. A second DC voltage
 level at which a photo coupler 17c is turned ON is set by a current
 limiting resistor 15c and a Zener diode 20b. For example, if the first DC
 voltage level is set to DC 170V (AC 120V) and the second DC voltage level
 is set to DC 283V (AC 200V), input voltage discrimination signals g1 and
 g2 become "H" when the input voltage is AC 100V, whereas the input voltage
 discrimination signal g1 becomes "L" and the input voltage discrimination
 signal g2 becomes "H" when the input voltage is AC 120V. The input voltage
 discrimination signals g1 and g2 become "L" when the input voltage is AC
 200V.
 In FIG. 3, R5 and R6 denote resistors.
 FIG. 4 shows an example of the exciting current signal generating circuit
 13, wherein an exclusive OR gate 22a generates a trigger pulse signal j
 when the zero crossing detecting signal f is changed to "H" from "L" and
 to "L" from "H".
 A monostable multivibrator 27 generates an exciting current signal h3
 determined by a combination of one of current exciting time setting
 resistors 24a to 24d and a capacitor 19b when the trigger signal j is
 generated.
 Analog switches 26a to 26f are turned ON when frequency setting signals i1
 and i2, and input voltage discrimination signals g2 and g4 are "H", and
 turned OFF when the frequency setting signals i1 and i2 and the input
 voltage discrimination signals g2 and g4 are ".sup.L".
 The analog switches 26a and 26c are turned ON when the frequency setting
 switch 12 is turned OFF and an input voltage of 50 Hz is applied.
 The analog switches 26b and 26d are turned ON when the frequency setting
 switch 12 is turned ON and an input voltage of 60 Hz is applied.
 Further, the input voltage discrimination signal g2 becomes "H" when the
 input voltage is AC 120V, and becomes "L" when the input voltage is AC
 200V. The analog switch 26e is turned ON when the input voltage is AC
 120V, and the analog switch 26f is turned ON when the input voltage is AC
 200V.
 Accordingly, the capacitor 19b is connected to the current exciting time
 setting resistor 24a when the input voltage is AC 120V/50 Hz, connected to
 the current exciting time setting resistor 24b when the input voltage is
 AC 120V/60 Hz, connected to the current exciting time setting resistor 24c
 when the input voltage is AC 200V/50 Hz, and connected to the current
 exciting time setting resistor 24d when the input voltage is AC 200V/60
 Hz.
 Specifically, each of the current exciting time setting resistors 24a to
 24d is determined according to the level of the input voltage and the
 frequency.
 A D-flip-flop 28 generates an exciting current signal h4 of which frequency
 is a half of the exciting current signal h3 at the heater exciting
 initiation time or when the leading edge of the exciting current signal h3
 is generated.
 An exclusive OR gate 22b generates an exciting current signal h5 when the
 exciting current signal h4 is changed to "L" from "H" and to "H" from "L".
 An inverter gate 23C generates an exciting current signal h6 which is an
 inverted signal of the exciting current signal h5.
 A NAND gate 21 receives the input voltage discrimination signals g1 and g2
 and generates an input voltage discrimination signal g3. The input voltage
 discrimination signals g1 and g2 are "H" only when the input voltage is AC
 100V, so that the input voltage discrimination signal g3 becomes "L".
 Further, either one of the input voltage discrimination signals g1 and g2
 is "L" when the input voltage is AC 120V or AC 200V, so that the input
 voltage discrimination signal g3 becomes "H".
 An AND gate 25 receives the exciting current signal h6 and the input
 voltage discrimination signal g3 and generates the exciting current signal
 h1. The exciting current signal h1 becomes "L" when the input voltage
 discrimination signal g3 is "L" (input voltage is AC 100V), so that the
 heater is excited by the full current. When the input voltage
 discrimination signal g3 is "H" (input voltage is AC 120V or AC 200V), the
 exciting current signal h6 is equal to the exciting current signal h1.
 In FIG. 4, R7 to R10 represent resistors, and C1 represents a capacitor.
 FIG. 5 shows a timing chart of an operation of the heater control apparatus
 according to the present invention, wherein hatched wave form portions in
 FIG. 5(a) show a heater current and solid line portions and dotted line
 portions show an input voltage. In FIG. 5(b), a reference symbol c
 designates the detected temperature processing signal, and d designates
 the temperature setting signal. In FIG. 5(c), a reference symbol f
 designates the zero crossing detecting signal. In FIG. 5(d), a reference
 symbol h2 designates the second exciting current signal. The first
 exciting current signal h1 is an inverted signal of the second exciting
 current signal h2 and not shown in FIG. 5.
 In FIG. 5(e), a reference symbol e designates the control signal.
 In FIG. 5(f), a reference symbol b designates the exciting current control
 signal.
 The function of the heater control apparatus according to the present
 invention will now be explained with reference to FIG. 5.
 As shown in FIG. 5, the zero crossing detecting signal f is changed to "H"
 and "L" alternately at the zero crossing points of the input voltage of
 the AC power source 2A.
 The exciting current signal h2 consists of pulses each generated after a
 time t from the leading edge and trailing edge of the zero crossing
 detecting signal f (zero crossing point of the input voltage). The time t
 is so determined that the value of the exciting current to the heater in
 case that the input voltage is a level other than the lowest level is
 substantially equal to that in case that the input voltage is the lowest
 level with respect to each frequency of the power supply 2A set by the
 frequency setting switch 12 and the input voltage discrimination signal.
 The time t is set previously in the exciting current signal generating
 circuit 13 for each level of the input voltage and the frequency. The
 non-zero crossing type power element driver 8a receives the exciting
 current signal h2 and the control signal e and outputs the (+) or (-)
 pulses of the exciting current control signal b at the leading edge of the
 exciting current signal h2 when the control signal e is "L", that is, the
 detected temperature signal c is lower than the temperature setting signal
 d. The heater current is flowed through the power element 2 until the next
 zero crossing point of the input voltage by the exciting current control
 signal b.
 In case that the input voltage is the lowest level, the exciting current
 generating circuit 13 receives the input voltage discrimination signal and
 generates the exciting current signal h1 of "L". Accordingly, the
 transistor 14 is turned OFF and the heater is excited by the maximum
 heater current.
 The heater control apparatus of the present invention can be used for at
 least two levels of the power supply. According to the heater control
 apparatus of the present invention, the level of the power supply is
 discriminated, and the heater current is automatically controlled in such
 a way that if the input voltage is lowest level the heater is excited by
 the maximum heater current and if the input voltage is a level other than
 the maximum level the heater current is reduced automatically to a
 predetermined value so that the heater is heated equally at both levels of
 the input voltage. Accordingly, it is not necessary to adjust the
 temperature setting for the different input voltages.
 In the embodiment mentioned above, the function of the exciting current
 generating circuit 13 shown in FIG. 4 may be achieved by any
 microcomputer.
 A further embodiment of this invention will be explained with reference to
 FIG. 6 and FIG. 7. In FIG. 6, parts of the apparatus shown in FIG. 6 which
 are similar to corresponding parts of the apparatus shown in FIG. 13 have
 been given corresponding reference numerals and need not be further
 redescribed.
 The heater control apparatus of the present invention comprises the
 exciting current control circuit 9 consisting of, as shown in FIG. 6, a
 bridge diode 18 for full-wave rectifying the input voltage applied by the
 AC power supply 2A, the current limiting resistor 15b for limiting the
 heater current from the AC power supply 2A, the Zener diode 20a for
 setting the input voltage discrimination value for discriminating the
 input voltage applied by the AC power supply 2A, the photo coupler 17b for
 generating the input voltage discrimination signal g1, the exciting
 current signal generating circuit 13 which receives the input voltage
 discrimination signal g1 and generates the exciting current signal h1, and
 the transistor 14 as an inversion switching element which receives the
 exciting current signal h1 and outputs the exciting current signal h2.
 The exciting current generating circuit 13 is composed of a logic circuit
 element which receives the input voltage discrimination signal g1 from the
 photo coupler 17b and forms the first exciting current signal h1 which is
 1/4 of the frequency of g1.
 FIG. 7 shows a timing chart of an operation of the heater control apparatus
 according to the present invention, wherein hatched wave form portions in
 FIG. 7(a) show a heater current and solid line portions and dotted line
 portions show an input voltage. Vz shows a voltage as an input voltage
 discrimination standard value. In FIG. 7(b), a reference symbol c
 designates the detected temperature processing signal, and d designates
 the temperature setting signal. In FIG. 7(c), a reference symbol g1
 designates the input voltage discrimination signal.
 In FIG. 7(d), a reference symbol h2 designates the second exciting current
 signal. The first exciting current signal h1 is an inverted signal of the
 second exciting current signal h2 and not shown in FIG. 7.
 In FIG. 7(e), a reference symbol e designates the control signal. In FIG.
 7(f), a reference symbol b designates the exciting current control signal.
 The function of the heater control apparatus according to the present
 invention will now be explained with reference to FIG. 7.
 As shown in FIG. 7(c), the input voltage discrimination signal g1 becomes
 the high level "H" if the absolute value of the input voltage applied by
 the AC power supply through the power element 2 is smaller than the input
 voltage discrimination standard value set by the Zener diode 20a in the
 term between t1 to t2, for example, whereas the input voltage
 discrimination signal g1 becomes the low level "L" as shown in FIG. 7(c)
 if the absolute value of the input voltage is higher than the input
 voltage discrimination standard value in the term between t2 to t3, for
 example.
 Accordingly, the input voltage discrimination signal g1 becomes a pulse
 signal which is changed to "H" and "L" alternately as shown in FIG. 7(c)
 when the maximum value (absolutely value) of the input voltage is higher
 than the input voltage discrimination standard value.
 The exciting current signal generating circuit 13 receives the input
 voltage discrimination signal g1 and outputs the exciting current signal
 h1 which is 1/4 of the frequency of g1. The transistor 14 receives the
 exciting current signal h1 and outputs the exciting current signal h2 as
 shown in FIG. 7(d).
 The power element driver 8a receives the exciting current signal h2 shown
 in FIG. 7(d) and the control signal e shown in FIG. 7(e) and outputs the
 (+) or (-) pulses of the exciting current control signal b as shown in
 FIG. 7(f) at such a timing that the input voltage (sine wave) applied by
 the AC power supply 2A becomes zero only when the exciting current signal
 h2 is high level "H" and the control signal e is low level "L". The power
 element 2 excites the heater 3 according to the exciting current signal b.
 Accordingly, in the heater control apparatus of this embodiment, the heater
 3 is excited by the heater current of 1/4 wave form of the wave form
 indicated by the solid line shown in FIG. 7(a).
 FIG. 8 shows an exciting current generating circuit 13A which is an
 embodiment of the exciting current signal generating circuit 13 shown in
 FIG. 6.
 As shown in FIG. 8, the exciting current signal generating circuit 13A is
 composed of an inverter gate 36, three D-flip-flops 37a, 37b, 37c, a
 resistor 38 and a capacitor 39.
 The D-flip-flop 37b receives the input voltage discrimination signal g1
 from the photo coupler 17b and generates a frequency divided signal i2
 which is a signal frequency divided the input voltage discrimination
 signal g1 by 2 at such a timing that the input voltage discrimination
 signal g1 is changed to the high level "H" from the low level "L" as shown
 in FIG. 9(c).
 The D-flip-flop 37c receives the frequency divided signal i2 and generates
 a frequency divided signal j which is a signal frequency divided the input
 voltage discrimination signal g1 by 4 at such a timing that the input
 frequency divided signal i2 is changed to the high level "H" from the low
 level "L" as shown in FIG. 9(d).
 The inverter gate 36 receives the input voltage discrimination signal g1
 and generates a signal h which is an inverted signal of the input voltage
 discrimination signal g1, as shown in FIG. 9(b).
 The D-flip-flop 37a receives the inverted signal h and the frequence
 divided signal j and generates an exciting current signal h1 which is a
 signal frequency divided the inverted signal h by 2 at such a timing that
 the inverted signal h is changed to the high level "H" from the low level
 "L" as shown in FIG. 9(e) only when the frequency divided signal j is the
 high level "H". The exciting current signal h1 becomes a low level "L"
 every four pulses of the inverted signal h so as to maintain a high level
 "H" when the frequency divided signal j is the low level "L".
 FIG. 10 shows an exciting current signal generating circuit 13B which is
 the other embodiment of the exciting current signal generating circuit 13
 shown in FIG. 6.
 As shown in FIG. 10, the exciting current signal generating circuit 13B is
 composed of the inverter gate 36, five D-flip-flops 37a, 37b, 37c, 37d,
 37e, the resistor 38 and the capacitor 39.
 The D-flip-flop 37c receives the input voltage discrimination signal g1
 from the photo coupler 17b and generates the frequency divided signal i2
 which is a signal frequency divided the input voltage discrimination
 signal g1 by 2 at such a timing that the input voltage discrimination
 signal g1 is changed to the high level "H" from the low level "L" as shown
 in FIG. 11(c).
 The D-flip-flop 37d receives the frequency divided signal i2 and generates
 a frequency divided signal j which is a signal frequency divided the input
 voltage discrimination signal g1 by 4 at such a timing that the input
 frequency divided signal i2 is changed to the high level "H" from the low
 level "L" as shown in FIG. 11(d).
 The D-flip-flop 37e receives the frequency divided signal j and generates a
 frequency divided signal k which is a signal frequency divided the input
 voltage discrimination signal g1 by 8 at such a timing that the input
 frequency divided signal j is changed to the high level "H" from the low
 level "L" as shown in FIG. 11(e).
 The inverter gate 36 receives the input voltage discrimination signal g1
 and generates a signal h which is an inverted signal of the input voltage
 discrimination signal g1, as shown in FIG. 11(b).
 The D-flip-flops 37a receives the inverted signal h and generates a
 frequency divided signal i1 which is a signal frequency divided the
 inverted signal h by 2 at such a timing that the inverted signal h is
 changed to the high level "H" from the low level "L" as shown in FIG.
 11(f).
 The D-flip-flop 37b receives the frequency divided signals i1 and k and
 generates a frequency divided signal h1 which is a signal frequency
 divided the inverted signal h by 4 at such a timing that the frequency
 divided signal i1 is changed to the high level "H" from the low level "L"
 as shown in FIG. 11(g), only when the frequency divided signal k is the
 high level "H". The exciting current signal h1 becomes a low level "L"
 every eight pulses of the inverted signal h so as to maintain a high level
 "H" when the frequency divided signal k is the low level "L".
 Accordingly, the heater control apparatus using the second exciting current
 signal generating circuit 13B exhibits a second control property shown in
 FIG. 12.
 In FIG. 12(a), hatched wave form portions show the heater current. The wave
 form of the heater current differs from that of the first control property
 shown in FIG. 7, and the exciting current is reduced by 1/4. Specifically,
 the exciting current of hatched portions shown in FIG. 12(a) are applied
 to the heater during only one cycle among four cycles of the input voltage
 applied by the AC power supply 2A indicated by the solid and dotted lines
 in FIG. 12(a), when the control signal e shown in FIG. 12(e) is low level
 "L", that is, the detected temperature processing signal c is lower than
 the temperature setting signal d.
 Further, in this embodiment, the exciting current is applied to the heater
 during the full-wave of the input voltage, whereas, in the former
 embodiment, the exciting current is applied to the heater during the
 half-wave of the input voltage, so that the higher harmonic current in the
 exciting current can be reduced.
 As stated above, the input voltage discrimination signal g1 is maintained
 at the high level "H" when the maximum value (absolute value) of the input
 voltage is lower than the input voltage discrimination standard value.
 The exciting current signal generating circuit 13 generates the exciting
 current h1 of low level "L" when the input voltage discrimination signal
 g1 is maintained at the high level "H" during a term more than one cycle
 of the AC power supply 2A, so that the transistor 14 is turned OFF and the
 heater is excited by the current of full-wave.
 Accordingly, the input voltage discrimination standard value is set to a
 value larger than the maximum value of the input voltage applied by the
 low voltage power supply, but lower than the maximum value of the input
 voltage applied by the high voltage power supply.
 As stated above, the voltage applied to the heater by the high voltage
 power supply can substantially be equal to that by the low voltage power
 supply by varying the applying manner to the heater.
 Specifically, an effective value Erms1 of the AC voltage is expressed by a
 following formula (1) if the maximum value is Em1.
EQU Erms1=Em1/2 (1)
 Further, an effective value Erms2 of 1/4 wave (solid line portions) of the
 AC voltage shown in FIG. 7 is expressed by a following formula (2) if the
 maximum value is Em2.
 ##EQU1##
 The effective value Erms2 in case of 1/4 wave of the AC voltage is
 expressed by a following formula (3).
EQU Erms2=2.multidot.Erms1/8=Erms1/2 (3)
 Accordingly, the effective value of full-wave of the AC voltage becomes
 equal to the effective value in case that the 1/4 wave of the AC voltage
 of two times.
 It can be applied similarly to the heater control apparatus using the
 exciting current signal generating circuit 13B shown in FIG. 10.
 Accordingly, in the heater control apparatus of the present invention, it
 is not necessary to adjust the temperature setting according to the level
 of the AC voltage on the contrary to the conventional apparatus, because
 the level of the input voltage is discriminated by the exciting current
 control circuit and in case of low level of the input voltage, the
 full-wave of the input voltage is applied to the heater and in case of
 high level of the input voltage, the 1/4 wave of the input voltage is
 applied to the heater, so that the effective values of the input voltage
 applied to the heater in both cases are substantially equal to each other.
 As stated above, the heater control apparatus according to the present
 invention has following merits.
 (1) It is possible to generate automatically the exciting current signal
 according to at least two levels of the input voltage by the exciting
 current control circuit.
 (2) The temperature control properties for different levels of the input
 voltage can be made equal without adjusting.
 (3) It is possible to generate automatically the exciting current signal
 which can be changed between the full-wave exciting and the 1/4 wave
 exciting according to the levels of the input voltage which are different
 about twice from each other by the exciting current control circuit.
 (4) The temperature control properties for different levels of the input
 voltage which are different about twice from each other can be made equal
 without adjusting.