Charging device for electrophotographic equipment

A scorotron charging device applicable to electrophotographic image forming equipment and having a main charger with discharge wires and a grid and applying voltages to the wires and the grid by respective power sources. The two power sources are controlled by the same control signal. In response to the control signal, a voltage starts being applied to the grid before a voltage is applied to said wires and ends being applied to the grid after the voltage application to the wires has ended. This allows the charge potential of a photoconductive element to rise and fall rapidly.

BACKGROUND OF THE INVENTION 
The present invention relates to a charging device for electrophotographic 
image forming equipment and, more particularly, to a scorotron charging 
device having a main charger with discharge wires and a grid and applying 
voltages to the wires and the grid by respective power sources. 
It is a common practice with electrophotographic image forming equipment, 
e.g., a copier, facsimile transceiver or laser printer to charge, before 
imagewise exposure, the surface or a photoconductive element or image 
carrier by a charging device which effects corona discharge. For the 
corona discharge, a voltage is applied to a discharge wire forming part of 
a main charger which is included in the discharging device. When the 
products of discharge, toner particles or similar impurities deposit on 
the discharge wires, the corona discharge and, therefore, the charge 
distribution on the surface of the photoconductive element becomes 
irregular. This prevents a toner from depositing in a constant amount on a 
latent image electrostatically formed on the photoconductive element in 
the event of development. A scorotron charging device is an effective 
implementation against such an occurrence and has a grid between the 
discharge wire and the photoconductive element. Specifically, a scorotron 
charger generally has a main charger made up of one or more discharge 
wires and a casing connected to ground, a grid interposed between the 
wires and the photoconductive element, a common power source for applying 
voltages to both of the wires and grid, a switch connected between the 
wires and grid and the common power source, a controller for controlling 
the switch, and a varistor intervening between the switch and the grid. 
When the controller closes the switch, part of the charge generated by the 
wires is released to ground via the casing while the other part is 
directed toward the grid and photoconductive element. Since a voltage 
controlled by the varistor is applied to the grid, the amount of charge to 
be deposited on the surface of the photoconductive element is also 
controlled. This type of scorotron charger is disclosed in, for example, 
Japanese Patent Laid-Open Publication No. 72177/1989. 
The conventional scorotron charger in which the discharge wires and grid 
share a single power source has a problem that a substantial period of 
time is necessary for the grid voltage to rise or fall. Assuming that the 
timing for starting charging the photoconductive element is delayed, then 
a bias voltage for development will rise first and cause a toner to 
deposit on the surface of the element other than an image area. This not 
only results in the waste of toner but also increases the load on a 
cleaning device for cleaning the photoconductive element. On the other 
hand, assuming that the wire voltage rises before the grid voltage, then 
the photoconductive element will be charged excessively to suffer from 
fast fatigue or to cause a toner or a carrier to deposit needlessly on the 
element. Further, since the power source for applying a voltage to the 
grid has a high impedance, it is likely that a current fails to flow from 
the wires to the grid and leaks from the wires to the photoconductive 
element to thereby burn the element 601. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a charging 
device for image forming equipment which eliminates the excessive charging 
of a photoconductive element and the burning of the element due to a leak 
which are ascribable to the time when the grid voltage starts rising or 
falling and the period of time necessary for the grid voltage to fully 
rise or fully fall. 
It is another object of the present invention to provide a charging device 
for image forming equipment which eliminates the deposition of needless 
toner or carrier on a photoconductive element by speeding up the rise or 
fall of the potential of the element or by starting and ending the 
application of a bias voltage for development in matching relation to the 
time when the charged area of the element passes a developing position. 
In accordance with the present invention, a scorotron charging device for 
electrophotographic image forming equipment comprises a main charger for 
discharging, a grid located in close proximity to the main charger, a main 
power source for applying a voltage to the main charger, a grid power 
source for applying a voltage to the grid, and a controller for 
controlling a timing for the main charger power source to apply a voltage 
and a timing for the grid power source to apply a voltage. 
Also, in accordance with the present invention, a scorotron charging device 
for electrophotographic image forming equipment comprises a main charger 
for discharging, a grid located in close proximity to the main charger, a 
controller for generating a trigger signal for commanding the start of 
application of voltages to the main charger and grid, a main charger 
applying section for applying a voltage to the main charger at a 
predetermined timing in response to the trigger signal, and a grid 
applying section for applying a voltage to the grid at a predetermined 
timing in response to the trigger signal. 
To better understand the present invention, a brief reference will be made 
to a conventional scorotron charging device, shown in FIG. 6. As shown, 
the charging device, generally 602, negatively charges a photoconductive 
element in the form of a drum 601 and has a main charger made up of wires 
602a and a casing 602b which is connected to ground, a grid 602c, a power 
source 603 for applying voltages to the wires 602a and grid 602c, and a 
varistor 604 connected between the grid 602c and a switch SO. A controller 
605 selectively turns on or off the switch SO for controlling the timing 
at which the power source should apply a voltage. The discharge wires 602a 
and grid 602c share the single power source 603. When the controller 605 
turns on the switch SO, part of the charge (negative) generated by the 
wires 602a is released to ground via the casing 602b while the other part 
is directed toward the grid 602c and the drum 60. Further, a voltage 
controlled by the varistor 604 is applied to the grid 602c so as to 
control the amount of charge to be deposited on the drum 601. 
The conventional charging device has various problems due to the fact that 
the wires 602a and grid 602c share a single power source, as discussed 
earlier. Specifically, a substantial period of time is necessary for the 
grid voltage to rise or fall. Assuming that the timing for starting 
charging the drum 601 is delayed, then a bias voltage for development will 
rise first and cause toner to deposit on the surface of the drum 601 at 
other than an image area. This not only results in the waste of toner but 
also increases the load on a cleaning device for cleaning the drum 601. 
Assuming that the wire voltage rises before the grid voltage, then the 
drum 601 will be charged excessively to suffer from fast fatigue or to 
cause a toner or a carrier to deposit needlessly on the element 601. 
Further, since the power source 603 for applying a voltage to the grid 
602c has a high impedance, it is likely that a current fails to flow from 
the wires 602a to the grid 602c and leaks from the wires 602a to the drum 
601 to thereby burn the drum 601. 
FIGS. 7A and 7B each show a relation of the voltages applied to the wires 
602a and grid 602c and the charge potential of the drum 601 to one 
another. In FIGS. 7A and 7B, horizontal dash-and-dot lines associated with 
the wire voltages are representative of a discharge start voltage while 
horizontal dash-and-dot lines associated with the drum potentials are 
representative of a target charge potential. Regarding time, the time 
t.sub.0 when the wire voltage starts rising is used as a reference. 
The relation shown in FIG. 7A holds when the voltage of the grid 602c 
starts rising later than the voltage of the wires 602a. In this condition, 
the wire voltage reaches the discharge start voltage at a time t.sub.1, 
i.e., on the elapse of a period of time T.sub.1 as counted from the time 
t.sub.0 and fully rises at a time t.sub.2 which is a period of time Tc 
later than the time t.sub.0. On the other hand, the grid voltage starts 
rising at a time t.sub.3 which is a period of time T.sub.2 later than the 
time t.sub.0 and fully rises on the elapse of a period of time Tg, i.e., 
at a time t.sub.4. Since the wire voltage does not reach the discharge 
start voltage up to the time t.sub.1, the charge having been accumulated 
on the drum 601 up to the time t.sub.1 is extremely small. During the 
interval between the times t.sub.1 and t.sub.3, the grid voltage does not 
rise despite the rise of the wire voltage to the discharge start voltage. 
This, coupled with the fact that the impedance between the grid 602c and 
ground is high and prevents a current from easily flowing from the wires 
602a to the grid 602c, charges the drum 601 excessively far beyond the 
target charge potential. Then, a current is apt to leak from the wires 
602a to the drum 601. As the grid voltage starts rising at the time 
t.sub.3, the charge potential once noticeably falls and then sequentially 
rises with the rise of the grid voltage. When a period of time Td from the 
time t.sub.0 at which the wire voltage starts rising to the time t.sub.4 
at which the grid voltage fully rises expires, the target charge potential 
is set up stably. 
The relation shown in FIG. 7B holds when the wire voltage and grid voltage 
start rising at the same time and the period of time Tg necessary for the 
grid voltage to fully rise is longer than the period of time Tc necessary 
for the wire voltage to do so. As shown, during the period of time T.sub.1 
in which the wire voltage reaches the discharge start voltage, only a 
small amount of charge is accumulated on the drum 601, as in the relation 
of FIG. 7A. Thereafter, although the wire voltage fully rises at the time 
t.sub.2, i.e., on the elapse of the period of time Tc, the charge 
potential of the drum 601 sequentially rises from the time t.sub.1 to the 
time t.sub.4 since the grid voltage fully rises at the time t.sub.4, i.e., 
on the elapse of the period of time Tg which is longer than Tc. During 
this period, the timing for starting charging of the drum 601 is delayed, 
i.e., the bias voltage for development rises first. Then, it is likely 
that a toner deposits on the surface of the drum 601 other than an image 
area, and that a toner or a carrier deposits on the image area on the drum 
601 with no regard to the direction of the bias voltage.

Referring to FIG. 1, a copier or similar image forming equipment of the 
type using a two-component developer is shown to which a scorotron 
charging device embodying the present invention is applied. As shown, a 
scorotron charging device, generally 102, negatively charges the surface 
of a photoconductive element or drum 101. A developing device 103 develops 
a latent image electrostatically formed on the charged surface of the drum 
101 by exposure by using a toner. An image density sensor 104 senses the 
density of reference toner images formed on the drum 101 in a 
predetermined pattern. A transfer charger 106 transfers the toner image 
formed on the drum 101 to a recording sheet 105 being transported. A 
separation charger 107 separates the recording sheet 105 from the drum 101 
after the image transfer. A separator in the form of a pawl 108 insures 
the separation of the recording sheet 105 from the drum 101. A cleaning 
device 109 removes the toner remaining on the drum 101 after the image 
transfer. A discharge lamp 110 dissipates the charge remaining on the drum 
101 after the drum 101 has been cleaned by the cleaning device 109. The 
developing device 103 has a toner hopper 111 storing fresh toner, not 
shown, a toner supply roller 112 for discharging the toner from the hopper 
111, a developing sleeve 115 for conveying the toner to the drum 101, a 
paddle 113 for agitating the developer, i.e., a mixture of toner and 
carrier to charge the toner by friction and conveying the developer to the 
developing sleeve 115, and a doctor blade 114 for regulating the amount of 
developer deposited on the developing sleeve 15. It is to be noted that a 
bias voltage for development is applied to the developing sleeve 115 for 
adjusting the amount of toner to deposit on the drum 101. In the figure, 
labeled Tb is the period of time necessary for the charged surface of the 
drum 101 to move from the charging device 102 to the developing sleeve 
115. 
In operation, while the drum 101 is rotated in a direction indicated by an 
arrow in the figure, the charging device 102 charges the surface of the 
drum 101 to a negative polarity by corona discharge. As the charged 
surface of the drum 101 is exposed imagewise, the charge is dissipated in 
matching relation to the intensity of light with the result that a latent 
image is electrostatically formed. The developing device 103 develops the 
latent image by the toner to convert it to a corresponding toner image. As 
the recording sheet 105 is transported at a predetermined timing, the 
toner image is transferred from the drum 101 to the sheet 105 by the 
transfer charger 106. The recording sheet 105 carrying the toner image 
thereon is separated from the drum 101 by the separation charger 107. The 
cleaning device 109 removes the toner remaining on the drum 101 after the 
image transfer, and then the discharge lamp 110 dissipates the charge 
remaining on the drum 101 by light. 
FIG. 2 shows the scorotron charging device 102 embodying the present 
invention. As shown, the charging device 102 has a main charger made up of 
discharge wires 102a and a casing 102b connected to ground, a grid 102c, a 
main power source 201 for applying a voltage to the wires 102a, a grid 
power source 202 for applying a voltage to the grid 102c, a switch S1 
connected between the wires 102a and the main power source 201, and a 
switch S2 connected between the grid 102c and the grid power source 202. A 
controller 203 selectively turns on or off the switches S1 and S2 at 
predetermined timings. In this manner, the wires 102a and the grid 102c 
are provided with respective power sources. For this reason, the 
conventional varistor does not exist between the grid 102c and the grid 
power source 202, i.e., a power source matching the voltage to be applied 
to the grid 102c is used. The controller 203 turns on or off the switches 
S1 and S2 such that the application of a voltage to the grid 102c starts 
earlier than the start of application of a voltage to the wires 102a and 
ends later than the end of application of the latter voltage, as shown in 
FIG. 3. This allows the voltage to the grid 102c and, therefore, the 
charge potential of the drum 101 to rise and fall rapidly. Further, the 
application of a bias voltage for development to the sleeve 115 is so 
controlled as to start and end the period of time Tb later than the start 
and end of the voltage to the wires 102a. This prevents the toner from 
depositing on the surface of the drum 101 other than an image area and 
prevents the drum 101 from being excessively charged or being burned by a 
leak. 
In the illustrative embodiment, the charge potential V.sub.d (background), 
the potential V.sub.l of an exposed area (image area), and the bias 
voltage V.sub.b for the developing sleeve 115 are selected to be -800 V, 
-100 V, and -600 V, respectively. The latent image is developed by the 
difference between the potential V.sub.l and the potential V.sub.b, i.e., 
V.sub.l -V.sub.b =500 V. 
The above-described advantages are attainable even with an arrangement 
wherein discharge wires and a grid share a single power source only if the 
voltages are applied at the timings shown in FIG. 3 via switches assigned 
to the wires and grid and a varistor associated with the grid. 
A reference will be made to FIG. 4 for describing an alternative embodiment 
of the present invention. In FIG. 4, the same parts and elements as those 
shown in FIG. 2 are designated by like reference numerals, and redundant 
description will be avoided for simplicity. As shown, the charging device 
102A has a controller 401 for generating signals commanding the start and 
end of discharge, and a composite power source unit 402. The power source 
unit 402 has a main power source section 402a and a grid power source 
section 402b for applying voltages to the discharge wires 102a and the 
grid 102c, respectively, in response to the signal from the controller 
401. As FIG. 4 indicates, the signals fed from the controller 401 to the 
main and grid power source sections 402a and 402b are identical. This 
implements the delivery of signals from the controller 401 to the power 
source unit 402 by a single line, thereby reducing noise ascribable to a 
harness. On receiving a signal commanding the start of voltage application 
from the controller 402a, the main power source section 402a starts 
applying a voltage while delaying it a predetermined period of time. On 
the other hand, the grid power source section 402b starts applying a 
voltage immediately in response to the signal. Conversely, on receiving a 
signal commanding the end of voltage application, the main power source 
section 402a ends the application immediately while the grid power source 
402b ends it a predetermined period of time later. 
FIG. 5 shows a relation of the voltage applied to the wires 102a by the 
charging device 102A, the voltage applied to the grid 102c, the bias 
voltage applied to the developing sleeve 115, and the potential of the 
drum 101 to one another. As FIG. 5 indicates, the grid voltage starts 
rising at a time t.sub.3 which is a period of time T.sub.2 earlier than 
the rise of the wire voltage and fully rises at a time t.sub.4 before the 
wire voltage reaches a discharge start voltage thereof, i.e., t.sub.1. 
Also, the charge potential of the drum 101 rises rapidly. The bias voltage 
for development rises at a time t.sub.5, i.e., on the elapse of the period 
of time Tb after the time t.sub.0. This is successful in achieving the 
same advantages as described in relation to the previous embodiment. 
The grid power source section 402b will be described more specifically. 
When a voltage is not applied to the grid 102c, a current to flow from the 
wires 102a to the drum 101 cannot be adjusted since the impedance between 
the grid 102c and ground is high, as stated earlier. In this condition, it 
is likely that the drum 101 is charged excessively or a current leaks from 
the wires 102a to the drum 101. In light of this, the grid power source 
section 402b is provided with two switches, although not shown in the 
figures. One of the two switches is used to apply the voltage to the grid 
102c (referred to as a switch A hereinafter), and the other is included in 
a circuit which shorts the circuiry between the grid 102c and ground 
(referred to as a switch B hereinafter). On receiving a signal commanding 
the start of application of a voltage to the grid 102c from the controller 
401, the grid power source section 402b closes the switch A to apply a 
voltage to the grid 102c and then opens the switch B to cancel the 
shorting between the grid 102c and ground. The short interval between the 
closing of the switch A and the opening of the switch B is provided since 
the voltage may fail to rise immediately after the switch A has been 
closed. To end the application of the voltage, the power source section 
402b closes the switch B and then opens the switch A. This surely frees 
the drum 101 from excessive charging and prevents a current from leaking 
from the wires 102a to the drum 101 while no voltages are applied to the 
grid 102c. 
While the above embodiment uses the switches A and B, they may be replaced 
with a lower impedance between the grid 102c and ground or circuitry 
capable of cancelling the shorting between the grid 102c and ground 
automatically in response to a voltage to the grid 102c. 
In summary, it will be seen that the present invention provides a charging 
device for image forming equipment which eliminates the excessive charging 
and burning of a photoconductive element ascribable to the time when a 
grid voltage starts rising or falling and the period of time necessary for 
it to fully rise or fully fall. In addition, the charging device of the 
invention causes the potential of the photoconductive element to rise and 
fall rapidly to prevent a toner or a carrier from depositing needlessly on 
the photoconductive element. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof.