U.S. patent number 3,909,614 [Application Number 05/510,336] was granted by the patent office on 1975-09-30 for scorotron power supply circuit.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Richard C. Marshall.
United States Patent |
3,909,614 |
Marshall |
September 30, 1975 |
Scorotron power supply circuit
Abstract
A scorotron for uniformly charging an electrostatographic
copying photoconductive surface has a coronode wire, a conductive
metal shield and a screen grid. The coronode wire is supplied
through a stabilizing resistor from an electrical inverter. The
screen and the shield are connected together and commonly connected
to ground through two series connected resistors. From a connection
point between the two resistors a feed-back loop is provided to
supply feed-back signals to a regulator connected at the input of
the inverter, which feed-back signals are indicative of the sum of
the screen and shield currents from the coronode wire corona
emission, but insensitive to proportional changes between the
screen current and the shield current.
Inventors: |
Marshall; Richard C.
(Harpenden, EN) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
10483953 |
Appl.
No.: |
05/510,336 |
Filed: |
September 30, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Dec 21, 1973 [GB] |
|
|
59538/73 |
|
Current U.S.
Class: |
250/324;
361/225 |
Current CPC
Class: |
G03G
15/0266 (20130101); G03G 15/0291 (20130101) |
Current International
Class: |
G03G
15/02 (20060101); G03G 015/00 () |
Field of
Search: |
;250/324,325,326
;317/262A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Church; Craig E.
Claims
What is claimed is:
1. In electrostatographic copying apparatus including scorotron
corona charging apparatus for charging a surface, wherein said
scorotron includes a conductive shield a conductive control grid
and at least one corona emitting electrode, the improvement in said
scorotron charging apparatus comprising:
adjustable electrical power supply means connected to said corona
emitting electrode to provide a current supply thereto;
sensing circuit means, including resistor means, for sensing a
voltage produced on said grid by said corona emitting
electrode,
said sensing circuit means being connected to said power supply
means to supply a control signal to said power supply means
proportional to said voltage produced on said grid from said corona
emitting electrode,
said control signal from said sensing circuit means controlling
said current supply from said power supply to said corona emitting
electrode to maintain said voltage at said grid substantially
constant,
wherein said conductive control grid and said conductive shield are
directly electrically connected together and to said resistor means
of said sensing current means,
said resistor means conducting the combined total current from said
control grid and said conductive shield therethrough,
said combined total current through said resistive means providing
said control signal to said power supply means.
2. The copying apparatus of claim 1 further including regulator
means connected to said resistor means, and inverter means
connected between said regulator means and said corona emitting
electrode.
3. In a scorotron for uniformly charging an electrostatographic
copying photoconductive surface comprising a corona emitting
coronode wire, a conductive metal shield and a conductive screen
grid, wherein the coronode wire is electrically supplied from a
regulator, the improvement wherein the screen grid and the shield
are connected together and commonly connected to ground through at
least two series connected resistors having a connection point
therebetween and a feed-back loop is provided connecting feed-back
signals from this connection point to the regulator, which
feed-back signals correspond to the sum of the screen and shield
currents from the coronode wire corona emission and are insensitive
to proportional changes between the screen grid current and the
shield current.
Description
This invention relates to electrostatography. More particularly
this invention relates to electrical circuitry for controlling a
corona generating device applying electrostatic charge onto a
suitable surface, such as a xerographic imaging plate.
The basic electrostatographic process is disclosed in the Carlson
U.S. Pat. No. 2,297,691. In this process an electrostatographic
plate comprising a photoconductive insulating material on a
conductive backing is given a uniform electric charge over its
surface and is then exposed to the subject matter to be reproduced
usually by conventional projection techniques. This exposure
discharges the plate areas in accordance with the radiation
intensity which reaches them and thereby creates an electrostatic
latent image on or in the plate coating which may then be developed
with an electroscopic material which electrostatically clings to
the plate in a visual pattern corresponding to the electrostatic
image. Thereafter the developed image usually transferred to a
support material to which it may be fixed by any suitable
means.
The charging of the electrostatographic plate in preparation for
the exposure step can be accomplished by means of a corona
generating device whereby electrostatic charge is applied to the
electrostatographic plate to raise it to a potential of
approximately 500 to 600 volts. One form of corona generating
device for this purpose is disclosed in the Walkup U.S. Pat. No.
2,777,957 wherein a plurality of parallel wires are connected in
series to a high voltage source and are supported in a conductive
shield that is arranged in closely spaced relation to the surface
to be charged. When the wires are energized, corona is generated
along the surface of the wires and ions are caused to be deposited
on the adjacent photoconductive surface. Suitable means are usually
provided to effect relative movement of the surface to be charged
and the corona generating device. A biased wire screen placed
between the corona wires and the electrostatographic plate permits
energizing the corona wires to a potential well above the corona
threshold potential thereof without causing damage to the
electrostatographic plate because the excess of corona current over
that required for proper charging of the plate is drained off by
the biased screen. This type of corona generating device is
referred to in the art as "scorotron."
As is well known, the corona threshold potential and the corona
current from an energized wire are functions of the wire diameter,
i.e., the corona threshold increases and the corona current for any
given potential decreases as the wire diameter is increased.
Variations in the potential applied to corona wires of a given
diameter will cause relatively large changes in corona current with
corresponding variations in the charging rate. In addition, the
corona threshold potential and corona current are also affected
directly by deposits of dust that may accumulate on the wire and by
variations of movement and ionized conditions of the air sheath
surrounding the wire. Thus, when operating at the corona threshold,
minute differences in wire diameter, slight accumulations of dust
on the wire and variations in air temperature or in the air
pressure, i.e., ambient conditions, can drastically affect the
corona generating capability of the wire causing non-uniform
electrostatic charge to be deposited on the electrostatographic
plate.
It has heretofore been established that consistent high quality
reproductions can best be obtained when uniform potential is
applied to the electrostatographic plate in preparation of the
plate for the exposure step. If the electrostatographic plate is
not charged to a sufficient potential, the electrostatic latent
image obtained upon exposure will be relatively weak and the
resulting deposition of an electroscopic material thereon will be
correspondingly small. If, however, the electrostatographic plate
is overcharged, the converse will occur, and if over-charged
sufficiently, the photoconductive layer of the electrostatographic
plate can be permanently damaged.
Since the contrast value, comparable to the contrast values
obtained from silver halide papers, of the electrostatic latent
image may be related directly to the potential charge on the
electrostatographic plate before exposure, it is apparent that if
the plate is not uniformly charged over its entire area, the
contrast value of the electrostatic latent image obtained upon
exposure will vary in different areas on the plate, and a mottled
effect will be visible on the image when developed.
A prior art improved scorotron device whereby a uniform
electrostatic charge can be deposited on the electrostatographic
plate is disclosed in the Codichini U.S. Pat. No. 3,062,956. The
scorotron device described therein consists of a backup shield,
corona generating electrode wires called the coronode, and screen
wires. The coronode wires, by corona discharge, charging the
photoconductive surface of the electrostatographic plate. The
potential applied to the plate surface is varied by changing the
screen potential. To ensure a constant charging current during
operation at any given contrast setting, the charging circuit
contains a current stabilizer and a regulated direct current power
supply. The current stabilizer is a device for controlling charging
current by automatically adjusting the screen potential when a
change is sensed in the current being supplied to the coronode.
In operation in the Codichini circuit, any change in the charging
current from the coronodes to the electrostatographic plate
produces a change in the applied voltage to a control valve, for
example, a high gain pentode, the output of said control valve
being applied to the screen wires of the scorotron device. In this
manner, any change in the charging current from the coronodes to
the electrostatographic surface results in a change in the control
valve resistance which, in turn, produces a change in the screen
potential. With this circuit, as a decrease in charging current
occurs, the resistance of the control valve increases thereby
increasing the screen voltage to permit the charging current to
increase back to its desired value and, of course, the converse is
true as the charging current increases above a desired value. In
this manner, by altering the screen voltage, the charging current
can be maintained at a substantially constant value and is not
adversely affected by any of the normal variables such as dirty
wires, atmospheric changes, etc., which otherwise would affect the
magnitude of the charging current.
Various other relevant corotron or scorotron current control
arrangements are known in the art. The following additionally noted
U.S. Patents are listed as exemplary: U.S. Pat. No. 2,956,487 to E.
C. Giaimo, Jr., (RCA), U.S. Pat. No. 3,335,275 to P. F. King
(Xerox), U.S. Pat. No. 3,688,107 to J. M. Schneider et al (Xerox),
U.S. Pat. No. 3,699,388 (Ricoh) (and its related U.K. Pat. No.
1,235,497), and U.S. Pat. No. 3,805,069 to D. H. Fisher (Xerox),
and U.S. Pat. No. 3,813,548 to M. Silverberg (Xerox).
In the present invention there is a scorotron power supply
arrangement comprising an electrical power supply means and circuit
means arranged to sense the voltage at the grid and to supply
control signals in dependence thereon to adjust the power supply to
alter the current supply to the coronode in a manner to maintain
said voltage substantially constant.
The FIGURE here illustrates a scorotron supply arrangement
according to the present invention, by way of example, in which a
schematic circuit diagram of the arrangement is shown.
Referring to the FIGURE, a scorotron 10 has a coronode wire 11, and
a conductive metal shield 12 directly electrically connected to a
screen 13. The coronode wire 11 is connected to be supplied through
a stabilizing resistor 14 from a conventional inverter 15. The
screen 13 is connected through series connected resistors 16 and
17, to ground. From a point between the resistors 16 and 17 a
feed-back loop is provided to supply signals to a conventional
regulator 18 connected at the input side of the inverter 15.
The inverter 15 is supplied from a transformer 19 through a
rectifier bridge network 20. A smoothing capacitor 21 is connected
across the bridge network 20.
In use, the current is supplied from the inverter output to the
coronode wire 11 and corona discharge takes place as has been
explained. In order to maintain uniform depositing of ions on the
photoconductive surface to provide in turn uniform charging of the
surface, the grid voltage is required to be kept substantially
constant at a predetermined value. The shield 12 and the screen 13
collect excess ions liberated at the surface of the coronode wire
11 to produce currents in the shield 12 and screen 13. The
summation of these currents, hereinafter referred to as the biasing
current, flows through the resistors 16 and 17 to ground.
The biasing current in the described embodiment is maintained
substantially constant, for maintaining the grid voltage
substantially constant, by feeding, via the feed-back loop, a
voltage signal generated by the biasing current to the regulator 18
to adjust the inverter output current to increase or decrease as
the case may be. Whenever the voltage in the feed-back loop drops
the output current is increased to raise the voltage to its
predetermined level. Similarly, if the voltage in the feed-back
loop rises the output current is reduced by action of the regulator
18.
The surface to be charged charges up to a voltage substantially
equal to the grid voltage provided that sufficient ions are
liberated by the corona discharge. In the described arrangement, we
ensure that the scorotron is capable of producing sufficient ions
per unit time to charge up a moving surface, of a photoreceptor
say, to be charged and that the power supply to the scorotron can
supply sufficient current to cause release of those ions.
Typically using a selenium coated photoreceptor approximately 25
cms long, we charge the surface up to 650 volts using a scorotron
of the type schematically illustrated in the drawing. The surface
speed of the photoreceptor is 4.8 inches per second and is
separated from the grid by 1.65 millimeters. The coronode voltage
and current are 5,700 volts and 170 milliamps and the biasing
current is 151 milliamps. In the specific embodiments, we choose
the circuit parameters such that for the typical values above the
voltage at the connection between resistors 16 and 17 is about 4
volts.
It will be noted that according to the present invention we control
the charging of the surface to be charged by maintaining the grid
potential constant. This is acheived by altering the coronode
current as and when required. We monitor the grid voltage by
measuring the current flowing through a resistor from the grid and
made such adjustments automatically using the circuit means. In a
scorotron of the type described, the grid current is found to be a
substantially fixed ratio, during normal use of the coronode
current so that by monitoring the grid voltage linearly, related
adjustments of the coronode current are made.
In some prior art arrangements, the charging of the surface to be
charged was controlled by altering the coronode current. This was
achieved, however, by varying the grid potential, one such
arrangement has already been mentioned above incorporating a
pentode valve supply circuit. In the present inventor's experience
such arrangements, in which the grid voltage is altered as a
controlling adjustment, are more speed dependent than embodiments
of the present invention. In other words, the present invention
provides scorotron supply arrangements which are comparatively
insensitive to variations of surface speed, from either side of an
optimum speed, of the surface to be charged. Also, the supply
arrangements of the present invention can be provided using a small
number of electrical components.
While a particular embodiment of the invention has been described
above, it will be appreciated that various modifications may be
made by one skilled in the art without departing from the scope of
the invention as defined in the following claims.
* * * * *