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.

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.

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.