Apparatus For Electrostatic Charging Of Paper In Electrophotographic Reproduction

Abstract

A corona discharge electrode is supplied with voltage from a high frequency power source to produce a corona discharge which strikes a sheet of reproducing paper to charge the same electrostatically. A detection electrode is positioned in the field of the corona discharge and is connected via a resistor and amplifier to the power source to control the same in accordance with the detected corona discharge to maintain the magnitude of the discharge field constant. A switch is connected in the power supply to interrupt connection thereof with the discharge electrode if a spark discharge is produced between the discharge electrode and the paper.

Parent Case Text

This application is a continuation-in-part of my earlier application, Ser. No. 736,054, filed June 11, 1968 and claiming the priority of my application 4351/67 filed in Japan on July 6, 1967.

Description

BRIEF SUMMARY OF THE INVENTION

The invention relates to apparatus and methods for controlling high voltage current supplied to a corona discharge electrode in electrophotographic reproduction apparatus.

Heretofore it has been difficult to stabilize the operation of a corona discharge electrode as its charging action is substantially affected by changes in power voltage, loads or atmospheric conditions.

An object of the present invention is to provide a charging device incorporating means for stabilizing the charging action and obviate any variations thereof.

According to the invention a corona current detecting means is incorporated in the charging device for controlling the change of corona current for eliminating instability and for maintaining a uniform charge so that a uniform copy may be obtained.

According to a specific embodiment of the invention there is provided a charge control circuit for electrophotographic reproducing apparatus which comprises a high voltage power source, a corona discharge electrode electrically connected to said source for producing a corona discharge current, a target plate for supporting a sheet of reproducing paper, said target plate being grounded and facing said discharge electrode to expose the sheet of reproducing paper to the corona discharge current from said corona discharge electrode whereby said sheet is electrostatically charged, a detection electrode positioned adjacent said discharge electrode to detect corona discharge current from said discharge electrode, and feedback means connected to said discharge electrode and to said high voltage power source to control said power source in accordance with the detected corona discharge current to maintain the value of said current constant.

The discharge electrode and detection electrode are placed adjacent one another in a hollow, grounded guide member which has an opening facing the target plate. Thus, the discharge between the discharge electrode and the detection electrode is substantially constant regardless of the presence or absence of a copy sheet on the target plate.

A further object of the invention is to provide a safety device which is operative to interrupt supply of power to the discharge electrode if a spark discharge should be produced between the discharge electrode and the paper due to contact therebetween, or the like, which could produce a fire.

In further accordance with the invention the safety device is operative when the detection electrode detects the absence of a corona discharge field upon the production of a spark discharge between the discharge electrode and the copy paper.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a circuit diagram of the charge control apparatus according to the invention; and

FIG. 2 is a perspective view, partly broken away, of a portion of the apparatus of FIG. 1.

DETAILED DESCRIPTION

Referring to the drawing, therein is seen a corona discharge electrode 1 constituted as a tungsten wire. Mounted adjacent electrode 1 is a detection electrode 2 also in the form of a tungsten wire. The detection electrode 2 is intended to detect the magnitude of the corona discharge field developed by the electrode 1. The electrodes 1 and 2 are mounted in a hollow guide member 3 which is connected to ground and is provided with an opening facing a grounded target plate 10 on which a sheet of reproducing paper 11 can be placed. The hollow member 3 serves to confine the corona discharge and direct the discharge to the paper 11 to charge the paper electrostatically. The intensity of the corona field is the same at the detection electrode 2 as at the plate 10 with the exception that the electrode 2 is not subject to the variable effects of the paper 11 as is the plate 10. Thus, the current detected in the electrode 2 is an accurate representation of the intensity of the corona field produced by the electrode 1. As will be seen hereafter the current in the electrode 2 will serve as a means to control the voltage applied to electrode 1 in order to stabilize the corona discharge and render the charge on the copy paper 11 uniform.

The detection electrode 2 is connected to a variable resistor 4 across which a voltage is produced related to the current in detection electrode 2. The resistor 4 is connected to the input of an amplifier 5 which produces an amplified output signal of the voltage in resistor 4. The output of the amplifier produced at the plate of a triode V.sub.2 is connected to the control grid of an oscillating vacuum tube 6 which serves to control the voltage input to the discharge electrode 1 by virtue of its connection via resonance condenser 7 to the primary winding of a high frequency transformer 8. The secondary of the transformer is connected to a high voltage rectifier 9 which is connected to discharge electrode 1.

In an oscillation circuit consisting of the oscillating vacuum tube 6, the resonance condenser 7 and the transformer 8, when resonance current is caused to flow in the primary winding of the transformer 8 through its output, sine wave high voltage is simultaneously induced in the secondary winding of the transformer 8. This sine wave high voltage is changed to negative high voltage by the high voltage rectifier 9 and applied to the discharge electrode 1. Thereby, a corona field is produced and corona current flows between the discharge electrode 1, the detection electrode 2, the corona guide member 3 and the paper 11. The corona current which flows from the electrode 1 to the paper 11 is approximately proportional to the corona current which flows from the electrode 1 to ground through the detection electrode 2 and the variable resistor 4.

When the negative corona current which flows from the discharge electrode 1 to the paper 11 is increased excessively due to changes of power voltage, changes in the paper and atmospheric conditions, the negative corona current which flows from the electrode 1 to ground through electrode 2 and the variable resistor 4 is also increased and the electric potential at the point "d " of the variable resistor 4 is lowered. Such a voltage change is amplified in the amplifier 5, and lowers the voltage of the screen grid of the vacuum tube 6. Accordingly, the oscillating action of the oscillation circuit including vacuum tube 6 is controlled, and the negative high voltage to be applied to the electrode 1 is lowered, whereby the corona current is controlled, and thus a balance is maintained. When the corona current flowing from the electrode 1 is decreased, a similar action is produced, namely, the corona current is increased contrary to the above-mentioned case and the balance is maintained. In this way, the corona current is always kept at a fixed value, and the charging operation is stabilized without any unevenness so that a uniform copy can be obtained by the electrophotographic reproduction apparatus. Thus, the resistor 4, amplifier 5 and oscillation circuit constitute a feedback means to control the power source of transformer 8 and rectifier 9 to maintain the value of the corona discharge current constant.

The same object is obtainable by providing the oscillation circuit of the invention with a pulse generator, thereby changing the pulse interval by using the detected voltage of the variable resistor 4 and controlling the high voltage applied to the discharge electrode 1. The detection electrode 2 need not be arranged adjacent to the electrode 1 and for example, a shielding box (similar to corona guide member 3) can be employed for electrode 1, but the shielding box is normally grounded directly. Preferably, however, the electrodes 1 and 2 are mounted in hollow guide member 3 with the electrode 2 remote from paper 11 and spaced further therefrom than is the electrode 1, in order to eliminate any effect of the paper 11 on the current detected in detection electrode 2.

From the above it is seen that a corona current detection produces a signal which is employed as a feedback to the corona generating power supply to maintain the corona current constant without any change so that a uniform copy may be obtained.

The circuit of the invention is also constructed to eliminate the possibility of a fire in the event that the copy paper should contact the discharge electrode 1 to produce a spark discharge therebetween.

Accordingly, a safety device is provided which detects the presence of a spark discharge to open a switch 40 connected between the primary winding of transformer 8 and the voltage source. As seen in the circuit in FIG. 1, a grid of a triode V.sub.3 is connected to the plate of triode V.sub.2 through a Zener diode Zn. A coil of a holding type relay 40' is connected to the plate of triode V.sub.3. Any holding type relay can be used if the open state of its contacts is maintained until the relay once operated is reset by hand.

A time constant circuit comprising a diode 30, a resistor 31, a condenser 32 and a resistor 33 is connected between the plate of the triode V.sub.2 and the Zener diode Zn. If a button is pushed to supply the charging device with power, a discharge will be effected in the discharge electrode 1 after a short delay of time during which the heater of the vacuum tube 6 is heated. Herein, if there is not provided this time lag mechanism, the triode V.sub.3 is operated to open the switch contacts 40 of the relay 40'. In the state of normal use, the potential of the plate of the triode V.sub.2 is lower than that of the Zener diode and the triode V.sub.3 is cut off. If a sheet of paper is caught in the charging device and a spark discharge is produced, the discharge between the discharge electrode 1 and the detecting electrode 2 will be interrupted and the voltage of the resistor 4 will become zero. With the increase of the potential of the grid of the triode V.sub.2 ', the potential of the plate of the triode V.sub.2 ' will be lowered, and the triode V.sub.2 ' will be cut off and the potential of its plate will be increased, the Zener diode Zn will be operated, the triode V.sub.3 will become conductive, and the normally closed switch contacts 40 of the relay coil 40' will be opened. These contacts can be closed by resetting them by hand after the trouble has been corrected.

Although the invention has been described with reference to a specific embodiment thereof, the invention is not limited thereto and modifications and variations will become evident to those skilled in the art without departure from the spirit and scope of the invention as defined in the appended claims.

Claims

What is claimed is:

1. A charge control circuit for electrophotographic reproducing apparatus, said charge control circuit comprising a high voltage power source, a corona discharge electrode electrically connected to said source for producing a corona discharge current, a target plate for supporting a sheet of reproducing paper, said target plate being grounded and facing said discharge electrode to expose the sheet of reproducing paper to the corona discharge current from said corona discharge electrode whereby said sheet is electrostatically charged, a detection electrode positioned adjacent said discharge electrode to detect corona discharge current from said discharge electrode, feedback means connected to said discharge electrode and to said high voltage power source to control said power source in accordance with the detected corona discharge current to maintain the value of said current constant, and means connected to said power source to interrupt the connection thereof to said discharge electrode upon elimination of discharge current between the discharge electrode and the detection electrode when a spark discharge is produced between the discharge electrode and the reproducing paper, said power source comprising a high voltage transformer with primary and secondary windings, and a high voltage rectifier connected to the secondary winding of the transformer and to the discharge electrode, said means to interrupt the connection of the power source to the discharge electrode comprising a switch means for interrupting power supply to the primary winding of the transformer, a relay controlling said switch means, and means including a timer circuit connected to said feedback means and to said relay to operate said relay and open said switch means if said feedback means detects lack of corona discharge current.

2. A circuit as claimed in claim 1 comprising a hollow grounded guide member having an opening facing said target plate, said discharge electrode and detection electrode being adjacent one another within said guide member.

3. A circuit as claimed in claim 2 wherein said discharge electrode is closer to said target plate than is said detection electrode.

4. A circuit as claimed in claim 1 wherein said feedback means comprises a variable resistor connected to said detection electrode, amplifier means having an input connected to said variable resistor, control means connected to the amplifier means at the output thereof and to the power source to adjust the latter in accordance with the variations produced by the output signal from the amplifier means, said means for operating the relay comprising a triode connected to said relay, a Zener diode connected between said triode and said amplifier means and normally inoperative when said variable resistor is subjected to corona discharge current, said Zener diode being operative when the variable resistor is not subjected to corona discharge current to render said triode conductive whereupon the relay is activated and said switch means is opened.

5. A circuit as claimed in claim 4 wherein said timer circuit comprises a diode, a resistor means and a condenser connected between said Zener diode and said amplifier means.