US3598991A

US3598991A - Electrostatic charging device having a spark gap voltage regulator between a corona source and a voltage source

Info

Publication number: US3598991A
Application number: US847865A
Authority: US
Inventors: Louis N Nost
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1969-08-06
Filing date: 1969-08-06
Publication date: 1971-08-10
Anticipated expiration: 1988-08-10
Also published as: SE359663B; DE2038742A1; JPS4820144B1; ES382404A1; GB1322378A; CS163219B2; CH513436A; NL7011480A; BE754426A; PL70053B1; FR2056699A5; SU442617A3

Abstract

A corona-charging device including a corona wire surrounded by a grounded conductive shield. The corona wire is connected to either a positive or negative high-potential source. Inserted in the circuit between the wire and the source is an airgap which draws an arc at high voltages.

Description

I Unlted States Patent 11113,598,991

[72] Inventor Louis N. Nest [56] References Cited Rochester, N.Y. UNITED STATES PATENTS "P 847365 2.221.338 1 1/1940 Wintermute 250/495 [221 3,084,06! 4/1970 Hall 250/495 12:21:: 22124 5 6 3,133,193 5/ 1964 Guillotte 250/49.5 Rochester 3.370.212 2/ 1968 Frank 250/495 Primary Examiner-James W. Lawrence Assistant Examiner-C. E. Church 1 ELECTROSTATIC CnARGING DEVICE HAVING A Attorneys-Donald F. Daley, James J. Ralabate and Thomas J.

SPARK GAP VOLTAGE REGULATOR BETWEEN A Wall CORONA SOURCE AND A VOLTAGE SOURCE 14 Claims, 3 Drawing Figs.

[52] US. Cl. 250/495 ABSTRACT: A corona-charging device including a corona ZC, 250/49.5 GC wire surrounded by a grounded conductive shield. The corona [51] Int. CIL 603g 15/00 wire is connected to either a positive or negative high-poten- [50] Field of Search ..250/49.560, tial source. Inserted in the circuit between the wire and the 49.561, 49.562 source is an airgap which draws an arc at high voitages.

HV-I

PATENTEUAUGIOISYI 3,598,991

BARE PLATE T F40 CURREN O 9 IO INVENTOR. LOUIS N. NOST woqwmg ATTORNEY ELECTROSTATIC CHARGING DEVICE HAVING A SPARK GAP VOLTAGE REGULATOR BETWEEN A CORONA SOURCE AND A VOLTAGE SOURCE BACKGROUND OF THE INVENTION This invention relates in general to an electrostatic charging apparatus and in particular to a novel electrical circuitfor a corona emission-charging device.

More specifically, the invention relates to a circuit for use with a corona discharge device which will produce more efficient and effective electrostatic chargingof a surface. This novel circuit includes an air or spark gap device in the circuit connecting the corona electrode to the high-potentialsource.

While not intended to be so limited, for convenience of iilustration the present invention is described for use in xerography. lnthe process of xerography, a xerographic plate comprising a layer of photoconductive material on a conductive backing is given a uniform electric charge over its surface and then exposed to the subject matter to be reproduced by various projection techniques. This exposure discharges the plate in accordance with the light intensity reachingit, thereby creating a latent electrostatic image on or in the plate. 7

Development of the image is effected by developers which comprise, in general, a mixture of suitable pigmented or dyed resin base powder, hereinafter referred to as toner, and a granular carrier material which functions to generate triboelectric charges on, and to carry the toner. More specifically, the function of the carrier material is to provide mechanical control of the toner, or to carry the toner to an image surface and simultaneously provide almost complete homogeneity of charge polarity. In the development of the image, the toner powder is brought into surface contact with the photoconductive coating and is held thereon electrostatically in a pattern corresponding to the latent electrostatic image. Thereafter, the developed xerographic image may be transferred to a support material to which it may be fixed by any suitable means such as heat fusing.

Various methods and devices have been utilized in the prior art to place a uniform electrostatic charge on the photoconductor plate used in the xerographic process. One such mode of charging is disclosed in the Vyverberg US. Pat. No. 2,836,725 issued May 27, 1958, wherein an electrode such as a wire (known as a coronode) surrounded by an electrically grounded conductive shield is located adjacent a grounded surface and is connected to a high-voltage source to emit a corona discharge along the surface of the wire. The corona discharge is formed, because the electric potential gradient between the wire and the grounded surface causes a breakdown of the air surrounding the wire to form a glow discharge. As a result ions are formed around the coronode which flow to a surface through the influence of the electric field. This flow of ions is utilized to deposit an electrical charge on the grounded surface which is adjacent the opening in the conductive shield. This type of charging apparatus is known in the art as a corotron. The efficiency of this device depends on such factors as the diameter of the corona wire or coronode, the distance from the wire to the grounded plate and the geometrical characteristics of the charging corona. The corona current flowing from the wire to the photoconductive plate is proportional to the potential difference between the two elements which changes during charging as a charge is built up on the insulated photoconductive surface.

The amount of the ion current between the wire and the plate controls the rate at which the photoconductive surface will charge. Also, if relativemovement occurs between the photoreceptor plate and corona charging device, as is conventional in xerography, the relative velocity therebetween will likewise control the charging rate of the photoreceptor. One

limitation in achieving higher speed xerographic equipment is the rapidity at which a photoconductive surface is uniformly charged by a corona discharge device. For example, a drumtype photoconductor rotating relative to the charging device has been utilized to produce higher speeds. Accordingly, higher rotation speeds of the drum to produce more efficient and faster duplicating can be accomplished by quicker charging of the photoreceptor surface. In practice it has been necessary to apply greater potential inputs to the coronode to achieve faster charging, which can be uneconomical. Therefore, it is desirable in order to provide greater economy for high-speed automatic xerographic reproducing machines to increase ion flow to the photoconductive surface for a given potential input to the charging apparatus.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to improve the electrical charging of a surface.

Another object of this invention is to improve charging by a corona discharge device of photoconductive surfaces used in the art of xerography.

A further object of this invention is to increase the rate of charging of a photoconductive surface for a particular applied voltage.

Still another object of this invention is to improve the rapidity and uniformity at which a positive electrostatic charge is deposited upon a photoconductive surface.

A still further object of this invention is to improve the rapidity and uniformity at which a photoconductive surface is negatively charged.

These and other objects are attained in accordance with the present invention wherein there is provided a novel spark gap circuit for use with a corotron for charging with greater efficiency and economy in the xerographic process by increasing ion current flow for a given wire potential, as compared to prior art devices, and furthermore is operable at higher input potentials than heretofore possible. Normally, in conventional corotron devices the connection of the corona wire to a positive electrical potential for providing a positive charge on the photoconductor plate will cause a corona to form in a sheath immediately around the corona wire. According to the present invention the provision of an electrode spark gap or are in the circuit connecting the high voltage to the coronode will cause the corona emission to occur throughout substantially the entire area within the shield surrounding the wire. Such a dispersed corona discharge has experimentally been found to improve charging of a plate at high potentials as compared to charging apparatus previously known, The present invention increases the amount and rate of charging of a surface by allowing the corotron wire to be connected to greater maximum applied voltages than possible with prior charging apparatus. The invention is also more efficient than conventional devices because it produces a greater charge on the plate for a given applied voltage.

Similar results have been found to occur upon placing a negative charge on a photoconductor plate when a corona charging device is connected to a high negative potential in the novel manner disclosed herein. In conventional corotrons a corona wire connected to a negative potential will produce beads of corona discharge alongthe wire which results in nonuniform negative charging of a photoconductive plate. Negative charging of a photoconductive surface is useful, for example, in reversal development, sometimes referred to as discharged area development, since the areas developed with toner are actually the areas of the image in which the charge has been dissipated. Such reversal development is useful in the xerographic process when negative microfilm is utilized to expose the photoreceptive surface. According to this invention the insertion of a spark gap in'the circuit between the high negative potential and the corona wire will disperse the corona beads along the surface of the'wire and form-a more substantially uniform corona discharge sheath around the wire. This will produce a more uniform negative charging of a plate than with conventional corotrons as well as producing greater charging at high potentials.

DESCRIPTION OF THE DRAWINGS Further objects of the invention, together with additional features contributing thereto and advantages accruing therefrom, will be apparent from the following description of conventional system. Referring below to table 1 there is shown a tabulation of experimental test results that demonstrate the greater efficiency of the invention over conventional prior art. The invention was tested by measuring the current flow to a sewn] embodimems f he invention when read in conjuno 5 grounded bare plate mounted 0.3 inch below the corotron tion with the accompanying drawings, wherein: unit. The bare plate current flowing therefrom to ground is an FIG. I is a schematic illustration of a suitable electrical mdlcanon of h amount of cilarging Produced y a corotron discharge appmms utilizing the invention f this li fi The corona wire of a conventional corotron was connected to H 2 i a graphical illustration f the emciency f the various positive DC voltages up to maximum for the device pamms Show in HQ 1 as compared to a conventional 10 and the bare plate current was measured. The maximum voltdevice; age that can be applied to a coronode is limited because of FIG. 3 isa schematic illustration of a second embodiment of arcmg that will occur between the wire and grolfnded R a suitable corona discharge apparatus utilizing the invention The same tests made on a form of this Invention f f of this application. 15 spark gap of inch inches and 0.035 inch inserted in a circuit DESCRWON OF THE PREFERRED EMBODIMENTS connecting the corona wire to the positive potential and these results are shown below in table 1. A graphical representation Referring in particular to FIG. I, there is illustrated a of the comparative results of these tests is shown in FIG. 2.

TABLE ISUMMARY OF TEST RESULTS Prior Art Corotron with .025 gap Corotron with .035 gap Plate current Applied Plate current Applied Plate current formed volt e formed .3 voltage formed .3 from(rnicro- HV-l (DC from(micro- HV-l (DC from-(microamps) volts amps) volts) amps) 1 Maximum.

corotron for depositing an electrostatic charge on a surface such as a photoconductive insulating material. Illustrated in the figure is a xerographic plate member 1 comprising a photoconductive insulating layer 2 on a grounded conductive substrate 3. A corotron unit 10 is positioned above the plate to deposit an electrical charge on the plate surface during relative movement therebetween. The corotron unit includes a shield 11 which surrounds one or more coronodes or corona.

discharge wires 12. The shield 11 comprises an electrically conductive material and is preferably grounded. A slit 13 is formed in the shield to allow ions to flow from the corona emission around the wire 12 to the photoconductive insulating surface 2 in order to deposit an electrical charge thereupon. This slit 13 is preferably located directly between the corona wire and the photoconductive surface. For further details regarding a suitable corotron, reference is made to the disclosure of the aforementioned Vyverberg patent.

The corona discharge wire is connected by suitable means such as electrical circuit 14 to a high potential source HV-l. The corona wire utilized in this embodiment is connected to the positive terminal of the high potential source HV-l, in order to place a positive charge on the plate I. The circuit 14 includes a spark gap 15 inserted in the line. The spark gap 15 comprises two

adjacent electrodes

16 and 17 which cause an are or spark to be generated when a high potential is applied therebetween. Any suitable electrodes can be utilized to generate such an arc, for example, an automobile spark plug. The distance of the gap can also be made adjustable for increasing or decreasing various outputs according to the result desired. An automobile spark plug is one example of an adjustable spark gap but other adjustable electrodes could be utilized.

The insertion of a spark gap 15 in the circuit of a corotron has been found to produce much greater efficiency than with a Referring in particular to FIG. 2, there is illustrated a graphic representation of the test results tabulated in table 1 as a plot of supply voltage to the coronode versus bare plate current. Comparing the curves for the conventional corotron and the two forms of the charging device according to the present invention. it is seen that greater plate current is formed by the conventional corotron at low voltages. However, above certain applied voltages the charge formed by the two forms of the corotron using an airgap according to this invention becomes significantly greater than the prior art device. This result indicates that the novel device disclosed herein will produce much greater efficiency at high voltage levels of operation. Further, it can be seen from the graph that a charge was deposited on the bare plate by the two forms of this invention at a greater voltage than with a conventional corotron, which would not operate at such potentials. Accordingly, the corotron utilizing the novel circuit of this application can operate at higher maximum potentials than prior art devices as well as produce more efficient charging at high voltages enabling, for example, xerographic drums to be more rapidly charged for faster operation.

Referring to FIG. 3, there is illustrated a second embodiment of the invention which places a negative charge on the photoconductive insulating layer or other surface. This form of the invention is the same as the device shown in FIG. 1 except that the corona wire 12 is connected by a circuit including a spark gap 15 to a high negative potential I-IV-2. This form of the invention produces similar results as demonstrated with reference to the device of FIG. 1 and produces more efficient charging than conventional corotrons. Further, as discussed earlier, the insertion of the spark gap in this form disperses the negative corona beads normally formed in negative potential corotrons and, therefore, it is possible to negatively charge a photoconductive insulating surface more uniformly than with the prior art devices.

In the above description there has been disclosed a more effective and efficient corotron charging means. The invention also may encompass two or more corona wires or include shields of other shapes and forms. It is further within the scope of this invention to place a charge on surfaces other than a photoconductive insulating plate and to utilize the invention in any other application where it is desired to produce an electrostatic charge by corona emission. The surface to be charged is not intended to be limited to a flat plate, but could be for example, a cylinder, web, or other form. Also, as is clear from the foregoing disclosure, the corotron of this invention could be mounted stationary with respect to a movable photoconductive surface.

While the invention has been described with reference to the structure disclosed herein it is not to be confined to the details set forth or the specific environments set forth. Therefore, this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

l claim:

1. A corona discharge device for depositing an electrostatic charge upon a surface comprising an electrode positioned adjacent to the surface to be charged, I a circuit means coupling a source of electrical potential to said electrode, and

spark gap means in series between said circuit means and said electrode for holding said electrode inactive below a predetermined potential, said predetermined potential being at least greater than the threshold potential at which said device emits corona, and to effect a continuous discharge of corona at a potential above said predetermined potential.

2. The device of claim 1 wherein said electrode is an elongated wire positioned substantially parallel to said surface to be charged.

3. The device of claim 2 wherein said elongated wire is substantially surrounded by a shield having an opening to permit ion current flow therefrom.

4. The device of claim 3 wherein said shield is electrically grounded.

5. The device of claim 1 wherein said source of electrical potential comprises a positive potential for depositing a positive charge upon said surface.

6. The device of claim 1 wherein said source of electrical potential comprises a negative potential for depositing a negative charge upon said surface.

7. Said device of claim 1 wherein said gap means includes two spaced electrodes. 3

8. The device of claim 7 wherein said spark gap means is adjustable to vary the magnitude of the corona discharge potential and the charging rate of the device.

9. A corona discharge device of the type having an electrode for depositing an electrostatic charge upon a chargereceiving surface and a grounded electrode shield partially surrounding said electrode, said electrode and shield being operatively connected to circuit means for providing a source of electrical potential to the electrode, said discharge device further including spark gap means in series with said electrode and said shield for holding the corona discharge device inactive below a predetermined potential, said predetermined potential being at least greater than the threshold potential at which said device emits corona, and to effect a continuous discharge of corona at a potential above said predetermined potential whereby the surface is efficiently and uniformly charged.

l0. The corona discharge device of claim 9 wherein said spark gap means includes two electrodes positioned in spaced parallel relation to each other.

11. The corona discharge device of claim 10 wherein said two parallel electrodes are spaced at between 0.025 inch and 0.035 inch apart.

12. The corona discharge device of claim 10 wherein the gap between the two spaced electrodes is adjustable whereby the magnitude of the sparking potential is controllable.

13. The device of claim 9 wherein a positive potential is applied to said electrode to deposit a positive charge upon said surface.

14. The device of claim 9 wherein a negative potential is applied to said electrode to deposit a negative charge upon said surface.

Claims

1. A corona discharge device for depositing an electrostatic charge upon a surface comprising an electrode positioned adjacent to the surface to be charged, a circuit means coupling a source of electrical potential to said electrode, and spark gap means in series between said circuit means and said electrode for holding said electrode inactive below a predetermined potential, said predetermined potential being at least greater than the threshold potential at which said device emits corona, and to effect a continuous discharge of corona at a potential above said predetermined potential.

4. The device of claim 3 wherein said shield is electrically grounded.

7. Said device of claim 1 wherein said gap means includes two spaced electrodes.

9. A corona discharge device of the type having an electrode for depositing an electrostatic charge upon a charge-receiving surface and a grounded electrode shield partially surrounding said electrode, said electrode and shield being operatively connected to circuit means for providing a source of electrical potential to the electrode, said discharge device further including spark gap means in series with said electrode and said shield for holding the corona discharge device inactive below a predetermined potential, said predetermined potential being at least greater than the threshold potential at which said device emits corona, and to effect a continuous discharge of corona at a potential above said predetermined potential whereby the surface is efficiently and uniformly charged.

10. The corona discharge device of claim 9 wherein said spark gap means includes two electrodes positioned in spaced parallel relation to each other.

11. The corona discharge device of claim 10 wherein said two parallel electrodes are spaced at between 0.025 inches and 0.035 inches apart.