|Publication number||US3370212 A|
|Publication date||Feb 20, 1968|
|Filing date||Aug 19, 1965|
|Priority date||Aug 19, 1965|
|Also published as||DE1563173B1|
|Publication number||US 3370212 A, US 3370212A, US-A-3370212, US3370212 A, US3370212A|
|Inventors||Lee F Frank|
|Original Assignee||Eastman Kodak Co|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (22), Classifications (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
Feb. 20, 1968 L. F. FRANK' 3,370,212
CORONA CHARG I NG APPARATUS Filed Aug. 19, 1965 LEE F F RANK INVENTOR.
BY flaw/M ATTORNEYS 3,370,212 Patented Feb. 20, 1968 United States Patent Ofiice 3,370,212 CORONA CHARGING APPARATUS Lee F. Frank, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey Filed Aug. 19, 1965, Ser. No. 480,980 Claims. (Cl. 317--262) ABSTRACT OF THE DISCLOSURE A unilaterally conducting circuit for rectifying an AC. corona current as a pulsed D.C. corona current is produced by modulating an alternating current corona discharge by interposing between the corona source and the grid plate of an electrographic plate a conductive grid connected to ground through a rectifier.
This invention relates primarily to electrograph'y and in a preferred embodiment thereof to a direct current corona source useful in charging electrographic plates.
It is usual in electrography to uniformly charge an in sulating plate (which becomes conductive upon activation), to imagewise activate the plate to produce an electrostatic image thereon and then to develop the electrosatic image. The imagewise activation can be by imagewise exposure to light as in xerography or by imagewise heating as in xerothermography, for example.
Conventional electrographic charging sources employ an A.C. voltage source connected to a corona discharge electrode through a series of rectifiers and capacitors to obtain a high DC. potential at the electrode. The use of such high voltage components to rectify the AC. current and to properly handle the total current demands of the system add to the expense of the electrographic device (whichmay be, for example, a document copier). One
corona charging source described in the literature employs, between the corona discharge electrode (in this case connected directly to an AC. potential source) and 'the plate to be charged, a control electrode having a DC. voltage impressed thereon to regulate the flow of ions.
It is an object of the present invention to provide a DC. corona source which is both simple and inexpensive.
It is a further object of the invention to provide an electrographic corona charging apparatus in which only low voltage rectifiers are required and in which only a fraction of the total current drawn by the power supply needs to be rectified.
Still another object of the invention is to provide a simple and economical D.C. corona source which additionally provides for the stabilization of the spatial distribution of the corona current.
It is a still further object of the invention to provide a simple and economical, pulsed D.C. corona source which additionally provides for the control of the phase and amplitude of the pulsed D.C. corona current.
These objects are accomplished by the present invention as follows. A pulsed D.C. corona discharge is produced, according to the present invention, by modulating an alternating current corona discharge by interposing, between the corona source and the ground plate of the electrographic plate or sheet, a conductive grid connected to ground through a rectifier. Only about 10% of the current drawn from the current source passes through the rectifier; thus, the rectifier must control only about 1% or less of the power that would be rectified if the rectifier were put in series with the alternating current source as is done in the above-described prior art. The above-described prior art which employs an AC. potential source connected directly to the corona electrode and a DC. voltage impressed on a control grid requires, in practice, more components than the simple rectifier of the subject invention, i.e., it requires a transformer, a capacitor, and an extra electrical lead to the corona electrode. Further, the rectifier of the subject invention does not cause a constant potential but rather a pulsating potential on the grid,-
varying between a couple of hundred volts when corona is being transmitted and zero volts when it is not. In the subject invention, the control voltage is derived from the current to be controlled instead of from an external power source. Important advantages of the present invention are (1) only low-power rectifiers are required, (2) the grid acts to stabilize the spatial distribution of the corona current and (3) the phase and amplitude of the pulsed D.C. corona can be controlled, In one embodiment of the present invention, instead of connecting the grid to ground through a rectifier, a rectifying coating, such as an unsensitized zinc oxide in resin binder layer, is coated on a grounded grid. In this embodiment, of course, only negative ions can be transmitted. Various types of rectifying coatings, however, may be employed to pro- I vide the transmission of the desired polarity of current.
These and other objects of the present invention will be more fully understood by reference to the following detailed description when read in connection with the accompanying drawings in which:
FIG. 1 is a schematic illustration of one embodiment of the present invention, and
FIG. 2 is a schematic illustration of another embodiment of the present invention.
FIG. 1 shows a high-voltage AC. power supply 2 connected through switch 4 to a corona discharge electrode 6. The other terminal of the power supply 2 is connected to a conductive backing 8 of a Xerographic sheet or plate 10 having a photoconductive coating 12 on the conductive backing 8. According to the present invention, a conductive grid 14 is positioned between the corona discharge electrode 6 and the plate 10. The grid 14 is connected to ground through a rectifier 18. When the corona discharge electrode 6 is energized by closing the switch 4, the grid 14 operates to rectify the alternating current which flows from the electrode 6 to the plate 10 whereby the photoconductive layer 12 is provided with a uniform electrostatic charge of a given polarity. Various methods are known for uniformly corona charging a photoconductive layer and anyof these methods, for example the use of control grids or shields or the relative movement of the corona discharge electrode and the plate 10, may be used in the present invention.
. During the positive half of the above described alternating current, the ions or charges incident on the grid 14 pass through the rectifier 18 to ground. When the corona current is negative, the rectifier will not conduct the current, and the grid 14 is quickly charged up to the space potential appropriate to its position and then immediately ceases to intercept current whereby the remaining current incident on the grid 14 passes through the grid 14 to charge the photoconductive layer 12.
Only pulsating D.C. corona current is transmitted through the grid 14 for most of the frequencies involved in alternating current. A grounded metal shield 20 partially surrounding the corona discharge electrode 6 may be used to stabilize the corona further at relatively high corona levels. The total potential across the rectifier 18 is limited to about 200300 volts. Usually only about 10% of the current drawn from the current source passes through the rectifier 18. It is easily seen that the rectifier 18 must control only about 1% or less of the power that would be required if it were put in series with the power supply 2. This system works equally well with the rectifier 18 reversed and produces the opposite charged current incident on the photoconductive layer 12. If it is found that the rectifier 18 does not completely 3 rectify the AC. corona current, a bias potential 22 which can be connected in the circuit by means of a switch 24 will guarantee that the A.C. corona current will be completely rectified.
FIG. 2 shows another embodiment of the invention, with like numerals indicating like elements. The only difference between the embodimentshown in FIG. 2 and that shown in FIG. 1 is the use of a rectifying coating 26 on the grounded grid 14- instead of the use of the rectifier 18 between the grid 14 and ground. This coating may be, for example, a layer of unsensitized zinc oxide in resin binder. Various rectifying coatings may be en1- ployed and the polarity of the current transmitted to the photoconductive layer 12 is dependent on the power characteristics of the material embodied in the coating. In the case of unsensitized zinc in resin, only negative current is transmitted. A bias potential 22 can be connected to the grid 14 by means of a switch 24 if desired.
As stated in the first paragraph of the specification, the present invention relates only primarily to electrography. The corona source of the present invention can be enclosed in a box with the grid 14- as the only opening to the air and this assembly can be used as a source of air ions of a given polarity, such as are used, for example, in those air conditioning units which have a dust trap. Various other uses of the subject invention will be obvious to those skilled in the art. Other modifications of the invention include the use of a silicon controlled rectifier to synchronize the partial phase of the pulse with a more complex circuit or to produce a pulse-width, modulated pattern. Further, a piezoeleoctric voltage source may be substituted for the customary high-voltage transformer in order to provide an even more economical system. A piezoelectric source of high-voltage may also be used for operation of the subject system away from sources of electrical power. Piezoelectric voltage sources are current alternating for continuous use.
The present invention has been described in considerable detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinbefore and as defined in the appended claims.
1. Apparatus for producing a DC. corona current comprising:
means for producing an AC. corona current, and
means for rectifying said current to produce a pulsed,
D.C. corona current comprising a grounded conductive grid having a rectifier in series circuit therewith interposed in said A.C. corona current, said grounded grid and rectifier forming a unilaterally conducting circuit for rectifying said A.C. corona current.
2. The apparatus according to claim 1 in which said rectifier comprises a coating of rectifying material on said grid.
3. The apparatus according to claim 1 in which said grid is connected to ground through said rectifier.
4-. Apparatus for charging a surface comprising:
means for directing an AC, corona current toward said surface, and
means for rectifying said A.C. corona current to produce a pulsed, D.C. corona current which impinges on said surface comprising a grounded conductive grid having a rectifier in series circuit therewith interposed in said A.C. current, said grounded grid and rectifier forming a unilaterally conducing circuit for rectifying said A.C. corona current.
5. The apparatus according to claim 4 in which said rectifier comprises a coating of rectifying material on said grid.
6. Apparatus according to claim 5 in which said grid is connected to ground through a rectifier.
7. An electrographic apparatus for charging the surface of an electrographic sheet having a grounded conductive backing comprising:
means for supporting said sheet in a charging plane,
a corona discharge electrode spaced from said surface,
means for applying an electric field between said electrode and said backing,
means for applying a corona generating A.C. potential to said electrode, and
a ground conductive grid positioned between said electrode and said surface and having a rectifier in series circuit therewith, said grounded grid and rectifier forming a unilaterally conducting circuit for rectifying said A.C. corona current to produce a pulsed, D.C. corona current.
8. Apparatus for producing a DC corona current comprising:
means for producing an AC. corona current, and
means for rectifying said current to produce a pulsed,
D.C. corona current comprising a conductive grid interposed in said A.C. corona current, said grid having a rectifier and a source of bias potential in series circuit therewith, said grid, rectifier, and source forming a unilaterally conducting circuit for rectifying said A.C. corona current.
9. The apparatus according to claim 8 in which said rectifier comprises a coating of rectifying material on said grid.
10. The apparatus according to claim 8 in which said grid is connected to ground through said rectifier and said source of bias potential.
References Cited UNITED STATES PATENTS 3,068,356 12/1962 Codichini 25049.5 2,982,647 5/1961 Carlson et al. 250-49.5 X 2,890,343 6/1959 Bolton 250-495 2,879,395 3/1959 Walkup 317-262 X 2,778,946 1/1957 Mayo 25049.5
MILTON O. HIRSHFIELD, Primary Examiner,
I. A. SILVERMAN, Assistant Examiner,
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|U.S. Classification||361/229, 361/235, 250/324, 250/326|