Alternating Current Corona Discharge Apparatus

1. Field of the Invention

The present invention relates to an alternating current corona discharge apparatus. More specifically, the present invention relates to an alternating current corona discharge apparatus particularly suited for an electrophotographic apparatus.

2. Description of the Prior Art

A corona discharge apparatus comprises a thin wire electrode and a shield plate enclosing the wire electrode and a high voltage of the order of several kV is applied between the wire electrode and the shield plate. It has been observed that generally uniform ion discharge is achieved along the wire electrode on the occasion of the positive corona discharge where the positive high voltage is applied to the wire electrode whereas a glow spot is likely to occur on the wire electrode on the occasion of the negative corona discharge where the negative high voltage is applied to the wire electrode so that a uniform ion discharge is not achieved. Such trend is seen even in an alternating current corona discharge apparatus wherein the positive corona discharge and the negative corona discharge are repeated alternately for each half cycle and thus uniform ion discharge is difficult to attain in the negative half cycle corona discharge. When a charge is imparted onto a member being charged such as a photosensitive member of an electrophotographic apparatus as a result of such non-uniform ion discharge, then an electric charge on the surface of the member being charged becomes non-uniform.

In order to eliminate such uneveness of the surface charge on the member being charged caused as a result of the negative corona discharge, it has been proposed that the negative high voltage being applied to the wire electrode be increased in the negative direction to make more dense the glow spot and also to increase the corona current so that uneveness of the surface potential on the member being charged is reduced. However, such an approach for increasing the voltage being applied to the wire electrode involves an upper limit voltage by virtue of a fear of a spark discharge and leakage. In addition, it has been observed that an increase of the voltage being applied as the discharge voltage causes a high frequency component of a large peak value in superposition on each peak in the positive direction and in the negative direction of the waveform of the corona discharge current. Such phenomenon uniquely occurs when a corona discharge apparatus is used as a load. Such high frequency component becomes a cause of an unstable discharge and an increased discharge voltage considerably increases the instability of the discharge.

Briefly described, the present invention comprises an alternating current power supply apparatus for applying a high alternating current voltage to a wire electrode of an alternating current corona discharge apparatus, which comprises means for limiting the alternating current waveform at a predetermined peak value.

According to the present invention, the alternating current waveform of a high alternating current voltage being applied to a wire electrode of an alternating current corona discharge apparatus is limited at a predetermined value without substantially varying the effective value of the high alternating current voltage being applied to the wire electrode. Therefore, an unfavorable influence caused by a high frequency component in superposition on the peak portion of the corona discharge current in the positive and negative directions can be effectively eliminated.

In a preferred embodiment of the present invention, the peak value as limited for the negative half cycle of the high alternating current voltage is selected to be relatively larger than the peak value as limited for the positive half cycle of the high alternating current voltage. Therefore, non-uniform ion discharge by the negative half cycle corona discharge is improved. If and when such alternating current corona discharge apparatus is employed in an electrophotographic apparatus, a clear copy without uneven density can be provided.

Therefore, a principal object of the present invention is to provide an improved alternating current corona discharge apparatus.

Another object of the present invention is to provide an alternating current corona discharge apparatus that is capable of achieving corona discharge of a high stability.

A further object of the present invention is to provide an electrophotographic apparatus employing an improved alternating current corona discharge apparatus.

These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1 is a diagrammatic view of one embodiment of an electrophotographic apparatus wherein the present invention can be advantageously employed;

FIG. 2 is a diagrammatic view showing in more detail a major portion of the FIG. 1 apparatus;

FIG. 3 is a schematic diagram of one embodiment of the present invention;

FIG. 4 shows a waveform of the output voltage of the power supply apparatus of the FIG. 3 embodiment;

FIGS. 5A and 5B are graphs for explaining the effect of the present invention, wherein FIG. 5A shows a surface potential of a member being charged without the present invention and FIG. 5B shows a surface potential of a member being charged with the present invention;

FIG. 6 is a graph showing a surface potential of a member being charged when the resistance of the variable resistor 107p is varied in the FIG. 3 embodiment;

FIGS. 7, 8 and 9 show schematic diagrams of further embodiments of the present invention;

FIG. 10 is a schematic diagram showing one example of the imbalancing circuit 108;

FIG. 11 shows waveforms of the output of the power supply apparatus employing the imbalancing circuit shown in FIG. 10;

FIG. 12 is a schematic diagram of another embodiment of the imbalancing circuit; and

FIG. 13 shows a waveform of the output of the power supply apparatus employing the FIG. 12 imbalancing circuit.

FIG. 1 is a diagrammatic view of one example of an electrophotographic apparatus in which the present invention can be advantageously employed. Since the structure and operaton of such an electrophotographic apparatus is well known to those skilled in the art, an outline of the structure and operation of the FIG. 1 electrophotographic apparatus will be described in the following. A full disclosure of such electrophotographic apparatus as shown in FIG. 1 is seen in U.S. Pat. No. 3,666,363, issued May 30, 1972 to Hiroshi Tanaka et al, entitled "ELECTROPHOTOGRAPHIC PROCESS AND APPARATUS", and in the corresponding British Pat. No. 1,165,406 and the corresponding German Pat. No. 1,522,568, and these patents are incorporated herein by reference thereto. The electrophotographic process and apparatus disclosed in the above referenced U.S. Pat. No. 3,666,363 comprises three major steps for forming an electrostatic image on a photosensitive member 1. As to be more fully described subsequently, the first step comprises the step of uniformly charging the photosensitive member 1 and to that end a direct current corona discharge apparatus 2 is provided close to the surface of the photosensitive member 1. The second step comprises the step of focusing an optical image of an original copy placed on an original copy table 4 through an optical system 5 on the photosensitive member 1 while removing the electric charge on the surface of the photosensitive member 1 by means of an alternating current corona discharge. To that end, an alternating current corona discharge apparatus 3 is provided. The third step comprises the step of uniformly illuminating the photosensitive member 1 and to that end a post lamp 6 such as a fluorescent lamp is provided.

The optical system 5 comprises a lamp 51 for illuminating the original copy table 4, a first movable mirror 52 for focusing an optical image of the original copy, a second movable mirror 53, a fixed mirror lens 54 and a fixed mirror 55. The lamp 51 and the first movable mirror 52 are provided to be movable in parallel with the original copy table 4, so that the lower surface of the original copy placed on the original copy table 4 is optically scanned. The second movable mirror 53 is provided to be movable in parallel with the first movable mirror 52 at the speed of a half of the speed of the first movable mirror 52, so that the length of the light path from the surface of the original copy placed on the original copy table 4 to the surface of the photosensitive member 1 may be constant. Such optical system 5 is disclosed in more detail in U.S. Pat. No. 3,330,181 issued July 11, 1978 to H. Jakobson, entitled "SURFACE EXPOSURE DEVICE FOR COPYING APPARATUS", and this patent is incorporated herein by reference thereto.

As better seen in FIG. 2, the photosensitive member 1 comprises a photoconductive layer 12 of such as CdS deposited on a conductive support member 11 and a transparent highly insulating film 13 coated thereon, with the conductive support member 11 grounded. As shown in the above referenced U.S. Pat. No. 3,666,363, a positive electric charge is first imparted on the highly insulating film 13 of the photosensitive member 1 through a positive corona discharge by means of the direct current discharge apparatus 2. At the same time, the photosensitive member 1 is irradiated with a weak light beam. As a result, the resistance value of the photoconductive layer 12 is decreased and a negative electric charge on the highly insulating film 13 as attracted by the positive electric charge reaches the surface of the highly insulating film 13 through the photoconductive layer 12 the resistance value of which has been partially decreased. The photosensitive member 1 is exposed to the light image through the optical system 5, while alternating current corona discharge is performed by means of the alternating current corona discharge apparatus 3. Then a portion of the photosensitve member 1 as exposed to the light, i.e. a light portion on the photosensitive member 1 gives rise to a decrease of the resistance value of the photoconductive layer 12, while a portion of the photosensitive member 1 as not irradiated with the light, i.e. a dark portion of the photosensitive member 1 maintains a large resistance value of the photoconductive layer 12 serving as an insulating layer. Therefore, an electrostatic capacitance is increased in the light portion and the quantity of a negative electric charge by the alternating current corona discharge increases as compared with that of the dark portion, whereby the positive electric charge as charged in the above described first step is removed. Although in this step an electric charge remains on the highly insulating film 13, as a result of the surface potential, a positive electric charge exists in the interface between the photoconductive layer 12 and the conductive support member 11 in the dark portion, for example, and therefore, no potential difference occurs, with the result that the surface potential becomes zero and there is no potential contrast.

In the third step, the photosensitive member 1 subjected to the second step is uniformly illuminated by means of the post lamp 6, when the light portion gives rise to no substantial variation, since the same has been once exposed, whereas the dark portion gives rise to a decrease of the resistance value of the photoconductive layer 12, which causes movement of the electric charge, so that the positive electric charge on the conductive support member 11 and the negative electric charge on the rear surface of the highly insulating film 13 are offset to cause a variation of a potential, with the result that a surface potential contrast of the light portion and the dark portion is established. Upon completion of these three steps, an electrostatic image associated with the original copy placed on the original copy table 4 is formed on the surface of the photosensitive member 1.

The photosensitive member 1 is then subjected to a developing process in a developing apparatus 7 by means of a magnetic brush dusted with a toner. The toner image thus developed is moved to a transfer corona discharge apparatus 8, where the toner image is transferred to a paper sheet as fed from a paper cassette 16 stocking paper sheets by means of upper and lower paper feed rollers 17 and 18. The paper sheet on which the toner image is transferred is separated from the photosensitive member 1 by means of a separating roller 14 and is then fed to a fixing roller 15. The fixing roller 15 serves to heat the toner image as transferred onto the paper sheet, thereby to fix the toner image to the paper sheet. Upon passage of the fixing roller 15, the paper sheet is taken out of the apparatus.

On the other hand, the photosensitive member 1 passing the transfer corona discharge apparatus 8 reaches a charge removing corona discharge apparatus 9, where any electric charge remaining in the photosensitive member 1 after transfer of the toner image is removed. At the same time, a toner remaining on the surface of the photosensitive member 1 after transfer of the toner image is also removed by means of a cleaning apparatus 10.

The present invention can be advantageously employed as an alternating current corona discharge apparatus for establishing an electrostatic image in an electrophotographic apparatus, with an excellent resultant effect; however, it is pointed out that the present invention can also be employed as a transfer corona discharge apparatus or an electric charge removing corona discharge apparatus. It is further pointed out that the present invention can also be employed in various applications, apart from the above described applications in an electrophotographic apparatus.

FIG. 3 is a schematic diagram of one embodiment of the present invention. An essential feature of the present invention resides in a power supply apparatus for applying a high alternating current voltage to a corona discharge apparatus as denoted by the reference numeral 3 in the FIG. 1 electrophotographic apparatus, for example. A power supply apparatus 100 of the embodiment shown in FIG. 3 is structured in the manner to be described in the following. A primary winding 102p of a power supply transformer 102 is connected to the commercial alternating current voltage source 101. A secondary winding 102s is coupled through an iron core 102c of the transformer 102 to the primary winding 102p. One end of the secondary winding 102s as well as the shield plate 32 of the discharge apparatus 3 and the conductive support member 1 of the photosensitive member 1 serving as a member being charged are connected to the ground. A non-grounded end i.e. a high voltage end of the secondary winding 102s is connected through a stablizing resistor 103 and an imbalancing circuit 108 to a wire electrode 31 of a corona discharge apparatus 3. A varistor circuit 104 is connected in parallel with the secondary winding 102s between the high voltage end and the grounded end of the secondary winding 102. The varistor circuit 104 comprises a series connection of, for example, eight varistors 104a to l04h. Each of these varistors 104a to 104h is a varistor of an operating voltage of about 1 kV. Such a varistor is commercially available as a high voltage varistor. Such a high voltage varistor may be a SiC varistor or any other type of a voltage responsive non-linear resistance characteristic device. A partial series connection including two varistors 104g and 104h of the varistor circuit 104 is shunted by a series connection 105p of a diode 106p, with the cathode at the ground side, and a variable resistor 107p. A partial series connection of the varistors 104e and 104f of the varistor circuit 104 is similarly shunted by a series connection 105n including a diode 106n, with the anode at the ground side, and a variable resistor 107n.

In operation, a high alternating current voltage as induced in the secondary winding 102s of the power supply transformer 102 shows a peak value as shown as V.sub.AC by a dotted line in FIG. 4. The peak value V.sub.AC is 11.3 kV, for example. However, since the secondary winding 102s is shunted by the varistor circuit 104, the voltage developed across the secondary winding 102s is subjected to an operation of the varistor circuit 104. More specifically, if and when the voltage developed across the secondary winding 102s exceeds a response voltage or an operating voltage of the varistor circuit 104, say approximately 8 kV in the embodiment shown, which voltage level may also be termed a threshold voltage, then the resistance value of the varistor circuit 104 suddenly decreases and accordingly the peak value of the voltage developed across the secondary winding 102s is limited as shown by a solid line in FIG. 4. Accordingly, a high frequency component or a pulsating component occurring in superposition on the peak portion in the positive or negative direction of the alternating current as shown by the dotted line in FIG. 4 is prevented from being applied to the corona discharge apparatus 3.

In the embodiment shown, portions of one or more varistors in the varistor circuit 104, such as the varistors 104g and 104h, and the varistors 104e and 104f, are each shunted by series connections 105p and 105n, respectively. In case where the non-grounded end of the secondary winding 102s is positive, i.e. during the positive half cycle of the alternating current voltage, the peak value V1 shown in FIG. 4 can be freely changed within a variable range of the variable resistor 107p by adjusting the variable resistor 107p included in the series connection 105p. Similarly, the grounded end of the secondary winding 102s is negative, i.e. during the negative half cycle of the alternating current voltage, the peak value V2 shown in FIG. 4 can be varied by adjusting the variable resistor 107n included in the series connection 105n.

Thus, according to the FIG. 3 embodiment, the high alternating current voltage obtained from the voltage source 100 is limited at a predetermined peak value and the high alternating current voltage as limited at a predetermined peak value is applied to the corona discharge apparatus 3. It is convenient to speak of the limiting of the alternating current voltage of the present invention as being "sliced off" to each predetermined peak value (as shown, for instance, by V1 and V2 in FIG. 4). As a result, any unfavorable influence caused by a high frequency component in superposition on the peak in the positive or negative direction of the corona discharge current is totally eliminated and stabilized corona discharge is performed. According to the FIG. 3 embodiment, the peak value V1 as limited for the positive half cycle and the peak value V2 as limited for the negative half cycle can be separately and independently adjustable. Therefore, if and when the FIG. 3 embodiment is employed in an electrophotographic apparatus, a charge efficiency of the surface of the member being charged, i.e. the photosensitive member 1 (see FIG. 3) can be readily adjusted to a desired value.

In case where the inventive alternating current corona discharge apparatus is employed in an electrophotographic apparatus, the discharge apparatus is preferably structured such that the negative half cycle corona discharge is more dominant as compared with the positive half cycle corona discharge. To that end, only the series connection 105p may be provided, while the other series connection 105n may be omitted. In such instance, the peak value V2 is constant and only the peak value V1 can be varied by means of the variable resistor 107p. Accordingly, adjustment may be made such that the peak value V1 be smaller than the peak value V2 by adjusting the variable resistor 107p.

Referring to FIGS. 5A and 5B, the effect of the present invention will be described. FIG. 5A is a graph showing a variation of the surface potential in case where an electric charge is imparted to a member being charged such as a photosensitive member of an electrophotographic apparatus, for example, by means of a conventional alternating current corona discharge apparatus, without employing the present invention, and FIG. 5B is a graph showing a variation of the surface potential on a member being discharged by means of an alternating current corona discharge apparatus employing the present invention. It has been observed that when an electric charge is imparted continually by an alternating current corona discharge the surface potential on the member being charged becomes saturated at the value associated with the output voltage of the alternating current voltage and when the output voltage of the alternating current voltage source fluctuates, the surface potential on the member being charged promptly fluctuates in association with the fluctuation of the output of the alternating current voltage source. Accordingly, the stability of the high alternating current voltage source can be observed by measuring the surface potential on the member being charged as shown in FIGS. 5A and 5B. Referring to FIGS. 5A and 5B, it is seen that although a conventional alternating current corona discharge apparatus shows a maximum fluctuation of about 160 V in terms of the surface potential, the alternating current corona discharge apparatus of the present invention restricts the maximum fluctuation to about 50 V in terms of the surface potential of the member being charged. From the foregoing description, it would be appreciated that the inventive alternating current corona discharge apparatus significantly stabilizes the discharge.

FIG. 6 shows a surface potential on a member being charged, with the resistance value of the variable resistor 107p as a parameter, by adjusting the variable resistor 107 constituting the series connection 105p shown in FIG. 3. The FIG. 6 graph shows that, by adjusting the variable resistor 107p or 107n, the surface potential on the member being charged can be freely adjusted with high stability.

More specifically, FIG. 6 shows a case where the present invention is employed in an electrophotographic apparatus forming an electrostatic image through the above described three steps described with reference to FIGS. 1 and 2, wherein the member being charged corresponds to the photosensitive member 1. Referring to FIG. 6, the ordinate denotes a surface potential of the photosensitive member 1 and the abscissa denotes a resistance value of the variable resistor 107p. The line L in FIG. 6 denotes the surface potential at the light portion as exposed in the second step and the line D denotes the surface potential at the dark portion as not exposed. Thus, FIG. 6 shows a variation of the surface potential when the resistance value of the variable resistor 107p is adjusted so that the peak value V1 shown in FIG. 4 is changed from 6.0 kV to 8.0 kV. In such instance, the variable resistor 107n is set such that the peak value V2 as shown in FIG. 4 may be -8.0 kV.

FIG. 7 is a schematic diagram of another embodiment of the present invention. The embodiment shown can be implemented using varistors of a smaller withstand voltage. More specifically, a third winding 102t is coupled to the iron core 102c of the power supply transformer 102. The voltage across the secondary winding 102s is applied through the imbalancing circuit 108 to the corona discharge apparatus 3. The third winding 102t is shunted by the varistor circuit 113 serving as a load of the third winding 102t. The varistor circuit 113 comprises a series connection of, for example, two varistors 113a and 113b, in which these varistors 113a and 113b may be of a withstand voltage which is as small as the order of 1/10 to 1/100 as compared with those varistors 104a to 104h described in conjunction with the FIG. 3 embodiment. One varistor 113b is shunted by a series connection 105p, while the other varistor 113a is shunted by a series connection 105n. According to the embodiment shown, the high alternating current voltage induced at the secondary winding 102s is limited to a predetermined peak value, as seen in FIG. 4, by means of the varistor circuit 113 serving as a load of the third winding 102t. More specifically, if and when the voltage as induced in the third winding 102t reaches the operating voltage of the varistor circuit 113, the output voltage of the third winding 102t is kept constant at the voltage. The above described limitation of the voltage as induced at the third winding 102t influences the voltage as induced at the secondary winding 102s, whereby the limiting function is performed.

It is pointed out that for the purpose of decreasing the withstand voltage of the varistor, a center tap may be provided to the secondary winding 102s, as in the case of the FIG. 7 embodiment, and the varistor circuit 104 and series connections 105p and 105n as shown in FIG. 3, for example, may be connected between the center tap and the grounded end of the secondary winding 102s. In other words, the embodiments shown in FIGS. 3, 7 and 8 are substantially the same in terms of an equivalent circuit configuration thereof. This means that substantially the same operation can be performed even when the primary winding of the transformer is shunted by such a varistor circuit. In other words, either in the case that varistor circuits 104 and 113 and the resistor 103 employed in the above described embodiments are connected to the primary winding of the power supply transformer 102 or in case where the same are connected to the secondary winding of the power transformer 102, or in the case where the same are connected to the third winding, substantially the same circuit operation and the limiting function are performed, except that the specific values of the voltage and current are slightly different. Accordingly, it is intended that the present invention covers such modifications.

As discussed above, FIG. 8 shows a schematic diagram of a further embodiment of the present invention. In the embodiment shown, the secondary winding 102s of the power supply transformer 102 is shunted by a series connection of a constant or reference voltage source 109p and a diode 110p, with the cathode at the ground side, and by a series connection of a constant voltage source 109n and a diode 110n, with the cathode at the non-grounded side. Both the constant voltage sources 109p and 109n comprise a capacitor 111p and a variable resistor 112p, and a capacitor 111n and a variable resistor 112n, respectively. The constant voltage sources 109p and 109n may comprise merely direct current voltage sources.

In operation, if and when the non-grounded end of the secondary winding 102s of the transformer 102 is positive, i.e. in the positive half cycle of the alternating current voltage, the capacitor 111p included in the constant voltage source 109p is charged through the diode 110p. When the capacitor 111p is charged and saturated, the electric charge is discharged through the variable resistor 112p. Accordingly, the output voltage of the voltage source 109p is limited at the value determined by the charge voltage of the capacitor 111p. Conversely, if and when the non-grounded end of the secondary winding 102s of the power supply transformer 102 is negative, i.e. in the negative half cycle of the alternating current voltage, the constant voltage source 109n and the diode 110n operate. Accordingly, the output of the power supply circuit 110, i.e. the alternating current voltage being applied to the corona discharge apparatus 3, is limited in the manner similar to that shown in FIG. 4. Thus, the peak value V1 can be adjusted by the variable resistor 112p and the peak value V2 can be adjusted by the variable resistor 112n, respectively, within given ranges.

FIG. 9 shows a schematic diagram of still a further embodiment of the present invention. The embodiment shown is different from the embodiments described previously in that a leakage transformer is employed as a power supply transformer. It is well known that such leakage transformer 114 exhibits a constant current characteristic wherein a constant current is caused to flow irrespective of fluctuation of a load thereof. Accordingly, a leakage transformer can be advantageously employed in a high alternating current voltage source for a corona discharge apparatus.

FIG. 10 is a schematic diagram of one example of the imbalancing circuit 108. The imbalancing circuit 108 of the embodiment shown comprises a parallel connection of a diode 108a and a varistor 108b. With such a circuit configuration, the output of the power supply circuit of the embodiments shown in FIGS. 3, 7, 8 and 9 gives rise to a voltage drop only in a positive half cycle of the alternating current voltage, as shown in FIG. 11, by virtue of the varistor 108b. Accordingly, the voltage is relatively dominant in the negative half cycle. As a result, employment of such power supply enables a stabilized operation of an alternating current corona discharge apparatus.

FIG. 12 is a schematic diagram of another embodiment of the imbalancing circuit 108. The embodiment shown comprises a parallel connection of a diode 108a and a variable resistor 108c. According to the embodiment shown, as shown in FIG. 13, a voltage drop is caused by means of the variable resistor 108c with respect to the positive half cycle voltage of the alternating current voltage obtained from the power supply circuit 100 shown in FIGS. 3, 7, 8 and 9, so that relatively the positive half cycle is less dominant with respect to the negative half cycle.

Although in the foregoing description, the embodiments were described as employing a commercial power supply as the alternating current voltage source, alternatively an alternating current voltage source of the frequency of say 200Hz may be provided in the case where the present invention is employed in a discharge apparatus for an electrophotographic apparatus. In such instance, a better electrostatic image of less charge uneveness can be achieved.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.