Charger

Abstract: A charger charges a recording medium (1) before irradiation of light to form an electrostatic latent image by irradiating light as an optical image onto a surface of the recording medium. The charger includes a discharging electrode (22) having a plurality of projections for generating corona discharge to charge the recording medium surface, a conductive shield case (21) having an opening face opposite to the recording medium and opposed to the discharging electrode, a holding member (24) for holding the discharging electrode in the shield case in a state in which the discharging electrode is electrically insulated from the shield case, a conductive grid electrode (23) electrically insulated and held between the discharging electrode and the recording medium, and a power supplying device (30) for supplying a high voltage for discharge to the discharging electrode and setting a ratio of electric currents flowing through the grid electrode and the shield case such that these electric currents are approximately equal to each other.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charger for charging or discharging a recording medium for forming an image in electrostatic electrophotography. More particularly, the present invention relates to a charger for uniformly charging a surface of a recording medium by corona discharge in an image forming apparatus such as a copying machine, a printer, etc., and having a discharging electrode having a plurality of projections in the direction of a rotational axis of the recording medium. The present invention further relates to a charger of an electrostatic electrophotographic system in which a current distribution ratio of electric currents flowing through an electrode of the charger and a grid electrode between this electrode and a photosensitive body can be adjusted and set to a predetermined current distribution ratio. 2. Description of the Related Art As is well known, an image forming apparatus using an electrostatic electrophotographic system is constructed by processing sections of charge, exposure, development, transfer, separation, cleaning and discharge. Namely, in a process for recording an image, a charger uniformly charges a surface of an image carrier as a recording medium formed on a conductive supporting body composed of e.g., an aluminum drum with respect to a rotated photoconductive layer. An optical image of an original image is next exposed onto the charged surface of the image carrier through an optical exposure device so that an electrostatic latent image according to this optical image is recorded on this carrier surface. Subsequently, toner is electrostatically attached to the electrostatic latent image on this image carrier and is then developed so that a toner image is formed on the image carrier surface. The toner image on the image carrier is then transferred onto a transfer material by a transfer device and is fixed by a fixing heater. Residual transfer toner left on the image carrier surface is removed therefrom by a cleaner and is collected in a predetermined collecting section. Residual charges are removed by a discharger from the image carrier surface after the cleaning operation to perform the next image forming operation. For example, the recording medium as the image carrier is constructed by a photosensitive body in which an organic photo conductor (OPC) as the photoconductive layer is formed on the conductive drum. A corona discharger is generally used as a charger for providing charges for a surface of this recording medium in many cases. In one corona discharger, a very thin conductive wire is covered with a conductive shield plate in a peripheral portion except for a shield portion opposite to the recording medium. A high voltage is applied to the wire so that corona discharge is caused to provide charges for charging by an electric current flowing through the recording medium. In another charger using corona discharge, a saw-toothed discharging electrode having many sharp projections arranged in line is disposed instead of the wire for corona discharge. A charging operation of this charger is performed by corona discharge from the sharp projections. For example, the charger using the above saw-toothed discharging electrode (which is called a saw-toothed electrode in the following description) is proposed in specifications of the following patents. (1) U.S. Patent No. 4,591,713 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 60-158582) (2) U. S. Patent No. 4, 725, 731 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-14176) (3) U. S. Patent No. 4, 725, 732 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-15272) (4) U. S. Patent No. 4, 792, 680 (corresponding to Japanese Patent Application Laying Open (KOKAI) No. 63-180977) In the general charger, a surface of the photosensitive drum is uniformly charged by corona discharge in an axial direction of this drum. A charging condition is slightly changed in accordance with various conditions of the corona discharge. When the charging condition is changed, charging irregularities on the surface of the photosensitive drum are caused so that the quality of an original image to be formed is influenced by the charging irregularities. A method for increasing a total electric current flowing through saw-toothed electrodes is considered as a simple improving method for reducing these charging irregularities. However, when the total electric current is increased, it is necessary to increase a voltage applied to the saw-toothed electrodes. A discharging electric current is increased when the voltage applied to the saw-toothed electrodes is increased. Therefore, an amount of ozone generated from a discharging portion is increased so that the surface of the photosensitive drum is influenced by this ozone, thereby reducing the quality of an original image. When the amount of ozone is increased, this ozone is bonded to various gases and foreign materials in the air floating within an image forming apparatus so that nitrogen oxides (NO X ), silicon oxides (SiO 2 ), etc. are generated. These oxides are attached onto surfaces of the saw-toothed electrodes and the grid electrode so that discharging ability of the saw-toothed electrodes and ability for controlling a charging potential of the grid electrode are reduced. Further, it is necessary to prevent leak discharge from tip portions of the saw-toothed electrodes to other unnecessary portions by an increase in applied voltage by increasing the total electric current. To prevent this leak discharge, it is necessary to excessively secure distances from discharging portions of the saw-toothed toothed electrodes to a shield case. Therefore, the shield case is large-sized so that the charger is large-sized. As is well known, a copying machine of an electrostatic electrophotographic system is constructed by unit processes of charge, exposure, development, transfer and fixing. Namely, in a copying process, uniform charges are given by a charger onto a surface of a photosensitive body. Reflected light of an original image is irradiated onto this photosensitive body surface through an optical system so that an electrostatic latent image is formed. This electrostatic latent image is developed by electrostatically attaching toner as developing powder to the electrostatic latent image so that a toner image is formed on the photosensitive body. The toner image is next transferred onto a piece of recording paper by static electricity of the photosensitive body as a transfer body and is thermally fixed onto the piece of recording paper as an image according to the original image. The photosensitive body is constructed by using a material such as selenium having a high resistance and a high optical carrier generation rate. The charges on the photosensitive body are given by corona discharge. A typical charger for generating the corona discharge is composed of a wire electrode charger and a saw-toothed electrode charger having a needle-shaped electrode. In the wire electrode charger, a charging line is made of tungsten or stainless steel having a thickness of 25 to 90 µm (micrometers) and is tensioned within a charger case. The charger case is opened onto a side of the metallic photosensitive body through an insulator. The charging line is connected to a power source and corona discharge is caused from this charging line. In contrast to the wire electrode charger having the charging line, the saw-toothed electrode charger has a saw-toothed electrode having a sharp tip portion and corona discharge is caused from this tip portion. In these chargers, a grid electrode having a predetermined potential is normally arranged between the photosensitive body and the charger to prevent a charging electric current from being changed. The charger gives uniform charges onto a photosensitive drum surface by the corona discharge. The corona discharge is influenced by an external environment such as atmospheric pressure, temperature, humidity, etc. Further, an electrode current is changed by wearing of a tip portion of the saw-toothed electrode so that no photosensitive drum is uniformly charged with ionic charges. The change in electrode current causes a change in impedance between the saw-toothed electrode and the photosensitive drum. A grid current and a case electric current respectively flow through the grid electrode and the charger case by the corona discharge from the saw-toothed electrode. A ratio of the grid current and the case electric current is an impedance ratio and is constant in a normal state. However, this impedance ratio is changed in accordance with the above external environment and operating states of the electrodes. The saw-toothed electrode is separated by a constant distance from the grid electrode and is fixedly arranged with respect to the photosensitive drum. Accordingly, when the impedance ratio is changed, charges on the photosensitive drum is not uniform and lack of uniformity on charges can not be corrected in the general charger. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a charger for stabilizing corona discharge using a saw-toothed electrode and performing a uniform charging operation without any discharging irregularities. Another object of the present invention is to provide a charger in which problems about a large-sized structure of the charger, leak discharge, etc. are solved without increasing a voltage applied to a saw-toothed electrode and charging characteristics are stabilized at any time by effectively generating a corona flow to discharge ozone from the charger so that no operation of the charger is influenced by this ozone. In accordance with a first structure of the present invention, the above objects can be achieved by a charger for charging a recording medium before irradiation of light to form an electrostatic latent image by irradiating light as an optical image onto a surface of the recording medium; the charger comprising a discharging electrode having a plurality of projections for generating corona discharge to charge the recording medium surface; a conductive shield case having an opening face opposite to the recording medium and opposed to the discharging electrode; a holding member for holding the discharging electrode in the shield case in a state in which the discharging electrode is electrically insulated from the shield case; a conductive grid electrode electrically insulated and held between the discharging electrode and the recording medium; and a power supplying device for supplying a high voltage for discharge to the discharging electrode and setting a ratio of electric currents flowing through the grid electrode and the shield case such that these electric currents are approximately equal to each other. In accordance with a second structure of the present invention, the shield case comprises at least one conductive auxiliary electrode plate arranged in parallel with a longitudinal direction of the discharging electrode for charging the recording medium surface, and the auxiliary electrode plate is located in a region located on a side of the recording medium from a tip portion of the discharging electrode and is also located within a discharging region of the discharging electrode. In accordance with a third structure of the present invention, a portion of the shield case is different in structure from the other portions of the shield case with respect to the discharging electrode for charging the recording medium surface, and a corona discharging portion on the recording medium surface is deflected in a predetermined direction. In the charger of the present invention, when a high voltage for generating corona discharge is applied to the discharging electrode, the corona discharge is generated from a projecting portion of the discharging electrode. Thus, an electric current of the corona discharge as a total electric current flows onto a side of the discharging electrode. At this time, a portion of the discharging electric current also flows onto the side of a recording medium so that a surface of the recording medium is charged with electricity having a predetermined polarity. In addition to the recording medium, a portion of the discharging current flows through the shield case and the grid electrode by the corona discharge. In the first structure of the charger, a ratio of a case electric current flowing through the shield case and a grid current flowing through the grid electrode is set such that the case electric current and the grid current are approximately equal to each other. Therefore. the corona discharge is approximately caused uniformly along an entire length of the discharging electrode in a longitudinal direction thereof. Accordingly, the entire recording medium can be uniformly charged without any charging irregularities. In particular, it is not necessary to increase a voltage applied to the discharging electrode. In the second structure of the charger, the shield case is constructed by at least one auxiliary electrode plate. Accordingly, it is not necessary to entirely cover the discharging electrode with the shield case so that the charger can be cheaply manufactured and can be made compact. Since the corona discharge is caused on a front projecting tip face of the discharging electrode, the shield case can be especially used as the auxiliary electrode on at least this front face. In the third structure of the charger, a portion of the shield case is different in structure from the other portions of the shield case so that a corona discharging direction can be deflected. Thus, the direction of a corona flow caused by the corona discharge is deflected so that ozone generated at a corona discharging time is discharged along this flowing direction. In this case, ozone can be easily discharged from the shield case by directing the corona discharging direction toward a position in which there are no other devices. Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the present invention as illustrated in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view showing a general charger; Fig. 2 is a view for explaining another general charger having a saw-toothed electrode; Fig. 3 is a block diagram showing a power supplying circuit including a high voltage generating circuit and applying a voltage to a charger in the present invention; Fig. 4 is an exploded perspective view showing an entire structure of the charger in the present invention; Fig. 5 is a cross-sectional view showing the internal structure of an image forming apparatus having the charger in the present invention; Fig. 6 is a characteristic graph for explaining the relation between an electric current distribution ratio and discharging characteristics on a surface of a photosensitive drum in the present invention, and evaluating uniform discharging characteristics on the photosensitive drum surface by changing a ratio of a grid current and a case electric current; Fig. 7 is an enlarged view of a main portion including the charger of the present invention shown in Fig. 5; Fig. 8 is a main enlarged view for explaining another structure of the charger in the present invention; Fig. 9 is a perspective view for explaining a charger in accordance with another embodiment of the present invention; and Fig. 10 is a view for explaining a charger in accordance with another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of a charger in the present invention will next be described in detail with reference to the accompanying drawings. Fig. 1 shows the schematic structure of a charger described in U.S. Patent No. 4,591,713. Two saw-toothed electrodes 102 and 103 are parallel to each other and are arranged and held within an insulating shield case 101. A photosensitive drum 107 is arranged as a recording medium on a front face of the shield case 101. The charger also has a conductive grid electrode 104 having a net shape and arranged in a position opposite to the photosensitive drum 107. The conductive grid electrode 104 is used to charge a surface of the photosensitive drum 107. A charger described in U.S. Patent No. 4,725,731 has a means for supporting the saw-toothed electrode 102 in the charging structure shown in Fig. 1. An opening portion is disposed in this supporting means to form a corona-like flow by corona discharge. In this charger, ozone caused by the corona discharge is collected within the shield case 101 so that charging irregularities are caused by deteriorations of the photosensitive drum 107 and the saw-toothed electrode 102, etc. The corona flow is generated to prevent such charging irregularities so that this ozone is discharged from the shield case. A charger described in U.S. Patent No. 4,725,732 shows a structure for supporting the saw-toothed electrode 102 and the shield case 101 by the same supporting member in the charging structure shown in Fig. 1. In a charger described in U.S. Patent No. 4,792,680, beryllium copper is used as the grid electrode 104 to stabilize discharging performance irrespective of a life of the charger. As mentioned above, in the charger 100 having the charging structure shown in Fig. 1, a predetermined voltage is applied by a power source 105 to each of the saw-toothed electrodes 102 and 103. A grid voltage Vg for controlling a corona electric current is applied by a power source 106 to the grid electrode 104. The corona electric current is discharged from a tip portion of each of the saw-toothed electrodes 102 and 103 onto a surface of the photosensitive drum 107. At this time, an electric current flowing through each of the saw-toothed electrodes 102 and 103 is a total electric current It. An electric current flowing through the grid electrode 104 is a grid current Ig. In the charger 100 shown in Fig. 1, a surface of the photosensitive drum 107 is uniformly charged by corona discharge in an axial direction of this drum. A charging condition is slightly changed in accordance with various conditions of the corona discharge. When the charging condition is changed, charging irregularities on the surface of the photosensitive drum 107 are caused so that the quality of an original image to be formed is influenced by the charging irregularities. A method for increasing a total electric current It flowing through the saw-toothed electrodes 102 and 103 is considered as a simple improving method for reducing these charging irregularities. However, when the total electric current It is increased, it is necessary to increase a voltage applied to the saw-toothed electrodes 102 and 103. A discharging electric current is increased when the voltage applied to the saw-toothed electrodes 102 and 103 is increased. Therefore, an amount of ozone generated from a discharging portion is increased so that the surface of the photosensitive drum 107 is influenced by this ozone, thereby reducing the quality of an original image. When the amount of ozone is increased, this ozone is bonded to various gases and foreign materials in the air floating within an image forming apparatus so that nitrogen oxides (NO X ), silicon oxides (SiO 2 ), etc. are generated. These oxides are attached onto surfaces of the saw-toothed electrodes and the grid electrode so that discharging ability of the saw-toothed electrodes and ability for controlling a charging potential of the grid electrode are reduced. Further, it is necessary to prevent leak discharge from the tip portions of the saw-toothed electrodes 102 and 103 to other unnecessary portions by an increase in applied voltage V by increasing the total electric current It. To prevent this leak discharge, it is necessary to excessively secure distances from discharging portions of the saw-toothed electrodes 102 and 103 to the shield case 101. Therefore, the shield case 101 is large-sized so that the charger 100 is large-sized. Fig. 2 is a view for explaining a general charger having a saw-toothed electrode. In Fig. 2, reference numerals 220, 221 and 222 respectively designate a charger, a charger case and a saw-toothed electrode. Reference numerals 223 and 224 respectively designate a photosensitive drum and a grid electrode. Further, each of reference numerals 225 and 226 designates a power source. In Fig. 2, the charger 220 is composed of the charger case 221 and the saw-toothed electrode 222. The saw-toothed electrode 222 is electrically insulated and fixed to an inner central portion of the charger case 221. The charger case 221 is constructed by an elongated metallic plate having a U-shape in cross section and one opening end 221a. The saw-toothed electrode 222 is constructed by a metallic plate having a sharp tip portion saw-toothed toward the opening portion 221a of the charger case 221. A negative voltage V 2 from the power source 225 is applied to the charger case 221 through a Zener diode D z . A negative voltage V 1 from the power source 226 is lower than the voltage V 2 and is directly applied to the saw-toothed electrode 222. An inner circumferential wall of the photosensitive drum 223 is connected to the ground and is opposed to the opening portion 221a of the charger 220. The grid electrode 224 is arranged between the photosensitive drum 223 and the charger 220. A distance d between the photosensitive drum 223 and the sharp tip portion of the saw-toothed electrode 222 is set to be constant. The grid electrode 224 is connected to the power source 225 having the negative voltage V 2 . In the above charger 220, a potential difference showing a constant voltage determined by the Zener diode D z is provided between the charger case 221 and the grid electrode 224. A voltage of the charger case 221 is held such that this voltage of the charger case 221 is lower by the potential difference than the voltage V 2 of the power source 225. The lower voltage V 1 is applied to the saw-toothed electrode 222 from the power source 226. Corona discharge is caused from the saw-toothed electrode 222 toward the photosensitive drum 223 so that the photosensitive drum 223 is charged with ionic charges. At this time, an electric current of the saw-toothed electrode 222 is set to It. The charger 220 gives uniform charges onto a surface of the photosensitive drum 223 by the corona discharge. The corona discharge is influenced by an external environment such as atmospheric pressure, temperature, humidity, etc. Further, the electrode current It is changed by wearing of a tip portion of the saw-toothed electrode 222 so that no photosensitive drum 223 is uniformly charged with ionic charges. The change in electrode current It causes a change in impedance between the saw-toothed electrode 222 and the photosensitive drum 223. A grid current Ig and a case electric current Ic respectively flow through the grid electrode and the charger case by the corona discharge from the saw-toothed electrode 222. A ratio of the grid current Ig and the case electric current Ic is an impedance ratio and is constant in a normal state. However, this impedance ratio is changed in accordance with the above external environment and operating states of the electrodes. The saw-toothed electrode 222 is separated by the constant distance d from the grid electrode 224 and is fixedly arranged with respect to the photosensitive drum 223. Accordingly, when the impedance ratio is changed, charges on the photosensitive drum 223 is not uniform and no lack of uniformity on charges can be corrected in the general charger. Fig. 3 is a block diagram showing a power supplying circuit including a high voltage generating circuit and applying a voltage to a charger in the present invention. Fig. 4 is an exploded perspective view showing a concrete structure of the charger in the present invention. Fig. 5 is a cross-sectional view showing the internal structure of an image forming apparatus such as a laser printer having the charger of the present invention. In the image forming apparatus shown in Fig. 5, a photosensitive drum 1 is arranged in a central portion on a left-hand side of the image forming apparatus. For example, the photosensitive drum 1 is formed by using a layer of an organic photo conductor (OPC) as a photoconductive layer on an aluminum drum as described before. Each of constructional units for forming an electrophotographic process is arranged around this photosensitive drum 1 as a center such that these constructional units are opposed to the photosensitive drum 1. A charger 2 in the present invention is arranged around the photosensitive drum 1 and uses corona discharge for uniformly charging the photosensitive drum 1. An optical recording section 3 irradiates a laser beam for exposing and recording an original image onto a surface of the photosensitive drum 1 uniformly charged by the charger 2. A developing device 4 develops an electrostatic latent image formed on the photosensitive drum 1 by the optical recording section 3 by using toner. A transfer device 7 transfers a toner image formed on the photosensitive drum 1 by the developing device 4 onto a surface of a transfer material such as paper fed from one of paper storing sections 5 and 6. A cleaner 8 removes and collects residual toner partially left on the photosensitive drum 1 without transfer of the toner image from the photosensitive drum 1 to the transfer material by the transfer device 7. The toner image as an original image transferred onto the transfer material by the transfer device 7 is fed to a fixing heater 9 and is fixed onto the transfer material by heat and pressure. Thus, the transfer material having the toner image is discharged through a paper discharging roller 10 onto a paper discharging tray 11 in an upper portion of the image forming apparatus. The paper storing sections 5 and 6 are detachably disposed in a body of the image forming apparatus. Paper feed rollers 12 and 13 are respectively opposed to the paper storing sections 5 and 6. The transfer material fed by each of the paper feed rollers 12 and 13 is fed toward a resist roller 16 by each of conveying rollers 14 and 15. The resist roller 16 temporarily stops a movement of the fed transfer material and controls a starting operation of conveyance of the transfer material in synchronization with rotation of the photosensitive drum 1. In particular, the resist roller 16 controls the starting operation of conveyance of a piece of paper such that a front end of the image formed on the photosensitive drum 1 is in conformity with a front end of the transfer material. Fig. 4 shows one example of the concrete construction of the charger 2 in the present invention. The charger 2 is constructed by a conductive shield case 21, a saw-toothed electrode 22, a grid electrode 23 and an insulating electrode holding member 24 for holding various kinds of electrodes. In Fig. 4, the shield case 21 is constructed by a conductive shield plate having a length approximately equal to a width of the photosensitive drum 1 in the direction of a rotational axis thereof. The shield case 21 is opened on a side opposite to a surface of the photosensitive drum 1. The saw-toothed electrode 22 has a plurality of sharp projections for discharge arranged in line at a predetermined pitch. The saw-toothed electrode 22 is constructed by a thin plate formed in the shape of a short strip and made of stainless steel such as an alloy of iron, chromium and nickel. For example, this alloy is constructed by SUS304 in Japanese Industrial Standard (JIS). Such a saw-toothed electrode 22 is formed by etching processing. The saw-toothed electrode 22 has a plurality of openings for fixing the saw-toothed electrode 22. Each of these openings is fitted onto a projecting portion 24b formed in a planar shape portion 24a of the electrode holding member 24 integrally formed by an insulating member. Thus, the saw-toothed electrode 22 is positioned, fixed and held by the shield case 21 in an electrically insulated state in the planar shape portion 24a of the electrode holding member 24. A grid electrode holding portion 25 is integrally formed in the electrode holding member 24. The grid electrode holding portion 25 electrically insulates and holds the grid electrode 23 with respect to the shield case 21 and the saw-toothed electrode 22. This grid electrode holding portion 25 has an engaging portion 25a having a returning portion for engagement and corresponding to an opening portion 23a formed at each of both ends of the grid electrode 23. When this grid electrode holding portion 25 is elastically deformed, the engaging portion 25a is inserted into the opening portion 23a of the grid electrode 23. When this elastic deformation of the grid electrode holding portion 25 is released, the grid electrode 23 is held by elastic force of the grid electrode holding portion 25 as predetermined tensile force. The above grid electrode 23 has openings having a mesh shape and uniformly formed by etching a thin plate. This thin plate is formed in the shape of a short strip and is made of stainless steel as in the above saw-toothed electrode 22. The grid electrode holding portion 25 integrally molded with the electrode holding member 24 is elastically deformed so that the engaging portion 25a is inserted into an opening formed in the grid electrode 23 and is engaged with this opening. Thus, the grid electrode holding portion 25 is tensioned by elastic force. A positioning member 26 is integrally molded with the electrode holding member 24 and is arranged in accordance with each of both end edges of the shield case 21. The positioning member 26 is used to position the electrode holding member 24 within the shield case 21. When a corona discharger having the above structure is assembled, a projection of the planar shape portion 24a of the electrode holding member 24 is first fitted into an opening formed in the saw-toothed electrode 22 so that the saw-toothed electrode 22 is held by this projection. The positioning member 26 is positioned and stored at an end edge of the shield case 21 in a predetermined position within the above shield case 21 in a state in which the saw-toothed electrode 22 is held. The engaging portion 25a of the grid electrode holding portion 25 is inserted into the opening portion 23a of the grid electrode 23 and is engaged with this opening portion 23a. A spring terminal 27 for power supply electrically comes in elastic contact with a tip portion of the saw-toothed electrode 22 located in the electrode holding member 24 and projected from the shield case. In the charger 2 having the above structure, as shown in Fig. 3, predetermined voltages are applied to the respective electrodes and the shield case 21 from a power supplying circuit 30. In Fig. 3, a predetermined voltage of + 24 V is supplied to the power supplying circuit 30. A high voltage generating circuit 31 is disposed within the power supplying circuit 30. The high voltage generating circuit 31 converts the supplied voltage + 24 V to a predetermined voltage and outputs the converted voltage. This high voltage generating circuit 31 generates voltages supplied to the shield case 21, the saw-toothed electrode 22 and the grid electrode 23 in the charger 2 of the present invention. Further, the high voltage generating circuit 31 generates a developing bias supplied to the developing device 4, a voltage supplied to the transfer device 7, etc. These generating voltages are outputted as predetermined voltages from respective output terminals of the power supplying circuit 30. As explained later, a voltage adjusting circuit 32 is disposed in the power supplying circuit 30 and adjusts voltages generated from the high voltage generating circuit 31 when the power supplying circuit 30 supplies voltages to the shield case 21 and the saw-toothed electrode 22 in the charger 2. The saw-toothed electrode 22 in the charger 2 is connected to an output terminal MC of the power supplying circuit 30 and receives a high voltage V from the power supplying circuit 30. The shield case 21 is connected to an output terminal CASE of the power supplying circuit 30 and receives a high voltage Vc from the power supplying circuit 30. Further, the grid electrode 23 is connected to an output terminal GRID of the voltage adjusting circuit 32 and receives a high voltage Vg from the voltage adjusting circuit 32. The voltage adjusting circuit 32 has a variable resistor VR1 for adjusting an output voltage supplied from the output terminal CASE to the shield case 21. The voltage adjusting circuit 32 also has a variable resistor VR2 for adjusting an output voltage supplied from the output terminal GRID to the grid electrode 23. Various kinds of voltages are supplied to the charger 2 by the power supplying circuit 30 having the above construction so that corona discharge is caused from a projecting tip portion of the saw-toothed electrode 22. An entire electric current as a total electric current It caused by this corona discharge flows through the saw-toothed electrode 22. A portion of the electric current caused by the corona discharge also flows onto a side of the photosensitive drum 1 so that a surface of the photosensitive drum 1 is charged with electricity having a specified polarity. At this time, a discharging electric current flows through the photosensitive drum by the corona discharge, but a charging potential of the photosensitive drum 1 is especially determined in accordance with the electric current flowing through the photosensitive drum. A drum current Id flowing through this photosensitive drum 1 can be controlled by controlling a voltage supplied to the grid electrode 23 so that a surface potential of the photosensitive drum 1 can be controlled and set to a predetermined potential. In this case, a grid current Ig flowing through the grid electrode 23 can be adjusted by suitably setting the output voltage of the output terminal GRID using the variable resistor VR2 of the adjusting circuit 32. Similarly, a case electric current Ic caused by the corona discharge flows through the shield case 21. The case electric current Ic can be also controlled by adjusting the output voltage of the output terminal CASE using the variable resistor VR1. The total electric current It is provided by the corona discharge caused by supplying a high voltage to the saw-toothed electrode 21. This total electric current It is equal to a sum of the case electric current Ic, the grid current Ig and the drum current Id respectively flowing through the shield case 21, the grid electrode 23 and the photosensitive drum 1. Namely, the total electric current It flowing through the saw-toothed electrode 22 by the corona discharge is distributed and flows through the shield case 21, the grid electrode 23 and the photosensitive drum 1. The total electric current It is distributed or divided into the case electric current Ic, the grid current Ig and the drum current Id and is represented by the following formula (1). It = Ic + Ig + Id Accordingly, when the total electric current It is constantly set, the drum current Id flowing through the photosensitive drum 1 can be constantly set so that the surface potential of the photosensitive drum 1 can be controlled and set to a constant potential. Therefore, similar to the general charger, the charger 2 has a constant current control section for constantly controlling the total electric current in the high voltage generating circuit 31 of the power supplying circuit 30. In the present invention, it is desirable to uniform discharging characteristics of the saw-toothed electrode 22 located in parallel with an axial direction of the photosensitive drum 1 so as to uniformly charge the entire surface of the photosensitive drum 1 in a rotational axis thereof. Therefore, the charging surface potential of the photosensitive drum 1 is measured along the axial direction thereof when the photosensitive drum 1 is charged with electricity by the charger 2, thereby knowing a charging characteristic state of the photosensitive drum. Fig. 6 is a characteristic graph showing charging characteristics of the charger in the present invention. This graph explains the relation between discharging characteristics and a current distribution ratio of the case electric current Ic and the grid current Ig. An axis of abscissa of this graph shows the current distribution ratio of the case electric current and the grid current. An axis of ordinate of this graph shows the discharging characteristics on the photosensitive drum as an image carrier. As can be seen from the characteristic graph in Fig. 6, uniform discharging characteristics are very bad when the case electric current Ic and the grid current Ig are greatly different from each other and the current distribution ratio is large. In this case, discharging irregularities are caused on the photosensitive drum surface so that the quality of a recorded image is influenced by the discharging irregularities. In contrast to this, when the case electric current Ic and the grid current Ig are approximately equal to each other, there are almost no discharging irregularities on the photosensitive drum surface. Accordingly, the photosensitive drum surface is uniformly charged and the quality of the recorded image is improved. When the case electric current Ic and the grid current Ig is especially equal to each other (1.0:1.0) and the current distribution ratio is equal to one, the corona discharging characteristics are very stabilized so that the photosensitive drum surface can be desirably charged uniformly. The photosensitive drum 1 is desirably charged uniformly in at least a range of the current distribution ratio of the case electric current Ic and the grid current Ig from 0.7:1.3 to 1.3:0.7. In this region, the case electric current Ic and the grid current Ig are approximately equal to each other. In the discharging characteristics shown in Fig 6, when the current distribution ratio of the case electric current Ic and the grid current Ig is set in a practical region, no black and white stripes can be seen as image irregularities with the naked eye. In contrast to this, in an unpractical region, the black and white stripes can be seen as image irregularities with the naked eye. With respect to the grid current Ig and the case electric current Ic, for example, a grid voltage Vg is fixedly set to a constant voltage such as - 600 V and a case voltage Vc is adjusted by suitably adjusting a resistance value of the variable resistor VR1. At this time, values of the grid current Ig and the case electric current Ic respectively flowing through the grid electrode and the shield case are measured. Charging characteristics at the current distribution ratio at this time are also shown in Fig. 6. At this time, the case voltage Vc is changed between 0 V and 1.5 kV. In the following experiments, the above charger is arranged in one image forming apparatus. In a first experiment, the image forming apparatus shown in Fig. 5 is set to a laser printer in which a rotational circumferential speed of the photosensitive drum 1 is set to 50 mm/second as a process speed. A high voltage V applied to the saw-toothed electrode 22 is set to about - 3.4 kV. At this time, a total electric current It flowing through the saw-toothed electrode 22 is equal to - 300 µA. Resistance values of the variable resistors VR1 and VR2 are adjusted such that a grid current Ig flowing through the grid electrode 23 and a case electric current Ic flowing through the shield case 21 are equal to each other. At this time, a voltage Vc of the shield case 21 is equal to - 600 V and a voltage Vg supplied to the grid electrode 23 is equal to - 500 V. Most preferable data about uniform discharging characteristics on the photosensitive drum 1 are obtained when each of the case electric current Ic and the grid current Ig is equal to - 145 µA. These experimental data obtained in the first experiment by using the image forming apparatus are used in the above formula (1) so that the following formula is obtained. It(300 µA) = Ic(145 µA)+Ig(145 µA)+Id At this time, the drum current Id is equal to 10 µA. From the above results, the case electric current Ic flows through the shield case 21 by applying the high voltage Vc to the shield case 21 of the charger 2. Further, the grid current Ig flows through the grid electrode 23 by applying the high voltage Vg to the grid electrode 23. At this time, the resistance values of the variable resistors VR1 and VR2 in the above voltage adjusting circuit 32 are preferably adjusted and set to provide these voltages Vc and Vg such that the case electric current Ic and the grid current Ig are approximately equal to each other. As mentioned above, the most preferable results of the uniform discharging characteristics can be obtained when the grid current Ig and the case electric current Ic are set to be equal to each other (1:1). A life of each of the above electrodes, an environment for arranging the charger 2, etc. can be considered with respect to the discharging characteristics of the charger 2. In this case, as can be seen from the characteristic graph in Fig. 6, the above current distribution ratio is practically set in the practical region in which the grid current Ig and the case electric current Ic are approximately equal to each other. As mentioned above, in this practical region, the grid current Ig and the case electric current are respectively jet in a region ranged from 0.7:1.3 to 1.3:0.7. Fig. 7 is a main enlarged view showing the charger 2 and the cleaner 8 in an electrophotographic processing section arranged around the photosensitive drum 1 in Fig. 5. The cleaner 8 and the charger 2 in the present invention are sequentially arranged from an upstream side of the photosensitive drum 1 in a rotational direction thereof. A laser optical path L is arranged in a lower portion of this charger 2. A laser beam is irradiated from the optical recording section 3 and is guided onto a surface of the photosensitive drum 1 as an image carrier along the laser optical path L. In Fig. 7, a corona portion caused by corona discharge from a discharging tip portion of the saw-toothed electrode 22 is discharged toward a surface of the photosensitive drum 1 in a discharging region shown by one-dotted chain line. This discharged corona portion acts on a photoconductive layer of the photosensitive drum 1 through a uniform opening screen of the grid electrode 23. At this time, an amount of the corona portion acting on the photoconductive layer on the surface of the photosensitive drum 1 is set by controlling the grid voltage Vg applied to the grid electrode 23. A waveform shown by a broken line in Fig. 7 shows a corona discharging portion discharged from the discharging tip portion of the saw-toothed electrode 22. A peak point P of this waveform shows most preferable charging characteristics of the photoconductive layer of the photosensitive drum 1. Normally, the peak point P of this waveform showing discharging characteristics is located in a region directly opposite to the tip portion of the saw-toothed electrode 22. However, as shown by a case portion 21a in Fig. 8, the peak point P of the waveform can be deflected in a direction shown by a broken line by bending a lower end of the shield case 21 upwards in an L-shape. The L-shaped lower end portion of the shield case 21 approaches the tip portion of the saw-toothed electrode 22 in comparison with another case portion 21b of the shield case 21. Accordingly, with respect to the waveform of the discharging characteristics shown in Fig. 8, the peak point P is deflected onto a side of the L-shaped lower end portion of the shield case 21 in accordance with a difference in impedance. In particular, corona discharge is caused on a front face of the charger 2 toward a surface of the photosensitive drum 1 from a projection of the saw-toothed electrode 22. This front face is located in front of one-dotted chain line shown in Fig. 8. Accordingly, it is sufficient to set at least the portion 21a of the shield case 21 to be conductive. The case portion 21b may be also set to be conductive. In this explanation, an end portion of the shield case 21 is partially bent in an L-shape to change impedances between this end portion and the other case portions. However, there is a method for changing an impedance between this case end portion and another discharging portion. In this method, an internal face of the shield case 21 is partially coated with a coating film, or a material of the shield case is partially changed. Preferable charging characteristics of the photoconductive layer of the photosensitive drum 1 can be also obtained by using this method. A corona portion discharged from the tip portion of the saw-toothed electrode 22 is desirably formed in a position in which no regions for the other processing portions are normally influenced by this corona portion. However, when an image forming apparatus is gradually made compact, there is no space for sufficiently arranging processing portions around the photosensitive drum 1. Therefore, it is necessary to arrange these processing portions in proximity to each other. Accordingly, a portion of the shield case 21 in the charger 2 is bent as above so that the peak point P showing discharging characteristics can be easily deflected on a desirable side of the charger on which no regions for the other processing portions or devices are influenced by the corona portion. When the corona discharge is performed by using the saw-toothed electrode 22, an air flow is generated in a corona discharging direction. A corona discharging portion from the tip portion of the saw-toothed electrode 22 has a flowing direction shown in Fig. 7 so that the air flow is formed in a specified direction. Therefore, the air flow can be deflected in an arbitrary direction by shifting the discharging peak point P using the above-mentioned method in accordance with a constructional arrangement of the charger. In Fig. 7, reference character a designates an air flow flowing into the charger 2 in accordance with the above discharging characteristics. Reference character b designates an air flow flowing out of the charger 2. Ozone generated by the corona discharge of the saw-toothed electrode 22 can be discharged by the air flow b from the charger 2. The reasons for this ozone discharge are as follows. The cleaner 8 is arranged in an upper portion of the charger 2. A cleaning blade 8a of this cleaner comes in press contact with a surface of the photosensitive drum 1. Accordingly, the air flow caused by the corona discharge is interrupted by this cleaning blade 8a so that no generated ozone, etc. are discharged from the shield case 21 along this air flow. In contrast to this, as shown in Fig. 7, there is no member for interrupting the air flow caused by the corona discharge in a lower portion of the charger 2. Accordingly, ozone generated by the corona discharge can be effectively discharged by directing the air flow toward the peak point P. Further, since the lower portion 21a of the shield case 2 is bent in an L-shape on an inner side thereof, a distance between the shield case 2 and the photosensitive drum 1 is increased in this bent portion so that the air flow can be effectively discharged from the charger 2. In a second experiment, the image forming apparatus shown in Fig. 5 is set to an electrophotographic copying machine in which a rotational circumferential speed of the photosensitive drum 1 is set to 200 mm/second as a process speed. A high voltage V applied to the saw-toothed electrode 22 is set to about - 4.2 kV. At this time, a total electric current It flowing through the saw-toothed electrode 22 is equal to - 700 µA. A voltage Vg supplied to the grid electrode 23 is set to - 700 V. The shield case 21 is held such that a voltage of the shield case 21 is equal to a ground potential. At this time, a current distribution ratio is adjusted such that a grid current Ig flowing through the grid electrode 23 is equal to - 340 µA and a case electric current Ic flowing through the shield case 21 is equal to - 340 µA. As shown in Figs. 7 and 8, this current distribution ratio is adjusted by bending an end tip 21a of the shield case 21 such that this end tip 21a approaches the saw-toothed electrode 22. Most preferable data of uniform discharging characteristics with respect to a photoconductive layer of the photosensitive drum 1 are obtained by setting the current distribution ratio to one as mentioned above. These experimental data obtained by using the image forming apparatus in the second experiment are used in the above formula (1) so that the following formula is obtained. It(700 µA)=Ic(340 µA)+Ig(340 µA)+Id At this time, the drum current Id is equal to 20 µA. Fig. 8 shows another construction of the charger 2 in the present invention. As mentioned above, corona discharge is caused in a specified direction from a tip portion of the saw-toothed electrode 22. Accordingly, it is not necessary to uniformly shield peripheral portions by the shield case as in a general wire corona discharger. Therefore, as shown in Fig. 8, one auxiliary electrode plate 21a is arranged in parallel with a longitudinal direction of the saw-toothed electrode 22 in the discharging tip portion thereof for generating the corona discharge. The auxiliary electrode plate 21a is arranged in a region located on a side of the photosensitive drum 1 from the discharging tip portion of the saw-toothed electrode 22. The auxiliary electrode plate 21a is further arranged within a discharging region from the tip portion of the saw-toothed electrode 22. Most preferable data of the uniform discharging characteristics can be also obtained when only a portion of the auxiliary eloctrode plate 21a is disposed within the corona discharging region from the tip portion of the saw-toothed electrode 22. Accordingly, it is not necessary to entirely cover the saw-toothed electrode with the shield case as shown in Figs. 5 and 7 so that the construction of the charger is simplified and the charger can be made compact. If the cleaner 8 is especially located in an upper portion of the charger, it is sufficient to dispose the auxiliary electrode plate 21a in at least a lower portion of the charger. An auxiliary electrode plate 21b may be disposed in the upper portion of the charger if other devices are close to the auxiliary electrode plate in the lower portion of the charger and there is no space for arranging the auxiliary electrode plate. In the above charger, a surface of a recording medium as the photosensitive drum can be uniformly charged. However, this charger can be also used as a discharger for removing residual charges from the recording medium surface. Namely. the charger 2 is used as a discharger for uniformly discharging the photosensitive drum 1. In particular, corona discharge is caused by applying a backward voltage reverse to a surface potential of the photosensitive drum 1 to the saw-toothed electrode 22 so as to discharge the photosensitive drum. In this case, the discharging operation can be similarly performed by applying an alternating voltage to the saw-toothed electrode 22. As mentioned above, in accordance with the charger of the present invention, a current distribution ratio is set such that electric currents flowing through the grid electrode and the shield case are approximately equal to each other, thereby uniformly performing a charging operation of the photosensitive drum. Further, the uniform charging operation can be performed without increasing a voltage applied to a discharging electrode. In particular, the voltage applied to the discharging electrode can be reduced so that it is not necessary to excessively secure a distance from a corona discharging portion to the shield case. Further, the shield case can be made compact so that a size of the charger can be reduced. Further, the direction of a corona flow caused by corona discharge can be freely set by adjusting the electric current flowing through the shield case. Accordingly, the corona flow can be efficiently discharged from the charger without any interruption of other devices. In particular, a discharging operation can be preferably stabilized for a long time by using the discharging electrode of the present invention irrespective of attachment of whiskers of silicon oxide, etc. caused in proportion to a using time of the discharging electrode. Further, since no discharging operation is easily influenced by erosion of the discharging electrode caused by ions of nitrogen, etc., the discharging operation can be continuously performed stably even when the discharging operation is performed for a long time. Fig. 9 is a perspective view for explaining a charger in accordance with another embodiment of the present invention. In Fig. 9, reference numerals 301, 302 and 303 respectively designate a charger, a charger case and a holding member for holding a saw-toothed electrode unit 304. Reference numerals 305, 306 and 307 respectively designate a saw-toothed electrode, a unit driving gear and a gear shaft. In Fig. 9, the charger case 302 in the charger 301 is constructed by an elongated metallic plate having a U-shape in cross section and an opening upper face. The charger case 302 has a bottom face 302a and side faces 302b, 302b. The saw-toothed electrode unit holding member 303 is constructed by a plate formed in the shape of a short strip. One long side of the holding member 303 is fixed to the charger case 302 in a state in which the holding member 303 is perpendicular to the bottom face 302a of the charger case 302 and is parallel to a side face 302b of the charger case 302. The saw-toothed electrode 305 has plural sharp ends having the same saw-toothed shape on one side of the elongated metallic plate. The saw-toothed electrode unit 304 is integrally formed by supporting the saw-toothed electrode 305 between insulating plates 304a and 304b. The saw-toothed electrode unit 304 is held such that the saw-toothed electrode unit 304 can be slidably moved on a face of the saw-toothed electrode unit holding member 303. A rack 304c is formed on a non-sliding face of the saw-toothed electrode unit 304 and is engaged with the unit gear 306. The unit gear 306 is fixed to the gear shaft 307. When the gear shaft 307 is rotated in the directions of arrows ±R, the saw-toothed electrode unit 304 can be moved in the directions of arrows ±P. Namely, the saw-toothed electrode 305 can be moved forward and backward with respect to an unillustrated photosensitive drum. Fig. 10 is a view for explaining a charger in accordance with another embodiment of the present invention. In Fig. 10, reference numerals 308 and 309 respectively designate a photosensitive drum and a grid electrode. Reference numerals 310 and 311 designate power sources. Reference numerals 312, 313 and 314 respectively designate a case electric current detector, a grid current detector and a current distribution ratio discriminator. Reference numerals 315, 316, 317 and 318 respectively designate a moving timing controller, a shift drive unit, a drive motor and a driving transmission mechanism. Constructional portions similar to those in Fig. 9 are designated by the same reference numerals as Fig. 9. In Fig. 10, a charger 301 has an unillustrated drive unit for operating a saw-toothed electrode 305 as shown in Fig. 9. The saw-toothed electrode 305 is opposed to the photosensitive drum 308. The grid electrode 309 is arranged between the photosensitive drum 308 and the charger 301. The grid electrode 309 is separated by a distance d from a sharp tip portion of the saw-toothed electrode 305. The grid electrode 309 is connected to the grid current detector 313 and the power source 310 having a negative voltage V 2 . A series circuit composed of a Zener diode D z and the case electric current detector 312 is connected to the charger case 302 and the power source 310. The saw-toothed electrode 305 is connected to the power source 311 having a negative voltage V 1 lower than the voltage V 2 . Outputs of the case electric current detector 312 and the grid current detector 313 are inputted to the distribution ratio discriminator 314. An output of the distribution ratio discriminator 314 is inputted to the shaft drive unit 316. An output of the moving timing controller 315 is also inputted to the shaft drive unit 316. The shaft drive unit 316 drives the drive motor 317 and moves the saw-toothed electrode 305 through the driving transmission mechanism 318 in the directions of arrows ±F. The driving transmission mechanism 318 has a transmitting function shown by the unit driving gear 306 and the rack 304c in the case of Fig. 9. An impedance of the charger 301 shown in Fig. 10 is changed by an environmental state between the charger 301 and the photosensitive drum, a change in shape of the sharp tip portion of the saw-toothed electrode 305, etc. The grid current detector 313 and the case electric current detector 312 detect a change in current distribution ratio of a grid current Ig flowing through the grid electrode and a case electric current Ic flowing through the charger case. A detected value of the current distribution ratio is inputted to the distribution ratio discriminator 314. The distribution ratio discriminator 314 has an arithmetic circuit for calculating a ratio of the grid current Ig and the case electric current Ic. The distribution ratio discriminator 314 also has a reference signal generating circuit for generating a reference signal as a reference of each of the grid current Ig and the case electric current Ic. The drive motor 317 and the driving transmission mechanism 318 are driven through the shaft drive unit 316 to move the saw-toothed electrode 315 through the driving transmission mechanism 318 such that a difference in voltage between the reference signal and an arithmetic output signal of the arithmetic circuit is set to zero. Thus, the distance d between the saw-toothed electrode 305 and the grid electrode 309 is set such that the current distribution ratio of the grid current Ig and the case electric current Ic is equal to a reference value shown by the reference signal. At this time, a moving period of the saw-toothed electrode 305 is controlled by the moving timing controller 315 such that no saw-toothed electrode 305 is moved while the charger 301 charges the photosensitive drum 308. In the above explanation, the charger is operated when the charger has a saw-toothed electrode. However, the present invention can be similarly applied to a wire electrode charger. As mentioned above, in accordance with the present invention, there is a case in which a distribution ratio of the grid current and the case electric current is abnormal by a change in impedance of a charging system caused by damage and wearing of electrodes of the charger, a change in environment such as atmospheric pressure, temperature, humidity, etc. In this case, the current distribution ratio is maintained at an optimum level by partially changing the impedance of the charging system by an operator so that the photosensitive drum can be stably charged. Many widely different embodiments of the present invention may be constructed without departing from the scope of the present invention as defined in the appended claims.

Claims: A charger for charging a recording medium (1) before irradiation of light to form an electrostatic latent image by irradiating light as an optical image onto a surface of said recording medium, characterized in that said charger comprises: a discharging electrode (22) having a plurality of projections for generating corona discharge to charge said recording medium surface; a conductive shield case (21) having an opening face opposite to said recording medium and opposed to said discharging electrode; a holding member (24) for holding said discharging electrode in said shield case in a state in which said discharging electrode is electrically insulated from said shield case; a conductive grid electrode (23) electrically insulated from said discharging electrode and said conductive shield and held between said discharging electrode and said recording medium; and a power supplying device (30) for supplying a high voltage for discharge to said discharging electrode and setting a ratio of electric currents flowing through said grid electrode and said shield case such that these electric currents are approximately equal to each other. A charger as claimed in claim 1, characterized in that said shield case (21) comprises at least one conductive auxiliary electrode Plate (21a) arranged in parallel with a longitudinal direction of said discharging electrode for charging said recording medium surface, and said auxiliary electrode plate (21a) is located in a region on a side of said recording medium with respect to a tip portion of said discharging electrode and is also located within a discharging region of said discharging electrode. A charger as claimed in claim 1, characterized in that a portion of said shield case (21) is different in structure from other portions of said shield case (21) with respect to said discharging electrode (22) for charging said recording medium surface, and a direction characteristics of a corona discharging to said recording medium surface is deflected in a predetermined direction.

Inventors: ANZAI SYUNJU, FUKUNAGA KEIZO, IMAGAWA SHINJI, KAWABATA ITARU, KAWASAKI YOSHIKAZU, KIDO EIICHI, MORI TOYOKAZU, TSUJI MASARU, YOSHIDA HIROAKI, YOSHIURA SYOICHIRO, YUI YUHI, ANZAI, SYUNJU, FUKUNAGA, KEIZO, IMAGAWA, SHINJI, KAWABATA, ITARU, KAWASAKI, YOSHIKAZU, KIDO, EIICHI, MORI, TOYOKAZU, TSUJI, MASARU, YOSHIDA, HIROAKI, YOSHIURA, SYOICHIRO, YUI, YUHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1993
Main classification: G03G15
IPC codes: G03G15