Device for charging a photosensitive body

Abstract: A rotating photosensitive body (11) has a plurality of charging members (12, 13) on its surface. A direct voltage whose polarity is opposite to the charging polarity of the photosensitive body (11) is applied to at least one (12) of the charging members (12, 13), and a direct voltage whose polarity is equal to the charging polarity thereof is applied to at least one (13) of the other charging members.

Description: The present invention relates to a method and a device for charging a photosensitive body in contact with a charging member in an electrophotographic apparatus such as a laser printer and a copying machine. An electrophotographic apparatus, for example, a laser printer includes a charging device, an exposure device having a laser beam emitter, a developing device, and a transfer device which are arranged around a photosensitive body. In the laser printer, when printing is performed, the charging device is operated to uniformly charge the photosensitive body, and then the exposure device is operated to scan the photosensitive body with a laser beam having recording information, thereby forming an electrostatic latent image thereon. After this, the developing device is operated to stick the toner on the electrostatic latent image, and the transfer device is operated to transfer a toner image formed by the operation of the developing device onto paper. There are two methods for charging the photosensitive body using the charging device of the laser printer. The first method is executed by corona discharge caused by a scorotron, and the second method is executed by bringing a member such as a charging roller, a charging brush, and a charging blade into contact with the photosensitive body. The first method requires a high voltage power supply of 5 to 10 KV and generates a great deal of ozone due to the corona discharge. The ozone degrades the material characteristics of the photosensitive body. To solve this problem, the first charging method is replaced with the second method. The second charging method has the advantage of requiring a relatively low voltage power supply and greatly reducing the ozone. The second charging method is disclosed in Published Unexamined Japanese Patent Application No. 56-91243. According to the second charging method, as shown in Fig. 6, first, second, and third charging rollers 2a, 2b, and 2c are arranged on a photosensitive drum 1 in contact with each other in the moving direction of the photosensitive drum (in the direction of arrow). As shown in Fig. 7, the charging rollers 2a, 2b, and 2c superimpose an alternating voltage on direct voltages of 200V, 350V, and 500V, respectively. The peak-to-peak value of the alternating voltage is 20% of each of the direct voltages. The voltages obtained by the superimposition are sequentially applied to the surface of the photosensitive drum 1. The potential of the surface of the photosensitive drum 1 gradually increases, and finally it is charged with a voltage of 500V necessary for exposure. In the second charging method, however, since the direct and alternating voltages are applied to the charging rollers 2a, 2b, and 2c, two power supplies, i.e., direct and alternating power supplies are required, thus complicating an arrangement of the power supplies. US-4,959,688 describes an image forming apparatus comprising charging and discharging means. This apparatus has a photosensitive drum (1) as the member to be charged with a charge transfer start voltage. The photosensitive drum (1) is rotated about its central axis at a predetermined peripheral speed. Furthermore, the apparatus includes a first charging means (7) and a conductive roller (2). During operation, the surface of the photosensitive drum (1) is first charged by the first charging means (7) to have a nonuniform surface potential and than discharged by the conductive roller (2) to have a predetermined uniform surface potential. Within this apparatus the voltage applied to the conductive roller (2) is restricted to a voltage not higher than the charge transfer start voltage, i.e. charging start voltage. It is an object of the present invention to provide a device for charging a photosensitive body at an uniform potential by bringing a charging member into contact with the photosensitive body, in which a power supply necessary for the charge is simple in arragement. According to the invention, the object is solved by the features of claims 1 or 3. The subclaims contain further preferred embodiments of the invention. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a view of an arrangement of the main part of a charging device according to an embodiment of the present invention; Fig. 2 is a graph showing a relationship between a voltage applied to each conductive brush and a potential of the surface of a photosensitive body in the charging device shown in Fig. 1; Fig. 3 is a view of an arrangement of the main part of a charging device according to another embodiment of the present invention; Figs. 4A and 4B are circuit diagrams showing the structures of direct power supplies of the charging device shown in Fig. 3; Figs. 5A to 5G are graphs each showing a variation in the potential of the surface of a photosensitive body due to an operation of the charging device shown in Fig. 3; Fig. 6 is a view of a conventional charging device; and Fig. 7 is a graph showing waveforms of voltages applied to charging rollers of the conventional charging device. Fig. 1 shows a charging device according to an embodiment of the present invention, which is applied to a laser printer. A drum type photosensitive body 11 rotates in the direction of the arrow, i.e., clockwise. First and second conductive brushes 12 and 13 are arranged with their ends in contact with the surface of the photosensitive body 11. The second conductive brush 13 is arranged after the first conductive brush 12 in the rotating direction of the photosensitive body 11. These conductive brushes 12 and 13 can be replaced with a conductive cloth, a conductive roller, a conductive rubber blade, a conductive sponge, or the like. The first and second conductive brushes constitute a charging device A. The laser printer further includes an exposure device B, a developing device C, and a transfer device D which are arranged around the photosensitive body 11. The exposure device B emits a laser beam onto the surface of the photosensitive body 11 charged by the charging device A to expose it, thereby recording image information in the form of an electrostatic latent image. In the developing device C, toner 15 is stuck to the electrostatic latent image formed by the exposure device B, thereby performing a developing operation. The transfer device D transfers a toner image formed by the developing device C to paper 16. The paper 16 is fed from a paper feeding device (not shown) in predetermined timing. The developing device C includes a developing roller 17. When the developing roller 17 rotates in the direction of the arrow shown in Fig. 1, the developing device C transmits the toner 15 to the surface of the photosensitive body 11. The transmitted toner 15 is stuck to the surface of the photosensitive body 11. A direct voltage V1 whose polarity is opposite to the charging polarity of the photosensitive body 11 is applied to the first conductive brush 12, and a direct voltage V2 whose polarity is equal to the charging polarity of the photosensitive body 11 is applied to the second conductive brush 13. A transfer voltage Vt is applied to the transfer device D. A development bias voltage Vb is applied to the developing roller 17. The direct voltages V1 and V2 are set to, for example, +500V and -1100V, respectively. More specifically, the direct voltage V1 is set to +500V substantially equal to a charge starting voltage which is the minimum voltage necessary for positively charging the toner in order to prevent the photosensitive body 11 from being degraded because of discharge between the photosensitive body 11 and the first conductive brush 12. The direct voltage V2 is set so that the surface of the photosensitive body 11 has a predetermined charging potential. Fig. 2 is a graph showing a relationship between the surface potential of the photosensitive body 11 and the direct voltage V2 when the direct voltage V1 is set to, for example, +500V. As is apparent from Fig. 2, if the charging potential of the photosensitive body 11 is -500V, the direct voltage V2 is set to -1100V. An operation of the charging device according to the above embodiment of the present invention will be described. The photosensitive body 11 rotates clockwise in Fig. 1 and is charged by the charging device A. An electrostatic latent image is formed on the photosensitive body 11 by the exposure device B based on image information. Toner is stuck to the electrostatic latent image by the developing device C to form a toner image on the photosensitive body 11, and the toner image is transferred to the paper 16 by the transfer device D. In this case, part of toner remains on the surface of the photosensitive body 11, and the remaining toner is conveyed to the developing device C to be used together with fresh toner and is positively and negatively charged. If the photosensitive body 11 is next charged only by a conductive brush whose polarity is, for example negative when the positively and negatively charged toner remains thereon in the form of a lump, it cannot be uniformly charged. To solve this problem, a voltage (V1 = +500V) whose polarity is opposite to the charging polarity of the photosensitive body 11 is applied from the first conductive brush 12 to the surface of the photosensitive body 11. Therefore, all the remaining toner on the surface of the photosensitive body is positively charged. A voltage (V2 = -1100V) whose polarity is equal to the charging polarity of the photosensitive body 11 is applied from the second conductive brush 13 to the surface of the photosensitive body 11. Therefore, the surface of the photosensitive body 11 is charged to have a predetermined charging potential of -500V. The direct voltages V1 and V2 are sufficient for the voltage used in the charging device A, and the direct voltages V1 and V2 can be extracted from a common direct power supply. The number of power supplies required for charging the photosensitive body is thus one, resulting in simplification of power supply arrangement and reduction in cost. Since the first and second conductive brushes 12 and 13 have a function of reducing the density of the remaining toner to uniform the toner, it is unnecessary to arrange a cleaning device exclusively for removing the remaining toner. Fig. 3 shows a charging device according to another embodiment of the present invention, which is applied to a laser printer. Like the first and second conductive brushes 12 and 13, third and fourth conductive brushes 18 and 19 are arranged with their ends in contact with the surface of the photosensitive body 11. These first to fourth conductive brushes 12, 13, 18, and 19 constitute the charging device A. The third conductive brush 18 is arranged after the second conductive brush 13 in the rotating direction of the photosensitive body 11, and the fourth conductive brush 19 is arranged after the third conductive brush 18 in the rotating direction of the photosensitive body 11. Direct voltages V1 of +500V and V2 of -1100V are applied to the first and second conductive brushes 12 and 13, respectively, as in the embodiment shown in Fig. 1. A direct voltage V3 whose value ranges from 0 to 10V is applied to the third conductive brush 18, and a direct voltage V4 of -1100V is applied to the fourth conductive brush 19. The direct voltage V1 of +500V and the direct voltage V3 of, e.g., +10V are extracted from the power supply circuit shown in Fig. 4A. In this power supply circuit, a reference diode 23 is connected in parallel to a direct power supply 21 of +550V through a resistor 22, and a reference diode 25 is connected in parallel to the direct power supply 21 through a resistor 24. The direct voltage V1 is extracted from both ends of the reference diode 23, and the direct voltage V3 is extracted from both ends of the reference diode 25. The direct voltage V2 of -1100V and the direct voltage V4 of -1100V are extracted from the power supply circuit shown in Fig. 4B. In this power supply circuit, a reference diode 28 is connected in parallel to a direct power supply 26 of -1200V through a resistor 27, and a reference diode 30 is connected in parallel to the direct power supply 26 through a resistor 29. The direct voltage V2 is extracted from both ends of the reference diode 28, and the direct voltage V4 is extracted from both ends of the constant voltage diode 30. An operation of the charging device according to the above embodiment shown in Fig. 3 will be described. The photosensitive body 11 rotates clockwise in Fig. 3 and is charged by the charging device A. An electrostatic latent image is formed on the photosensitive body 11 by the exposure device B based on image information. Toner is stuck to the electrostatic latent image by the developing device C to form a toner image on the photosensitive body 11, and the toner image is transferred to the paper 16 by the transfer device D. In this case, part of toner remains on the surface of the photosensitive body 11, and the remaining toner is positively and negatively charged. The surface potential of the photosensitive body 11 is shown in Fig. 5A. The photosensitive body 11 further rotates and the remaining toner reaches the first conductive brush 12. The direct voltage of +500V is applied to the first conductive brush 12, as indicated by a dotted line in Fig. 5B. The surface potential of the photosensitive body 11 is changed by the conductive brush 12 as shown in Fig. 5C. All the remaining toner is charged positively. The photosensitive body 11 further rotates and the remaining toner reaches the second conductive brush 13. The direct voltage of -1100V is applied to the second conductive brush 13, as indicated by a dotted line in Fig. 5D. The toner positively charged on the photosensitive body 11 is absorbed by the second conductive brush 13, and the toner absorbed and negatively charged by the second conductive brush 13 is then absorbed by the developing device C. The surface potential of the photosensitive body 11 varies from -470V to -530V by the second conductive brush 13, as shown in Fig. 5E. The photosensitive body 11 further rotates and the remaining toner reaches the third conductive brush 18. The direct voltage of +10V is applied to the third conductive brush 18. The surface potential of the photosensitive body 11 is changed to around -500V by the third conductive brush 18, as shown in Fig. 5F. The photosensitive body 11 further rotates and the remaining toner reaches the fourth conductive brush 19. The direct voltage of -1100V is applied to the fourth conductive brush 19. The surface potential of the photosensitive body 11 is changed to -500V by the fourth conductive brush 19, as shown in Fig. 5G, and the photosensitive body is charged in substantially uniform fashion. Finally the photosensitive body 11 is uniformly charged at its surface potential of -500V. The direct voltage V1 of +500V and the direct voltage of V3 of +10V can be generated from the power supply circuit shown in Fig. 4A, and the direct voltage V2 of -1100V and the direct voltage V4 of -500V can be generated from the power supply circuit shown in Fig. 4B. The direct power supplies 21 and 26 can be obtained from a single direct power supply. In the embodiment shown in Fig. 3, the number of power supplies required for charging the photosensitive body is also one, resulting in simplification of power supply arrangement and reduction in cost. If three or more conductive brushes are arranged, the first one of them does not have to have a polarity opposite to the polarity of the photosensitive body 11. Even though the polarity of the first conductive brush is the same as that of the photosensitive body, the surface potential of the photosensitive body can be uniformly charged.

Claims: A device for charging a photosensitive body, comprising: a plurality of charging means for charging a photosensitive body in contact with a surface of the photosensitive body which is rotating; first applying means for applying a first direct voltage whose polarity is opposite to a charging polarity of the photosensitive body to at least one of said plurality of charging means; and second applying means for applying a second direct voltage whose polarity is equal to the charging polarity of the photosensitive body to at least one of said plurality of charging means other than said at least one of the plurality of charging means to which the first direct voltage is applied by said first means, characterized in that said plurality of charging means include a first charging member (12) and a second charging member (13) arranged after the first charging member (12) in a rotating direction of the photosensitive body (11), said first applying means applies the first direct voltage to said first charging member (12), and said second applying means applies the second direct voltage to said second charging member (13), said second direct voltage being not lower than a charge starting voltage at which the photosensitive body (11) starts to be charged. The device according to claim 1, characterized in that said first direct voltage applied to said first charging member is +500V and said second direct voltage applied to said second charging member is -1100V. A device for charging a photosensitive body, comprising: a plurality of charging means for charging a photosensitive body in contact with a surface of the photosensitive body which is rotating; first applying means for applying a first direct voltage whose polarity is opposite to a charging polarity of the photosensitive body to at least one of said plurality of charging means; second applying means for applying a second direct voltage whose polarity is equal to the charging polarity of the photosensitive body to at least one of said plurality of charging means; third applying means for applying a third direct voltage whose polarity is opposite to the charging polarity of the photosensitive body to at least one of said plurality of charging means; and fourth applying means for applying a fourth direct voltage whose polarity is equal to the charging polarity of the photosensitive body to at least one of said plurality of charging means, wherein said plurality of charging means includes first, second, third and fourth charging members arranged in sequence in a rotating direction of the photosensitive body (11), said first and third applying means apply the first and third direct voltages to said first and third charging members, respectively, and said second and fourth applying means apply the second and fourth direct voltages to said second and fourth charging members, respectively, each of said second and fourth direct voltages being not lower than a charge starting voltage at which the photosensitive body (11) starts to be charged. The device according to claim 3, characterized in that said first direct voltage applied to said first charging member is +500V, said second direct voltage applied to said second charging member is -1100V, said third direct voltage applied to said third charging member is +10V, and said fourth direct voltage applied to said fourth charging member is -1100V.

Inventors: ENDOU MITSUHARU, OHTAKA YOSHIMITSU, ENDOU, MITSUHARU, OHTAKA, YOSHIMITSU
Applicants: TOKYO ELECTRIC CO LTD, KABUSHIKI KAISHA TEC
Application year: 1992
Main classification: G03G15
IPC codes: G03G15, G03G13, G03G21