Charging device disposed close to member to be charged and image forming apparatus using same

Abstract

 A charging device includes a charging member (2) opposed to a member to be charged (1) with a smaller clearance therebetween; voltage application means (3) for applying an oscillating voltage between the member to be charged (1) and said charging member (2); wherein said charging member (2) is provided with a resistance layer, and a reactance of the resistance layer against the oscillating voltage is smaller than a resistance of the resistance layer.

Description

FIELD OF THE INVENTION AND RELATED ART

[0001] The present invention relates to an image forming apparatus such as an electrophotographic copying machine or electrostatic recording machine and to a charging device for uniformly charging or discharging a surface of a member to be charged or discharged such as an image bearing member in the form of a photosensitive member, dielectric member or the like.

[0002] As for the means for uniformly charging the member to be charged such as the image bearing member to a predetermined potential and polarity, there is widely used a corona discharger providing good uniformity of charging, such as corotron or scorotron, which has a wire electrode and a shield electrode.

[0003] The corona discharger involves the drawbacks that it requires an expensive high voltage source, that a large space is required by the corona discharger and the high voltage source with the shielding space, that the corona products such as ozone or the like result, and therefore, additional means and mechanisms against them are required and that the size and cost thereof are increased thereby.

[0004] Recently, therefore, charging means or devices of a contact charging type is considered in place of the corona dischargers.

[0005] In this type of charging, a charging member supplied with a voltage such as a DC voltage of approximately 1 - 2 KV, for example or an AC biased DC voltage, is used to charge the member to be charged such as an image bearing member, by which the member to be charged is charged to a predetermined potential. U.S. Patent No. 4,851,960 which has been assigned to the assignee of this application has proposed that the charging member is supplied with an oscillating voltage having a peak-to-peak voltage which is not less than twice a charge starting voltage which is the voltage at which the charging of the member to be charged starts only when the DC voltage is applied, by which the member to be charged is uniformly charged.

[0006] When such a contact type charging device is used in an image forming apparatus such as a laser beam printer as the charging member therefor in which the image bearing member in the form of the photosensitive member is charged and scanned with image information to form an electrostatic latent image, the following problems arise. When an image pattern in which the laser projection and non-projection are repeated at regular intervals at high density in the direction of sub-scan of the photosensitive member, interference stripes may be formed if the spatial frequency of the image pattern approaches the frequency of the AC voltage applied to the type charging member. This problem may be solved by sufficiently increasing the frequency of the AC voltage. This, however, tends to production of noise by the oscillating electric field vibrating the charging member and the photosensitive member because they are contacted.

[0007] Additionally, if the charging member is kept contacted to the photosensitive member for a long period of time in the contact type charging device, the charging member and/or the photosensitive member is deteriorated. Particularly when the charging member is made of rubber material, the plasticizer may ooze out of the rubber to be deposited on the photosensitive member with the result of deterioration of the photosensitive member, of the toner fusing by the charging member pressing the toner, and of the blurness of the image.

[0008] It would be considered to provide a space between the charging member and the member to be charged. However, then, the charged potential is not stabilized.

SUMMARY OF THE INVENTION

[0009] Accordingly, it is a principal object of the present invention to provide a charging device, charging method and image forming apparatus capable of stably and uniformly charging a member to be charged.

[0010] It is another object of the present invention to provide a charging device, a charging method and an image forming apparatus in which noise during the charging action can be suppressed.

[0011] It is a further object of the present invention to provide a charging device, a charging method and an image forming apparatus in which deterioration of the charging device and/or the member to be charged is prevented.

[0012] It is a further object of the present invention to provide a charging device, a charging method and an image forming apparatus wherein deterioration of the member to be charged or the image blurness is prevented which are otherwise caused by deposition of the plasticizer oozed out of the charging member on the member to be charged.

[0013] These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is a sectional view of an image forming apparatus according to an embodiment of the present invention.

[0015] Figure 2A is a sectional view of a charging device according to an embodiment of the present invention.

[0016] Figure 2B is a front view of the charging device shown in Figure 2A.

[0017] Figure 3 is a graph showing a relation between an AC voltage applied between the charging member and the photosensitive drum and a surface potential of the photosensitive drum.

[0018] Figure 4 is an equivalent circuit of the charging device according to the embodiment of the present invention.

[0019] Figure 5 is a front view of a charging device according to another embodiment of the present invention.

[0020] Figure 6 is a sectional view of a charging device of Figure 5.

[0021] Figure 7 is a side view of an image forming apparatus according to another embodiment of the present invention.

[0022] Figure 8 is a graph showing a relation between a peak-to-peak voltage of an AC voltage component applied to the charging member and a surface potential of the photosensitive drum.

[0023] Figure 9 is a graph showing a relation between an AC current and a surface potential of a photosensitive drum.

[0024] Figure 10 is a graph showing a relation between a peak-to-peak voltage of an AC voltage component applied to the charging member and a surface potential of a photosensitive drum.

[0025] Figure 11 is a graph showing a relation between an AC current and a surface potential of the photosensitive drum.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Referring to the accompanying drawings, the embodiments of the present invention will be described in detail.

[0027] Referring to Figure 1, there is shown an exemplary image forming apparatus incorporating a charging device according to an embodiment of the present invention as the charging means for charging an image bearing member thereof.

[0028] The image bearing member is in the form of a photosensitive drum 1 comprising an aluminum base member 1b and a photosensitive layer 1a of organic photoconductor (OPC) on the outer periphery of the base member. The outer diameter is 30 mm in this example. The photosensitive drum 1 rotates in a direction indicated by an arrow a at a predetermined speed. A charging member 2 in the form of a roller is opposed to the surface of the photosensitive drum with a small clearance Z therebetween.

[0029] As shown in Figure 2B, the charging roller 2 is provided, adjacent longitudinally opposite ends thereof, with spacer ring layers 16 extending along the circumferential of the roller, the spacer ring layer 16 being made of insulative material such as nylon, Teflon (trade names) or the like integral with the roller.

[0030] The charging roller 2 has a core metal 2a, and the longitudinally opposite end portions of the core metal are supported by unshown bearings so that the charging roller 2 extends substantially parallel with the generating line of the photosensitive drum 1. The end portions of the charging roller are pressed by urging means 10 and 10 by urging springs or the like so that the spacer ring layers 2c and 2c are press-contacted to the photosensitive drum 1 surface.

[0031] Therefore, the portion of the charging roller between the spacer ring layers 2c and 2c is maintained out of contact with the surface of the photosensitive drum 1 with a clearance d corresponding to the thickness of the spacer ring layers 16 and 16. The spacer ring layers 2c and 2c are disposed out side the charging region of the photosensitive drum 1 by the charging roller 2 in the longitudinal direction, that is, out side the image formation area of the photosensitive drum 1.

[0032] As shown in Figure 2A, the charging roller 2 is rotated following the photosensitive drum 1 rotation. It may be positively rotated in the codirectional peripheral movement direction relative to the photosensitive drum 1, or it may be rotated in the opposite peripheral direction. Further alternatively, it may not be rotated.

[0033] The charging member 2 is supplied with a voltage from a voltage source 3 by way of sliding contact electrodes 14 contacted to the end portions of the roller core metal 2a.

[0034] The photosensitive drum 1 uniformly charged by the charging member 2 is exposed to image light L in accordance with the image information. In this embodiment, the exposure means is in the form of a laser beam scanner (not shown) providing a laser beam modulated in accordance with electric signals on the basis of the information of the image to be recorded. The photosensitive drum 1 on which an electrostatic latent image is formed by the image exposure is developed with toner by a developing device 4. The toner image thus formed on the photosensitive drum 1 is transferred onto a transfer material S in the form of a sheet by transfer means in the form of a transfer roller. During the image transfer action, the transfer roller 5 is supplied with a transfer voltage. The toner image is fixed on the transfer material S by a fixing device 7 after the image transfer. On the other hand, the photosensitive drum 1 after the image transfer operation, is cleaned by a cleaning device 6 so that the residual toner is removed from the photosensitive drum 1, so that the photosensitive drum 1 is prepared for the next image forming operation. As shown in Figure 1, the charging roller 2 for charging the photosensitive drum 1, the cleaning device 6 and the developing device 4 are supported in a process unit 8, and the process unit 8 is detachably mountable to the main assembly of the image forming apparatus. When the process unit 8 is mounted to or demounted from the main assembly, the process unit slides on guides 9 of the main assembly of the image forming apparatus. The developing device 4 may be an element separate from the process unit 8. The process unit 8 includes at least the image bearing member (photosensitive drum 1) and the charging roller 2 (charging member).

[0035] The charging device will be further described. Figure 2A is a sectional view of the charging device. Between the charging roller 2 and the photosensitive drum 1, there is formed a predetermined clearance which is not more than 1 mm. The charging roller 2 comprises an electrically conductive core metal 2a made of iron, aluminum, stainless steel or the like, a resistance layer 2b made of rubber, plastic resin material containing or not containing conductive fine particles, and spacer rings 2c of insulative material integrally mounted at longitudinally opposite end portions of the resistance layer 2b. The resistance layer 2b is faced to the photosensitive drum 1. The voltage source 3 supplies to the core metal 2a an oscillating voltage (VDC + VAC) which is an AC biased DC voltage.

[0036] The oscillating voltage supplied to the charging roller 2 from the voltage source 3 preferably has a peak-to-peak voltage which is not less than twice the charge starting voltage, which is the voltage at which the charging starts when only a DC voltage is applied to the charging roller 2.

[0037] For the determination of the charge starting voltage, a member to be charged having zero potential is prepared. The charging member is faced to the member to be charged, and only a DC voltage is supplied to the charging roller 2, and the voltage is increased. When the charging of the photosensitive member starts, the voltage applied at this time is the charge starting voltage. The charge starting voltage differs, depending on the material thickness or the like of the photosensitive layer of the photosensitive member (the material of the member to be charged) and the clearance between the charging member and the member to be charged. By the application of the voltage having the peak-to-peak voltage which is not less than twice the charge starting voltage, an oscillating electric field is formed between the charging member and the member to be charged, so that uniform charging property can be provided. The formation of the oscillating electric field is effective to cause transfer and back-transfer of the electric charge. If the peak-to-peak voltage is smaller than twice the charge starting voltage, the member to be charged may be non-uniformly charged with spot patterns.

[0038] Here, the oscillating voltage is a voltage having voltage levels periodically changes. The waveform thereof may be sine, triangular or rectangular. It may be provided by periodically switching on and off a DC voltage source (rectangular wave voltage).

[0039] With the above structure, the clearance between the charging roller 2 and the photosensitive member 1 is set 80 microns, and the photosensitive member 1 is rotated at a peripheral speed of 24 mm/sec. The voltage source provided -600 V DC voltage and an AC voltage having the frequency of 1 KHz. The surface potentials of the photosensitive member 1 were measured when the charging roller 2 is provided with the resistance layer 2b and when it is not provided with the resistance layer 2b, while changing the peak-to-peak voltage of the AC component. The resistance layer 2b of the charging roller 2 is made of cellulose having a thickness of 500 microns. The resistance between the core metal 2a and the surface of the resistance layer 2b was 26 m.ohm/cm².

[0040] Figure 3 shows that the surface potential increases with increase of the peak-to-peak voltage irrespective of the presence or absence of the resistance layer.

[0041] However, without the resistance layer, the charge is not uniform even if the peak-to-peak voltage is increased, and the variation in the surface potential is large. With the resistance layer, the variation in the surface potential decreases with increase of the peak-to-peak voltage, and therefore, the uniform charging is accomplished.

[0042] With the increase of the resistance of the resistance layer 2b, the uniformity of the charging is increased. Correspondingly, however, the high peak-to-peak voltage is required, because the peak-to-peak voltage reduces at the surface of the charging roller 2 by the existence of the resistance layer 2b.

[0043] In order to prevent this, the electrostatic capacity between the core metal 2a of the resistance layer 2b and the surface of the resistance layer 2b of the charging roller 2 is increased.

[0044] Figure 4 shows a model of equivalent circuit of the charging device, wherein Rr is a resistance of the charging roller, Cr is an electrostatic capacity of the charging roller, C_A is an electrostatic capacity of the clearance, and Cd is an electrostatic capacity of the drum.

[0045] Here, the reactance of the resistance layer of the charging roller against the oscillating voltage is smaller than the resistance of the resistance layer, that is,





where f is a frequency of the oscillating voltage.

[0046] Then, the oscillating voltage (AC voltage) VAC effective to make the charging of the photosensitive drum uniform can be applied on the photosensitive drum 1 substantially without loss by the resistance layer 2b. In addition, the large resistance of the resistance layer is effective to prevent local charge failure in the form of a stripe by the leakage of the current through a pin hole of the photosensitive drum 1. Furthermore, the AC voltage effective to provide uniformity of the drum charging by the electrostatic capacity of the resistance layer can be applied to the photosensitive drum 1 without attenuation by the charging member. Further preferably,





   when the electrostatic capacity of the charging member is larger than the electrostatic capacity of the photosensitive drum, the reactance of the charging member against the AC voltage becomes smaller than the reactance of the drum, and therefore, the AC voltage can be applied to the drum without attenuation by the charging member, and therefore, it is preferable. In other words, the following is preferable:

[0047] As shown in Table 1, if the resistance layer 2b of the charging roller 100 is produced by PVdF (polyfluorinated vinylidene) having a thickness of 30 microns, for example, the roller resistance is 53 m.ohm/cm², and the electrostatic capacity is 1000 pF/cm².

[0048] The charging roller 101 provided with the cellulose film having a thickness of 500 microns has a resistance of 26 m.ohm/cm² and an electrostatic capacity of 40 pF/cm². A minimum AC voltage providing the uniform charging at the AC frequency of 1 KHz is 2.6 KVp-p in the case of the PVdF charging roller 100, and 3.0 KVp-p for the charging roller 101 with the cellulose film.

[0049] As will be understood, the uniform charging can be obtained with lower AC voltage if the electrostatic capacity of the resistance layer 2b is large.

Table 1

Roller	Material of resistance layer (µm)	Cr (pF/cm²)	Rr (MΩ/cm²)	1/(2πfCr) (MΩ/cm²)	AC voltage capable of uniform charging (kVp-p)
100	PVdF 30	100	53	0.16	2.6
101	cellulose 500	40	26	4.0	3.0

[0050] In the case of PVdF, the resistance of the resistance layer 2b is preferably 1 - 500 m.ohm/cm². If it is smaller than 1 m.ohm/cm², non-uniform charging occurs, and if it is larger than 500 m.ohm/cm², a large voltage source is required, and the risk of leakage becomes significant. The frequency of the voltage source 3 is preferably 100 Hz - 10 KHz. If it is smaller than 100 Hz, the non-uniform charging easily occurs along the length of the roller. If it is larger than 10 KHz, the voltage source becomes difficult.

[0051] Figures 5 and 6 show a charging member according to another embodiment. In this embodiment, the charging member is in the form of a blade. The charging blade 11 comprises a base member 11a of metal and a resistance layer 11b.

[0052] The longitudinal end portions of the charging blade 11 are fixedly mounted on the associated ones of the spacer members 12 and 12 by screws 13 and 13. The spacer members 12 and 12 are supported on an unshown non-movable member. The clearance d between the charging blade 11 and the photosensitive drum 1 is defined by the spacer members 12 and 12. The voltage to the charging blade 11 from the voltage source 3 is directly applied through the lead wire 15.

[0053] Since the charging member 11 is not movable, and therefore, the noise attributable to the electric contacts can be avoided.

[0054] In this embodiment, the reactance of the resistance layer of the charging blade against the oscillating voltage is smaller than the resistance of the resistance layer, and therefore, the oscillating voltage can be applied on the photosensitive drum 1 substantially without loss by the resistance layer 11b.

[0055] Similarly to the case of the charging roller, the charging member of this embodiment provides stabilized and uniform charging action.

[0056] Referring to Figure 7, a further embodiment of the present invention will be described. The exemplary image forming apparatus with which the charging device of this embodiment is used is the same as the image forming apparatus of Figure 1, and therefore, the description thereof is omitted for simplicity by assigning the same reference numerals in Figure 1 to the elements of Figure 7 having the corresponding functions.

[0057] The charging member is in the form of a charging roller 22, which comprises an electrically conductive core metal rod 22a made of steel, aluminum or the like, an elastic layer 22b thereon of rubber material such as EPDM or the like which is provided with electric conductivity by carbon or the like (not more than 10 ⁶ ohm.cm), and a surface layer 22c having a relatively high resistance 10⁶ ohm.cm - 10⁹ ohm.cm which is higher than that of the elastic layer 2b.

[0058] The charging roller, similarly to the case of Figure 2B, is provided with unshown spacer rings made of insulative material integrally mounted on the longitudinally end portions of the surface layer 22c. A predetermined small clearance z is provided between the charging roller 22 and the photosensitive drum 1.

[0059] The charging roller 22 is supplied from a voltage source 3 with an oscillating voltage (the voltage level changes periodically with time) in the form of a sum of an AC voltage (oscillating component) and a DC voltage (non-oscillating component). The voltage is supplied through sliding contact electrodes 14 contacted to the end portions of the roller core metal 22a.

[0060] A voltage source 3 comprises a constant current source V_AC. An AC constant current control means G functions to provide a predetermined AC current component (oscillating component), 800 micro-amperes, for example in this embodiment. It also comprises a constant voltage DC source V_DC. A DC constant voltage control means H functions to provide a predetermined voltage of the DC current component (non-oscillating component), -650 V, for example in this embodiment. The charge potential on the photosensitive drum 1 is determined.

[0061] In this embodiment, between the member to be charged and the charging member, a voltage having a peak-to-peak voltage which is not less than twice the charge starting voltage therebetween, is applied. Between the charging member and the discharging member, an oscillating electric field is formed by transfer and back-transfer of the electric charge.

[0062] Figure 8 shows the potential (Vs) of an OPC photosensitive drum 1 (the member to be charged) when the peak-to-peak voltage (Vpp) which is the oscillating voltage applied to the charging member in the form of the charging roller 22 under the normal temperature and normal humidity conditions (23 ^oC and 60 %). The clearance between the charging roller 22 and the photosensitive drum 1 is Z = 80 microns or Z = 150 microns. The DC current component voltage was -650 V, and the AC frequency was 1000 Hz.

[0063] As will be understood, when the clearance Z is 80 microns, the peak-to-peak voltage becomes equal to or larger than twice the charge starting voltage (-900 V) when the AC component voltage Vpp is equal to or larger than 1800 V. Under these conditions, the charging and reverse-charging (transfer and back-transfer of the electric charge) repeatedly occur through the clearance between the charging roller 2 and the photosensitive drum 1, so that the photosensitive drum is uniformly charged. When, on the other hand, the clearance Z is 150 microns, the uniform charging is possible when the AC component voltage Vpp is equal to or larger than 2600 V (the charge starting voltage of the photosensitive drum is -1300 V). This is because with increase of the clearance, the impedance against the voltage applied by the voltage source 3 increases, and therefore, a higher voltage of the voltage source is required to maintain the discharging electric field.

[0064] When the voltage Vpp is smaller than 1800 V (Z = 80 microns) or when the voltage Vpp is smaller than 2600 V (Z = 150 microns), the spot-like non-uniformity occurs on the photosensitive drum 1, and the charge potential is lower than the desired potential of -650 V (reduction of the charging efficiency).

[0065] Figure 9 shows a relation between the charge potential vs. of the photosensitive drum 1 and an AC current I_AC. It will be understood that when I_AC is not less than 750 micro-ampere, the voltage Vs is stabilized, and the uniform charging is possible in both of the cases of Z = 80 microns and Z = 150 microns.

[0066] Therefore, the constant current control with a predetermined current 750 micro-ampere and with the peak-to-peak voltage applied to the charging roller which is not less than twice the charge starting voltage, the uniform charging is always possible irrespective of the change in the clearance between the charging roller and the photosensitive drum.

[0067] Figure 10 shows the potential Vs of the photosensitive drum 1 when the peak-to-peak voltage Vpp of the AC voltage (oscillating voltage) applied to the charging roller 2, under the normal temperature and normal humidity conditions (23 ^oC and 60 %) and under the low temperature and low humidity conditions (15 ^oC and 10 %), with a constant clearance (Z = 80 microns) between the charging roller 2 and the photosensitive drum 1. Similarly to the case of Figure 8, the AC component voltage required for uniformly charging the photosensitive drum changes particularly by the humidity in this case (2500 V under the low temperature and low humidity condition, and 1800 V under the normal temperature and normal humidity condition). This is because, the electric resistance of the charging roller changes with humidity, more particularly, the impedance thereof against the voltage source is low under the high humidity condition, and is high under the low humidity condition.

[0068] Figure 11 shows the relation between the surface potential of the photosensitive drum 1 and the AC current I_AC. As will be understood from this Figure, the charged potential Vs of the photosensitive drum is stabilized and uniform irrespective of the humidity, if the AC current I_AC is not less than 750 micro-ampere.

[0069] Therefore by the constant current control with an AC component voltage not less than 750 micro-ampere and with the peak-to-peak voltage applied to the charging roller which is not less than twice the charge starting voltage, the uniform charging is possible irrespective of the humidity change with the necessary and sufficient peak-to-peak voltage level. In order to accomplish the constant current control so as to provide the peak-to-peak voltage which is twice the charge starting voltage, the effective current is made constant if the waveform and the frequency of the oscillating voltage are constant.

[0070] On the other hand, the DC component is preferably controlled to be constant because when it is superposed with the AC voltage with I_AC ≧ 750 micro-ampere, the photosensitive drum is charged to the voltage level which is substantially the center between the peak levels of the peak-to-peak voltage.

[0071] As will be understood from Figures 8 and 10, in order to maintain the uniformity of the charging irrespective of the variation in the clearance between the photosensitive drum 1 and the charging roller 2 and further irrespective of variation in the humidity of the ambience, it is desired that the peak-to-peak voltage is larger than the inflection point in Figures 8 and 10. However, if the constant voltage control is carried out with the peak-to-peak voltage Vpp not less than 2600 V with Z = 80 microns or 150 microns as shown in Figure 8, the excessive voltage application occurs with the result of local charging failure due to the leakage current through a pin hole of the photosensitive drum 1.

[0072] By the constant current control of the AC component voltage applied between the photosensitive drum 1 and the charging roller 2, the excessive voltage application and the resultant charging failure due to the leakage current can be prevented irrespective of the distance between the photosensitive drum 1 and the roller 2 and irrespective of the variation in the humidity condition.

[0073] The charging roller shown in Figure 7 may be in the form of a charging blade shown in Figures 5 and 6, and further alternatively, it may be in the form of a brush, belt or the like. On the charging blade of Figures 5 and 6, a high resistance layer having a resistance higher than the blade may be provided.

[0074] It is desirable that the reactance of the resistance layers 22b and 22c of the charging layer against the oscillating or vibrating voltage is smaller than the resistance of the resistance layers, as described in the embodiment described hereinbefore.

[0075] As described in the foregoing, since there is a small clearance between the charging member and the member to be charged (photosensitive member), the charging noise is hardly produced, and the deformation of the charging member does not occur, and in addition, the image blurness attributable to the deposition of the plasticizer in the charging member onto the photosensitive member.

[0076] If the clearance exceeds 1 mm, the peak-to-peak voltage between the photosensitive member and the charging member required for the uniform charging becomes significantly high, and therefore, the dielectric break down of the photosensitive member tends to occur with the result of current leakage, and therefore, the clearance is preferably not more than 1 mm.

[0077] The photosensitive member may be of, selenium, amorphous silicon or the like as well as OPC.

[0078] As described in the foregoing, according to the present invention, an oscillating voltage is applied between a member to be charged and a charging member with a small clearance therebetween, and the charging member is provided with a resistance layer, so that the charging noise can be reduced with the advantage of uniform charging operation.

[0079] By making the reactance of the resistance layer against the oscillating voltage smaller than the resistance of the resistance layer, the uniform charging operation is possible with lower peak-to-peak voltage of the oscillating voltage.

[0080] Furthermore, by the constant current control of the AC component applied between the member to be charged and the charging member, the leakage of the current to the member to be charged can be prevented even when the clearance between the charging member and the member to be charged and the ambient condition are changed, and therefore, the uniform and stabilized charging is possible without improper charging.

[0081] While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

[0082] A charging device includes a charging member opposed to a member to be charged with a smaller clearance therebetween; voltage application means for applying an oscillating voltage between the member to be charged and said charging member; wherein said charging member is provided with a resistance layer, and a reactance of the resistance layer against the oscillating voltage is smaller than a resistance of the resistance layer.

Claims

1. A charging device, comprising:
   a charging member opposed to a member to be charged with a smaller clearance therebetween;
   voltage application means for applying an oscillating voltage between the member to be charged and said charging member;
   wherein said charging member is provided with a resistance layer, and a reactance of the resistance layer against the oscillating voltage is smaller than a resistance of the resistance layer.

2. A device according to Claim 1, wherein 10 times said reactance is smaller than the resistance of said resistance layer.

3. A device according to Claim 1, further comprising a clearance maintaining member in contact with the member to be charged outside a region where said charging member is effective to charge the member to be charged, so as to maintain a clearance between the member to be charged and said charging member in the region.

4. A device according to Claim 1 or 3, wherein the clearance is not more than 1 mm.

5. A device according to Claim 1, wherein the oscillating voltage is in the form of a sine wave.

6. A device according to Claim 1, further comprising constant current control means for providing a predetermined constant current of an oscillating component of said oscillating voltage through said charging member.

7. A device according to Claim 1 or 6, wherein said oscillating voltage includes non-oscillating component, and said apparatus further comprises constant voltage control means for providing a predetermined voltage of the non-oscillating component supplied through said charging member and the member to be charged.

8. A device according to Claim 1, wherein said charging member has an electrostatic capacity which is larger than that of the member to be charged.

9. An image forming apparatus, comprising:
   an image bearing member;
   a charging member disposed with a small clearance from said image bearing member;
   voltage application means for applying an oscillating voltage between said image bearing member and said charging member;
   wherein said charging member is provided with an resistance layer, and a reactance of said resistance layer against the oscillating voltage is smaller than a resistance of said resistance layer.

10. A device according to Claim 9, further comprising a process unit detachably mountable to said image forming apparatus, said process unit containing said image bearing member and said charging member.

Drawing