JPH0619280A - Electrifier and image forming device - Google Patents

Abstract

PURPOSE:To prevent the occurrence of the electrified memory of the surface of a body to be electrified when an alternating voltage applied to an electrifying member is off, in an AC electrifying system electrifier, and the degradation of the quality of an output image and the scattering of toner in a machine, caused by the occurrence of the electrified memory of the surface of the body to be electrified when the alternating voltage applied to the electrifying member is off, in an image forming device using the AC electrifying system electrifier. CONSTITUTION:In a contact system electrifier allowing the electrifying member to contact with the body to be electrified or approach it, and applying the alternating voltage to the electrifying member, to execute the electrification processing of the body to be electrified, or the image forming device using the electrifier, when an applied alternating voltage is off, the drop ratio of the peak voltage of one side of the alternating voltage is set <=400V/1 cycle or below, and the voltage is off to fall.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a charging member (contact electrode).
Relates to a contact-type charging device for contacting or approaching an object to be charged and applying an alternating voltage to the charging member to charge the object to be charged (including static elimination processing). Further, the present invention relates to an image forming apparatus using the charging device as a charging step means in an image forming process.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric is charged to a predetermined polarity and potential. As a processing means, a high voltage is applied to a thin discharge wire made of tungsten or the like to generate corona discharge, and the surface of the image bearing member is exposed to the corona discharge to charge the surface of the image bearing member. The device, a corona discharge device, has been commonly used.

However, in recent years, the contact type charging device has been put into practical use because it has advantages such as a low power supply voltage, less ozone generation, and cost reduction. .

FIG. 5 shows a schematic structure of an example of the contact type charging device.

Reference numeral 1 denotes a member to be charged, which is, for example, a rotating drum type photosensitive member of an electrophotographic apparatus. This photoconductor is rotationally driven in the direction of the arrow at a predetermined peripheral speed.

Reference numeral 2 denotes a charging roller as a charging member which is arranged in parallel with the photosensitive member 1 and is in pressure contact therewith.
It is composed of a conductive elastic layer 2b which is concentrically and integrally formed in a roller shape on the outer periphery thereof. The charging roller 2 may be driven to rotate in accordance with the rotation of the photoconductor 1, may be rotationally driven in the forward direction or the reverse direction of the rotation of the photoconductor 1, or may be a non-rotating roller body.

By applying a charging bias from the charging bias applying power source 3 to the charging roller 2 through the core metal 2a, the peripheral surface of the rotating photosensitive member 1 is charged to a predetermined polarity and potential by a contact method. .

Although not shown in the figure, in addition to the charging means 2 and 3, the necessary image forming process equipment such as an exposing means, a developing means, a transferring means, a cleaning means, a fixing means and the like are provided around the photosensitive member 1. Are arranged to form an image forming apparatus,
Image formation is executed.

As the mechanism of contact charging, dielectric breakdown of air occurs mainly in the minute gaps d and d in the vicinity of the contact portion N between the charging roller 2 and the photosensitive member 1, which causes the charging roller 2 to move to the photosensitive member 1. The electric charge is transferred to.

The charging roller 2 to be brought into contact with the photoconductor 1 preferably has an appropriate elasticity (hardness) in order to achieve uniform contact with the photoconductor 1, and therefore the conductive elastic layer 2b is formed, for example. EPDM, EPT, EPM, NBR, BR, CR
Synthetic rubber, natural rubber, etc., such as various thermosetting or thermoplastic elastomers, etc. are mixed with conductive particles such as carbon black, plasticizers, etc. to provide appropriate elasticity and conductivity. It

The charging roller 2 is adjusted to have an appropriate resistance value. If the resistance is too low, when the photoconductor 1 has a pinhole, the charging current concentrates here and causes a voltage drop.
Since the pinhole portion is not charged in the longitudinal direction (the contact direction of the charging member), it is output as a white belt in general, and as a black belt in the reversal development system that has become common in laser beam printers in recent years. The image quality is significantly deteriorated. On the other hand, if the resistance value is too high, on the contrary, the current required for charging does not flow, which causes charging failure.
Therefore, the resistance value of the charging roller 2 must be within a certain range.

The charging member 2 is not limited to the roller type described above,
Various shapes and forms such as blade type, pad type, block type, rod type, and brush type can be used.

The charging member 2 may be arranged in a non-contact floating state from the surface of the charged body 1 as long as there is a minute gap d between the charged body 1 and the charged body 1 which may cause dielectric breakdown of air. Of course, in the present invention, such a configuration is also included in the contact type charging device.

The bias application method for the charging member 2 includes the following DC charging method and AC charging method.

(1) DC charging system This is a system in which only a direct current (DC) voltage is applied to the charging member 2 to charge the charged body 1.

For example, when the member 1 to be charged is a negatively charged OPC photosensitive member, when a DC voltage is applied to the photosensitive member 1 as shown in FIG.
From a certain voltage V _TH (about -550V)
Charging is started, and thereafter, the applied voltage is proportional to the photoreceptor surface potential (charging potential) V _D.

Therefore, the surface potential of the photoconductor is set to the desired potential V _D.
When it is desired to charge the battery, the applied DC voltage should be V _TH + V _D. For example, when V _D = -700 V, a DC voltage of about -1250 V (about -550 V-700 V) should be applied.

Although the DC charging method can sufficiently charge the battery,
Since the potential focusing ability is small, if the charged body is an electrophotographic photoreceptor, the exposure memory cannot be completely removed,
There is a problem in that it is not possible to completely remove image roughness due to poor charging in a minute area.

(2) AC charging method A method in which an alternating voltage (oscillating voltage, pulsating voltage; voltage whose voltage value periodically changes with time) is applied to the charging member 2 to charge the charged body 1. Yes, alternating voltage AC
The voltage component has the effect of converging the potential of the body to be charged to the DC voltage component voltage.

In particular, the present applicant has previously proposed (Japanese Patent Laid-Open No. 63-
No. 149669), and a peak-to-peak voltage that is more than twice the charging start voltage V _TH of the surface of the member to be charged 1 when only the DC voltage V _DC corresponding to the V _D voltage and the DC voltage is applied to the charging member 2. A method of applying an alternating voltage superposed with an alternating voltage V _PP having a voltage to the charging member 2 to charge the surface of the member to be charged 1 enables uniform charging, and the charging of the photosensitive member 1 in the image forming apparatus can be performed. The effect of removing the exposure memory and the transfer memory can be obtained.

The waveform of the alternating voltage (or AC voltage component) is a sine wave (sine wave), a rectangular wave, a triangular wave, a sawtooth wave, a pulse wave, or the like, and is formed by periodically turning on and off a DC power supply. It may be a rectangular wave voltage.

In this AC charging method, an AC of an applied alternating voltage is applied.
Although the member to be charged potential by the voltage component V _PP oscillates, the level with no image on no problem by increasing the frequency f of the AC voltage component V _PP to beyond a certain point in the charging process of the photosensitive member 1 of the image forming apparatus It is possible to

The higher the peak-to-peak voltage of the AC voltage component V _PP , the higher the potential convergence effect, but the charging member 2
Since an alternating electric field is generated between the charging member 2 and the conductive layer of the photosensitive member base layer of the photosensitive member 1 as the member to be charged, the charging member 2 and the photosensitive member 1 vibrate and abnormal noise is generated. It is necessary to select an appropriate peak-to-peak voltage because it causes an adverse effect on the cleaning property.

The transition of the potential of the body to be charged during the charging process in this AC charging system will be described with reference to FIG. FIG. 7 shows a discharge area portion d formed by the charging roller 2 and the photoconductor 1, and an arrow indicates the rotation direction or the plane movement direction of the charging roller 2 and the photoconductor 1.

In order to charge the photoconductor 1 to the potential V _DC , a DC voltage component V _DC AC voltage component V _PP is applied to the charging roller 2. Then, the negative and positive voltages of − (V _PP / 2 + V _DC ) and + (V _PP / 2 + V _DC ) are alternately applied to the charging roller 2.

In the above structure, when a voltage on the negative side is first applied and the initial photosensitive member potential is V _{PO at} point P ₁ which is the minimum discharge gap, V _PO + {(− V _PP / 2 + V _DC ) _{-V PO - (- 550)}} = V PP / 2 + V DC + 550 = V P1 ‥‥‥ (1) is caused to charge the equation.

Next, when the voltage of the positive side is applied to the charging roller 2, P ₁ point is moved to the _two points P, since the gap of the charging roller 2 and the P ₂ point increases, the discharge starting voltage V _TH consists of the potential difference of 550 + α (V).

Therefore, at the point P ₂ , V _P1 + {(V _PP / 2 + V _DC ) −V _P1 − (550 + α ₁ )} = V _PP + V _DC −550−α ₁ = V _P2 (2) When a voltage on the negative side is applied, it becomes the P ₃ point, and V _P2 + {(− V _PP / 2 + V _DC ) −V _P2 − (− 550 −α ₂ )} = − V _PP / 2 + V _DC + 550 + α ₂ = V _P3 equation (3) After that, equations (1) and (2) are repeated, and α ₁ and α ₂ gradually increase, and V _P2 and V _P3 become approximately the same value (potential converges). The charging process is completed.

The potential at this time is expressed by the following equation (2) = (3): V _PP / 2 + V _DC −550 −α ₁ = −V _PP / 2 + V _DC + 550 + α ₂ V _PP −1100 = α ₁ + α ₂ Actually, Is the process speed (photosensitive surface moving speed)
On the other hand, since the charging frequency is set sufficiently high, α ₁ ≈α ₂ . _{Therefore, V PP -1100 = 2α 1 α} 1 = V PP / 2-550 are substituted into this equation _{(1), V P2 '= V PP /} 2 + V DC -550- (V PP / 2-550) = V _It becomes _DC and the potential converges on V _DC .

Therefore, when the photosensitive member 1 is desired to be charged to -700V, _VDC = -700V, and in order to obtain good charging, the AC voltage V _PP may be applied at V _PP ≈2 KV.

After the image forming (printing) operation is completed, if the potential is left on the photoconductor 1, the charging memory may be discharged or the photoconductor potential may become indefinite. When the photoconductor is rotated in front, it develops in an unnecessary place.

Therefore, when the photoconductor is rotated backward, DC of an alternating voltage applied to the charging roller 2 for primary charging of the photoconductor 1 is applied.
It has been devised to set the voltage component to V _DC = 0V and apply only the AC voltage component to uniformly converge the photoconductor potential to 0V. This eliminates the need for pre-exposure for converging the photoreceptor potential to 0 V, reducing costs,
The size of the device can be reduced.

[0033]

As described above, in the case of the AC charging type charging device, the DC voltage component V _DC of the alternating voltage applied to the charging member 2 is turned off and only the AC voltage component V _PP is applied. As a result, the potential of the body to be charged can be uniformly converged to 0V.

Therefore, in the image forming apparatus, the photoconductor 1
At the time of the subsequent rotation, the alternating voltage applied to the charging member 2 is turned off with the DC voltage component V _DC and only the AC voltage component V _PP is applied so that the photoconductor potential is uniformly converged to 0 V, and the exposure memory and the transfer memory are stored. The effect of removal is obtained, but in this case, in the actual machine, the charging memory may appear linearly in a minute area, or a black line may occur in the photoconductor surface area that should not be developed during pre-rotation of the photoconductor. I got it.

As a result of repeated studies, the inventors of the present invention have found that these linear defects occur at the end of charging.

At the end of charging, as shown in FIG.
Since the surface portion of the photosensitive member at the position P ₁ of the photosensitive member 1 as the member to be charged is charged to the potential V _P1 shown by the formula (1) and completed, the potential remains at the position P ₁ . This is because, for example, when V _PP = 2KV _VDC = 0V, the battery remains charged to −450V or + 450V.

FIG. 8 shows how the AC voltage component falls when it is turned off. According to this, the voltage drops immediately after the AC voltage component-off start time A '. In such a case, when the voltage is applied at the point A ′, the voltage is charged to +450 V at the point P ₁ , and the charging (static elimination) ends.

Specifically, when an organic photoconductive photosensitive member (OPC) is used as the photosensitive member 1 as a member to be charged and a reversal developing method for developing a negative latent image with negatively charged toner is taken as an example, a charging memory is used. Of course, for example, when the photoconductor portion charged to + 450V shifts to the next image forming operation,
Until the non-charged portion of the photoconductor surface passes the developing position, the developing bias is set to 0V so that toner does not adhere to the surface of the photoconductor (DC component is 0V), but when passing + 450V, development is performed with this contrast. However, there is a problem in that the inside of the machine is contaminated by toner scattering by rotating without the transfer material.

In view of this, the present invention relates to an AC charging type charging device, which prevents the generation of the above-mentioned charging memory on the surface of the body to be charged when the alternating voltage applied to the charging member is turned off.
Further, regarding an image forming apparatus using an AC charging type charging device, deterioration in output image quality and occurrence of toner scattering inside the machine due to generation of the above-mentioned charging memory on the surface to be charged when the alternating voltage applied to the charging member is turned off. The purpose is to prevent.

[0040]

The present invention is a charging device and an image forming apparatus characterized by the following configurations.

(1) In a contact type charging device in which a charging member is brought into contact with or close to a member to be charged and an alternating voltage is applied to the charging member to charge the member to be charged, when the applied alternating voltage is off. The rate of decrease of the peak voltage on one side of the alternating voltage is 4
A charging device characterized in that the voltage is lowered to 00V / 1 cycle or less to turn off.

(2) An image forming apparatus for forming an image by an image forming process including a step of charging a charged body, wherein the charging means of the charged body contacts the charging member to the charged body. Alternatively, the charging device is a contact type charging device in which an alternating voltage is applied to the charging member to charge an object to be charged, and when the applied alternating voltage is off, the rate of decrease of one side peak voltage of the alternating voltage is 400 V / An image forming apparatus characterized in that the voltage is lowered to reach off in one cycle or less.

[0043]

In the AC charging type charging device or the image forming apparatus using the charging device, when the alternating voltage applied to the charging member is off as described above, the rate of decrease of one side peak voltage of the alternating voltage is 400 V / 1. By lowering the voltage with a time constant below the cycle and turning it off,
As will be described later, when the applied alternating voltage is turned off, the voltage of the applied alternating voltage gradually decreases at a predetermined decrease rate and the charging potential converges, and the potential is not high even immediately before it falls below the discharge start voltage. There is no large potential on any part of the surface of the body to be charged.

By keeping the potential low in this way, the charging memory does not occur. Therefore, in the image forming apparatus using the charging device, the output image does not have a stripe due to the charging memory, and a good image can be obtained. Further, since the potential is low, when the image bearing member as a member to be charged is rotated forward, even if that portion faces the developing portion, an illegal phenomenon does not occur and the toner is not scattered inside the machine.

The above-mentioned lowering rate is determined by an experiment as described later, and the toner scattering inside the machine starts at a lowering rate of about 500 V / cycle, and an image shows a lowering rate of 600 V.
/ A horizontal line (charging memory) begins to appear on the image from the cycle. If the descending rate is 400 V / cycle or less, a good image can always be obtained without charging memory, and toner scattering in the machine is prevented.

[0046]

【Example】

<Embodiment 1> FIG. 1 is a schematic configuration diagram of an example of a charging device according to the present invention or an image forming apparatus using the charging device.

Reference numeral 1 denotes a rotary drum type electrophotographic photosensitive member as a member to be charged (image bearing member), which is 30 mm in the direction of the arrow.
It is driven to rotate at a process speed of / sec.

Reference numeral 2 denotes a charging roller as a contact charging member, which is composed of a cored bar 2a and a conductive elastic layer 2b which is concentrically and integrally formed on the outer periphery of the cored bar and is arranged in parallel with the photosensitive member 1 to form a cored bar. Both ends are rotatably supported by bearings and pressed against the photoconductor 1 with a predetermined pressing force by a pressurizing means.
The photoconductor 1 is driven to rotate.

Reference numeral 3 is a power supply for applying a charging bias to the charging roller 2, and in this embodiment, The superposed alternating voltage is applied to the charging roller 2 via the core metal 2a.

The photosensitive member 1 starts to rotate in response to the image forming operation start command (print signal), and after the rotation is stabilized (≈1
The above-mentioned alternating voltage is applied to the charging roller 2 from the power source 3 after about 00 msec.

As a result, the surface of the rotary photosensitive member 1 is contact-charged by the AC charging method. The charging potential of the photoconductor 1 is -700V immediately after charging, and when it reaches the developing means 5 which will be described later, it becomes about -680V due to dark decay.

Exposure of desired image information 4 is carried out by exposing means (not shown) such as an image forming slit exposing means for a document image, a laser beam scanning exposing means, etc. on the surface of the above-mentioned charged photosensitive drum 1. An electrostatic latent image corresponding to the target image information is formed on the surface of the body 1.

This electrostatic latent image is visualized as a toner image by the developing means 5, and at the transfer portion between the photoconductor 1 and the transfer means 6, the electrostatic latent image is fed to the transfer portion at a predetermined timing from a sheet feeding means (not shown). The images are sequentially transferred onto the surface of the transfer material 9 fed in.

The transfer material 9 having undergone the toner image transfer is separated from the surface of the photosensitive member 1 and is introduced into the fixing means 8, where it is subjected to image fixing and discharged as an image formed product (copy, print) to the outside of the apparatus.

Photoreceptor 1 after Transferring Toner Image to Transfer Material 9
The cleaning means 7 removes adhered contaminants such as residual toner and paper dust on the surface to clean the surface, and the surface is repeatedly used for image formation.

At the end of the image forming operation, the voltage applied to the charging roller 2 is only the AC voltage component V _PP , and D
The C voltage component V _DC is 0V. By rotating the photosensitive member 1 after one rotation or more in such a state, the surface potential of the photosensitive member 1 can be set to approximately 0 V over the entire circumference.

Finally, the AC voltage component V _PP applied to the charging roller 2 is stopped, and thereafter, substantially at the same time or after a time delay, the rotation of the photosensitive member 1 is stopped.

At this time, the AC applied to the charging roller 2
The way of turning off the voltage component V _PP is to gradually decrease the voltage with a certain time constant as shown in FIG.

In this embodiment, it takes about 67 ms from the point A, which is the starting point of OFF, to the point Z, which is completely OFF, in FIG.
ec.

In this way, the AC voltage component V _PP is completely turned off after about 10 cycles from the start of turning off. Therefore, the rate of decrease of the peak voltage on one side of the AC voltage component V _PP is about 100 V / 1 cycle.

The surface potential of the photoreceptor 1 in this case will be described with reference to FIG. FIG. 7 shows the discharge area formed by the charging roller 2 and the photoconductor 1 as described above, and the charging roller 2 and the photoconductor 1 are each surface-moved in the arrow direction.

Point P ₁ is the portion of the photosensitive member below the minimum discharge area, and point P ₁₀₀ is the surface portion of the charging roller 2 that opposes it. As described in the conventional example, the potential contrast of ₁ point and P ₁₀₀ points P begins to start discharging from above 550 V, since then the charging amount in proportion to the contrast is performed, AC voltage component V _PP in FIG. 7 In the case of only steady charging (static elimination), the point P _{1 in} FIG. 7 is −450 V at the one-side peak (−1 KV) of point A in FIG. 2.

Next, the voltage at point B in FIG. 2 (+950 V)
To when it becomes, because P ₁ point in FIG. 7 has been moved to the _two points P, and substituting the value of (2) to the next above, there arises a potential V _P2 'of P ₂ points.

V _PP / 2 = + 950 V _DC = 0 ∴V _P2 ′ = 950 + 0-550-α ₁ = 400-α ₁ (α ₁ > 0) Next, the voltage at point C in FIG. 2 (−900 V) When it becomes, it moves to the point P ₃ and in the above formula (3), -V
_{When PP} / 2 = −900 V and V _DC = 0 V, V _P3 ″ = −900 + 550 + α ₂ = −350 + α ₂ (α ₂ > 0), and by repeating this, it gradually converges.

On the other hand, since the surface portion of the photosensitive member moves to the point corresponding to the _first point P ₁ with time, the surface potential changes one after another and changes to P ₁ ′ and P ₁ ″.

These points are also charged one after another, but the charging voltage at this time is decreased by 50 V (ΔV) on one side peak voltage, so that the charging potential converges one after another,
Since the potential is not high even immediately before the voltage drops below the discharge start voltage, the potential does not become high on any part of the photosensitive member surface.

As a matter of course, since the charging memory is not generated by keeping the potential low as described above, there is no streak and a good image can be obtained. Further, since the potential is low, even when the portion is opposed to the developing portion when the photoconductor is rotated forward, an illegal phenomenon does not occur, and the toner is not scattered in the machine.

In this embodiment, the intermittent print is set to 500.
0 sheets were taken, and the print interval was changed to see the charging memory, and the images and the toner scattering inside the machine were observed and evaluated.

As a comparative example, when the AC voltage component V _PP was turned off and the voltage was lowered as shown in FIG. 8 without taking a time constant, the same observation and evaluation as above were carried out.

The results are shown in Table 1.

[0071]

[Table 1] <Example 2> In Example 1, the same image as in Example 1 was obtained by changing the time constant of the AC voltage component-off time, that is, changing the falling rate (V / cycle) of the peak voltage on one side of the AC voltage component. Then, the toner scattering inside the machine was observed and evaluated.

The results are shown in Table 2 and FIG.

[0073]

[Table 2] FIG. 3 is a correlation graph of the descent rate and the toner scattering inside the machine. From Table 2 and FIG. 3, the toner scattering inside the machine starts from the descending rate of 500 V / cycle, and a horizontal line (charging memory) begins to appear in the image from the descending rate of 600 V / cycle.

Therefore, if the descending rate is 400 V / cycle or less, it is always stable, no toner is scattered in the machine, and a good image can be obtained.

<Third Embodiment> FIG. 4 shows a charging blade 2A in place of the charging roller 2 as a charging member in the image forming apparatus shown in FIG.

In this embodiment, the process speed is 2
AC component V _PP of the alternating voltage applied to A Set to.

An image forming apparatus similar to the apparatus shown in FIG. 1 except for the above, was subjected to the same tests as those in Examples 1 and 2.
As a result, good results were obtained when the falling rate was 400 V / 1 cycle or less.

That is, regardless of the shape and form of the charging member, the process speed, the peak-to-peak voltage and frequency of the AC voltage component, the AC voltage component of the alternating voltage minus the AC when off.
Decrease rate of one side peak voltage of voltage component V _PP is 400V / 1
By setting the cycle time or less, a good image forming apparatus can be obtained.

<Embodiment 4> In this embodiment, the alternating voltage applied to the charging roller 2 or the charging blade 2A or the alternating voltage in the image forming apparatus of the above-described Embodiment 1 or 3 (FIGS. 1 and 4). The AC voltage component of was a rectangular wave.

Also in this case, as in the case of the above-described Example 2, a good image was obtained at a descending rate of 400 V / cycle or less.

The same applies not only when the alternating voltage or the AC voltage component of the alternating voltage is a sine wave or a rectangular wave, but also when it is a triangular wave, a sawtooth wave, a pulse wave, or the like.

[0082]

As described above, according to the present invention, in the AC charging type charging device, it is possible to prevent the generation of the charging memory on the surface to be charged when the alternating voltage applied to the charging member is turned off, and the AC charging type charging device is used. In the image forming apparatus using the charging device, it is possible to prevent the deterioration of the image quality of the output image and the scattering of the toner inside the machine due to the generation of the charging memory on the surface of the body to be charged when the alternating voltage applied to the charging member is turned off.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment.

FIG. 2 is a waveform graph when the alternating voltage (AC voltage component) is turned off by lowering it with a time constant.

FIG. 3 is a measurement graph showing the relationship between the descending rate (V / cycle) and the toner scattering inside the machine.

FIG. 4 is a schematic configuration diagram of an image forming apparatus according to another embodiment.

FIG. 5 is a schematic configuration diagram of an example of a contact type charging device.

FIG. 6 is a graph showing the relationship between the DC voltage applied to the charged body and the potential of the charged body.

FIG. 7 is an explanatory diagram of a charging process of the AC charging method.

FIG. 8 is a waveform graph when the alternating voltage (AC voltage component) is turned off without having a time constant.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Rotating drum type electrophotographic photosensitive member as a member to be charged 2.2A A charging roller or charging blade as a charging member 3 Charging bias applying power source 4 Exposure light 5 Developing means 6 Transfer means 7 Cleaning means 8 Fixing means 9 Transfer material

Claims (2)

[Claims] 1. A contact-type charging device in which a charging member is brought into contact with or close to a member to be charged and an alternating voltage is applied to the charging member to charge the member to be charged, wherein when the applied alternating voltage is off, A charging device characterized in that the rate of decrease of one-sided peak voltage of an alternating voltage is set to 400 V / 1 cycle or less to turn off the voltage and turn off. 2. An image forming apparatus for forming an image by an image forming process including a step of charging a charged body, wherein the charging means of the charged body contacts a charging member to the charged body or It is a contact type charging device in which an alternating voltage is applied to the charging member so as to charge the body to be charged, and when the applied alternating voltage is off, the rate of decrease of one side peak voltage of the alternating voltage is 400 V / 1. An image forming apparatus characterized in that the voltage is lowered and turned off in a cycle or less.