JPH0143304B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image forming apparatus for stably forming a proper image.

For example, an exposure source, a corona charger, and a developer are generally used to form an image on a photoreceptor. Since corona charging occurs in the air, the state of corona discharge changes due to environmental changes, such as humidity, temperature, atmospheric pressure, or contamination of the discharge wire by floating objects in the air. A change in the amount of current directed toward the member changes the potential of the member to be charged. This means that in electrophotography in which an electrostatic latent image is formed on a charged member and visualized, the developed image changes significantly. Therefore, in order to stabilize the image state, it is necessary to stabilize the charge on the charged member due to corona discharge, or otherwise compensate for changes in the latent image potential due to changes in the charge on the charged member. must be taken.

A method described in USP 2956487 is known as a method for obtaining a stable image. Furthermore, it is conventionally known to use a constant current power source as a power source for a corona charger to apply a constant amount of corona discharge current to the material (constant current control).

However, if the surface potential of the charged member before the charging is not constant, or if the capacitance between the charged member and the ground is not constant, it is not possible to charge the charged member to a constant potential. Can not.

To explain the reason, in general, if the capacitance of the charged member is C, the change in surface potential of the charged member due to charging is △V, and the charge given to the charged member due to charging is △Q, then △Q It can be expressed as =C△V. Further, in a charging method in which a fixed amount of effective corona discharge current is applied to the member to be charged, ΔQ becomes a constant value determined by the charging time and the effective corona discharge current.

Here, if the capacitance C of the charged member changes due to changes over time, etc., since △Q is constant, the amount of change △V in the surface potential of the charged member caused by charging changes, and after charging The surface potential of the charged member is not constant. Furthermore, even if the amount of change in surface potential due to charging △V is constant, if the surface potential of the charged object before charging is not constant, the surface potential of the charged object after charging will not be constant. .

Therefore, a charging method (constant current control) that applies a certain amount of effective corona discharge current to the charged member
In this case, if the capacitance of the member to be charged and the surface potential before charging are not constant, charging cannot be performed to a constant surface potential, and a proper image cannot be obtained.

In addition, when corona discharge is performed on a charged member by using a constant voltage power supply as the power source of a corona discharge device, if the charged member is charged with a constant voltage for a certain period of time, the charged member before charging is Regardless of the surface potential, the surface potential of the charged object can be made constant (constant voltage control).

However, even if the charging voltage is kept constant, the corona resistance varies depending on environmental conditions such as humidity, so the charging current also varies. For example, in the case of high humidity,
As the corona resistance increases, the charging current decreases,
In the case of low humidity, corona resistance decreases and charging current increases.

Therefore, even if the charging voltage is kept constant, the surface potential of the charged object after charging changes due to fluctuations in environmental conditions, making it impossible to obtain a proper image.

The present invention has been made in view of the above points, and its purpose is to form an image that makes it possible to obtain high-quality images regardless of changes in environmental conditions, and that has a simple configuration and is easy to handle. The goal is to provide equipment.

That is, the present invention provides an image forming apparatus that has a charging means and forms an image on a transfer paper by forming an electrostatic latent image on a rotating body, developing it, and then transferring it.
a primary winding to which an AC input voltage is applied; a secondary winding coupled to a charging electrode of the charging means;
a transformer comprising a tertiary winding magnetically coupled to a secondary winding; command means for commanding the start of image formation; a driving means for performing one preparatory rotation to bring the rotating body to a uniform potential, and after the completion of the preparatory rotation, continuing to rotate the rotating body for an image forming operation without stopping the rotating body; a detection means for detecting a difference between a positive component and a negative component of a charging current generated by the charging means; a detection means for comparing the detection output with a reference value during the preparatory rotation; and a current flowing through the tertiary winding or the A control means for setting the difference to a predetermined value by controlling a voltage applied to a tertiary winding, and storing a control value for setting the difference to the predetermined value obtained by a control operation of the control means. , a storage means for storing data during the image forming operation, and starts the image forming operation during the rotation of the rotating body that is performed subsequent to the preparatory rotation;
During the image forming operation, the current value flowing through the tertiary winding or the third
By maintaining the voltage value applied to the next winding at a constant value corresponding to the control value held in the storage means, the charging voltage applied to the charging electrode of the charging means is maintained constant during the image forming operation. The present invention provides an image forming apparatus characterized by:

As a specific example, in the first step, a member to be charged is placed whose capacitance and surface potential are maintained at a standard state, and at this time, a high voltage is applied so as to bring the corona discharge current effective for charging to a predetermined value. Automatically adjust the state of the output voltage or output voltage waveform of the power supply. In AC corona discharge, the corona discharge current effective for charging is expressed by the difference between the absolute values of the positive current and negative current, and in DC corona discharge, it is expressed by the current value. The reference state corresponds to a case where the member to be charged is uniformly exposed to light, uniformly dimmed, uniformly neutralized, or uniformly charged to a predetermined value.

Next, as a second step, the state of the high voltage power source achieved in the first step or a control signal corresponding to the state of the high voltage power source is stored by the storage means, and the state of the high voltage power source is fixed.

Although the high-voltage output voltage or the high-voltage output voltage waveform will be automatically adjusted in accordance with the environmental conditions if the desired charged member is charged after completing the first and second stages. First, the target member to be charged is charged with a constant high-voltage output voltage or high-voltage output voltage waveform. the result,
It becomes possible to obtain a substantially constant and uniform surface potential without reducing the process speed, regardless of changes in environmental conditions or the state of the member to be charged.

This will be explained below using electrophotography as an example.

The following two methods are currently widely used as typical electrophotographic methods.

The first method is to perform primary charging of positive or negative polarity on a two-layer photoconductor consisting of a photoconductive layer and a conductive substrate, and then imagewise expose it to form an electrostatic latent image.
A visible image is obtained through a further development process.

In the second method, a three-layer photoreceptor consisting of a transparent insulating layer, a photoconductive layer, and a conductive substrate is primarily charged to a positive or negative polarity. Subsequently, image exposure and secondary charging are performed, and by further uniform exposure, an electrostatic latent image is formed, and then a visible image is obtained through a development process.

FIG. 1 shows the latter process, where 1 is a photoreceptor which rotates in the direction of the arrow. 2 is a primary charger,
3 is the optical axis of a light image when the original 12 is exposed with the lamp 10, and the light image is obtained by scanning the original by reciprocating mirrors 13 and 14 in synchronization with the rotation of the photoreceptor. 4 is a secondary charger, 5
A visible image was transferred to transfer paper 8 using a full-surface exposure source, 6 a developing device, and a transfer charger 7. A blade cleaner 9 cleans the photoreceptor after the visible image has been transferred to the transfer paper 8.

The charging method used in this electrophotographic process utilizes DC corona discharge or AC corona discharge. For example, in FIG. A common method is to use AC corona discharge as a secondary charger.

A conventional example of a charger having the simplest structure is shown in FIG. 2A, in which 21 represents a high-voltage power source 22, and corona discharge wire 11 represents a photoreceptor.

By using an AC power supply or a DC power supply as the high voltage power supply 21 and applying a voltage higher than the corona discharge starting voltage Vc to the corona discharge electrode 22,
It generates a corona discharge current to impart charge to the surface of the photoreceptor.

In electrophotography, it is important to always obtain electrostatic latent images with a constant surface potential depending on the image with good reproducibility. Corona charging has a large effect on the electrostatic latent image, and therefore, in order to stabilize the surface potential, the charger shown in Figure 2A, the corona discharger, the opening width of the shield case of the corona discharger, the corona discharge line and the photosensitive Not only must the distance from the body be kept constant, but also the environmental conditions such as temperature and humidity must be kept constant.

Figure 2 (b) and (c) show a conventional charging device that attempts to reduce changes in surface potential when the above conditions fluctuate;
are inserted in series, and C is one in which a grid 25 is arranged between the corona discharge wire and the photoreceptor 1, but in both cases, changes in the atmospheric condition or the corona discharge wire and the photoreceptor surface Fluctuations in corona resistance due to variations in the distance between the two surfaces are not sufficiently compensated for, and the stability of the obtained surface potential and, by extension, the stability of the finally obtained visible image are unsatisfactory. For example, a change in surface potential due to a change in the atmosphere from normal temperature and humidity to high temperature and humidity causes problems such as fogging in the visible image after development.

Therefore, when charging the surface of the photoreceptor, if a charging method is applied that applies a certain amount of effective corona discharge current to the surface of the photoreceptor, the surface potential of the photoreceptor will be able to change easily due to environmental changes such as temperature, humidity, and atmospheric pressure. It is possible to obtain a constant surface potential in an extremely stable manner.

However, a case will be considered in which a charging method that applies a certain amount of effective corona discharge current is applied to the primary charging of the first and second electrophotographic methods described above. When primary charging is applied, the capacitance of the photoreceptor is considered to be constant. However, the surface potential of the photoreceptor before primary charging is generally extremely non-uniform. For example, in the electrophotographic apparatus shown in FIG. 1, the photoreceptor 1 is charged by the transfer charger 7 before being charged by the primary charger 2. However, the corona current generated by the transfer charger 7 is blocked by the transfer paper 8 during transfer, and while only a small amount of corona current reaches the photoreceptor 1, the transfer paper 8 is supplied in front of the transfer charger 7. When there is no charger, most of the corona current from the transfer charger 7 reaches the photoreceptor 1. Since the transfer is generally performed intermittently as necessary, the surface of the photoreceptor 1 that has passed in front of the transfer charger 7 has extremely non-uniform potential unevenness.

As an example, when the transfer paper 8 is in front of the transfer charger 7, the potential of that part before the next primary charging is +300V, but the transfer paper 8 is not in front of the transfer charger 7. At this time, the potential of that part before the next primary charging was +800V.

In addition, the potential before primary charging changes depending on the influence of the previous electrostatic latent image and the like.

Therefore, when the above-mentioned charging method of applying a certain amount of effective corona discharge current to the charged object is used for the primary charging in the electrophotographic method, the surface potential after the primary charging becomes extremely non-uniform.

Furthermore, when a charging method that applies such a certain amount of effective corona discharge current is applied to charge removal that is performed simultaneously with image exposure in the second electrophotographic method, it is difficult to form an electrostatic latent image with sufficient contrast. It is.

That is, in this electrophotographic method, static electricity is removed at the same time as image exposure, but the photoconductive layer in the photoreceptor essentially acts as a conductor or insulator depending on the brightness of the image light, and the apparent capacitance changes. do. The electrophotographic method creates a difference in the amount of charge on the surface of the photoreceptor depending on the brightness or darkness of the image light by setting the photoreceptor at a uniform potential regardless of the brightness or darkness of the image light during charge removal, and then applying a uniform charge to the photoreceptor. The photoconductive layer in the photoreceptor is irradiated with light in a similar manner to make the photoconductive layer in the photoreceptor conductive, and an electrostatic latent image with high contrast is formed depending on the charge present on the surface of the photoreceptor. Therefore, in the state after charge removal, there is a large difference in the amount of charge depending on the brightness and darkness of the image light, and the surface potential remains constant regardless of the brightness and darkness of the image light, that is, the difference in the apparent capacitance of the photoreceptor. desirable.

When applying a charging method that applies a certain amount of effective corona discharge current as described above, a certain amount of charge is applied to the surface of the photoreceptor regardless of changes in the apparent capacitance of the photoreceptor, so that In the state of
It is difficult to form a difference in the amount of charge depending on the brightness and darkness of the image light, and therefore it is impossible to obtain an electrostatic latent image with high contrast.

A specific example will be explained with reference to FIGS. 3 to 9.

FIG. 3 is a block diagram of the charge control device.

The high voltage output generated by the high voltage generator 31 is guided to the discharge electrode 22 and applies a corona discharge charge to the charged body 37 .

On the other hand, the effective corona discharge current is determined by the current detection unit 3
2, and the detected signal is sent to the comparison amplification section 33.
is compared with a reference signal to generate a control signal. The control signal is sent to the storage section 35 via the switch 34 and stored in the storage section 35.

At the same time, the control signal is sent to the control section 36, and the control section 36 controls the high voltage output according to the control signal so that the corona discharge current is appropriate.

In charging, as a first step, a member kept in a standard state is placed as the member to be charged 37 and corona discharge is performed, and the corona discharge current detected by the current detection unit 32 is determined in advance. After reaching the value, proceed to the second stage. In the second step, the switch 34 is turned off to cut off the control signal from the detection unit 32, and the high voltage output voltage level or high voltage output voltage waveform is kept constant by the control signal stored in the storage unit 35 for performing a predetermined corona discharge. This allows corona discharge to be carried out and a member to be charged at an arbitrary potential to be charged to a constant value. Since the detection section 32 is provided on the low-pressure side of the high-pressure section 31, multiple chargers with different high-pressure sections are provided on a common photoreceptor, and even if they are operating simultaneously, they can be controlled independently and accurately. .

4 to 8 show circuit configurations of charging devices in which the charging control device shown in FIG. 3 is used in various corona dischargers.

Figure 4 shows an example of application to positive corona discharge. It is also possible to apply to negative corona discharge in the same way.

In FIG. 4, 38 is a well-known oscillator whose oscillation output voltage changes depending on the input voltage, 311 is a step-up transformer, 312 is a rectifier for positively charging, 3
21 is a resistor that detects the current effective for charging due to corona discharge as a voltage drop; 331 is an operational amplifier that compares the voltage drop with a reference voltage source 332 and outputs an output according to the difference; and 351 is the amplifier 331. A capacitor 362 that samples and holds the output is an amplifier that controls the amount of current flowing through the control transistor 361 based on its hold value.

Discharge current 2 when the environment is lower temperature and higher humidity than usual
2, the corona discharge current decreases and the charging potential drops below a predetermined value. When the object to be charged is in the reference state, the detection resistor 321 detects the change, and the amplifier 331 charges the capacitor 351 via the switch 34 and increases the output of the amplifier 362 according to the change. The input voltage of the oscillator 38 is increased by increasing the amount of current supplied to the oscillator. Therefore, the output of the high voltage transformer 31 is increased to increase the discharge current and return to the predetermined charging potential. Then, the switch 34 is turned off before the charged object is no longer in the standard state, and thereafter the output of the amplifier 362 is held by the charging potential of the capacitor 351, and corona discharge is continued by the amount of current supplied by the transistor 361. Furthermore, the output of the transistor 361 is connected to the amplifier 36.
Since this returns to step 2, a constant output is maintained and the holding effect is further enhanced.

FIG. 5 shows an example of application to corona charging using a power source that combines an AC power source and a DC power source. 39 is an oscillation circuit that generates a constant output independently of 38; 31
3 is a diode for increasing the negative component in the AC waveform.

This method does not determine the charging potential based on the total current to the discharge electrode 22, but rather determines the charging tendency (polar direction) and Determine the surface potential.

Here, the current difference has a negative charging tendency due to the diode 313, and charges the object to be charged negatively. Then, the current difference due to the AC corona discharge is detected by the detection resistor 321 that detects the AC difference, and the comparator 3
The difference detected by the amplifier 31 is compared with a reference value by the power supply 332, and the amplifier 331 charges the capacitor 351 via the switch 34 according to the detected value, and outputs a control signal to the amplifier 362.

Then, the input of the oscillator 38 is controlled by the control transistor 361 via the amplifier 362 so that the current difference becomes a predetermined constant value. Next, the switch 34 is opened by an external timing signal, and the memory circuit 35 is configured so that the current difference becomes constant.
Based on the stored signal, a DC signal is applied to the oscillator 38 to continue corona discharge with a constant current difference.

FIG. 6 shows an example of application to a charging device using AC corona discharge. It can also be used to uniformly remove surface charges.

FIG. 7 shows an embodiment of the present invention, in which the present invention is applied to AC corona discharge. It is characterized by having a separate output winding. The current flowing through the output control winding 41 distorts the waveform of the voltage generated in the high voltage output generation winding 40, thereby changing the efficiency of positive and negative corona discharge.

Therefore, the circuit shown in the figure controls the difference between the positive and negative absolute values of the corona discharge current by controlling the current flowing through the output control winding 41 using the detection circuit 32 and the memory circuit 35.

The output control winding 41 may be provided independently from the high voltage output generation winding 40 as shown in this figure, or a part of the high voltage output generation winding 40 may be provided as the output control winding 41.
It is also possible to serve as

FIG. 8 is similar to FIG. 7, but instead of controlling the current of the output control winding 41, the voltage between the terminals of the output control winding 41 is controlled. Therefore, compared to the circuit shown in FIG. 7, this has the advantage of providing a power supply with excellent constant voltage characteristics in the state after the switch 34 is opened.

In addition, in FIGS. 7 and 8, the transistors 366,
Since the output of 368 is returned to amplifiers 367 and 369, the control output becomes constant.

Next, the operation timing of the switch 34 will be explained.

Whether or not the object to be charged is in the reference state in the first stage of charging can be indirectly known from a signal from a device in which the charger is installed, such as a copying machine. Therefore, the control mode can be changed by turning off the switch 34 using this signal. For example, when the photoreceptor is pre-exposed and pre-charged to a uniform potential, it is preferable to turn it off when the processing period ends.

FIG. 10 is an example of the circuit, and FIG. 11 is a time chart thereof. In the figure, V is the oscillator 3 in Figures 3 to 9.
8 is the DC power supply, M is the photosensitive drum (first
The motor that rotates 1) in the figure, MS ₁ is a switch that is turned on by a cam that corresponds to the drum position installed on the drum, K is a relay that is turned on by power switch SW, L is a relay that is turned on by MS ₁ , and CL is a document It is a moving clutch.

When the power switch SW is turned on, the drum is rotated by contacts _k1 , _k2 , and _k3 , which are turned on by relay K.
The drum 1 is rotated in preparation for pre-exposure and pre-charging to bring the drum surface to a uniform potential. During this time, corona discharge is started, and the detection section 22 detects the difference between positive and negative corona discharge currents, compares this detected value with a reference value, and controls the high voltage control section 36 according to the comparison output. When the drum rotates approximately one revolution, MS ₁ is turned on, and contacts l ₁ and l ₂ turned on by relay L turn on document table reciprocating clutch CL to start exposure scanning of the original image and start the copying process. At the same time, the switch 34 that was in the on state is turned off. As a result, during the copying process, the high voltage control section 36 operates based on the control signal stored in the storage section 35 to perform corona discharge at a constant voltage, and pre-exposure and pre-charging can also be performed by further providing a lamp and a charger. However, it is easier to do this using an existing image exposure lamp and charger.

When the copy end signal END is output, relay K,
L is turned off, and accordingly, corona discharge is performed according to the control signal stored in the storage section 35 from the start to the end of the copying process.

It is also possible to use a contact switch such as a thyristor as the switch 34.

Further, the switch 34 does not necessarily need to be operated in response to an externally applied signal.

For example, if a state in which the effective corona current is maximized or minimized is selected as the reference state of the charged member 37 in the first stage of charging, the switch 34 can simply function as a rectifier. can. An example according to FIG. 9 is shown.

The example shown in FIG. 9 is applied when the difference in the absolute values of positive and negative corona currents is maximum when the charged member 37 is in the standard state in FIG. 5.

Further, the resistor 43 is a resistor 4 that automatically discharges and erases the control signal stored in the storage section 35.
The duration of storage in the storage unit 35 determined by the 3 and 44 capacitors 45 is sufficiently long compared to the time that this charging device operates with one storage.
Further, it is necessary to make the time sufficiently shorter than the time required for environmental changes such as temperature, humidity, and atmospheric pressure to affect corona charging.

Fig. 14 shows the case where the shield 23 surrounding the wires 22 in Figs. 3 to 8 is made conductive and is grounded, and Fig. 15 shows the case where the shield 23 surrounding the wire 22 in Figs. 3 to 8 is connected as shown in the figure so that no shield current flows. be. Even when a grid is provided between the wire 22 and the photoreceptor, the grid can be connected as shown in the figure. If the shield 23 is made of an insulating material and the corona current contains an AC component, then the configuration shown in FIG. 3 is sufficient and the configuration is simple.

In the above example, an A-D converter that converts the detected current into a digital quantity, a comparator that compares the conversion signal with a reference quantity, and an output control quantity of the comparator that is to achieve a predetermined charging potential are stored as digital quantities. memory. It is also possible to have an inverter that converts the controlled amount into a DC potential amount, and to provide a switch 34 to control the current at a constant level and maintain the potential in the reference state using the method described above.

Also, instead of controlling the input of the oscillator, transformer 3
It is also possible to provide a servo motor that slides the primary tap 11 or a servo motor that slides a resistor connected to the primary line, and to operate this motor with a control signal to obtain a predetermined charging potential.

When the charging method of the present invention is applied to a static elimination process that is performed simultaneously with irradiation of original image light in an electrophotographic apparatus using a three-layered photoreceptor shown in FIG. 1, in order to bring the photoreceptor 1 into a reference state, After performing the primary charging, instead of irradiating with the original image light, static electricity may be removed in a dark area.

Since the detection is performed during the pre-rotation and one rotation and the appropriate output is set, the desired photoreceptor surface can be checked and the subsequent image formation can be stably controlled without significantly reducing the process speed.

In addition, by providing a part of the photoreceptor that is not used for image formation, providing a certain electrode or insulator in place of the photoreceptor in that part, and periodically facing the charger, the electrode or insulator can be removed. can be used as a charged member in a reference state.

An example of the operation of each part in FIG. 1 in the former case is shown in the time chart of FIG. 12. M is a signal to operate the main motor to rotate photosensitive drum 1, V _AC is
A signal for operating AC charger 4, LA is a signal for lighting lamp 5, V _DC is a signal for operating DC charger 2, 7, LI, CL ₁ ,
CL ₂ is a signal for operating each lamp 10, the optical system forward clutch, and the optical system reverse clutch. Switch X corresponds to switch 34, and switches BP and HD are provided in the optical system passage. When the switch is activated by the movement of the optical system, the signal that turns on is used to reverse and stop each optical system. MS ₁ is a pulse signal output every rotation of the drum.

When the drum is rotated by turning on the SW, the AC charger 4 and lamp 5 are turned on to uniformly eliminate static electricity from the drum and the blade 9 is used to clean the drum. Turn on the copy button CPB and turn on the lamp 10.
Rotate once while exposing uniformly, then turn on CL ₁ and start scanning. During the forward rotation after turning on the copy button, switch X is closed to perform a detection operation. or
If switch X is closed or opened during rotation after SW is turned on,
Detection operation in dark areas is possible. The figure shows the case of two copies. Note that this operation can be executed by the control circuit shown in FIG. 61, 62 are and gates, 63, 6
4 is an OR gate, 65 to 69 are flip-flops that output 1 from Q as a set signal to S, and are reset by a signal to r; 65 to 68 are triggers for switching elements that turn on each load lamp via an amplifier. 69 is connected to a relay for turning on the switch 34 via an amplifier.

As described above, according to the present invention, by taking advantage of the advantages of both constant current charging and constant voltage charging, the potential of the charged body is uniformized during the preparatory rotation to bring the rotating body to a uniform potential prior to the image forming operation. In this state, charging is performed by the output of the secondary winding, and during this period, the difference between positive and negative charging currents is detected, and the current or voltage of the tertiary winding is controlled according to the detected value, so that the amount of charging reaches a predetermined amount. The charging voltage is kept constant by determining a control value and maintaining the tertiary winding current or voltage constant based on this control value during the subsequent image forming operation. It is possible to obtain a charging voltage according to conditions, etc., and it is possible to obtain a high-quality image regardless of fluctuations in environmental conditions, etc.

Furthermore, since the charging voltage is made constant by current or voltage control of the tertiary winding, a DC high voltage power source is not required, and the configuration is simplified. In addition, the output of the secondary winding can be controlled by distorting the waveform every half wave of AC.
Compared to controlling the peak-to-peak of the AC output of the secondary winding, the total amount of current can be suppressed,
Furthermore, charging unevenness is less than when direct current is superimposed on alternating current.

[Brief explanation of drawings]

FIG. 1 is a process explanatory diagram of a copying machine to which the present invention can be applied, and FIG. 2 is a schematic diagram of a conventional corona discharge device.
FIG. 3 is a block diagram showing the basic configuration of the present invention, FIGS. 4 to 6 and 9 are diagrams showing the circuit configurations of various charging devices, and FIG. 7 is a circuit diagram showing one embodiment of the present invention. , FIG. 8 is a circuit diagram showing another embodiment of the present invention, and FIG. 10 is a circuit diagram showing the switch 34 in FIGS. 4 to 9.
11 is an operation time chart of FIG. 10, FIG. 12 is a time chart showing an example of the operation of FIG. 1, FIG. 13 is a circuit diagram for the operation of FIG. 12, and FIG. 14 and 15 are cross-sectional views of other chargers, in which 1 is a photosensitive drum, 2 is a primary corona discharger, and 32 is a discharge current detector.

Claims (1)

[Claims] 1. In an image forming apparatus that has a charging means and forms an image on a transfer paper by forming an electrostatic latent image on a rotating body, developing it, and then transferring it, an AC input voltage is applied. a secondary winding coupled to a charging electrode of the charging means; and a tertiary winding magnetically coupled to the secondary winding. a commanding means for instructing, in response to an image formation start command from the commanding means, the rotary body is rotated one preparatory rotation before the start of image formation, the rotary body is brought to a uniform potential, and after the preparatory rotation is completed, the preparatory rotation is made one rotation, and after the completion of the preparatory rotation, the a driving means for rotating a rotating body for an image forming operation without stopping; a detecting means for detecting a difference between a positive component and a negative component of a charging current by the charging means during the preparatory rotation; during the preparatory rotation a control means that compares the detection output with a reference value and sets the difference to a predetermined value by controlling the current flowing through the tertiary winding or the voltage applied to the tertiary winding according to the comparison result; storing a control value for making the difference the predetermined value obtained by the control operation of the control means;
storage means for holding during the image forming operation, the image forming operation is started during the rotation of the rotating body that is performed subsequent to the preparatory rotation, and the image forming operation is started based on the control value held in the storage means during the image forming operation. By maintaining the current value flowing through the tertiary winding or the voltage value applied to the tertiary winding at a constant value corresponding to the control value held in the storage means, the charging electrode of the charging means An image forming apparatus characterized in that a charging voltage applied to the image forming apparatus is maintained constant during an image forming operation.