Transferring apparatus with two or more voltage output modes

To provide a transferring apparatus, it is possible to perform switching between a high mode and a low mode in a transfer voltage generator circuit of a transferring apparatus by using a microcomputer. The high mode is a mode in which a positive transfer voltage generator circuit is operated independently, and a voltage generated by the positive transfer voltage generator circuit is output as a transfer voltage. The low mode is a mode in which the positive transfer voltage generator circuit and a negative transfer voltage generator circuit are both operated, and the voltages generated by each of the circuits are superimposed and output as the transfer voltage.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transferring apparatus that is used in an image forming apparatus.

2. Related Background Art

A transferring apparatus is used in an image forming apparatus according to an electrophotographic process in order to transfer a toner image borne on an image bearing member, a so-called photosensitive drum, to a transferring material, a so-called sheet. There are several types of apparatuses known as transferring apparatuses.

Among the transferring apparatuses, a system is widely used for transferring a toner image borne on a photosensitive drum to a sheet by applying a transfer voltage to a cylindrical transferring member, a so-called transfer roller, and passing a sheet between the transfer roller and the photosensitive drum. The transfer roller and the photosensitive drum are in contact in this system in a state in which a sheet has not passed between the transfer roller and the photosensitive drum. The sheet therefore remains within the apparatus, and the transfer roller may be contaminated by the toner when the sheet is removed manually, or the like. Accordingly, this system has a function for cleaning the transfer roller by applying a voltage having a polarity opposite to that of the transfer voltage to the transfer roller at a predetermined timing, and rotating the photosensitive drum and the transfer roller.

A circuit that generates a positive transfer voltage and a circuit that generates a negative transfer voltage are provided in a transfer voltage generator circuit. Direct current high voltage output circuits that are structured by an inverter transformer and a rectifying circuit are generally used as the circuits that respectively generate the positive transfer voltage and the negative transfer voltage. A positive electric potential voltage is variably output as the transfer voltage with this type of transfer voltage generator circuit, the voltage varying according to the environment and transfer roller characteristics. On the other hand, an output voltage used when cleaning the transfer roller is a negative voltage in order to achieve a function for promoting the toner to move from the transfer roller to the photosensitive drum. High precision is not demanded for the negative voltage, and therefore variable voltage control is not necessary. The negative voltage is a fixed output voltage.

The transfer voltage generator circuit is explained while referring toFIGS. 6 to 8.FIG. 6is a schematic diagram that shows a transfer voltage generator circuit for a case where the toner is negative toner, andFIG. 7shows a pulse waveform that is output from a pulse output port DPLS10of a microcomputer IC201ofFIG. 6.FIG. 8is a graph that shows a relationship between an output voltage of a positive transfer voltage generator circuit202and a PWM (pulse width modulation) signal output from the microcomputer IC201ofFIG. 6.

A photosensitive drum105that is scanned and exposed by a laser light109is provided in an image forming apparatus as shown inFIG. 6, and the photosensitive drum105is grounded. A charging roller107, a developing sleeve108, and a transfer roller106are disposed in the periphery of the photosensitive drum105. Predetermined voltages are applied to the charging roller107and to the developing sleeve108by a charging voltage generator circuit (not shown) and a developing voltage generator circuit (not shown), respectively. A transfer voltage that is output from the transfer voltage generator circuit201is applied to the transfer roller106.

The photosensitive drum105is rotated in a direction of an arrow inFIG. 6when forming an image, and a surface of the photosensitive drum105is charged uniformly to a predetermined electric potential by the charging roller108. The surface of the photosensitive drum105is then scanned and exposed by the laser light109. An electrostatic latent image is thus formed on the photosensitive drum105. The electrostatic latent image is then made into a visible image as a toner image by toner supplied from the developing sleeve108. The toner image borne on the photosensitive drum105is transferred by the transfer roller106onto a sheet110that is nipped and conveyed between the photosensitive drum105and the transfer roller106.

The transfer voltage generator circuit201has the microcomputer IC201, the positive transfer voltage generator circuit202that generates the positive transfer voltage, a negative transfer voltage generator circuit103that generates the negative transfer voltage, and a transfer current detector circuit104that detects current flowing in the transfer roller106. The microcomputer IC201has two independent output ports DPLS10, one port PWM, and one A/D port CRINT. Pulses having the same waveform are output from the two pulse output ports DPLS10. Both of the waveforms are waveforms having an ON duty of 10%, for example, as shown inFIG. 7. The two pulses serve as drive signals for the positive transfer voltage generator circuit201and the negative transfer voltage generator circuit103, and drive inverter transformers T101and T102, respectively. Outputs from the inverter transformers T101and T102are changed into the positive transfer voltage and the negative transfer voltage through a latter stage quadruple rectifying circuit and a latter stage rectifying circuit, respectively. That is, the microcomputer IC201turns on the pulse output port DPLS10that is connected to the positive transfer voltage generator circuit201when outputting the positive transfer voltage. The microcomputer IC201turns on the pulse output port DPLS10that is connected to the negative transfer voltage generator circuit103when outputting the negative transfer voltage.

The PWM port is connected to the positive transfer voltage generator circuit202, and the A/D port is connected to the transfer current detector circuit104. A current value detected by the transfer current detector circuit104is input to the microcomputer IC201through the A/D port, and the microcomputer IC201determines the transfer voltage based on the current value. The PWM signal is changed and sent to the positive transfer voltage generator circuit202through the port PWM to obtain the determined transfer voltage. A driver voltage of the transformer T101of the positive transfer voltage generator circuit202is changed according to the PWM signal, and the desired output voltage (transfer voltage) is obtained. For example, a relationship between the output voltage of the positive transfer voltage generator circuit202and the value set for the PWM signal is shown inFIG. 8when the PWM signal is variable to256levels.

The positive transfer voltage generator circuit202specifically includes a switching portion that drives the transformer T101based on the pulse signal from the pulse output port DPLS10of the microcomputer IC201, a constant voltage control portion that controls the switching state of the transformer T101, and a quadruple rectifier portion that rectifies and smoothes the output voltage of the transformer T101. The switching portion is constituted of transistors Q101and Q102, resistors R101and R102, a capacitor C202, and a diode D101.

The constant voltage control portion is constituted of a comparative operational amplifier IC202, a transistor Q201, resistors R201, R202, R203, R204, R205, and R103, and a capacitor C201. A voltage to be input to the comparative operational amplifier IC202is generated in the constant voltage control portion based on the PWM signal from the microcomputer IC201. An operation of the transistor Q201is controlled based on the results of the comparison operation of the comparative operational amplifier IC202.

The quadruple rectifier portion is constituted of capacitors C101, C102, C103, and C104, diodes D102, D103, D104, and D105, and a resistor R104. The output voltage of the rectifier portion is a positive voltage, and the output voltage is applied to the transfer roller106, which is a load.

The negative transfer voltage generator circuit103specifically includes a switching portion that drives the transformer T102based on the pulse signal from the pulse output port DPLS10of the microcomputer IC201, and a rectifier portion that rectifies and smoothes the output voltage of the transformer T102. The switching portion is constituted of transistors Q103and Q104, and resistors R105, R106, and R107. The resistor R107is connected to a reference power source (24 V) here, and the output voltage of the transformer T102is set by the reference power source. The rectifier portion is constituted of a capacitor C105, a diode D107, and a resistor R108. The output voltage of the rectifier portion is a negative voltage, and the output voltage is applied to the transfer roller106, which is a load.

The transfer current detector circuit104detects the value of the current that flows in the transfer roller106when the positive output voltage of the positive transfer voltage generator circuit202is applied to the transfer roller106. The detected current value is sent to the microcomputer IC201. The transfer current detector circuit104is specifically constituted of a comparative operational amplifier IC102, capacitors C106and C107, and resistors R109, R110, R111, R112, R113, R114, R115, and R116. Output from the comparative operational amplifier IC102is input to the microcomputer IC201as a signal (CRNT) that shows the detected current value.

Further, it is also possible to use a circuit as disclosed in Japanese Patent Application Laid-Open No. H08-140351, which changes a driving frequency of an inverter transformer as the positive transfer voltage generator circuit. A transfer voltage generator circuit that adopts the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 as the positive transfer voltage generator circuit is explained while referring toFIGS. 9 to 11.FIG. 9is a diagram that shows a circuit configuration of a transfer voltage generator circuit that adopts the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 as a positive transfer voltage generator circuit.FIG. 10shows a pulse waveform output from a port DPLSVAR of a microcomputer IC301ofFIG. 9.FIG. 11is a graph that shows a relationship between an output voltage of a positive transfer voltage generator circuit102and the pulse output from the port DPLSVAR of the microcomputer IC301ofFIG. 9. It should be noted that elements shown inFIG. 9which are identical to the circuits, components, and members shown inFIG. 6are denoted by the same reference symbols as those used inFIG. 6.

Specifically, the transfer voltage generator circuit301has the positive transfer voltage generator circuit102, the negative transfer voltage generator circuit103, the transfer current detector circuit104, and the microcomputer IC301as shown inFIG. 9. The positive transfer voltage generator circuit102includes a switching portion that drives the transformer T101based on the pulse signal from the port DPLSVAR of the microcomputer IC301, and a quadruple rectifier portion that rectifies and smoothes the output voltage of the transformer T101. The switching portion is constituted of the transistors Q101and Q102, the resistors R101, R102, and R103, and the diode D101. The resistor R103is connected to a reference power source (24 V) here, and the output voltage of the transformer T102is set by the reference power source.

The microcomputer IC301has the port DPLSVAR for outputting a pulse with a variable frequency and fixed on-time, one pulse output port DPLS10for outputting a pulse, and one A/D port CRINT. The port PWM shown inFIG. 6is not provided in the microcomputer IC301.

The pulse output from the port DPLSVAR of the microcomputer IC301is generated by frequency division using a digital circuit counter. A pulse having one of 256 frequencies is output from the port DPLSVAR, for example, as shown inFIG. 10. The pulse has a waveform with the ON duty varying from 25% to approximately 1%. With respect to the variations in the pulse output from the port DPLSVAR of the microcomputer IC301, the output voltage of the positive transfer voltage generator circuit102changes as shown inFIG. 11.

The positive transfer voltage generator circuit102thus has fewer components when constituted of the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 as compared with the transfer voltage generator circuit202shown inFIG. 6. The transfer voltage generator circuit301can therefore be configured at low cost.

However, when the circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 is used as the positive transfer voltage generator circuit102, the pulse frequency for driving the inverter transformers becomes low in the transfer voltage generator circuit101in a case where the required transfer voltage becomes low. An output ripple in the transfer voltage therefore becomes large. Furthermore, the digital circuit counter must be added in order to generate low frequency pulses for driving the inverter transformers.

SUMMARY OF THE INVENTION

The present invention has been made in view of the problems described above, and an object of the present invention is to provide an improved image forming apparatus.

Further, another object of the present invention is to provide a transferring apparatus including:

a transferring member applied with a transfer voltage for transferring a toner image on an image bearing member to a recording material;

a positive voltage generating portion that generates a positive-polarity voltage that is applied to the transferring member;

a negative voltage generating portion that generates a negative-polarity voltage that is applied to the transferring member; and

a control portion that controls the transfer voltage applied to the transferring member, the control portion controlling the positive voltage generating portion and the negative voltage generating portion, in which:

the control portion performs control in a first mode adapted to generate the transfer voltage by superimposing the negative-polarity voltage and the positive-polarity voltage in a case where the transfer voltage applied to the transferring member is smaller than a predetermined threshold voltage; and

the control portion performs control in a second mode adapted to generate the transfer voltage from the positive-polarity voltage, without superimposing the negative-polarity voltage, in a case where the transfer voltage applied to the transferring member is larger than the predetermined threshold voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

FIG. 1is a circuit diagram that shows a configuration of a main portion of a transferring apparatus according to a first embodiment of the present invention. The circuit disclosed in Japanese Patent Application Laid-Open No. H08-140351 is used as a positive transfer voltage generator circuit in this embodiment. Elements withinFIG. 1that are identical to the circuits, components, and members shown inFIG. 9are identified by the same reference characters as those used inFIG. 9.

A photosensitive drum105that is scanned and exposed by a laser light109is provided in an image forming apparatus as shown inFIG. 1, and the photosensitive drum105is grounded. A charging roller107, a developing sleeve108, and a transfer roller106are disposed in the periphery of the photosensitive drum105. Predetermined voltages are applied to the charging roller107and to the developing sleeve108by a charging voltage generator circuit (not shown) and a developing voltage generator circuit (not shown), respectively. A transfer voltage that is output from the transfer voltage generator circuit101is applied to the transfer roller106.

The photosensitive drum105is rotated in a direction of an arrow inFIG. 1when forming an image, and a surface of the photosensitive drum105is charged uniformly to a predetermined electric potential by the charging roller107. The surface of the photosensitive drum105is then scanned and exposed by the laser light109. An electrostatic latent image is thus formed on the photosensitive drum105. The electrostatic latent image is then made into a visible image as a toner image by toner supplied from the developing sleeve108. The toner image being held on the photosensitive drum105is transferred by the transfer roller106onto a sheet110which is a recording material that is nipped and conveyed between the photosensitive drum105and the transfer roller106.

The transfer voltage generator circuit101has the positive transfer voltage generator circuit102, the negative transfer voltage generator circuit103, the transfer current detector circuit104, and the microcomputer IC101, as shown inFIG. 1.

The positive transfer voltage generator circuit102contains a switching portion that drives the transformer T101based on the pulse signal from the port DPLSVAR of the microcomputer IC101, and a quadruple rectifier portion that rectifies and smoothes the output voltage of the transformer T101. The switching portion is constituted of transistors Q101and Q102, and resistors R101, R102, and R103, and a diode D101. The resistor R103is connected to a reference voltage source (24 V) here, and the output voltage of the transformer T101is set by the reference voltage source. Further, the quadruple rectifier portion is constituted of capacitors C101, C102, C103, and C104, diodes D102, D103, D104, and D105, and a resistor R104. The output voltage of the rectifier portion is a positive voltage, and the output voltage is applied to the transfer roller106, which is a load.

The negative transfer voltage generator circuit103specifically contains a switching portion that drives the transformer T102based on the pulse signal from the pulse output port DPLS10of the microcomputer IC101, and a quadruple rectifier portion that rectifies and smoothes the output voltage of the transformer T102. The switching portion is constituted of transistors Q103and Q104, and resistors R105, R106, and R107. The resistor R107is connected to a reference voltage source (24 V) here, and the output voltage of the transformer T102is set by the reference voltage source.

Further, the rectifier portion is constituted of a capacitor C105, a diode D107, and a resistor R108. The output voltage of the rectifier portion is a negative voltage, and the output voltage is applied to the transfer roller106, which is a load.

The transfer current detector circuit104detects the value of the current that flows in the transfer roller106when the positive output voltage of the positive transfer voltage generator circuit102is applied to the transfer roller106. The detected current value is sent to the microcomputer IC101. The transfer current detector circuit104is specifically constituted of a comparative operational amplifier IC102, capacitors C106and C107, and resistors R109, R110, R111, R112, R113, R114, R115, and R116. An output from the comparative operational amplifier IC102is input to the microcomputer IC101as a signal (CRNT) that shows the detected current value.

The microcomputer IC101has one pulse output port DPLS10that outputs a pulse, a port DPLSVAR for outputting a pulse having a fixed on-time, and an A/D port for inputting a current value detected by the transfer current detector circuit104.

The pulse output from the port DPLS VAR of the microcomputer IC101becomes a drive signal of the positive transfer voltage generator circuit102here, in accordance with which the inverter transformer T101is driven. An output from the inverter transformer T101becomes a positive transfer voltage through a latter stage quadruple rectifying circuit. That is, the microcomputer IC101turns on the pulse output port DPLSVAR connected to the positive transfer voltage generator circuit102when outputting the positive transfer voltage. It should be noted that the pulse output from the port DPLSVAR of the microcomputer IC101is generated by frequency division using a digital circuit counter. A pulse having one of 256 frequencies is output from the port DPLSVAR, for example, as shown inFIG. 10. The frequency of the pulse varies from an ON duty of 25% to approximately 0.1%. In practice, it is not necessary to vary the frequency of the pulse from an ON duty of 25% to approximately 0.1%. The frequency may be varied from an ON duty of 25% to a percentage that corresponds to a DPLSVAR set value192described later. It therefore becomes possible to reduce the number of frequency division circuits compared to that in the conventional art.

Further, the pulse output from the pulse output port DPLS10becomes a drive signal of the negative transfer voltage generator circuit103, in accordance with which the inverter transformer T102is driven. An output of the inverter transformer T102becomes a negative transfer voltage through a latter stage rectifier circuit. That is, the microcomputer IC101turns on the pulse output port DPLS10connected to the negative transfer voltage generator circuit103when outputting the negative transfer voltage.

In addition, the A/D port is connected to the transfer current detector circuit104. The current value detected by the transfer current detector circuit104is input to the microcomputer IC101through the A/D port, and the microcomputer IC101determines the transfer voltage based on the current value. The pulse that is output from the port DPLSVAR and the pulse that is output from-the port DPLS10are changed and sent to the positive transfer voltage generator circuit102and to the negative transfer voltage generator circuit103, respectively, to obtain the determined transfer voltage. The drive voltage of the transformer T101of the positive transfer voltage generator circuit102thus changes by this operation, and a desired output voltage (transfer voltage) is obtained.

The operations for setting the pulse that is output from the port DPLSVAR and the pulse that is output from the port DPLS10in order to apply the desired transfer voltage to the transfer roller106are described below.

In the first embodiment, it is possible to perform switching between a high mode and a low mode by using the microcomputer IC101. The high mode is a mode in which the positive transfer voltage generator circuit102is operated independently, and in which a positive voltage generated by the positive transfer voltage generator circuit102is output as the transfer voltage. The low mode is a mode in which the positive transfer voltage generator circuit102and the negative transfer voltage generator circuit103are both operated, and the voltages thus generated are superimposed and output as the transfer voltage.

Each of the modes described above will be explained while referring toFIG. 2.FIG. 2is a diagram that shows a relationship between the frequency (DPLSVAR set value) of a pulse output from the port DPLSVAR of the microcomputer IC101and an output transfer voltage in the transfer voltage generator circuit101ofFIG. 1. A curve A inFIG. 1shows a relationship between the frequency (DPLSVAR set value) of the pulse that is output from the port DPLSVAR and the output voltage (transfer voltage) when only the positive transfer voltage generator circuit102is operated. A curve B inFIG. 1shows a relationship between the DPLSVAR set value and the output voltage (transfer voltage) when both the positive transfer voltage generator circuit102and the negative transfer voltage generator circuit103are operated.

The output voltage shown by the curve B (the output voltage during the low mode) becomes less than the output voltage shown by the curve A (the output transfer voltage during the high mode) by an amount equal to the output voltage of the negative transfer voltage generator circuit103. The output voltage shown by the curve B becomes 0 V when the DPLSVAR set value is approximately 85. The transfer voltage can therefore be controlled from 0V to a maximum voltage by setting the low mode in a case where a low transfer voltage is required, and setting the high mode when a high transfer voltage is necessary, without adding any components to the circuit shown inFIG. 9.

In this embodiment, in a case where the transfer voltage is controlled by a constant current, first the microcomputer IC101turns on the port DPLS10and the port DPLSVAR, thus starting up the transfer voltage in the low mode. The DPLSVAR set value is set to 85 at this point. The microcomputer IC101then reduces the DPLSVAR set value until the target current value (current detected by the transfer current detector circuit104) is input to the A/D port of the microcomputer IC101. That is, the frequency of the pulse that is output from the port DPLSVAR is increased. Even if the DPLSVAR set value is decreased to 24, the microcomputer IC101will switch to the high mode in a case where the detected current value does not reach a predetermined value (a point a withinFIG. 2). The port DPLS10of the negative transfer voltage generator circuit103therefore turns off, and the DPLSVAR set value switches to 154 (a point b inFIG. 2). The microcomputer IC101then reduces the DPLSVAR set value until the target current value (current detected by the transfer current detector circuit104) is input to the A/D port of the microcomputer IC101. It should be noted that whether or not the detected current value input to the microcomputer IC101from the transfer current detector circuit104becomes the target current value can be determined by, for example, whether or not an inequality Ia−α<the detected current value<Ia+α is satisfied, where Ia is taken as the target current value and α is a predetermined current value.

It should be noted that the value of the current detected by the transfer current detector circuit104can be changed according to the environment in which the image forming apparatus is placed, or by the operating state of each portion of the image forming apparatus, after the detected current value input to the microcomputer ICl01from the transfer current detector circuit104becomes the target current value. In this case as well, the DPLSVAR set value is changed according to the detected current value as described above. The microcomputer IC101controls the DPLSVAR set value so that the predetermined target current value flows in the transfer roller106. For example, the DPLSVAR set value is increased when a current value that is larger than the target current value Ia is input to the A/D port of the microcomputer IC101. In a case where the operating mode is the high mode at this point, and a current value that is larger than the target current value is input to the A/D port of the microcomputer IC101even after the DPLSVAR set value reaches 154 (the point b inFIG. 2), the operating mode is switched from the high mode to the low mode. Further, the DPLSVAR value is reduced in a case where a current value that is smaller than the target current value Ia is input to the A/D port of the microcomputer IC101. In a case where the operating mode is the low mode at this point, and a current value that is smaller than the target current value is input to the A/D port of the microcomputer IC101even after the DPLSVAR set value reaches 85 (the point a inFIG. 2), the operating mode is switched from the low mode to the high mode.

Further, the transfer voltage always changes under the constant current control in a case where there are resistance value irregularities in a rotary circumferential direction of the transfer roller106. In this case it is preferable to provide a hysteresis as shown inFIG. 3in switching between the low mode and the high mode in order to stabilize the control.FIG. 3is a diagram that shows a relationship between the frequency (DPLSVAR set value) of the pulse output from the port DPLSVAR of the microcomputer IC101and the output transfer voltage when a hysteresis is provided in switching between the low mode and the high mode.

In a case of switching from the low mode to the high mode inFIG. 3, the switchover to the high mode occurs at a point where the DPLSVAR set value is 8 (a point c inFIG. 3), and the DPSLVAR set value is set to 86 (a point d inFIG. 3). Further, in a case of switching from the high mode to the low mode, the switchover occurs when the DPLSVAR set value reaches 175 (a point e inFIG. 3), and the DPLSVAR set value is set to 28 (a point f inFIG. 3).

It should be noted that, although the control modes of the transferring apparatus (the low mode and the high mode) by the microcomputer IC101explained above are control modes in a case of transferring a toner image on the photosensitive drum105to the recording material sheet110, a cleaning mode for cleaning the transfer roller106also exists as another control mode.

As described above, the transfer roller may be contaminated by toner when a set remains within the apparatus and the sheet is removed manually or the like. In the cleaning mode, the microcomputer IC101performs a control so that a voltage having the same polarity as the toner polarity (negative polarity) is applied from the negative transfer voltage generator circuit103to the transfer roller106, causing the toner on the transfer roller106to transit to the photosensitive drum105. It should be noted that the microcomputer IC101performs a control in the cleaning mode so that a positive-polarity voltage, which has the opposite polarity to that of the toner polarity (negative polarity) is not applied from the positive transfer voltage generator circuit102to the transfer roller106.

Switching thus occurs in this embodiment between the mode in which the positive transfer voltage generator circuit102is operated independently, and the mode where the positive transfer voltage generator circuit102and the negative transfer voltage generator circuit103are both operated together. Therefore, even in a case when a required transfer voltage is low, the required low transfer voltage can be generated by employing a low cost circuit configuration, without making a ripple larger.

Second Embodiment

A second embodiment of the present invention is explained next while referring toFIGS. 4 and 5.FIG. 4is a diagram that shows a relationship between an output voltage of the negative transfer voltage generator circuit103, and the frequency (DPLSVAR set value) of the pulse output from the port DPLSVAR of the microcomputer IC101, when there is variation in the output voltage in a transferring apparatus according to the second embodiment of the present invention.FIG. 5is a diagram that shows a relationship between the output voltage and the frequency (DPLSVAR set value) of the pulse output from the microcomputer IC101when there is load variation in the transferring apparatus according to the second embodiment of the present invention.

A case of controlling the transfer voltage by constant current control is explained in the first embodiment described above. a DPLSVAR set value at which the voltage output during the low mode becomes 0 V is very important for systems in which the transfer voltage is determined by computing the voltage during constant current control after the transfer voltage is controlled by constant current control. This is because, from the start, high precision is not necessary for outputting the output voltage of the negative transfer voltage circuit103, and a relationship between the DPLSVAR set value in the low mode and the output voltage is not uniqueely determined due to large variations in the output voltage. Therefor, if the DPLSVAR set value that gives an output voltage of 0 V in the low mode is known in advance, a correction can be incorporated into the relationship between the DPLSVAR set value and the output voltage, and the output voltage can be found from the DPLSVAR set value.

In the low mode the relationship between the output voltage and the DPLSVAR set value when there is variation in the output voltage of the negative transfer voltage generator circuit103is as shown inFIG. 4, for example. A curve B inFIG. 4shows a standard relationship, and curves B′ and B″ each show a relationship in which there is a deviation in the output voltage of the negative transfer voltage generator circuit103. The DPLSVAR set value that makes the output voltage 0 V becomes 64 for the case of the curve B′, and becomes 112 for the case of the curve B″.

As shown by the curves B′ and B″, the relationships denoted by the curves B′ and B″ only shift up and down with respect to the standard relationship shown by the curve B when the output voltage of the negative transfer voltage generator circuit103has a deviation. Therefore, in a case where the output voltage is estimated when performing constant current control, it is easy to correct the transfer voltage by using a DPLSVAR set value at which the output voltage becomes 0 V.

The DPLSVAR set value at which the output voltage becomes 0 V can be set as a value at which the transfer current value detected by the transfer current detector circuit104becomes 0 A. The detection operation is performed by setting the target current value Ia for the detected current to 0 in the first embodiment. The DPLSVAR set value is made smaller in a case where the detected current value is smaller than the target current value, and the DPLSVAR set value is made larger in a case where the detected current value is larger than the target current value. Then, when the detected current value becomes the target current value (for example, in a case when the inequality Ia−α<the detected current value<Ia+α is satisfied, where Ia is taken as the target current value and α is a predetermined current value), the DPLSVAR set value that is set at that time is stored in a memory (not shown) that is provided to the microcomputer IC101or the like. The stored DPLSVAR set value is a value at which the transfer voltage is 0 V, and therefore the DPLSVAR set value can be set to this value when applying a desired transfer voltage based on this value. It should be noted that the transfer current is not influenced by the state of the photosensitive drum105at this time. Accordingly, it is necessary to perform the detection described above according to predetermined conditions. This is discussed later.

Further, the slope of the curve B changes when there is variation in the resistance value of the transfer roller106, that is, when there is variation in load. However, the transfer current is 0 A when the transfer voltage is 0 V, regardless of the load, and therefore the DPLSVAR set value at which the transfer voltage becomes 0 V in the low mode does not change. In a case where the load in the low mode changes from the standard value, the relationships between the transfer voltage and the DPLSVAR set value become relationships like those shown by curves B″′ and B″″ ofFIG. 5. In this case the curve B changes, and therefore it is necessary to predict the changes in the curve B in advance according to the load variation, and correct the output voltage from the DPLSVAR set value during constant current control. However, the DPLSVAR set value at which the transfer voltage becomes 0 V does not change, and it is thus not necessary to consider this point.

The conditions described above for detecting the DPLSVAR set value at which the transfer voltage becomes 0 V will be explained. There are no problems when detecting the DPLSVAR set value at which the transfer voltage becomes 0 V, provided that the electric potential on the photosensitive drum105is the same as the ground electric potential. The photosensitive drum105, however, is not limited to always being in that state. At a minimum, the transfer voltage will not become 0 V, even if the transfer current value is 0 A, in a case where a surface that is charged by the charging roller107contacts the transfer roller106. This is because charges on the photosensitive drum105flow to the transfer roller106when the charged photosensitive drum105contacts the transfer roller106. Accordingly, it is necessary that a surface of the photosensitive drum105that contacts the transfer roller106be not charged by the charging roller107when detecting the DPLSVAR set value at which the transfer voltage becomes 0 V. Furthermore, charges on the photosensitive drum105dissipate by irradiating the laser light109to the photosensitive drum105. It is therefore preferable to detect the DPLSVAR set value at which the transfer voltage becomes 0 V at a timing where a surface of the photosensitive drum105that has been irradiated by the laser light109contacts the transfer roller106.

Further, the sheet110is, of course, not allowed to be present between the transfer roller106and the photosensitive drum105when detecting the DPLSVAR set value at which the transfer voltage becomes 0 V.

The process for developing a toner image on the photosensitive drum105by reversal development is explained above. It should be noted that reversal development is a development process in which toner that is charged with the same polarity as the polarity of an electrostatic latent image adheres to regions of the latent image having a small electric potential absolute value, thus visualizing the latent image. With the reversal development process, negative-polarity toner adheres to the electrostatic latent image on the photosensitive drum105, and a positive-polarity transfer voltage is applied to the transfer roller106, thus transferring the toner on the photosensitive drum105to the sheet110. The microcomputer IC101performs a control so that a positive-polarity voltage and a negative-polarity voltage are superimposed, generating a transfer voltage, in a case where the transfer voltage applied to the transfer roller105has an absolute value that is smaller than a predetermined threshold voltage. The microcomputer IC101performs a control so that the transfer voltage is generated from a positive-polarity voltage, without superimposing a negative-polarity voltage, in a case where the transfer voltage applied to the transfer roller105has an absolute value that is larger than the predetermined threshold voltage.

On the other hand, the present invention is also applicable to a process in which a toner image is developed on the photosensitive drum105by normal development.

It should be noted that normal development is a development process in which toner that is charged with a polarity that is opposite to the polarity of an electrostatic latent image adheres to regions of the latent image having a large electric potential absolute value, thus visualizing the latent image. With the normal development process, positive-polarity toner adheres to the electrostatic latent image on the photosensitive drum105, and a negative-polarity transfer voltage is applied to the transfer roller106, thus transferring the toner on the photosensitive drum105to the sheet110. The microcomputer IC101performs a control so that a positive-polarity voltage and a negative-polarity voltage are superimposed, generating a transfer voltage, in a case where the transfer voltage applied to the transfer roller105has an absolute value that is smaller than a predetermined threshold voltage. Alternatively, the microcomputer IC101performs a control so that the transfer voltage is generated from a negative-polarity voltage, without superimposing a positive-polarity voltage, in a case where the transfer voltage applied to the transfer roller105has an absolute value that is larger than the predetermined threshold value.

It should be noted that the present invention is not limited to the embodiments described above. Various modifications may be made within the scope of the appended claims.