Abstract:
An image forming apparatus includes a latent image forming unit that forms a latent image on an image bearing member, a developing unit that develops the latent image to obtain a developer image, and a transfer unit that transfers the developer image to a recording medium. A first supply unit supplies a voltage to a charging unit and the transfer unit, with the first supply unit including a transformer, and a second supply unit supplies a voltage to the transfer unit, with the second supply unit including a transformer and supplying a voltage supplied opposite in polarity to the voltage supplied from the first supply unit. In addition, a detection unit detects current flowing through the transfer unit, and a control unit is configured to control power supply. When a power is supplied from the first supply unit to the charging unit, the control unit sets a discharge start voltage in which discharging starts between the image bearing member and the charging unit is based on a current detected by the detection unit, and when power is supplied from the second supply unit to the transfer unit, the control unit sets one or more adjusted voltages by calculating one or more voltages to be supplied from the transfer unit so that a current detected by the detection unit is to be a predetermined value. The first supply unit supplies a voltage to the charging unit based on the discharge start voltage and the adjusted voltage set by the control unit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic image forming apparatus including an electrophotographic copy machine, an electrophotographic printer (for example, LED printer or laser beam printer), and an electrophotographic facsimile apparatus. 
     2. Description of the Related Art 
       FIG. 10A  illustrates a schematic structure illustrating a charge bias circuit of a conventional image forming apparatus. A charge DC bias circuit part (hereinafter, referred to as charge bias application circuit unit)  1201  includes a voltage set circuit part (unit)  1202 , a transformer drive circuit part  1203 , a high-voltage transformer part (unit)  1204 , and a feedback circuit part (unit)  1205 . The voltage set circuit part (unit)  1202  may change a voltage value set based on an input PWM signal. The transformer drive circuit part (unit)  1203  drives the high-voltage transformer part (unit)  1204 . The feedback circuit part (unit)  1205  uses a resistor R 1201  to detect a voltage value applied to a charge roller  1206  (load) which is a charging member, and transfers the detected voltage value as an analog value to the voltage set circuit part (unit)  1202 . The voltage set circuit part (unit)  1202  controls to apply a constant voltage (voltage indicated by PWM signal) to the charge roller  1206  which is the charging member based on a value of the PWM signal and a feedback value. When an applied voltage is controlled using such a structure, a constant voltage value may be applied to the charge roller  1206  (see, for example, Japanese Patent Application Laid-Open No. H06-003932). 
     However, a voltage for starting discharge between the charging member (charge roller) and an image bearing member changes depending on a circumstance temperature and humidity in the image forming apparatus or a film thickness of a photosensitive drum (hereinafter, simply referred to as drum film thickness). Therefore, as illustrated in  FIG. 10B , even when control is performed to apply a predetermined voltage (controlled PWM value), a variation in potential on the photosensitive drum is caused by temperature-humidity (such as low temperature and low humidity (L/L) or high temperature and high humidity (H/H)). The variation causes a change in image density. In order to correct the change in image density, it is necessary to provide a density detection sensor for detecting an image density and a temperature-humidity sensor for detecting a temperature and a humidity in the image forming apparatus, and control the image density based on a result obtained by detection by the density detection sensor. When the density detection sensor and the temperature-humidity sensor are provided in the image forming apparatus, an apparatus cost may increase. When the density detection sensor and the temperature-humidity sensor are not provided, it is difficult to adequately correct the change in image density. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the problem described above. 
     A purpose of the invention to provide a feature that a variation in potential of an image bearing member may be reduced in an apparatus having a low-cost structure. 
     Another purpose of the present invention is to provide an image forming apparatus including a charging unit that charges an image bearing member on which a latent image is formed, a latent image forming unit that forms the latent image on the image bearing member charged by the charging unit, a transfer unit that develops the latent image formed on the image bearing member to obtain a developer image and transferring the developer image to a recording medium; a first application unit that applies a voltage to the charging unit and the transfer unit; a second application unit for applying, to the transfer unit, a voltage opposite in polarity to the voltage applied from the first application unit; a detection unit that detects a current flowing through the transfer unit; and a control unit that determines whether or not discharge starts between the charging unit and the image bearing member based on the current detected by the detection unit when the voltage is applied from the first application unit to the charging unit. 
     A further purpose of the present invention will become apparent from the following descriptions of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional diagram illustrating a main body of an image forming apparatus according to a first embodiment of the present invention. 
         FIG. 2A  is a block diagram illustrating the main body of the image forming apparatus according to the first embodiment. 
         FIG. 2B  is a schematic diagram illustrating a cartridge of the image forming apparatus according to the first embodiment. 
         FIG. 3  is a circuit diagram illustrating a bias generation circuit in the first embodiment. 
         FIG. 4A  illustrates a V-I characteristic in a case where a charge bias is applied in the first embodiment. 
         FIG. 4B  illustrates correction of a drum potential after a discharge start voltage is detected. 
         FIG. 5  is a flow chart illustrating processing for controlling the drum potential to a constant value in the first embodiment. 
         FIG. 6A  illustrates a V-I characteristic in a case where a transfer bias is applied in a second embodiment. 
         FIG. 6B  illustrates a V-I characteristic in a case where the charge bias is applied. 
         FIG. 7  is a flow chart illustrating processing for controlling the drum potential to a constant value in the second embodiment. 
         FIG. 8  illustrates correction of the drum potential after the discharge start voltage is detected in a third embodiment. 
         FIG. 9  is a flow chart illustrating processing for controlling the drum potential to a constant value in the third embodiment. 
         FIG. 10A  illustrates a charge bias generation circuit according to a conventional example. 
         FIG. 10B  illustrates a variation in drum potential. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, structures and operations in the present invention are described. Note that, embodiments described below are merely an example, and are not intended to limit the technical scope of the present invention thereto. 
     Hereinafter, the embodiments of the present invention are described with reference to the attached drawings. 
     First, the first embodiment is described. An image forming apparatus according to the first embodiment has a structure in which high voltages are applied as a charge bias and a transfer cleaning bias from a single transformer for high-voltage generation (hereinafter, referred to as high-voltage transformer). The charge bias is a high voltage applied to a charge roller in order to uniformly charge a surface of a photosensitive drum serving as an image bearing member. The transfer cleaning bias is a negative high voltage for transferring, to an intermediate transfer belt, a developer deposited on a transfer roller for transferring an image (hereinafter, referred to as negative transfer bias). A constant voltage source capable of applying a desired high voltage as the charge bias is provided. A current value flowing through the transfer roller in a case where a gradually increased charge bias is applied is detected by a current detection circuit provided for transfer bias (hereinafter, referred to as positive transfer) output. An output voltage of the constant voltage source for the charge bias in a case where the detected current value reaches a desired value is detected. A potential on the photosensitive drum (hereinafter, referred to as drum potential) serving as the image bearing member is controlled to a predetermined value based on the detected voltage. 
     [Structure of Image Forming Apparatus] 
     First, a laser beam printer which is an example of the image forming apparatus according to this embodiment is described with reference to  FIG. 1 . A laser beam  106  is emitted from a semiconductor laser  103  serving as a light source. The laser beam  106  is used to scan a photosensitive drum  101  serving as the image bearing member by a rotating polygonal mirror  105  rotated by a scanner motor  104 . An interchangeable cartridge  122  includes a charge roller  102  for uniformly charging the photosensitive drum  101  and a developing device  107  for developing, with toner serving as a developer, an electrostatic latent image formed on the photosensitive drum  101  by exposure with the laser beam  106 . A transfer roller  108  is provided to transfer, to a sheet serving as a recording medium, a toner image which is a developer image obtained by development by a developing roller  124  of the developing device  107 . A fixing device  109  fuses the toner transferred to the sheet by heat to fix the toner image to the sheet. The sheet is set in a manual feed tray  116 . The sheet is fed from the manual feed tray  116  to a conveyance path by one turn of sheet feed rollers  110 . A top sensor  114  is provided to synchronize the image writing (recording/printing) on the photosensitive drum  101  for the fed sheet with the transfer of the recording sheet and measure a length of the fed sheet in a conveyance direction. Delivery rollers  111  are provided to deliver, to a delivery tray  117 , the sheet to which the toner image is fixed. A delivery sensor  115  detects the presence or absence of the sheet to which the toner image is fixed. An engine controller  112  (also referred to as engine control unit) includes a CPU  113  and controls a series of image formation operations described above. That is, the engine controller  112  controls respective units of an engine of the laser beam printer and controls a printing operation in response to an instruction of a printer controller  118 . The engine controller  112  sends state information indicating an internal state of the laser beam printer to the printer controller  118 . The state information is information indicating a sheet conveyance state, the presence or absence of the sheet, and an abnormal state. The printer controller  118  serves to decode image code data sent from an external device, for example, a host computer (not shown) into bit data required for printing of the laser beam printer, and serves to read the internal information of the laser beam printer and display the internal information thereof. 
       FIG. 2A  is a block diagram illustrating a structure of a control unit of the entire laser beam printer including the engine controller  112  and the printer controller  118 . A structure of a printer main body  201  is as follows. A high-voltage control part  203  controls high voltage outputs during respective processes including charging, development, and transfer in response to an instruction of the engine controller  112 . An optical system control part  204  controls the start and stop of the scanner motor  104  for rotating the rotating polygonal mirror  105  and the turn-on operation of the semiconductor laser  103  in response to the instruction of the engine controller  112 . A fixing device temperature control part  200  controls the start and stop of supply of power to a fixing heater  120  based on temperature information from a thermistor  121  for detecting a fixing temperature in response to the instruction of the engine controller  112 . A sensor input part  205  sends, to the engine controller  112 , signals from sensors including the top sensor  114  and the delivery sensor  115 , for detecting the presence or absence state of the sheet in the laser beam printer. A sheet conveyance control part  202  performs the start and stop of motors and rollers to convey the sheet in response to the instruction of the engine controller  112 , and controls the start and stop of the sheet feed rollers  110 , rollers of the fixing device  109 , and the delivery rollers  111 . 
       FIG. 2B  illustrates a schematic structure of the cartridge  122  illustrated in  FIG. 1  in a case where voltages are applied. A negative-polarity bias is applied from an application circuit  125  to the charge roller  102  and the transfer roller  108 . A positive-polarity high voltage is applied from an application circuit  126  to the transfer roller  108 . 
     [High-Voltage Generation Circuit of Image Forming Apparatus] 
       FIG. 3  is a schematic structural diagram illustrating a charge bias generation circuit, a negative-transfer bias generation circuit, and a positive-transfer bias generation circuit in this embodiment. A voltage setting circuit part  216  may change a charge bias value and a negative transfer bias value based on a common PWM signal  207 . A common transformer drive circuit part  209  drives a common high-voltage transformer  210  (first application unit). The common high-voltage transformer  210  is connected to a charge rectification circuit part  212  and a negative-transfer rectification circuit part  213 . A charge bias of an output voltage value Vout 1  and a negative transfer bias of an output voltage value Vout 2  are supplied to the charge roller  102  and the transfer roller  108 , respectively. A feedback circuit part  217  monitors the output voltage value Vout 1  through a resistor R 201  and performs feedback to obtain the output voltage value Vout 1  corresponding to the setting of the common PWM signal  207 . In this case, the output voltage value Vout 1  corresponding to the setting of the common PWM signal  207  is output as the output voltage value Vout 2  of the negative transfer bias from the common high-voltage transformer  210 . A transfer current detection circuit part  214  (detection unit) detects a current I 203  flowing through the transfer roller  108  and transmits the detected current value as an analog value from a terminal J 201  to the CPU  113  of the engine controller  112 . The current I 203  flows before the start of discharge between the photosensitive drum  101  and the charge roller  102 . A current I 204  flows after the start of discharge between the photosensitive drum  101  and the charge roller  102 . A positive-transfer transformer drive circuit part  208  drives a positive-transfer high-voltage transformer  211  (second application unit) based on a positive transfer PWM signal  206 . The positive-transfer high-voltage transformer  211  is connected to a positive-transfer rectification circuit part  215 . 
     [Detection of Discharge Start Voltage Value] 
     Before the start of discharge between the photosensitive drum  101  and the charge roller  102 , the photosensitive drum  101  and the charge roller  102  are insulated from each other. Therefore, before the start of discharge, a load of the common high-voltage transformer  210  is only the resistor R 201 . Therefore, a step-up voltage corresponding to a value of the resistor R 201  is output from the common high-voltage transformer  210  to the charge rectification circuit part  212 . At this time, the step-up voltage corresponding to the value of the resistor R 201  is also output from the common high-voltage transformer  210  to the negative-transfer rectification circuit part  213 , and hence the current I 203  flows through a detection resistor R 202 . 
     When the discharge starts between the photosensitive drum  101  and the charge roller  102 , the load of the common high-voltage transformer  210  becomes a value obtained in a case where the resistor R 201  and the charge roller  102  are connected in parallel. The load of the common high-voltage transformer  210  has a relationship “[R 201 ]&gt;[combined resistance value in the case where resistor R 201  and charge roller  102  are connected in parallel]”, and hence the voltage output from the common high-voltage transformer  210  to the charge rectification circuit part  212  increases. With the increase in voltage, the voltage output from the common high-voltage transformer  210  to the negative-transfer rectification circuit part  213  becomes larger, and hence the current I 204  (I 204 &gt;I 203 ) flows into the detection resistor R 202 . In other words, as indicated by Line 1  illustrated in  FIG. 4A , before the start of discharge, the set-up voltage corresponding to the load of the resistor R 201  is output to the negative-transfer rectification circuit part  213 , and hence the current I 203  flows into the detection resistor R 202 . However, when the discharge starts between the photosensitive drum  101  and the charge roller  102 , the voltage corresponding to the load of “[combined resistance value in the case where resistor R 201  and charge roller  102  are connected in parallel]” is output to the negative-transfer rectification circuit part  213 , and hence the current I 204  flows into the detection resistor R 202 . In other words, as indicated by Line 2  illustrated in  FIG. 4A , a straight line having a branch point at the time of the start of discharge is exhibited. Therefore, a discharge current is calculated as a delta (Δ) value obtained by subtracting Line 1  from Line 2 , and hence a voltage calculated when the Δ value becomes a desired current value is determined as a voltage at which discharge starts (hereinafter, referred to as discharge start voltage). After discharge start voltages for respective circumstances (V 1  (circumstance is high temperature and high humidity: H/H), V 2  (circumstance is normal temperature and normal humidity: N/N), and V 3  (circumstance is low temperature and low humidity: L/L)) are detected, as illustrated in  FIG. 4B , a predetermined voltage value (ΔPWM) is added to each of the discharge start voltages. Therefore, the photosensitive drum may be maintained at a constant potential without depending on a change in circumstance. 
     [Processing for Maintaining Photosensitive Drum at Constant Potential] 
       FIG. 5  is a flow chart illustrating control in this embodiment. Upon receiving a print command (Step A 501 ), the engine controller  112  enters a forward rotation operation to start rotating the photosensitive drum  101  and the charge roller  102  (Step A 502 ). After that, the voltage setting circuit part  216  applies a predetermined charge bias to the charge roller  102  based on PWM[ 1 ] (Step A 503 ). The transfer current detection circuit part  214  detects the current I 203  flowing through the transfer roller  108  and transmits the detected current value as an analog value from the terminal J 201  to the CPU  113  (Step A 504 ). The CPU  113  calculates a value corresponding to the Δ value obtained by subtracting Line 1  from Line 2  illustrated in  FIG. 4A , based on the current value detected by the transfer current detection circuit part  214  (hereinafter, the calculated value is referred to as calculation value) (Step A 505 ). The CPU  113  compares the calculation value with a reference Δ value and determines whether or not the calculation value is within a tolerance of the Δ value ((lower tolerance of Δ)&lt;(calculation value)&lt;(higher tolerance of Δ)) (Step A 506 ). When the CPU  113  determines that the calculation value is larger than the higher tolerance of the Δ value, it is determined that the discharge start voltage is a lower voltage, and hence the PWM value for bias setting is set to a low value (Step A 507 ) and processing returns to Step A 504 . When the CPU  113  determines that the calculation value is smaller than the lower tolerance of the Δ value, it is determined that the discharge start voltage is a higher voltage, and hence the PWM value is set to a higher value (Step A 508 ) and processing returns to Step A 504 . The CPU  113  performs the control as described above. When the calculation value is within the tolerance of the Δ value, an obtained bias set value is set as the PWM value corresponding to the discharge start voltage, that is, PWM[ 2 ] (Step A 509 ). The CPU  113  adds the bias value (ΔPWM) corresponding to the potential on the photosensitive drum to the set discharge start voltage (PWM[ 2 ]) (Step A 510 ) and determines a bias value for image formation (PWM[ 3 ]=PWM[ 2 ]+ΔPWM) (Step A 511 ). After the completion of the setting described above, printing starts (Step A 512 ). 
     As described above, in this embodiment, the discharge start voltage is accurately detected and the bias value corresponding to the drum potential is added to the detected discharge start voltage. Therefore, even when circumstances vary, the drum potential may be controlled to the constant value. That is, according to this embodiment, a variation in drum potential may be reduced using a low-cost structure without providing a density detection sensor or a temperature-humidity sensor. The structure is described in which the high voltage outputs are supplied from the single high-voltage transformer in order to output the charge bias and the negative transfer bias. However, the present invention is not limited to this structure of this embodiment. For example, as long as the structure capable of similarly performing the current detection is provided, another structure for applying the same-polarity high voltage may be shared. 
     Next, the second embodiment is described. In the second embodiment, the current value to determine the discharge start voltage at the application of the charge bias is adjusted based on the resistance value of the transfer roller. In this embodiment, the parts corresponding to the same constituent elements as in the first embodiment are denoted by the same reference symbols in the drawings and the description thereof is omitted. 
     In  FIG. 3 , when the positive transfer bias reversed in polarity from the charge bias is applied, a current I 205  flows through the transfer roller  108 . The current I 205  flowing through the transfer roller  108  is detected by the transfer current detection circuit part  214  and transmitted as an analog value from the terminal J 201  to the CPU  113  of the engine controller  112 . Then, the CPU  113  detects the current I 205  flowing through the transfer roller  108  and controls the positive transfer bias so that the current value flowing through the transfer roller  108  becomes a desired value. 
       FIG. 6A  illustrates a V-I characteristic (relationship between voltage and current) corresponding to each circumstance (temperature and humidity) in a case where constant current control is performed so that a current of 2.5 μA flows into the transfer roller  108 . An applied voltage in the case where the constant current control is performed on the transfer roller  108  is changed in a range of 500 V to 3,000 V while a circumstance is changed from a high-temperature high-humidity circumstance (for example, 35° C./90% (also referred to as H/H)) to a low-temperature low-humidity circumstance (for example, 5° C./10% (also referred to as L/L)). That is,  FIG. 6A  illustrates a change in resistance value of the transfer roller  108  due to a change in circumstance. In  FIG. 6A , a set value “A” of the positive transfer bias indicates a threshold value for distinguishing between the L/L circumstance and a normal-temperature normal-humidity circumstance (for example, 20° C./50% (also referred to as N/N)). Similarly, in  FIG. 6A , a set value “B” of the positive transfer bias indicates a threshold value for distinguishing between the N/N circumstance and the H/H circumstance. 
       FIG. 6B  illustrates V-I characteristics in cases where the charge bias is applied in the respective circumstances. In  FIG. 6B , Line 3 , Line 5 , and Line 7  exhibit V-I characteristics before discharge starts in the respective circumstances. In  FIG. 6B , Line 4 , Line 6 , and Line 8  exhibit V-I characteristics after discharge starts in the respective circumstances. That is, a Δ value is calculated by subtracting Line 3  from Line 4 . When the Δ value becomes a desired current value Δ 1 , a voltage at which discharge starts in the L/L circumstance is determined. A Δ value is calculated by subtracting Line 5  from Line 6 . When the Δ value becomes a desired current value Δ 2 , a voltage at which discharge starts in the N/N circumstance is determined. A Δ value is calculated by subtracting Line 7  from Line 8 . When the Δ value becomes a desired current value Δ 3 , a voltage at which discharge starts in the H/H circumstance is determined. A gradient of a line (after start of discharge) extending from a branch point joining a line (before start of discharge) with the line (after start of discharge) is changed depending on each circumstance. Therefore, the values Δ 1 , Δ 2 , and Δ 3  calculated in the respective circumstances are different from one another. 
       FIG. 7  is a flow chart illustrating the control in this embodiment. Upon receiving a print command (Step A 1601 ), the engine controller  112  enters a forward rotation operation to start rotating the photosensitive drum  101  and the charge roller  102  (Step A 1602 ). The positive-transfer transformer drive circuit part  208  applies the positive transfer bias corresponding to the PWM[ 3 ] signal to the transfer roller  108  (Step A 1603 ). The transfer current detection circuit part  214  detects the current I 205  flowing through the transfer roller  108  and transmits an analog value of the current from the terminal J 201  to the CPU  113 . Therefore, the CPU  113  detects the current I 205  (Step A 1604 ). The CPU  113  calculates a current value based on the value detected by the transfer current detection circuit part  214  and determines whether or not the calculated current value is equal to 2.5 μA (Step A 1605 ). When the CPU  113  determines that the calculated current value is larger than 2.5 μA, the PWM value (PWM[ 3 ]), which is the set value of the positive transfer bias, is set to a low value (Step A 1607 ) and processing returns to Step A 1604 . When the CPU  113  determines that the calculated current value is smaller than 2.5 μA, the PWM value (PWM[ 3 ]), which is the set value of the positive transfer bias, is set to a high value (Step A 1606 ) and processing returns to Step A 1604 . 
     When the CPU  113  determines in Step A 1605  that the calculated current value is equal to 2.5 μA, processing goes to Step A 1608 . In Step A 1608 , the CPU  113  compares the set value of the positive transfer bias with the threshold values “A” and “B” illustrated in  FIG. 6A  to set the Δ value corresponding to each circumstance. That is, when the CPU  113  determines that the set value of the positive transfer bias is larger than the threshold value “A” (L/L of  FIG. 6A ), it is determined that Δ=Δ 1 . When the set value of the positive transfer bias is equal to or smaller than the threshold value “A” and equal to or larger than the threshold value “B” (N/N of  FIG. 6A ), it is determined that Δ=Δ 2 . When the CPU  113  determines that the set value of the positive transfer bias is smaller than the threshold value “B” (H/H of  FIG. 6A ), it is determined that Δ=Δ 3 . The positive-transfer transformer drive circuit part  208  stops the application of the positive transfer bias (Step A 1609 ). 
     The common transformer drive circuit part  209  applies, to the charge roller  102 , the predetermined charge bias set based on the PWM[ 1 ] signal (Step A 1610 ). Processing of Step A 1611  to Step A 1617  is the same as processing of Step A 504  to Step A 510  illustrated in  FIG. 5  in the first embodiment and thus the description thereof is omitted. Note that, the Δ value used in Step A 1613  is any one of the values Δ 1 , Δ 2 , and Δ 3  set corresponding to the respective circumstances in Step A 1608 . The CPU  113  determines a bias value for printing (PWM[ 4 ]=PWM[ 2 ]+ΔPWM) (Step A 1618 ). After the completion of the setting described above, printing starts (Step A 1619 ). 
     According to this embodiment, a variation in drum potential due to a variation in resistance value of the transfer roller may be prevented and the variation in drum potential may be suppressed using a low-cost structure without providing a detection part including a density detection sensor or a temperature-humidity sensor. 
     Next, the third embodiment is described. In the third embodiment, the PWM value added to the discharge start voltage is adjusted based on the resistance value of the transfer roller. In this embodiment, the parts corresponding to the same constituent elements as in the second embodiment are denoted by the same reference symbols in the drawings and the description thereof is omitted. 
       FIG. 8  is a schematic diagram illustrating correction of the drum potential after the discharge start voltage is detected in this embodiment.  FIG. 8  illustrates ΔPWM[ 3 ], ΔPWM[ 2 ], and ΔPWM[ 1 ] which are PWM values added to the charge discharge voltages V 1 , V 2 , and V 3 , respectively, in the respective circumstances. A relationship among the ΔPWM values to be added satisfies “ΔPWM[ 1 ]&gt;ΔPWM[ 2 ]&gt;ΔPWM[ 3 ]”. 
       FIG. 9  is a flow chart illustrating the control in this embodiment. Processing of Step A 1801  to Step A 1816  is the same as processing of Step A 1601  to Step A 1616  illustrated in  FIG. 7  in the second embodiment and thus the description thereof is omitted. The CPU  113  determines the bias value (ΔPWM) corresponding to the drum potential which is added to the discharge start voltage (PWM[ 2 ]), based on the set value of the positive transfer bias which is obtained in Step A 1808  in the case where the constant current control is performed so that the current value flowing into the transfer roller  108  is 2.5 μA. When the set value of the positive transfer bias is larger than the threshold value “A”, the CPU  113  sets “ΔPWM=ΔPWM[ 1 ]”. When the set value of the positive transfer bias is equal to or smaller than the threshold value “A” and equal to or larger than the threshold value “B”, the CPU  113  sets “ΔPWM=ΔPWM[ 2 ]”. When the set value of the positive transfer bias is smaller than the threshold value “B”, the CPU  113  sets “ΔPWM=ΔPWM[ 3 ]” (Step A 1817 ). The CPU  113  determines the bias value for printing (PWM[ 4 ]=PWM[ 2 ]+ΔPWM) (Step A 1818 ). After the completion of the setting described above, printing starts (Step A 1819 ). 
     According to this embodiment, a variation in drum potential due to a variation in resistance value of the transfer roller may be prevented and the variation in drum potential may be suppressed using a low-cost structure without providing a detection part including a density detection sensor or a temperature-humidity sensor. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2009-255098, filed Nov. 6, 2009 which is hereby incorporated by reference herein in its entirety.