Abstract:
An image-forming device which facilitates, with a simple structure, the realization of suitable transfer condition control in more situations is disclosed. While the transfer voltage applied to a transfer member such as a transfer roller is detected by a transfer voltage detector, the transfer current is detected by a transfer current detector, and these are transmitted to a CPU. The CPU determines the target current which is to be the control target from the transfer voltage and transfer current, and drives a step-up circuit so as to perform constant current control based on the target current which has been determined. By calculating the target current using calculation formulas which differ by transfer voltage range, suitable transfer condition control under more environments and conditions can be realized.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to image-forming devices. 
   2. Description of the Prior Art 
   An image-forming device employing an electrophotographic process, such as laser printer, forms an image on a recording medium while transferring a toner image formed on a photosensitive drum surface onto the recording medium. 
   Constant voltage control, in which a constant transfer voltage is maintained, and constant current control, in which a constant transfer current is maintained, are well utilized in order to transfer the toner image formed on the photosensitive drum surface onto the recording medium. On the other hand, the resistance of the recording paper often changes due to humidity changes. When the constant voltage control is utilized, the transfer current is decreased if the resistance is increased. As a result, faulty transfers often occur due to the insufficient transfer current. Accordingly, it is preferable that the constant current control is utilized. 
   Meanwhile, current flows directly from the transfer roller to the photosensitive drum, when the transfer roller is contact with the photosensitive drum. When the resistance of the recording medium is reduced, the current that flows directly from the transfer roller to the photosensitive drum is increased without charging the recording paper. As a result, faulty transfer occurs due to the insufficient transfer current. In order to resolve this problem, Japanese patent unexamined publication 2002-202671 discloses a technique that assumes that the transfer current flowing directly from the transfer roller to the photosensitive drum is increased when the transfer voltage drops below a prescribed value, and switches from the constant current control to the constant voltage control in order to increase the transfer current. 
   However, the transfer current flowing directly from the transfer roller to the photosensitive drum is not is increased necessary when the transfer voltage drops below the prescribed value. Therefore, Japanese patent unexamined publication 2002-202671 cannot take desired amount of current to the photosensitive drum necessarily. 
   SUMMARY OF THE INVENTION 
   An object of the present invention, which represents the result of taking the problems in question into consideration, is to provide an image-forming device which facilitates, with a simple structure, transfer condition control which is suitable to more situations. 
   In view of the above-described drawbacks, it is an objective of the present invention to provide an image-forming device capable of controlling a transfer condition with a simple structure corresponding to more situations. 
   In order to attain the above and other objects, the present invention provides an image-forming device having: an image-bearing body, a transfer unit, a transfer voltage applying unit, a transfer current detection unit, a transfer voltage detection unit, and a control unit. A toner image is capable of being bore on the image-bearing body. The transfer unit faces the image-bearing body. The transfer voltage applying unit applies transfer voltage to the transfer unit so that transfer current flows between the image-bearing body and the transfer unit. The transfer current detection unit detects the transfer current. The transfer voltage detection unit detects the transfer voltage. The control unit determines, based on both the detected transfer voltage and the detected transfer current, target current to be flowed between the image-bearing body and the transfer unit, and controls the transfer current to match the target current while adjusting the transfer voltage output from the transfer voltage applying unit. 
   The present invention further provides an image-forming device having: an image-bearing body, a transfer unit, a transfer voltage applying unit, a transfer current detection unit, a transfer voltage detection unit, and a control unit. A toner image is capable of being bore on the image-bearing body. The transfer unit faces the image-bearing body. The transfer voltage applying unit applies transfer voltage to the transfer unit so that transfer current flows between the image-bearing body and the transfer unit. The transfer current detection unit detects the transfer current. The transfer voltage detection unit detects the transfer voltage. The control unit determines, based on both the detected transfer voltage and the detected transfer current, target voltage applied between the image-bearing body and the transfer unit, and controls the transfer voltage to match the target voltage while adjusting the transfer voltage output from the transfer voltage applying unit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
       FIG. 1  is a perspective view showing the major structural parts of a laser printer of a preferred embodiment according to the present invention; 
       FIG. 2  is a simplified sectional side view of the laser printer; 
       FIG. 3  is a circuit diagram of the transfer voltage controller; 
       FIG. 4  is a diagram for describing the determination of the target current; and 
       FIG. 5  is a flowchart showing the transfer condition control processing by the CPU  100 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A laser printer according to a preferred embodiment of the present invention will be described while referring to the accompanying drawings wherein like parts and components are designated by the same reference numerals to avoid duplicating description.  FIG. 1  is a perspective view of main components of the laser printer  1000 .  FIG. 2  is a cross-sectional view of the laser printer  1000 . 
   The laser printer  1000  is provided with a main casing  1 , a scanner unit  2 , a process unit  3 , a fixing unit  4 , a paper supply unit  5 , a drive unit  6 , a top cover  7 , a paper delivery tray  8 , and a control unit (not shown) to control each unit. 
   The main casing  1 , which is made of plastic, has the main frame  1   a , a main cover  1   b , an operating panel  1   c , and an accommodation recess  1   d . The main covers  1   b  covers the outer surfaces on all four sides (the front, rear, and left and right sides) of the main frame  1   a . The main frame  1   a  and the main cover  1   b  are formed integrally by an ejection molding, for example. The operating panel  1   c  protrudes upward from the right side of the main frame  1   a . The accommodation recess  1   d  is formed in the left side of the main frame  1   a  and the main cover  1   b  formed integrally. 
   The drive unit  6  includes a main motor and a gear train (not shown). The drive unit  6  is inserted from the lower side of the main casing  1  into an accommodation recess  1   d  and attached thereto. 
   The top cover  7 , which is made of plastic, covers the upper surfaces of the main frame  1   a  and the main cover  1   b . In the top cover  7 , a through-hole  7   a  and a through-hole  7   b  are formed. The through-hole  7   a  passes through the operating panel  1   c . The through-hole  7   b  passes through the paper supply unit  5 . Further, the top cover  7  is provided with brackets  9  that protrude respectively from the left and right sides of the front edge of the top cover  7  (although only one bracket  9  is shown in  FIG. 1 ). 
   The paper delivery tray  8  is attached to the brackets  9 , where the brackets  9  are able to move up and down. When the paper tray  8  is not in use, the paper delivery tray B can be folded up toward the upper surface of the top cover  7 . 
   The paper supply unit  5  is provided with a feeder case  5   a  and a manual insertion opening  5   b . Within the feeder case  5   a , recording paper P is set in a stacked state. As shown in  FIG. 2 , the leading edge of the recording paper P is pressed toward a paper supply roller  11  by a support plate  10 , wherein the support plate  10  is pressed toward upper by screws  10   a  within the feeder case  5   a . Therefore, sheets of the recording paper P can be separated one at a time by a combination of a separation pad  12  and the paper supply roller  11 , where the paper supply roller  11  is rotating due to the power transmitted from the drive unit  6 . The sheets can be sent on to a pair of upper and lower registration rollers  13  and  14 . 
   The manual insertion opening  5   b  opens diagonally upward of the paper supply unit  5 . A recording medium that is different from the recording paper P within the feeder case  5   a  can be inserted into the manual insertion opening  5   b  fox recording. 
   The process unit  3  forms an image (toner image) by a developer (toner) on the surface of the recording paper P that is sent through the registration rollers  13  and  14 . 
   The fixing unit  4  is provided with a heating roller  15  and a pressure roller  16 . The recording paper P on which the toner image is formed is heated while being sandwiched between a heating roller  15  and a pressure roller  16 , in order to fix the toner image onto the recording paper P. 
   The heating roller  15  has a fixing heater  15   a  that is inserted into an aluminum tube coated with fluorine. A substantially central portion of an outer surface of the heating roller  15  is in contact with a thermistor  41 . The pressure roller  16  is a rubber roller having a surface covered with a fluoroplastic. 
   A paper delivery unit is provided with a paper delivery roller  17  and a pinch roller  18  that are disposed on the downstream side within the casing of the fixing unit  4 . The paper delivery unit delivers the recording paper P, where the toner image has been fixed, to the paper delivery tray  8 . A recording medium conveying path is configured by a portion from the paper supply roller  11  to the paper delivery portion. 
   An upper support plate  2   a  of the scanner unit  2  is fixed to stays by screws or the like at a part that is below the process unit  3  disposed in a substantially central portion of the main frame  1   a , where the stays are formed integrally on the upper surface side of the base plate of the main frame  1   a.    
   The scanner unit is provided with a light-emitting portion (not shown), a polygon mirror  20 , a lens  21 , and a reflective mirror  22 , on the lower surface side of a plastic upper support plate  2   a . The polygon mirror  20  is rotated at high speed by a scanner motor  86  driven by a motor drive circuit  90 . A laser emitted from the light-emitting portion is deflected by the polygon mirror  20  and passes through a glass plate  24 , and exposes the outer peripheral surface of the photosensitive drum  23 , where the glass plate  24  covers a long, thin scanner hole, which is pierced through the upper support plate  2   a  so as to extend along the axial line of the photosensitive drum  23 . 
   The process unit  3  is provided with the photosensitive drum  23 , a transfer roller  25 , a scorotron type of charger  26 , a developer device, a removable toner cartridge  29 , a cleaning roller  30 , and a charge removal lamp  30   a.    
   The transfer roller  25  is in rotatable contact with the upper surface of the photosensitive drum  23 . The scorotron type charger  26  is disposed below the photosensitive drum  23 . The developer device has a developer roller  27  and a supply roller  28 , which are disposed on the upstream side of the photosensitive drum  23  in the paper supply direction. The developer (toner) supplier, in other words, a removable toner cartridge  29  is disposed further upstream of the developer device. The cleaning roller  30  is disposed downstream from the photosensitive drum  23 . The charge removal lamp  30   a  is disposed further downstream from the cleaning roller  30 . 
   A latent electrostatic image is formed on the outer peripheral surface of the photosensitive drum  23  when the laser beam emitted from the scanner unit  2  is scanned over the surface of the photosensitive drum  23  that has been charge uniformly by the charger  26 . The developer (toner) within the toner cartridge  29  has been agitated by an agitator  31  and discharged. Then, the developer (toner) is carried on the outer peripheral surface of the developer roller  27  via the supply roller  28 , and the thickness of the toner layer thereon is regulated by a blade  32 . 
   The latent electrostatic image formed on the surface of the photosensitive drum  23  is developed into a visible image when developer from the developer roller  27  adheres to the latent image. The image (toner image) formed by this developer is transferred to the recording paper P that passes between the photosensitive drum  23  and the transfer roller  25  to which is applied a transfer voltage whose potential is opposite to a potential of the photosensitive drum  23 . The transfer voltage is controlled by a transfer voltage controller  2000  ( FIG. 3 ). For the control unit (not shown) and the transfer voltage controller  2000 , separate CPUs may be utilized or, alternatively, a single CPU may be utilized. The toner remaining on the photosensitive drum  23  is collected temporarily by the cleaning roller  30 . The toner corrected is returned to the process unit  3  for re-use. 
   Note that a toner sensor  33  having a paired light-emitting portion and light-receiving portion is mounted on the upper support plate  2   a  of the scanner unit  2  so as to protrude upward from the upper support plate  2   a . The toner sensor  33  faces a recess in the lower surface of the toner cartridge  29  in the process unit  3  so that the toner sensor  33  can detect the presence or absence of toner within the toner cartridge  29 . 
   The process unit  3  is formed as a cartridge that is inserted into a plastic case  34 . The thus-packaged process unit  3  can be mounted removably in the main frame  1   a . An accommodation portion  36  for accommodating a cooling fan  35  and a ventilation duct  37  extending in the lateral direction crossing the direction of travel of the recording paper P are connected on a lower surface side linking a forward position of the main frame  1   a  and a forward position of the main cover  1   b . An upper surface plate  37   a  of the ventilation duct  37  is formed to have an inverted V-shape in section. This upper surface plate  37   a  is positioned between the process unit  3  and the fixing unit  4  to shield the process unit  3  from direct transmission of heat emitted from the heating roller  15  in the fixing unit  4 . 
   The cooling air flow produced by the cooling fan  35  passes through the ventilation duct  37  and is transferred to the lower surface on one side of the main frame  1   a , and cools a power source  39  at the rear and the main motor within the drive train unit  6 . Also, the cooling air flow is blown from a plurality of slits that open on the process unit  3  side so that the cooling air flow passes upward between the process unit  3  and the fixing unit  4 , and is exhausted out of the device from a plurality of exhaust holes  40  that pierce through the top cover  7 . 
   Next, the configuration of the transfer voltage controller  2000  that controls the transfer voltage applied to the aforementioned transfer roller  25  will be described.  FIG. 3  is a diagram of the configuration of the transfer voltage controller  2000 . The transfer voltage controller  2000  is provided with a CPU  100 , a PWM signal smoother  110  having a capacitor, a transformer driver  120 , a step-up circuit  130  having a transformer including a primary coil  131 , a secondary coil  132 , and a detection coil  133 , a resister  140 , a transfer current detector  150  having a resister  151 , and a transfer voltage detector  160 . 
   The CPU  100  outputs a transfer voltage control signal to the PWM signal smoother  110 , where the transfer voltage control signal indicates the duty ratio to control the transfer voltage. The PWM signal smoother  110  smoothes the transfer voltage control signal and transfers it to the transformer driver  120  as an analog signal. The transformer driver  120  controls the amount of current flowing into the primary coil  131  of the step-up circuit  130  based on the analog voltage. 
   An electromotive force corresponding to the amount of current flowing into the primary coil  131  is generated at the secondary coil  132 , and applied to the roller shaft in the transfer roller  25  as the transfer voltage. 
   The resister  140  stabilizes the voltage output from the step-up circuit  130 . The transfer current detector  150  has a resistor element  151  that detects the amount of the transfer current flowing between the transfer roller  25  and the photosensitive drum  23 . The transfer voltage detector  160  detects the transfer voltage applied to the transfer roller  25 . 
   The transfer current value detected by the transfer current detector  150  is sent to the CPU  100 . Also, the transfer voltage value detected by the transfer voltage detector  160  is sent to the CPU  100 . 
   The CPU  100  determines a target current based on the transfer current detected by the transfer current detector  150  and the transfer voltage detected by the transfer voltage detector  160  as described later. In the present embodiment, the constant current control, which controls the transfer voltage in order to maintain the constant transfer current, is utilized. Therefore, the CPU  100  outputs the PWM signal to the PWM signal smoother  110  in accordance with the determined target current, where the PWM signal prescribes the amount of current flowing into the primary coil  131  for outputting the transfer voltage. 
   Next, the method that determines the target current from the detected transfer current and transfer voltage will be described referring to  FIG. 4 .  FIG. 4  is an explanatory diagram showing the target current Ic corresponding to the transfer voltage V 0 . 
   X-axis indicates the transfer voltage V 0  detected by the transfer voltage detector  160 , and Y-axis indicates the target current Ic. Each mark A, B, and C indicates the target current Ic in a prescribed voltage range. Any of the mark A, B, and C can be selected based on various settings, such as the size, thickness, and consistency of the recording paper, and whether it is recording paper or overhead projector sheets, for example. 
   When the transfer voltage V 0  is within the range from the first prescribed value V 1  (1.5 kV in  FIG. 4 ) to the second prescribed value V 2  (5.5 kV in  FIG. 4 ), the target current Ic is maintained at any of the mark A, B, and C while the constant current control is performed. 
   On the other hand, when the transfer voltage V 0  drops below the first prescribed value V 1  and when the transfer voltage V 0  rises above the second prescribed value V 2 , the target current Ic should be changed to a value calculated using a prescribed formula. 
   More specifically, when the transfer voltage V 0  drops below the first prescribed value V 1 , the target current Ic is calculated while applying the detected transfer voltage V 0  and the detected transfer current I 0  to Formula 1 below,
 
 Ic=Io max/(1− a×V 0/ I 0)  (Formula 1)
 
   Here, the Iomax is a prescribed maximum current that the laser printer  2000  can output, and is shown in  FIG. 4  as the target current Ic when the transfer voltage is 0. The “a” represents a slope of a straight line connecting ( 0 , Iomax) and (V 1 , I 1 ) in  FIG. 4 , where the V 1  is 1.5 kV and the target current I 1  indicates any of the A, B, or C. Specifically, the “a” is (I 1 −Iomax)/V 1 . In the present embodiment, the V 1  is fixed at 1.5 kV, while the target current Ic in the range between 1.5 kV and 5.5 kV (hereinafter referred to “constant current region”) is any of the A, B, or C. Therefore, the value of the “a” will also change depending on which of the A, B, and C is selected. 
   Normally, when a resistance of the recording medium between the transfer roller  25  and the photosensitive drum  23  is reduced, the current that flows directly from the transfer roller  25  to the photosensitive drum  23  is increased without charging the recording medium. In that case, the detected transfer current I 0  will not indicate the current charging the recording medium precisely. In the present embodiment, the current charging the recording medium is estimated by experiments based on the detected voltage V 0  and the detected current I 0 , and the formula 1 indicates the proper transfer current Ic based on the current charging the recording medium. Thus, the current Ic is determined properly. 
   The target current Ic that does not match precisely the target current Ic calculated by the Formula 1 may be utilized. Furthermore, the Formula 1 may be not necessarily utilized. For example, an adjusted Formula 1 can be utilized, or another formula can be utilized. 
   Meanwhile, when the transfer voltage V 0  exceeds the second prescribed value V 2 , the target current Ic is calculated by Formula 2 below.
 
 Ic=Vo max/( V 0/ I 0− b )  (Formula 2)
 
   Here, the “b” represents a slope of a straight line connecting (V 2 , I 2 ) and (Vmax, Ith) in  FIG. 4 , where the Vmax is a prescribed maximum voltage that the laser printer  2000  can output, the V 2  is 5.5 kV, the target current I 2  represents any of the A, B, or C, and the target current Ith is fixed at −10 μA. 
   Specifically, the “b” is (I 2 −Ith)/(V 2 −Vmax). In the present embodiment, the Ith, V 2 , and Vmax are fixed, while the target current Ic in the constant current region is any of A, B, or C. Therefore, the value of the “b” will also change depending on which of the A and B is selected. 
   The Vomax represents the transfer voltage V 0  which is indicated when the transfer current is 0, if the slope “b” is extended in the direction of the X-axis. Therefore, Vomax will also changes depending on which one among A and B is selected. 
   In  FIG. 4 , the value of target current  12  is the same as for the aforementioned  12 . Accordingly, if the A has been selected, the target current  12  when the transfer voltage V 0  is the second prescribed value V 2  (in the example in  FIG. 4 , 5.5 kV) will be −12 μA. The Vmax is 6.5 kV in  FIG. 4 . 
   The current charging the recording medium will flow sufficiently when the transfer voltage V 0  exceeds the second prescribed value V 2 . However, when the resistance of the recording medium is increased, the transfer voltage V 0  exceeds the Vmax greatly if the constant current control is performed. Accordingly, the formula 2 indicates that the target current Ic is controlled to be decreased as the detected transfer current I 0  is closed to the Vmax. 
   The target current Ic that does not match precisely the target current Ic calculated by Formula 2 may be utilized. Furthermore, the Formula 2 may be not necessarily utilized. For example, an adjusted Formula 2 can be utilized, or another formula can be utilized. 
   Note that when the target current Ic in the constant current region is below the prescribed current Ith (for the C in  FIG. 4 ), the target current Ic will not be changed even if the transfer voltage V 0  exceeds the transfer voltage V 2 . Even if the resistance of the recording medium is increased, the transfer voltage V 0  does not exceed the Vmax greatly. In addition, since Vmax is the largest value that the laser printer  2000  can output, after the transfer voltage V 0  reaches the Vmax for any one of the A, B, or C, the constant voltage control is performed. 
   Next, the processing of the CPU  100  that performs transfer condition control in the present embodiment will be described referring to  FIG. 5 .  FIG. 5  is a flowchart of the processing of the CPU  100 . Upon the starting of the processing, the CPU  100  sets any of the A, B, or C in  FIG. 4  as the target current Ic (S 101 ) based on, for example, the specified paper size and consistency. The target current Ic may be selected either manually or automatically. 
   The CPU  100  determines whether the transfer current V 0  detected by the transfer current detector  150  has reached the target current Ic while controlling the transfer voltage V 0  corresponding to the target current Ic (S 102 ). When the transfer current I 0  has not reached the target current Ic (S 102 : NO), the CPU  100  determines continuously whether the transfer current I 0  has reached the target current Ic. 
   When the transfer current I 0  has reached the target current Ic (s 102 : YES), then the CPU  100  determines whether printing has ended (S 103 ). When printing has ended (S 103 : YES), the processing of the CPU  100  ends. When printing has not ended (S 103 : NO), the CPU  100  reads in the transfer voltage V 0  detected by the transfer voltage detector  106  and the transfer current I 0  detected by the transfer current detector  105  (S 104 ), and sets a timer for control (S 105 ). The value to which the timer is set can be determined freely based on the laser printer  2000 &#39;s specifications. 
   Next, the CPU  100  determines whether the transfer voltage V 0  has dropped below the first prescribed value V 1  (1.5 kV) (S 106 ). When the transfer voltage V 0  has dropped below 1.5 kV (S 106 ; YES), then the CPU  100  calculates the target current Ic using the aforementioned Formula 1 (S 107 ), and changes the pulse duty of the PWM signal in accordance with the difference between the calculated target current Ic and the detected transfer current I 0  in order to match the transfer current I 0  with the target current Ic (S 108 ). Then, the CPU  100  determines whether the timer has finished counting the value set at S 105  (S 109 ). When the timer has finished counting the value set at S 105  (S 109 : YES), the CPU  100  returns to step S 103 . When the timer has not finished counting the value set at S 105  (S 109 : NO), the CPU  100  determines continuously whether the timer has finished counting the value set at S 105 . 
   In step  106 , when the detected transfer voltage V 0  is 1.5 kV or greater (S 106 : NO), the CPU  100  determines whether the transfer voltage V 0  has reached the maximum voltage (Vmax. 6.5 kV) (S 110 ). When the transfer voltage V 0  has reached the maximum voltage (Vmax: 6.5 kV) (S 110 : YES), the CPU  100  switches the control method from the constant current control to the constant voltage control since the transfer voltage V 0  cannot be raised any higher due to the laser printer  2000 &#39;s specifications (S 111 ). 
   When the transfer voltage V 0  has not reached 6.5 kV (S 110 : NO), the CPU  100  determines whether the transfer voltage V 0  has exceeded the second prescribed value V 2  (5.5 kV) (S 112 ). When the transfer voltage V 0  has exceeded 5.5 kV (S 112 : YES), the CPU  100  calculates the target current Ic using the aforementioned Formula 2 (S 114 ), and changes the pulse duty of the PWM signal in accordance with the difference between the calculated target current Ic and the detected transfer current I 0  in order to match the transfer current I 0  with the target current Ic (S 108 ). Then, the CPU  100  goes to  8109  and S 103 . 
   When the transfer voltage V 0  has not been exceeded 5.5 kv (S 112 : NO), the cPU  100  resets the target current Ic to the target current Ic set at S 101  (S 113 ). Then, the CPU  100  goes to S 108  and S 109 , since the target current Ic is in the constant region (between 1.5 kV and 5.5 kV). Note that when the target current has never been changed, the CPU goes to S 109  without performing the steps of S 113  and S 108 . 
   As described above, with the present embodiment, the target current Ic is changed flexibly based on the detected transfer voltage V 0  and transfer current I 0 , and the constant current control is performed in order to match the transfer current I 0  with the target current Ic. The target current Ic indicates proper current to be flowed between the transfer roller  25  and the photosensitive drum  23  since the current flowing without charging the recording medium is made consideration. Thus, suitable transfer can be performed. 
   The need to change transfer conditions—that is, the size and consistency of the recording medium, double-sided printing, changes in humidity other changes in environmental conditions which give rise to changes in the state of the recording medium, changes in the transfer roller  25  over time, changes in the photosensitive drum  23  over time, changes in the toner over time, print duty, etc.—manifests itself as the ratio between the transfer voltage V 0  and the transfer current I 0 —i.e., as a variation in resistance. Accordingly, the target current Ic is determined based on a ratio between the detected transfer voltage V 0  and the detected transfer current Ic. 
   Since the formulas are utilized, less resources such as memory are required. 
   While the invention has been described in detail with reference to the specific embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
   In the above-described embodiment, when the target current Ic is calculated using formulas such as the Formula 1 and the Formula 2. However, it is also possible to determine the target current Ic while referencing a table that maintains the correspondence relationship between the detected transfer voltage V 0 , transfer current I 0 , and the target current Ic. 
   In the above-described embodiment, the constant current control is performed in the constant region. However, the constant voltage control may be performed. 
   The target voltage Vc may be selected based on the laser printer  2000 &#39;s specifications, expected conditions, etc. For example, when the slope in  FIG. 4  is utilized, the target voltage Vc is calculated below. 
   When the detected transfer voltage V 0  has dropped below 1.5 kv, the target voltage Vc can be determined based on Formula 3 below.
 
 Vc=Io max/( I 0/ V 0− a )  (Formula 3)
 
   Here, the same values as those shown in  FIG. 4  can be utilized for Iomax and the slope “a”. When determining the target voltage Vc as well, for example with A in  FIG. 4  being selected, the target voltage Vc can be calculated by applying, in Formula 3, the slope “a” corresponding to the A in  FIG. 4 . For the slope “a”, a fixed value calculated beforehand may be utilized, or the slope “a” can be calculated by appropriately setting the value of I 1  described in the aforementioned embodiment to be a prescribed current value. If the detected transfer voltage V 0  exceeds 5.5 kV, the target voltage V 0  can be determined based on Formula 4 below. The calculation of, for example, Vomax and the slope “b” can be considered equivalent to that described in the aforementioned embodiment.
 
 V=V 0max/(1− b×I 0/ V 0)  (Formula 4)
 
   It is also acceptable to maintain the correspondence relationship between the detected transfer voltage V 0  and the transfer current I 0  in a table, and to determine the target voltage Vc while referencing the table. 
   The formula to determine the target current Ic or the target voltage Vc is of course not limited to the specific examples described above; based on the environment, conditions, etc., the formulas may be modified as appropriate to determine the target current.