Patent Publication Number: US-8538282-B2

Title: Image forming apparatus and method for controlling charger

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2010-190989, which was filed on Aug. 27, 2010, the disclosure of which is herein incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     Apparatuses and devices consistent with the present invention relate to an image forming apparatus and a method for controlling a charger, and more particularly, to a technique of controlling a plurality of chargers used for an image forming apparatus when a common charging voltage is applied to the plurality of chargers. 
     BACKGROUND 
     For example, Patent Document 1 discloses a technique which controls a plurality of chargers when a common charging voltage is applied to the plurality of chargers. More specifically, Patent Document 1 discloses a technique in which power is supplied from one high voltage power supply unit to a plurality of corona chargers. 
     RELATED ART DOCUMENT 
     Patent Document 
     
         
         [Patent Document 1] JP-H03-142483-A 
       
    
     SUMMARY 
     The technique disclosed in Patent Document 1 provides a common circuit to apply a charging voltage to a plurality of corona chargers in common, which may result in an inexpensive and compact high voltage power supply circuit. When the voltage applying circuit is provided in common, each grid voltage is made constant and a grid current is controlled to be made constant based on one grid current value rather than a plurality of grid current values. However, to this end, there is a need for making the grid current and the grid voltage constant with high precision. 
     The invention provides a technique which is capable of making a grid current and a grid voltage constant with high precision in a configuration where a voltage applying circuit applies a charging voltage to a plurality of chargers in common. 
     According to a first illustrative aspect of the present invention, there is provided an image forming apparatus comprising: one or a plurality of photosensitive drums; a plurality of chargers each having a grid, which are provided for the one photosensitive drum or are respectively provided for the plurality of photosensitive drums and charge the one or plurality of photosensitive drums; a voltage applying unit that generates a charging voltage and applies the generated charging voltage to the plurality of chargers in common; a plurality of grid constant voltage circuits which are respectively provided for the plurality of chargers, each of the plurality of grid constant voltage circuits including: a voltage detecting unit that detects a voltage based on a grid voltage which is generated in the grid in accordance with the applied charging voltage; a first current detecting unit that detects a first current flowing into the voltage detecting unit; a voltage control line that makes the grid voltage constant; and an operation control device that performs a feedback control through the voltage control line such that the detected voltage detected by the voltage detecting unit has a predetermined voltage value; at least one of a second current detecting units which are respectively provided for at least one of the grid constant voltage circuits, and detect a second current flowing into the voltage control line; and a controller that controls the voltage applying unit such that a sum of the first current and the second current corresponding to one of the plurality of chargers becomes a predetermined current value. 
     With this configuration, each grid voltage is made constant by feedback control using an operation control device instead of a plurality of constant voltage elements having an element imbalance. In this case, although some of the grid current is flowed into a feedback circuit, the grid current is made constant in consideration of the flowed current (first current). Accordingly, in the configuration where the voltage applying circuit applies the charging voltage to the plurality of chargers in common, it is possible to make each grid voltage constant with high precision. In addition, any charger can be controlled to have a constant current with high precision. For example, by controlling a charger having the most contaminated discharging wire, that is, a charger having the minimal grid current, to have a constant current, it is possible to charge the photosensitive drums sufficiently even with the charger having the most contaminated discharging wire. 
     According to a second illustrative aspect of the present invention, in addition to the first aspect, a plurality of the second current detecting units are respectively provided for the plurality of grid constant voltage circuits, and wherein the controller determines whether or not the second current detecting unit corresponding to one of the chargers detects a minimal second current, and controls the voltage applying unit such that the sum of the first current and the second current corresponding to the charger in which the minimal second current is detected has the predetermined current value. 
     With this configuration, it is possible to charge the photosensitive drums sufficiently even with the charger having the most contaminated discharging wire, that is, the charger having the minimal grid current. 
     According to a third illustrative aspect of the present invention, in addition to the first aspect or the second aspect, each of the grid constant voltage circuits is connected to an output side of the respective operation control device and includes a transistor which controls a voltage of the respective voltage control line, and wherein each of the second current detecting units detects the respective second current between the transistor and a ground. 
     With this configuration, for example, by performing a feedback control to adjust a base voltage of a bipolar transistor to a predetermined voltage by an output of the operation control device, a collector-emitter voltage can be adjusted to a predetermined voltage. Accordingly, the grid voltage can be made constant with higher precision. 
     According to a fourth illustrative aspect of the present invention, in addition to the third aspect, the transistor includes a control terminal, wherein each of the second current detecting units includes a first resistive element which generates a voltage detection signal for detecting the second current, and wherein the controller makes the voltage of the voltage control line constant by controlling a voltage of the control terminal of the transistor based on a voltage value of the voltage detection signal. 
     With this configuration, since a voltage value of the voltage detection signal is varied depending on the grid current, the grid voltage can be made constant as the predetermined voltage by changing the collector-emitter voltage of the transistor based on the grid current. That is, a surface potential of the photosensitive drums can be changed to a predetermined value based on the grid current. 
     According to a fifth illustrative aspect of the present invention, in addition to the third aspect or the fourth aspect, each of the grid constant voltage circuits includes a phototransistor as the transistor. 
     With this configuration, since a base-emitter current of the transistor can be reduced, the second current, that is, the grid current, can be detected with high precision. 
     According to a sixth illustrative aspect of the invention, in addition to any one of the third to fifth aspects, the transistor includes a first terminal and a second terminal, and wherein each of the grid constant voltage circuits includes: a second resistive element, which is interposed between the grid and the first terminal of the transistor, in the voltage control line; and a third resistive element or a constant voltage element, which is interposed between the first terminal and the second terminal of the transistor, in the voltage control line. 
     With this configuration, a first terminal-second terminal voltage of the transistor can be limited to a withstanding voltage, which may result in improved reliability of the transistor. 
     According to a seventh illustrative aspect of the invention, in addition to any one of the third to fifth aspects, each of the grid constant voltage circuits includes a constant voltage element, which is interposed between the grid and the transistor, in the voltage control line. 
     With this configuration, a collector-emitter or source-drain voltage of the transistor can be limited to a withstanding voltage, which may result in improved reliability of the transistor. 
     According to an eighth aspect of the invention, in addition to any one of the first to seventh aspects, the controller controls each of the grid constant voltage circuits such that as a second current detected by the respective second current detecting unit increases, a predetermined constant voltage decreases. 
     Typically, since as a grid current increases a surface potential of the photosensitive drums increases, by reducing the grid voltage as the grid current increases, it is possible to prevent the surface potential of the photosensitive drums from being unbalanced, which may result in prevention of print image quality from being deteriorated. 
     According to a ninth aspect of the present invention, in addition to any one of the first to eighth aspect, each of the chargers is a scorotron type charger that includes a discharging wire and the grid. 
     According to a tenth illustrative aspect of the present invention, there is provided a method for controlling a plurality of chargers in an image forming apparatus including a plurality of photosensitive drums, a plurality of chargers each having a grid, which are respectively provided for the plurality of photosensitive drums and charge the plurality of photosensitive drums, a voltage applying unit which generates a charging voltage and applies the generated charging voltage to the plurality of chargers in common, and a plurality of grid constant voltage circuits which are respectively provided for the plurality of chargers, each of the plurality of grid constant voltage circuits including a voltage detecting unit, an operation control device, and a voltage control line, the method comprising the steps of: detecting a voltage by the respective voltage detecting unit, based on a grid voltage generated in the respective grid in accordance with the charging voltage; making the respective grid voltage constant by the respective operation control device by performing a feedback control through the respective voltage control line such that the detected voltage detected by the respective voltage detecting unit has a predetermined voltage value; detecting a first current flowing into the respective voltage detecting unit; detecting a second current flowing into at least one of voltage control lines from the second current flowing into the respective voltage control line; and controlling the voltage applying unit such that a sum of the first current and the second current corresponding to one of the plurality of chargers has a predetermined current value. 
     With this configuration, like the first aspect of the invention, in the configuration where the voltage applying circuit applies the charging voltage to the plurality of chargers in common, it is possible to make each grid voltage and each grid current constant with high precision. 
     According to the image forming apparatus and the method of controlling the chargers, in the configuration where the voltage applying circuit applies the charging voltage to the plurality of chargers in common, it is possible to make each grid voltage and each grid current constant with high precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative aspects of the invention will be described in detail with reference to the following figures wherein: 
         FIG. 1  is a schematic sectional view showing an internal structure of a printer according to a first embodiment of the invention; 
         FIG. 2A  and  FIG. 2B  show a schematic block diagram of a high voltage power supply of the printer; 
         FIG. 3  is a circuit diagram showing a grid constant voltage circuit according to a second embodiment of the invention; and 
         FIG. 4  is a circuit diagram showing another grid constant voltage circuit. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION 
     First Embodiment 
     A first embodiment of the invention will be described with reference to FIGS.  1  and  2 . 
     1. The Entire Structure of Printer 
       FIG. 1  is a schematic sectional view showing an internal structure of a color printer  1  (an example of an image forming apparatus) according to a first embodiment. In the following description, subscripts such as Y (yellow), M (magenta), C (cyan) and K (black) are appended to each of the elements if they are to be differentiated from each other, but otherwise, no subscript is appended. In addition, an image forming apparatus is not limited to the color printer but may be, for example, a multifunction copier having FAX and copying functions. 
     The color printer (hereinafter abbreviated as a “printer”)  1  includes a paper feeding section  3 , an image forming section  5 , a conveyance mechanism  7 , a fixing section  9 , and a high voltage power supply  50 . For example, the printer  1  forms toner images formed of unicolor or multicolor (four colors of yellow, magenta, cyan, and black in this embodiment) toner (developer), depending on image data input from the outside, on sheets  15  (paper, an OHP sheet, or the like). 
     The paper feeding section  3  is arranged on the bottom of the printer  1  and includes a tray  17  which accommodates the sheets  15 , and a pickup roller  19 . The sheets  15  accommodated in the tray  17  are taken one by one out of the tray  17  by means of the pickup roller  19  and are sent to the conveyance mechanism  7  through a conveyance roller  11  and a registration roller  12 . 
     The conveyance mechanism  7  serves to convey the sheets  15  and is, for example, removably mounted on a mount (not shown) formed within the printer  1 . The conveyance mechanism  7  includes a driving roller  31 , a driven roller  32 , and a belt  34  which spans the driving roller  31  and the driven roller  32 . When the driving roller  31  is rotated, a surface of the belt  34 , which faces photosensitive drums  44 , moves in the direction from the right side to the left side in  FIG. 1 . Accordingly, the sheets  15  sent from the registration roller  12  are conveyed to the image forming section  5 . The conveyance mechanism  7  further includes four transfer rollers  33 . 
     The image forming section  5  includes four process units  40 Y,  40 M,  40 C and  40 K and four exposure units  45 . Each process unit  40  includes a charger  41 , a photosensitive drum (an example of a photosensitive drum)  44 , a unit case  46 , a developing roller  47 , and a feeding roller  48 . Each process unit  40 Y,  40 M,  40 C, and  40 K is removably mounted on a mount (not shown) formed within the printer  1 . 
     The photosensitive drum  44  includes, for example, an aluminum base, which is, for example, connected to a ground line of the printer  1  via a conductive shaft  44   a , and a positively-charged photosensitive layer formed on the aluminum base. The charger  41  is, for example, a scorotron type charger and includes a discharging wire  42  and a grid  43 . When a charging voltage CHG is applied to the discharging wire  42 , a grid voltage GRID of the grid  43  is controlled such that a surface of the photosensitive drum  44  has substantially the same potential (for example, +700 V). 
     The exposure units  45  includes, for example, a plurality of light emitting devices (for example, light emitting diodes (LEDs)) arranged in a row in a rotation axial direction of the photosensitive drum  44  and forms an electrostatic latent image on the surface of the photosensitive drum  44  by controlling emission of the plurality of light emitting devices based on externally-input image data. In addition, the exposure units  45  are fixed within the printer  1 . The exposure units  45  may employ a laser. 
     The unit case  46  accommodates toner for each color (for example, positively-charged nonmagnetic one-component toner) and includes the developing roller  47  and the feeding roller  48 . The toner is fed to the developing roller  47  with a rotation of the feeding roller  48  and is positively charged by friction between the feeding roller  48  and the developing roller  47 . In addition, when the toner is fed onto the photosensitive drum  44  to form a uniform toner layer thereon, the developing roller  47  develops an electrostatic latent image to form a toner image on the photosensitive drum  44 . 
     The transfer rollers  33  are arranged to face the respective photosensitive drums  44  with the belt  34  interposed therebetween. When a transfer voltage having a polarity (a negative polarity in this example) reverse to the charged polarity of the toner is applied between the transfer rollers  33  and the photosensitive drums  44 , the transfer rollers  33  transfer the toner image formed on the photosensitive drums  44  onto the sheet  15 . Thereafter, the sheet  15  is conveyed to the fixing section  9  by the conveyance mechanism  7 , thermally fixed with the toner image by the fixing section  9 , and then discharged to the top side of the printer. 
     2. Configuration of High Voltage Power Supply 
     Next, an electrical configuration of the printer  1  of the invention will be described with reference to  FIG. 2  ( FIG. 2A  and  FIG. 2B ).  FIG. 2  shows a schematic block diagram of the high voltage power supply  50  mounted on a circuit board (not shown) and a connection configuration of the high voltage power supply  50 . 
     The high voltage power supply  50  includes an ASIC (Application Specific Integrated Circuit: an example of a controller)  51 , a high voltage power supply circuit  52  connected to the ASIC  51 , a ROM  53 , and a RAM  54 . The ASIC  51  controls the entire printer including the high voltage power supply circuit  52 . ROM  53  stores various operation programs to be executed by the ASIC  51  and the RAM  54  stores image data to be used for a printing process. The controller is not limited to the ASIC but may be, for example, a CPU. 
     The high voltage power supply circuit  52  includes a charging voltage generating circuit (an example of a voltage applying unit)  60 , grid constant voltage circuits  71 , and line current detecting circuits (an example of a second current detecting unit)  72 . In this embodiment, the charging voltage generating circuit  60  is provided for chargers  41 K to  41 C in common and the grid constant voltage circuits  71 K to  71 C and the line current detecting circuits  72 K to  72 C are provided to correspond to the respective chargers  41 K to  41 C. Without being limited thereto, the line current detecting circuits  72  may be provided to correspond to at least one charger  41 , that is, at least one grid constant voltage circuit  71 . For example, one line current detecting circuit  72  may be provided to correspond to one particular charger  41 , that is, one particular grid constant voltage circuit  71 . 
     The charging voltage generating circuit  60  includes, for example, a PWM signal control circuit  61 , a transformer driving circuit  62 , a boosting circuit  63 , and an output voltage detecting circuit  68 . 
     The charging voltage generating circuit  60  generates a charging voltage CHG which is applied to the discharging wires  42 K to  42 C of the chargers  41 K to  41 C in common. Respective grid voltages GRID are generated by the common charging voltage CHG and the grid constant voltage circuits  71 . The charging voltage CHG is, for example, about 5.5 kV to 7 kV and each grid voltage GRID is, for example, about 700 V. 
     The PWM signal control circuit  61  includes, for example, resistors and capacitors (not shown) and smoothes a PWM (Pulse Width Modulation) signal Sp 1  from a port PWM 1  of the ASIC  51  and supplies the smoothed PWM signal Sp 1  to the transformer driving circuit  62 . 
     The transformer driving circuit  62  is, for example, configured to flow an oscillating current into a primary winding  64   a  of a transformer  64  of the boosting circuit  63  based on the smoothed PWM signal received from the PWM signal control circuit  61 . In this embodiment, a value of the charging voltage CHG is controlled based on a duty cycle of the PWM signal Sp 1 . For example, the charging voltage CHG generated by the boosting circuit  63  is controlled to increase with an increase of the duty cycle of the PWM signal Sp 1 . 
     The boosting circuit  63  includes, for example, the transformer  64 , a rectifying diode  65 , a smoothing capacitor  66 , and an output resistor  67 . The transformer  64  includes the primary winding  64   a , a secondary winding  64   b , and an auxiliary winding  64   c.    
     With this configuration, a voltage of the primary winding  64   a  is boosted and rectified by the boosting circuit  63  and is applied, as the charging voltage CHG, to the discharging wires  42 K to  42 C of the chargers  41 K to  41 C. 
     The output voltage detecting circuit  68  is connected between the auxiliary winding  64   c  of the transformer  64  and the ASIC  51 . The output voltage detecting circuit  68  includes, for example, a smoothing circuit and a voltage dividing resistor. The output voltage detecting circuit  68  detects an output voltage v 1  generated between the output voltage detecting circuit  68  and the auxiliary winding  64   c  as the charging voltage CHG as an output voltage is generated, and smoothes and divides the output voltage v 1  to generate an output voltage detection signal Sv 1 . The output voltage detection signal Sv 1  is supplied to a port A/D 1  of the ASIC  51 . 
     Each grid constant circuit  71  includes a voltage detecting circuit (an example of a voltage detecting unit)  73 , a shunt current detecting circuit (corresponding to a first current detecting unit)  74 , a voltage control line Ln, and an operational amplifier OP 1  (an example of an operation control device). The grid constant voltage circuits  71  have the same configuration and therefore only the grid constant voltage circuit  71 K corresponding to a K (black) color will be explained in the following description for the purpose of simplicity. 
     The voltage detecting circuit  73 K includes voltage dividing resistors R 7  and R 8  and detects a voltage Vgr 1  based on the grid voltage GRID 1  of the grid  43 K by means of the voltage dividing resistors R 7  and R 8 . The detected voltage Vgr 1  is input to a non-inverting input terminal of the operational amplifier OP 1 . 
     In this embodiment, the shunt current detecting circuit  74 K is constructed by the voltage dividing resistor R 8  of the voltage detecting circuit  73 K and detects a shunt current (corresponding to a first current) Id 1  flowing into the voltage detecting circuit  73 K. More specifically, the shunt current detecting circuit  74 K generates a shunt detection signal Sid 1  as a terminal voltage (equal to the detected voltage Vgr 1 ) of the voltage dividing resistor R 8  and supplies the shunt detection signal Sid 1  to a port A/D 2  of the ASIC  51 . The ASIC  51  calculates the shunt current Id 1  based on a resistance value of the voltage dividing resistor R 8  and a voltage value of the shunt detection signal Sid 1 . 
     In addition, the ASIC  51  may calculate the grid voltage GRID 1  based on the voltage value of the shunt detection signal Sid 1  and a voltage dividing ratio between resistance values of the voltage dividing resistors R 7  and R 8 . That is, the ASIC  51  may detect the grid voltage GRID 1  based on the shunt detection signal Sid 1 . 
     The voltage control line Ln 1  is a circuit which makes the grid voltage GRID 1  constant, and includes resistors R 1 , R 2 , and R 3  connected in series. 
     The operational amplifier OP 1  performs a feedback control through the voltage control line Ln 1  such that the detected voltage Vgr 1  detected by the voltage detection circuit  73 K becomes a reference voltage (corresponding to a “predetermined voltage”) Vth. In this embodiment, for example, the reference voltage Vth is a voltage obtained by dividing a power supply voltage of 5 V by means of voltage dividing resistors R 9  and R 10  and is input to an inverting input terminal of the operational amplifier OP 1 . An output terminal and the inverting input terminal of the operational amplifier OP 1  are connected by a resistor R 6  and a capacitor C 2 . 
     In addition, a transistor Q 1  is connected to the output terminal of the operation amplifier OP 1 . The transistor Q 1  includes a collector (corresponding to a “first terminal”) connected between the resistor R 1  (corresponding to a “second resistive element”) and the resistor R 2  (corresponding to a “third resistive element”) and an emitter (corresponding to a “second terminal”) connected between the resistor R 2  and the resistor R 3 . The transistor Q 1  changes a collector-emitter voltage as a base current is controlled by the operational amplifier OP 1 . Accordingly, the grid voltage GRID is adjusted. 
     In this embodiment, the resistor R 2  is provided between the collector and the emitter of the transistor Q 1  and resistance values of the resistors R 1 , R 2 , and R 3  are set such that a voltage across the resistor R 2  falls within a withstanding voltage between the collector and the emitter. Accordingly, reliability of the transistor Q 1  is improved and the grid constant voltage circuit  71  is improved. In addition, the transistor Q 1  is not limited to a bipolar transistor but may be an FET (Field Effect Transistor). In addition, a constant voltage element such as a Zener diode or the like may be used instead of the resistor R 2 . A voltage of the constant voltage element is set to fall within the withstanding voltage between the collector and the emitter of the transistor Q 1 . 
     That is, the operational amplifier OP 1  changes the grid voltage GRID by changing a base voltage (corresponding to a “control terminal voltage”) such that the detected voltage Vgr 1  becomes the reference voltage Vth in the feedback control. In addition, as the detected voltage Vgr 1  becomes the reference voltage Vth by the feedback control, the grid voltage GRID 1  is made constant as a predetermined voltage. 
     In addition, the line current detecting circuit  72 K (corresponding to a “second current detecting unit”) which detects a line current (corresponding to a “second current”) Ir 1  flowing into the voltage control line Ln 1  between the transistor Q 1  and GND is provided between the transistor Q 1  and GND. In this embodiment, the line current detecting circuit  72 K is constructed by the resistor R 3  provided in the voltage control line Ln 1 . The line current detecting circuit  72 K generates a line current detection signal (corresponding to a “voltage detection signal”) Sir 1  as a terminal voltage across the resistor R 3  (corresponding to a “first resistive element”) and supplies the line current detection signal Sir 1  to a port A/D 3  of the ASIC  51 . The ASIC  51  calculates the line current Ir 1  based on a resistance value of the resistor R 3  and a voltage value of the line current detection signal Sir 1 . In addition, the ASIC  51  obtains a grid current Ig 1  by adding the shunt current Id 1  to the line current Ir 1 . 
     Capacitors C 1 , C 3 , C 4 , and so on are charging capacitors which delay voltages generated in the respective resistors. 
     The ASIC  51  controls the charging voltage generating circuit  60  such that the sum (the grid current Ig) of the shunt current Id and the line current Ir corresponding to one of the four chargers  41 K to  41 C has a predetermined current value. In this embodiment, the ASIC  51  determines whether or not the line current detecting circuit  72  corresponding to any charger  41  detects a minimal line current Ir, and controls the charging voltage generating circuit  60  in a constant current mode such that the sum (the grid current Ig) of the shunt current Id and the line current Ir corresponding to the charger  41  in which the minimal line current Ir is detected has a predetermined current value, for example, 250 μA. In this embodiment, it is assumed that the charger  41  in which the minimal line current Ir is detected is a charger  41  having the most contaminated discharging wire  42 . This is because a discharging current, that is, the grid current Ig, is typically decreased depending on a degree of contamination of the discharging wire  42 . 
     One charger  41  controlled in a constant current mode is not limited to the charger  41  in which the minimal line current Ir is detected, but may be appropriately selected according to the use situation of the printer  1 . 
     3. Control Operation of Charger 
     Next, a control operation of the plurality of (four in this embodiment) chargers  41  in the printer  1  including the charging voltage generating circuit  60  and the plurality of (four in this embodiment) grid constant voltage circuits  71  as configured above will be described. In this embodiment, a control operation related to making the grid voltage GRID of the chargers  41  constant will be described. 
     First, based on application of a predetermined charging voltage CHG to each charger  41  by the charging voltage generating circuit  60 , each voltage detecting circuit  73  of each grid constant voltage circuit  71  detects the voltage Vgr according to the grid voltage GRID of each grid  43  by means of the voltage dividing resistors R 7  and R 8 . 
     Subsequently, each grid voltage GRID is made constant as each operational amplifier OP 1  performs a feedback control through each voltage control line Ln such that the detected voltage Vgr detected by each voltage detecting circuit  73  becomes the reference voltage Vth. At this point, each shunt current detecting circuit  74  and the ASIC  51  each detects each shunt current Id flowing into each voltage detecting circuit  73 . 
     In addition, each line current detecting circuit  72  and the ASIC  51  each detects the line current Ir flowing into each voltage control line Ln. 
     In addition, the ASIC  51  controls the charging voltage generating circuit  60  such that the sum of the shunt current Id and the line current Ir corresponding to one of the four chargers  41  has a predetermined current value, for example, 250 μA. Preferably, the ASIC  51  determines whether or not the line current detecting circuit  72  corresponding to any charger  41  detects a minimal line current Ir, and controls the charging voltage generating circuit  60  such that the sum of the shunt current Id and the line current Ir corresponding to the charger  41  in which the minimal line current Ir is detected has a predetermined current value, for example, 250 μA. At this time, the charging voltage CHG generated by the charging voltage generating circuit  60  is applied to the chargers  41  in common. 
     In this manner, in this embodiment, the charging voltage generating circuit  60  performs a constant current control operation such that the grid current Ig of one of the four chargers  41  is made constant. On the other hand, the grid voltage GRID of each charger  41  is constant voltage-controlled by each grid constant voltage circuit  71 . At this point, grid voltages GRID controlled to be made constant may have the same or different voltage values. 
     4. Effects of Embodiment 
     In this embodiment, each grid voltage GRID is made constant by the feedback control using the operation amplifier OP 1  instead of a plurality of constant voltage elements having an element imbalance, for example a plurality of Zener diodes. Making the grid voltage GRID constant is less affected by the element imbalance in using the feedback control than using the constant voltage elements. In this case, although some (shunt current) Id of the grid current is flowed into a feedback circuit such as the voltage detecting circuit  73  or the like, the grid current Ig is made constant as a predetermined current value in consideration of the flowed current Id. 
     Accordingly, with the configuration where the charging voltage generating circuit  60  applies the charging voltage CHG to the plurality of chargers  41  in common, it is possible to make each grid voltage GRID constant with high precision. In addition, since a charger  41  having the most contaminated discharging wire  42 , that is, a charger  41  having the minimal grid current Ig, is controlled to have a constant current with high precision, it is possible to charge the photosensitive drums  44  sufficiently even with the charger  41  having the most contaminated discharging wire  42 . 
     Second Embodiment 
     Next, an image forming apparatus according to a second embodiment of the invention will be described with reference to  FIG. 3 . The second embodiment has the same configuration as the first embodiment except the configuration of the grid constant voltage circuit. Therefore, only the grid constant voltage circuit  71  will be explained for the purpose of simplicity. 
       FIG. 3  is a circuit diagram showing a configuration of a grid constant voltage circuit according to the second embodiment. The grid constant voltage circuit for each color has the same configuration as a grid constant voltage circuit  71 A shown in  FIG. 3 . 
     The grid constant voltage circuit  71 A of the second embodiment includes a reference voltage adjusting circuit  75 A which adjusts the reference voltage Vth, in addition to the grid constant voltage circuit  71  of the first embodiment. The reference voltage adjusting circuit  75 A includes, as main components, a transistor Q 2  and smoothing circuits R 12  and C 5 . 
     The transistor Q 2  is turned on/off by a PWM signal Sp 2  supplied from a port PWM 2  of the ASIC  51 . The smoothing circuit R 12  and C 5  includes a resistor R 12  and a capacitor C 5  and generates the reference voltage Vth by smoothing a connector output of the transistor Q 2 . That is, the reference voltage Vth is adjusted (changed) by changing a pulse width of the PWM signal Sp 2  and, accordingly, a value of the grid voltage GRID to be made constant is adjusted (changed) with the adjustment of the reference voltage Vth. Resistors R 13  and R 14  serve to adjust a base current of the transistor Q 2  to an appropriate level. 
     At this point, an ASIC  51  generates the PWM signal Sp 2  based on a line current detection signal Sir generated by a line current detecting circuit  72 A. That is, the ASIC  51  adjusts the reference voltage Vth based on the line current detection signal (voltage value) Sir and adjusts (changes) a voltage (grid voltage GRID) of the voltage control line to be made constant by controlling the base voltage of the transistor Q 1  based on the adjusted reference voltage Vth. 
     Accordingly, although the line current detection signal Sir is changed with the grid current Ig, the grid voltage GRID can be made constant as a predetermined voltage by changing a collector-emitter voltage of the transistor based on a voltage value by the line current detection signal Sir. That is, the grid voltage GRID can be made constant as the predetermined voltage based on the grid current Ig. 
     In this embodiment, the ASIC  51  controls the grid constant voltage circuit  71 A such that a higher line current Ir (grid current Ig) detected by the line current detecting circuit  72 A provides a lower grid voltage GRID (predetermined constant voltage) to be made constant. At this point, the PWM signal Sp 2  is generated based on the line current detection signal Sir and is supplied to the reference voltage adjusting circuit  75 A. 
     It is known in the related art that a higher grid current Ig, that is, a higher charging current Ichg, provides a higher surface potential to the photosensitive drums  44 . Accordingly, by setting a grid voltage GRID of a color having a higher grid current Ig to be lower, it is possible to prevent the surface potential of the photosensitive drums  44  corresponding to the respective colors from being unbalanced, which may result in prevention of print image quality from being deteriorated. In addition, it is assumed that a relationship between the grid current Ig and the surface potential of the photosensitive drums  44  and a relationship between the grid current Ig and the grid voltage GRID are known by prior experiments or the like. 
     In addition, the reference voltage adjusting circuit  75 A is not limited to the configuration shown in  FIG. 3 . The reference voltage adjusting circuit  75 A may be configured to allow the ASIC  51  to change the reference voltage Vth based on the line current detection signal (voltage signal) Sir. 
     Other Embodiments 
     The invention is not limited to the embodiments described in the above description and shown in the drawings. For example, the following embodiments are also intended to fall within the spirit and scope of the invention. 
     (1) In the above embodiments, as shown in  FIG. 4 , the grid constant voltage circuit  71  may include a phototransistor PCI as the transistor. In this case, since a current can be prevented from being flowed from the operational amplifier OP 1  into the current detection resistor R 3 , it is possible to detect the line current Ir (second current), that is, the grid current Ig with higher precision. In addition, in the configuration of the grid constant voltage circuit  71  shown in  FIG. 2 , the phototransistor PC 1  may be replaced for the transistor Q 1  and may be provided along with the transistor Q 2  and a varistor VR 1  which are Darlington-connected, as shown in  FIG. 4 . 
     (2) In addition, as shown in  FIG. 4 , the grid constant voltage circuit  71  may include a constant voltage element such as, for example, the varistor VR 1 , provided between the grid  43  and the transistor in the voltage control line Ln. In this case, a collector-emitter or source-drain withstanding voltage of the transistor can be limited, which may result in improved reliability. In addition, in the configuration of the grid constant voltage circuit  71  shown in  FIG. 2 , the constant voltage element may be replaced for the resistor R 1  and may be connected to the transistor Q 2  which is Darlington-connected to the phototransistor PC 1 , as shown in  FIG. 4 . 
     (3) Although it has been illustrated in the above embodiments that the photosensitive drums  44  correspond to the chargers  41  in a one-to-one correspondence (in other words, a photosensitive drum  44  is provided for each color), the invention is not limited thereto. For example, the invention may be applied to a printer (image forming apparatus) in which a plurality of chargers  44  correspond to one photosensitive drum, that is, toner images of various colors are overlapped on one photosensitive drum  44  and are then collectively transferred onto a sheet.