Patent Publication Number: US-9835969-B2

Title: Image forming apparatus having a plurality of selectively charged photosensitive bodies and controlling method for same

Description:
This application claims priority under 35 U.S.C. §119 from Japanese Patent Application No. 2015-154763 filed on Aug. 5, 2015. The entire subject matter of the application is incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosures relate to an image forming apparatus, and a controlling method of an image forming apparatus. 
     Related Art 
     There has been known an image forming apparatus which is provided with multiple photosensitive bodies corresponding to multiple colors (e.g., black, yellow, magenta and cyan) of developing agents, and multiple chargers configured to charge the multiple photosensitive bodies, respectively. In such an image forming apparatus, the multiple chargers are electrically connected in parallel, and a single voltage outputting circuit is provided to apply a common voltage to the multiple chargers. According to such a configuration, the number of parts can be reduced and the image forming apparatus can be downsized. 
     SUMMARY 
     When monochrome printing is executed with use of the image forming apparatus having multiple photosensitive bodies, only one of the multiple photosensitive bodies is used to form a monochrome image. According to the above-described conventional technique, even when only one of the photosensitive bodies is used, the common voltage applied to all of the multiple chargers, some of which may not be used for printing. 
     According to aspect of the disclosures, there is provided an image forming apparatus, which is provided with a first photosensitive body, a second photosensitive body, a first charger configured to charge the first photosensitive body, a second charger configured to charge the second photosensitive body, a voltage outputting circuit, a voltage dropping circuit, and a controller. The controller is configured to selectively execute a first charging control to apply a first voltage, which is an output of the voltage outputting circuit, to the first charger and the second charger, which are connected in parallel, and a second charging control to apply the first voltage to the first charger and a second voltage to the second charger, the second voltage being less than the first voltage, the first voltage being an input of the voltage dropping circuit and the second voltage being an output of the voltage dropping circuit. 
     According to aspects of the disclosure, there is also provided a controlling method of an image forming apparatus having a first photosensitive body, a second photosensitive body, a first charger configured to charge the first photosensitive body, a second charger configured to charge the second photosensitive body, a voltage outputting circuit and a voltage dropping circuit. The method includes a first charging step to apply a first voltage to the first charger and the second charger. The second voltage is less than the first voltage and is an output of the voltage dropping circuit while the first voltage is an input of the voltage dropping circuit. 
     It is noted that the technique disclosed in the present specification can be realized in various ways. For example, the technique may be realizes in forms of an image forming apparatus, a control method of an image forming apparatus, computer programs prepared to realize such a method or functions of such an apparatus, a non-transitory computer-readable medium storing such computer programs (i.e., computer-executable instructions). 
    
    
     
       BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS 
         FIG. 1  schematically shows an entire configuration of a printer according to an embodiment of the disclosures. 
         FIG. 2  shows a configuration of a charger according to the embodiment of the disclosures. 
         FIG. 3  shows a configuration of a charger according to the embodiment of the disclosures. 
         FIG. 4  is a block diagram showing an electrical configuration of the printer according to the embodiment of the disclosures. 
         FIG. 5  shows a circuit configuration of a charging power source according to the embodiment of the disclosures. 
         FIG. 6  is a flowchart illustrating a charge controlling process according to the embodiment of the disclosures. 
         FIG. 7  shows a circuit configuration of a first modification of the charging power source according to the embodiment of the disclosures. 
         FIG. 8  shows a circuit configuration of a second modification of the charging power source according to the embodiment of the disclosures. 
         FIG. 9  is a block diagram showing an electrical configuration of a printer according to a modified embodiment of the disclosures. 
         FIGS. 10A-10C  illustrate a configuration of a switching mechanism according to the embodiment of the disclosures. 
         FIGS. 11-13  show a relationship between a linear moving cam and a switching element according to the embodiment of the disclosures. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENT AND MODIFICATIONS 
     Hereinafter, a printer  10  according to an embodiment of the disclosures, and its modifications will be described with reference to the accompanying drawings. It is noted that mutually orthogonal three axes (i.e., X, Y and Z axes) are indicated in  FIG. 1  to identify directions in the description below. In the following description, for the sake of convenience, directions are referred to as indicated below. That is, positive and negative directions along the Z axis will be referred to as upper and lower directions, respectively, positive and negative directions along the X axis will be referred to as front and rear directions, respectively, and positive and negative directions along the Y axis will be referred to as right and left directions, respectively. The above definition will apply in the remaining drawings as well. 
     The printer  10  is an electrophotographic printer configured to form an image on a sheet W with four colors of toner (or developing agents) of black (hereinafter, abbreviated as K), yellow (hereinafter, abbreviated as Y), magenta (hereinafter, abbreviated as M) and cyan (hereinafter, abbreviated as C). It is noted that the printer  10  is an example of an image forming apparatus set forth in the claims. 
     In the following description, when components of the printer  10  are provided for each of the four colors and respective components are to be distinguished by color, the above abbreviation letters K, Y, M and C are suffixed to reference numbers/letters of respective components, while when the components are described without being distinguished by color, such a suffix will be omitted. Further, when components provided for respective colors are described, one of the components for a particular color will be described representatively, and description on the remaining components will be omitted for brevity. 
     As shown in  FIG. 1 , the printer  10  has a casing  100  which accommodates a sheet supplier  200 , a sheet conveyer  300 , and an image forming unit  400 . On an upper surface of the casing  100 , a discharge port  110  and a discharge tray  120  are formed, and a discharge roller pair  130  is arranged inside the casing  100  and in the vicinity of the discharge port  110 . 
     The sheet supplier  200  has a tray  210  and a pickup roller  220 . The tray  210  is a container configured to accommodates the sheets W. The pickup roller  220  is configured to pick up the sheets W accommodated in the tray  21  one by one, and feed the picked up sheet toward the sheet conveyer  300 . 
     The sheet conveyer  300  has a conveying roller pair  310 , a registration roller pair  320  and a belt unit  330 . The conveying roller pair  310  is configured to covey the sheet W supplied form the sheet supplier  200  toward the registration roller pair  320 . The registration roller pair  320  is configured to correct a skew to the sheet W conveyed from the conveying roller pair  310 , and further convey the sheet W toward the belt unit  330 . The belt unit  330  has an endless belt  331 , a driving roller  332  and a driven roller  333 . The driving roller  332  and the driven roller  333  are rotatable about respective rotation axes, which are arranged in parallel to each other and extend in the Y axis direction. The endless belt  331  is stretched between the driving roller  332  and the driven roller  333 , and rotates in association with rotation of the driving roller  332 . The sheet W conveyed by the registration roller pair  320  is placed on a sheet conveying surface, which is a part of an outer circumferential surface of the endless belt  331  and facing the multiple photosensitive bodies  610 , and conveyed toward a fixing unit  700  as the belt  331  rotates. It is noted that transferring rollers  640 , which constitute process units  600 , respectively, are provided inside a course of the endless belt  331 . 
     The image forming unit  400  has an exposing unit  500 , four process units  600  (i.e.,  600 K,  600 Y,  600 M and  600 C) corresponding to the four colors (i.e., K, Y, M and C colors), and the fixing unit  700 . The exposing unit  500  is configured to emit laser light L (i.e., four laser beams) to photosensitive bodies  610  which are provided to the process units  600 , respectively. 
     The four process units  600  are arranged along a conveying direction of the sheet W which is conveyed by the endless belt  331  (i.e., a front-rear direction). In the following description, the process unit  600 K for black color will be described. As mentioned above, the process unit  600 K is described as a representative one of the four process units  600  (i.e.,  600 K,  600 Y,  600 M and  600 C), and the other process units  600  have the same configuration as the process unit  600 K. 
     Each process unit  600  includes the photosensitive body  610 , the charger  620 , a developing unit  630  and the transferring roller  640 . The photosensitive body  610  is a cylindrical drum-shaped member rotatable about an axis which extends in the Y axis direction. 
     The charger  620  is a scorotron type corona charger, and includes a shield case  621 , a wire electrode  623  and a grid electrode  625  as shown in  FIGS. 2 and 3 . The shield case  621  has a U-shaped cross section extending in a direction of the rotation axis of the photosensitive body  610 , and opens facing the photosensitive body  610 . The wire electrode  623  is made of metal, and stretched inside the shield case  621  in a direction of the rotation axis of the photosensitive body  610 . The grid electrode  625  includes a plurality of slits or holes arranged in a matrix manner. The grid electrode  625  is attached to the shield case  621  such that the grid electrode  625  is arranged between the wire electrode  623  and the photosensitive body  610  without any part of the shield case  621  between the wire electrode  623  and the photosensitive body  610 . The charger  620  further includes a wire cleaner  627 . The wire cleaner  627  is arranged so as to be slidable along the wire electrode  623 . When the wire cleaner  627  is slid along the wire electrode  623 , contaminants adhered to the wire electrode  623  are removed therefrom. 
     The developing unit  630  ( FIG. 1 ) includes a toner box  631  accommodating toner, and a developing roller  632  which supplies the toner from the toner box  631  onto the surface of the photosensitive body  610 . The transferring roller  640  is arranged to face the surface of the photosensitive body  610  with the endless belt  331  therebetween, and is used to transfer the toner on the surface of the photosensitive body  610  toward the endless belt  331 . 
     When a voltage is applied to the wire electrode  623  of the charger  620 , corona discharge is generated, and due to ions generated by the corona discharge, a surface of the photosensitive body  620  is uniformly charged to a positive polarity. At this stage, a charge potential of the photosensitive body  610  is controlled by controlling voltage applied to the grid electrode  625 . Thereafter, as the laser beam L from the exposing unit  500  is applied on the charged surface of the photosensitive body  610 , an electrostatic latent image is formed on the surface of the photosensitive body  610 . As the toner is supplied, by the developing unit  630 , onto the surface of the photosensitive body  610 , the electrostatic latent image formed on the surface of the photosensitive body  610  is developed and a toner image is formed. The toner image formed on the surface of the photosensitive body  610  is transferred onto the sheet W or the sheet conveying surface of the belt  331  passing a position where the photosensitive body  610  and the transferring roller  640  face each other. 
     According to the present embodiment, the process unit  600 K, the process unit  600 Y, the process unit  600 M and the process unit  600 C are arranged in this order from an upstream side to a downstream side in the conveying direction of the sheet W. Therefore, a black toner image, a yellow toner image, a magenta toner image and a cyan toner image are sequentially transferred onto the sheet W in an overlapped manner. 
     According the present embodiment, the photosensitive body  610 K corresponding to the black color is an example of a first photosensitive body, and the photosensitive bodies  600 Y,  600 M and  600 C respectively corresponding to the Y, M and C colors are examples of second photosensitive bodies. Further, the charger  620 K corresponding to the black color is an example of a first charger, and the chargers  620 Y,  620 M and  620 C respectively corresponding to the Y, M and C colors are examples of second chargers. Further, the developing unit  630 K corresponding to the black color is an example of a first developing device, and the developing units  630 Y,  630 M and  630 C respectively corresponding to the Y, M and C colors are examples of second developing devices. 
     The fixing unit  700  is arranged on a downstream side, in the conveying direction of the sheet W, with respect to all the photosensitive bodies  610 , and serves to fix the toner images, which are transferred onto the sheet W, permanently to the sheet W, thereby an image being formed on the sheet W. The discharge roller pair  130  discharges the sheet W passed through the fixing unit  700  to the discharge tray  120  via the discharge port  110 . 
       FIG. 4  is a block diagram showing an electrical configuration of the printer  10 . The printer  10  includes a controller  800 , a controller  800 , a driving unit  810 , a display  820 , an operation unit  830 , a communication interface (I/F)  840  and a charging power source  900  as well as the sheet supplier  200 , the sheet conveyer  300  and the image forming unit  400  described above. 
     The controller  800  has a CPU  801 , a ROM  802 , a RAM  803 , a non-volatile memory  804  and ASIC  805 . The ROM  802  stores control programs, setting information and the like, which are used to control operation of the printer  10 . The RAM  803  is used as a work area and a temporary storage for data when the CPU  801  executes programs. The non-volatile memory  804  is a rewritable memory such as a NVRAM, a flash memory, an HDD, EEPROM or the like. The ASIC  805  is a hardware circuit mainly used for image processing. The CPU  801  controls respective components of the printer  10  by executing control programs retrieved from the ROM  802  in accordance with signals transmitted from sensors. The controller  800 , the CPU  801  or the ASIC  805  is an example of a controller. 
     The driving unit  810  includes one or more motors (not shown), and drives the pickup roller  220 , the registration roller pair  320 , the driving roller  332 , the photosensitive body  610 , the developing roller  632  and the like to rotate with use of driving force of the one or more motors. The display  820  may be a liquid crystal display (LCD) and displays various information in accordance with instructions by the controller  800 . The display  820  is an example of a notifying unit. The operation unit  830  is provided with keys acquiring user operations. The communication I/F  840  enables the printer  10  to communicate with external devices. The communication I/F  840  may be a network interface, a serial communication interface, a parallel communication interface or the like. 
     The charging power source  900  is a circuit configured to supply electrical power to respective chargers  620 . As shown in  FIG. 5 , the charging power source  900  has a single voltage outputting circuit  60 . The voltage outputting circuit  60  generates and outputs a voltage. The voltage is to be applied to the wire electrode  623  of each charger  620 K,  620 Y,  620 M and  620  C where each wire electrode  623  is electrically connected in parallel. The voltage outputting circuit  60  includes a PWM (pulse width modulation) signal controlling circuit  61 , a transformer driving circuit  62 , a voltage boosting circuit  63 , and an output voltage detecting circuit  68 . 
     The PWM signal controlling circuit  61  includes a resistor and a capacitor (not shown) for smoothing the PWM signal Sp 1  from the controller  800 . Then, the PWM signal controlling circuit  61  outputs the smoothed PWM signal Sp 1  to the transformer driving circuit  62 . The transformer driving circuit  62  causes flow of an oscillating current through a primary winding  64   a  of the transformer  64  included in the voltage boosting circuit  63  based on the smoothed PWM signal Sp 1 . The voltage boosting circuit  63  includes the transformer  64 , a rectifier diode  65 , a smoothing capacitor  66  and an output resistor  67 . The transformer  64  has a primary winding  64   a , a secondary winding  64   b  and an auxiliary winding  64   c . A number of turns of the second winding  64   b  is greater than a number of turns the primary winding  64   a . In the voltage boosting circuit  63 , a voltage across the primary winding  64   a  is boosted in accordance with a winding ratio of the primary winding  64   a  and the secondary winding  64   b . The voltage across the secondary winding  64   b  is rectified and smoothed by the rectifier diode  65  and the smoothing capacitor  66 , and the rectified and smoothed voltage is an output voltage CHG of the voltage boosting circuit  63 . The output voltage CHG is output from an output terminal T 1  of the voltage outputting circuit  60 . Incidentally, when the voltage is boosted by the transformer  64 , a voltage v 1  correlated to the output voltage CHG is generated across the auxiliary winding  64   c.    
     The output voltage detecting circuit  68  includes a smoothing circuit and a voltage dividing resistor, and is connected to the auxiliary winding  64   c  of the transformer  64 . The output voltage detecting circuit  68  smoothes and divides the voltage v 1  generated across the auxiliary winding  64   c  to generate a voltage detection signal Sv 1  corresponding to the amplitude of the output voltage CHG. The voltage detection signal Sv 1  thus generated is supplied to the controller  800 . 
     The charging power source  900  further includes a voltage output line Lv connected to the output terminal T 1 , a first branch line Lb 1  connecting a first point P 1  on the voltage output line Lv with the wire electrode  623  of the charger  620 K, a second branch line Lb 2  connecting the first point P 1  with a second point P 2 , and third branch lines Lb 3  connecting the second point P 2  with wire electrodes  623  of the chargers  620 Y,  620 M and  620 C, respectively. Specifically, the third branch lines Lb 3  include the branch line Lb 3 (Y) connecting the second point P 2  with the wire electrode  623  of the charger  620 Y, the branch line Lb 3 (M) connecting the second point P 2  with the wire electrode  623  of the charger  620 M, and the branch line Lb 3 (C) connecting the second point P 2  with the wire electrode  623  of the charger  620 C. 
     On the second branch line Lb 2  a voltage dropping circuit  20  is arranged. The voltage dropping circuit  20  includes a resistor  32  and a switching element  34 , which is connected to the resistance element  32  in parallel. The switching element  34  is configured to switch a connection between a connecting state and a disconnecting state. The switching element  34  may be a mechanical switch or a semiconductor switch. 
     Since the voltage outputting circuit  60  is connected to respective chargers  620  as described above, the output voltage CHG output by the output terminal T 1  of the voltage boosting circuit  63  is applied to the wire electrode  623  of the charger  620 K via the voltage output line Lv and the first branch line Lb 1 . Then, corona discharge is generated in the charger  620 K and the corona discharge charges the surface of the photosensitive body  610 K. 
     When the switching element  34  is in the connecting state, the output voltage CHG output by the voltage boosting circuit  63  bypasses the resistor  32  and is directly applied to the wire electrodes  623  of the respective chargers  620 Y,  620 M and  620 C via the second branch line Lb 2  and the third branch line Lb 3 . Then, corona discharge is generated in each of the chargers  620 Y,  620 M and  620 C, and the corona discharge charges the surfaces of the respective photosensitive bodies  610 Y,  610 M and  610 C. 
     When the switching element  34  is in the disconnecting state, the output voltage CHG output by the voltage boosting circuit  63  is dropped by the resistor  32  on the second branch line Lb 2  to a dropped output voltage dCHG. The dropped output voltage dCHG is applied to the wire electrodes  623  of the respective chargers  620 Y,  620 M and  620 C via the third branch line Lb 3 . It is noted an expression “the voltage is dropped” in the specification means that the absolute value of the voltage is decreased. When the dropped voltage dCHG is applied to the wire electrodes  623  of the respective chargers  620 Y,  620 M and  620 C, corona discharge is generated in each of the chargers  620 Y,  620 M and  620 C, and the corona discharge charges the surfaces of the respective photosensitive bodies  610 Y,  610 M and  610 C. The above configuration can be achieved by employing the resistor  32  having an appropriate resistance value of the voltage dropping circuit  20 . It is noted, however, since the absolute value of the dropped output voltage dCHG is less than the absolute value of the output voltage CHG, the absolute value of the charged potential of each photosensitive body  620  when the dropped output voltage dCHG is applied is less than that when the output voltage CHG is applied. It is noted that the output voltage CHG is an example of a first voltage, and the dropped output voltage dCHG is an example of a second voltage. 
     The charging power source  900  further includes four gird voltage applying circuits  71  respectively corresponding to the four chargers  620 . Since the four grid voltage applying circuits  71  have the same configurations, a configuration of the grid voltage applying circuit  71 K corresponding to black color (K) will be representatively described, and description of the other grid voltage applying circuits  71  corresponding to the other colors is omitted. It is also noted that, the configuration of the grid voltage applying circuit  71 K is shown in  FIG. 5  and configurations of the other grid voltage applying circuits  71 Y,  71 M and  71 C are omitted in  FIG. 5 . 
     The grid voltage applying circuit  71 K includes a voltage detecting circuit  73 K, a voltage controlling circuit Ln 1 , and a feedback circuit  75 K. In the following description, to distinguish the voltage controlling circuits Ln corresponding to K, Y, M and C colors from each other, one of the numerals 1-4 is suffixed after the reference letters “Ln”, respectively. So are in indicating grid voltages GRID, grid currents Ig, divided currents Id, line currents Ir, voltages Vgr, divided current detection signals Sid, and line current detection signals Sir. 
     The voltage detecting circuit  73 K includes voltage dividing resistors R 7  and R 8 , and with use of the voltage dividing resistors R 7  and R 8 , detects a voltage Vgr 1  corresponding to the grid voltage GRID 1  of the grid electrode  625 . 
     The voltage dividing resistor R 8  of the voltage detecting circuit  73 K also serves as a divided current detecting circuit  74 K which detects the divided current Id 1  flowing through the voltage detecting circuit  73 K. That is, the voltage dividing resistor R 8  as the divided current detecting circuit  74 K generates a divided current detection signal Sid 1 , which is a terminal voltage of the voltage dividing resistor R 8  (which is equal to the voltage Vgr 1 ) and supplies the thus generated divided current detection signal Sid 1  to the controller  800 . The controller  800  calculates the divided current Id 1  based on a resistance value of the voltage dividing resistor R 8  and the divided current detection signal Sid 1 . Further, the controller  800  calculates the grid voltage GRID 1  based on the divided current detection signal Sid 1  and a voltage dividing ratio defined by the resistance values of the voltage dividing resistors R 7  and R 8 . 
     The voltage controlling circuit Ln 1  is configured to adjust the grid voltage GRIM and includes a resistor R 1 , a Zener diode D 1 , a transistor Q 1  and a resistor R 3 . A cathode of the Zener diode D 1  is connected with the resistor R 1 , an anode of the Zener diode D 1  is connected with a collector of the transistor Q 1 , and an emitter of the transistor Q 1  is connected with the resistor R 3 . 
     The feedback circuit  75 K includes an operational amplifier OP 1 , and performs a feedback control via the voltage controlling circuit Ln 1  such that the voltage Vgr 1  detected by the voltage detecting circuit  73 K is equal to a reference voltage Vth. To a non-inverted input terminal of the operational amplifier OP 1 , the voltage Vgr 1  is input. Further, to an inverse input terminal of the operational amplifier OP 1 , the reference voltage Vth, which is generated, by dividing a power source voltage Vcc (e.g., 5V) with use of voltage dividing resistors R 9  and R 10  is input. An output terminal of the operational amplifier OP 1  and the inverted input terminal thereof are connected via a resistor R 6  and a capacitor C 2 . 
     Further, the output terminal of the operational amplifier OP 1  is connected to the base of the transistor Q 1  via a resistor R 4 . Between the resistor R 4  and the base of the transistor Q 1 , one terminal of a resistor R 5  is connected, while the other terminal of the resistor R 5  is grounded. As the base current of the transistor Q 1  is controlled by the operation amplifier OP 1 , a voltage between the collector and emitter of the transistor Q 1  is controlled, thereby the grid voltage GRID 1  being adjusted. That is, the feedback circuit  75 K varies the base current of the transistor Q 1  so that the detection voltage Vgr 1  coincides with the reference voltage Vth, thereby controlling the grid voltage GRID 1 . 
     The resistor R 3  provided in the voltage controlling circuit Ln 1  also serves as a line current detecting circuit  72 K which detects a line current Ir 1  flowing through the voltage controlling circuit Ln 1  between the transistor Q 1  and the GND. The line current detecting circuit  72 K generates a line current detection signal Sir 1  which is a terminal voltage of the resistor R 3 , and supplies the line current detection signal Sir 1  to the controller  800 . The controller  800  calculates a line current Ir 1  based on the resistance value of the resistor R 3  and the line current detection signal Sir 1 . Further, the controller  800  calculates a grid current Ig 1  flowing through the grid electrode  625  by adding the above-described divided current Id 1  to the line current Ir 1 . It is noted that the capacitors C 1 , C 3  and C 4  are charging capacitors, respectively, which delays the voltage generated across the corresponding resistors. 
     As described above, according to the present embodiment, the grid voltage GRID 1 -GRID 4  of the grid electrodes  625  of respective chargers  620  are applied by the grid voltage applying circuits  71  which are provided corresponding to the respective chargers  620 . Further, the controller  800  calculates the grid current Ig 1 -Ig 4  flowing through the respective grid electrodes  625  of the chargers  620  based on divided current detection signals Sid 1 -Sid 4  output by divided current detecting circuits  74  included in the respective grid voltage applying circuits  71 . Generally, the wire current flowing through the wire electrode  623  of each charger  620  is divided into the discharge current for charging the photosensitive body  610  and the grid current Ig at a particular ratio. Therefore, the amplitude of the grid current Ig would be regarded as an index indicating the amplitude of the discharge current. It is noted that the divided current detecting circuit  74  and the line current detecting circuit  72  are examples of an electrical current detecting circuit that detects the grid current Ig. 
     Next, a charge controlling process will be described. The charge controlling process is part of an image forming process to form an image on the sheet W. The charge controlling process is to control a charging status of each photosensitive body  610  by controlling the voltage applied to respective chargers  620 . When the controller  800  receives a print instruction to form an image on the sheet W through the communication I/F  840  or the operation unit  830 , the controller  800  starts the charge controlling process. It is noted that processes included in the image forming process other than the charge controlling process are well-known processes and detailed description thereof is omitted. 
       FIG. 6  is a flowchart illustrating the charge controlling process. When the charge controlling process is started, the controller  800  firstly determines whether a received print instruction is an instruction of color printing to form an image using four colors (K, Y, M and C) or an instruction of monochrome printing to form an image using only the black color (S 110 ). When it is determined in S 110  that the instruction is of the color printing, the controller  800  brings the switching element  34  in the connecting state (S 120 ). Further, the controller  800  supplies the PWM signal Sp 1  to the voltage outputting circuit  60  so that the voltage outputting circuit  60  starts outputting the output voltage CHG (S 130 ). At this stage, the output voltage CHG output by the voltage outputting circuit  60  is applied to wire electrodes  623  of the chargers  620 K,  620 Y,  620 M and  620 C, respectively. As the output voltage CHG is applied to the wire electrodes  623 , each of the chargers  620 K,  620 Y,  620 M and  620 C generates corona discharge, thereby the photosensitive bodies  610 K,  610 Y,  610 M and  610 C are charged, respectively. At this stage, the grid voltages GRID 1 -GRID 4  are substantially the same. 
     Thereafter, the controller  800  calculates the grid currents Ig 1 -Ig 4  in the chargers  620  corresponding to the four colors, respectively (S 140 ). Then, based on the least grid current Ig among the four grid currents Ig 1 -Ig 4 , the controller  800  adjusts the output voltage CHG of the voltage outputting circuit  60  (S 150 ). Specifically, the controller  800  adjusts output voltage CHG by controlling the duty ratio of the PWM signal so that the least grid current Ig is equal to a particular value. On the wire electrodes  623  of the chargers  620 , contaminants are adhered as each electrode  623  generates corona discharge. There is a tendency that the grid currents Ig of the chargers  620  decrease as contamination increases. Therefore, it is understood that the charger  620  having the least grid current Ig has the wire electrode  623  be contaminated most. By adjusting the output voltage CHG so that the least grid current Ig becomes the particular value, the gird currents Ig in all chargers  620  can be maintained to be equal to or greater than the particular value, thereby the absolute values of the charged potentials of the photosensitive bodies  610  can be maintained to be equal to or greater than a particular value. 
     If the degree of contamination of the wire electrode  623  varies largely among the chargers  620 , adjusting the output voltages CHG based on the least grid current Ig as in S 150  may cause the grid current Ig of the charger  620  having the wire electrode  623  be relatively less contaminated to be excessively large since the output voltage CHG is adjusted based on the grid current Ig of the charger  620  having the wire electrode  623  be relatively more contaminated. If the grid current Ig in the charger  620  corresponding to a certain color is excessively large, the discharge current corresponding to the charger  620  corresponding to the certain color may also be excessively large. Therefore, in such a case, an error notifying process is executed as described below. 
     In S 160 , the controller  800  determines whether the greatest grid current Ig among the grid current Ig 1 -Ig 4  of the chargers  620  is equal to or greater than a first threshold value. 
     When it is determined that the greatest gird current Ig is equal to or greater than the first threshold value (S 160 : YES), the controller  800  executes an error notifying process (S 170 ). The error notifying process is a process of displaying a message encouraging the user to clean the wire electrodes  623  of the chargers  620  with the wire cleaners  627 . If the controller  800  determines that the greatest grid current Ig is smaller than the first threshold value (S 160 : NO), the controller  800  skips the error notifying process. 
     Next, the controller  800  determines whether the image forming process has been completed (S 180 ). While it is determined that the image forming process has not been completed (S 180 : NO), the controller  800  repeats the above described processes S 140 -S 170  until the controller determines that the image forming process has been completed. When it is determined that the image forming process has been completed (S 180 : YES), the controller  800  terminates the charge controlling process. 
     When it is determined that the print instruction is of the monochrome printing in S 110 , the controller  800  brings the switching element  34  to the disconnecting state (S 220 ). Further, the controller  800  supplies the PWM signal Sp 1  to the voltage outputting circuit  60  so that the voltage outputting circuit  60  starts outputting the output voltage CHG (S 230 ). In this state, the output voltage CHG output by the voltage outputting circuit  60  is applied to the wire electrode  623  of the charger  620 K to be used for printing, while the dropped output voltage dCHG, which is generated by the voltage dropping circuit  20  is applied to the wire electrodes  623 Y,  623 M and  623 C. In the charger  620 K, the corona discharge is generated as the output voltage CHG is applied to the wire electrode  623 , thereby the corresponding photosensitive body  610  is charged. In the chargers  620 Y,  620 M and  620 C, the corona discharge is generated as the dropped output voltage dCHG is applied to the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C, thereby the corresponding photosensitive bodies  610  are charged. It is noted that, since the dropped output voltage dCHG is less than the output voltage CHG, and the grid voltage applying circuits  71 Y,  71 M and  71 C cannot maintain the grid voltage GRID 2 -GRID  4  as the same value as the grid voltage GRID 1 , the absolute values of the charged potentials of the photosensitive bodies  610 Y,  610 M and  610 C are less than that of the photosensitive body  610 K. 
     Thereafter, the controller  800  calculates the grid currents Ig 1 -Ig 4  corresponding to respective colors (S 240 ), and adjusts the output voltage CHG of the voltage outputting circuit  60  (S 250 ) based on the grid current Ig 1  in the charger  620 K. That is, the controller  800  adjusts the output voltage CHG by adjusting the duty ratio of the PWM signal Sp 1  so that the grid current Ig 1  coincides with the particular value. By this control, the grid current Ig 1  in the charger  620 K is maintained to be the particular value, and the absolute value of the charged potential of the photosensitive body  610 K is maintained to an appropriate value. It is noted that each of the grid currents Ig in the other chargers  620 Y,  620 M and  620 C has a value equal to or less than the grid current Ig 1  of the charger  620 K. 
     The controller  800  is configured to determine whether the least current Ig among the four grid currents Ig 1 -Ig 4  is equal to or less than the second threshold value (S 260 ). If it is determined that the least current Ig is equal to or less than the second threshold value (S 260 : YES), the controller  800  executes an error notifying process (S 270 ). The error notifying process is a process of displaying a message encouraging a user to clean the wire electrodes  623  with the wire cleaners  627 . Some of the wire electrodes  623  of the chargers  620  may be less contaminated and some may be more contaminated while the output voltage CHG is adjusted based on the grid voltage Ig of the charger  620 K in S 250  Thus the grid current Ig of the more contaminated wire electrode  623  may be very little. When the grid current Ig in one of the charger  620 Y,  620 M and  620 C is very little, the discharge current is also very little, and the absolute value of the charged potential of the photosensitive body  610 K becomes very little. When the absolute value of the charged potential of one of the photosensitive bodies  610 Y,  610 M and  610 C is very little, toner may be unnecessarily adhered to the one of the photosensitive bodies  610 Y,  610 M and  610 C. Therefore, in such a case, the error notifying process (S 270 ) described above is executed. When it is determined that the least grid current Ig is greater than the second threshold value (S 260 : NO), the controller  800  skips the error notifying process (S 270 ). 
     Next, the controller  800  determines whether the image forming process has been completed (S 280 ). When it is determined that the image forming process has not been completed (S 280 : NO), the controller  800  repeats the above-described processes S 240 , S 250 , S 260  and S 270  until the controller  800  determines that the image forming process has been completed (S 280 : YES). When it is determined that the image forming process has been completed (S 280 : YES), the controller  800  terminates the charge controlling process. 
     As described above, in the printer  10  according to the present embodiment, a single voltage outputting circuit  60  is provided for the four chargers  620  which are connected in parallel, and the output voltage CHG of the voltage outputting circuit  60  is applied to each of the four chargers  620 . Therefore, the printer  10  may include less number of parts rather than the conventional printer, thereby being downsized. Further, the printer  10  according to the embodiment is available for a first charging control and a second charging control, where the first charging control is that the controller  800  applies the output voltage CHG to each of the wire electrodes  623  of the chargers  620  K,  620 Y,  620 M and  620 C, and where the second charging control is that the controller  800  applies the output voltage CHG to the wire electrode  623  of the charger  620 K, and applies the dropped output voltage dCHG to each of the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C. 
     As described above, in the printer  10  according to the present embodiment, the controller  800  executes the first charging control when the four colors are used for image formation, each photosensitive body  610  used for the image formation can be appropriately charged. Further, when only one color (i.e., the K color) is used for image formation, the controller  800  executes the second charging control. Therefore, in the second charging control, the photosensitive body  610 K used for the image formation can be appropriately charged, while the dropped output voltage dCHG is applied to the chargers  620 Y,  620 M and  620 C which are not used for the image formation, thereby power consumption by these chargers  620 Y,  620 M and  620 C may be reduced. 
     Further, in the printer  10  according to the present embodiment, as the controller  800  executes the second charging control, the voltage applied to the chargers  620 Y,  620 M and  620 C which are not used for image formation is dropped, deterioration of the chargers  620 Y,  620 M and  620 C can be reduced, and generation of ozone by the chargers  620 Y,  620 M and  620 C can be reduced. 
     It is noted that, if the chargers  620 Y,  620 M and  620 C, which are not used for image formation, are disconnected from the voltage outputting circuit  60  in some way when the monochrome printing is executed, the wire electrode  623  of the charger  620 K is only contaminated. Therefore, the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C may not be contaminated when the monochrome printing is executed. Then, when the color printing is executed after the monochrome printing is executed, the discharge potentials of the photosensitive bodies  610 K,  610 Y,  610 M and  610 C may be largely vary, thereby causing quality of the color images to be poor. In the printer  10  according to the present embodiment, corona discharge generated in the chargers  620 Y,  620 M and  620 C may cause the contamination of the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C when the monochrome printing is executed. Therefore, difference of degrees of contaminations of the wire electrodes  623  among the chargers  620 K,  620 Y,  620 M and  620 C can be reduced, thereby deterioration of image quality can be reduced. 
     In the printer  10  according to the embodiment, the charging power source  900  has the voltage output line Lv connected to the output terminal T 1  of the voltage outputting circuit  60 , the first branch line Lb 1  which connects a first point P 1  on the voltage output line Lv with the charger  620 K, the second branch line Lb 2  connecting the first point P 1  on the voltage output line Lv with a second point P 2 , and a third branch line Lb 3  connecting the second point P 2  with each of the chargers  620 Y,  620 M and  620 C respectively. Further, the voltage dropping circuit  20  is arranged on the second branch line Lb. Therefore, according to the printer  10 , the voltage dropping circuit  20  is arranged on an upstream side (i.e., on the voltage outputting circuit  60  side) with respect to the chargers  620 Y,  620 M and  620 C, which are connected in parallel, thereby the circuit configuration being simplified. 
     In the printer  10  according to the embodiment, the voltage dropping circuit  20  includes a resistor  32 , and a switching element  34  connected in parallel with the resistor  32 . The controller  800  causes the switching element  34  to be in the connecting state in the first charging control, while causes the switching element  34  to be in the disconnecting state in the second charging control. Therefore, in the printer  10  according to the embodiment, the controller  800  applies the output voltage CHG to the chargers  620 Y,  620 M and  620 C as well as the charger  620 K in the first charging control. Further, the controller  800  applies the output voltage CHG to the charger  620 K, while applies the dropped output voltage dCHG to the chargers  620 Y,  620 M and  620 C in the second charging control. 
     In the printer  10  according to the embodiment, each charger  620  includes the wire electrode  623  serving as a discharging electrode, and the grid electrode  625  facing the wire electrode  623 . Further, the charging power source  900  includes the divided current detecting circuit  74  and the line current detecting circuit  72 , which serve as a current detecting circuit to detect the grid current Ig flowing through the grid electrode  625 . 
     The controller  800  changes the output voltage CHG of the voltage outputting circuit  60  based on the grid current Ig in the first charging control and the second charging control. Therefore, in the printer  10  according to the embodiment, by adjusting the grid current Ig to be an appropriate value, the discharge current can also be adjusted to be an appropriate value. Accordingly, the charging potential of each photosensitive body  610  can be adjusted to be an appropriate value. 
     Specifically, the controller  800  causes the voltage outputting circuit  60  to change the output voltage CHG based on the least grid current Ig of the grid currents Ig 1 -Ig 4  in the chargers  620 K,  620 Y,  620 M and  620 C in the first charging control. Further, the controller  800  causes the voltage outputting circuit  60  to change the output voltage CHG based on the grid current Ig 1  in the charger  620 K in the second charging control. 
     According to the above configuration, the printer  10  is enabled to adjust the voltage applied to each charger  620  so that each of the photosensitive bodies  610  is charged appropriately in the first charging control, and is also enabled to adjust the voltage applied to the charger  620 K so that the photosensitive body  610 K is charged appropriately. 
     In the printer  10  according to the present embodiment, the controller  800  executes the error notifying process in which a message encouraging the user to clean the wire electrodes  623  of the chargers  610  with the wire cleaners  627  when it is determined, in the second charging control, that the least grid current Ig of the grid currents Ig 1 -Ig 4  in the chargers  620 K,  620 Y,  620 M and  620 C is equal to or less than the second threshold value. Therefore, in the printer  10  according to the present embodiment, even if only the K color is used for image formation, when one of the grid currents Ig in the chargers  620 Y,  620 M and  620 C is the least value, the message for encouraging cleaning of the wire electrodes  623  is displayed. Therefore, the grid current Ig in one of the chargers  620 Y,  620 M and  620 C is very little, and then the absolute value of the charge potential of the photosensitive body  610  of the one of the chargers  620 Y,  620 M and  620 C is very little, thereby the toner is less adhered on the photosensitive body  610  of the one of the chargers  620 Y,  620 M and  620 C, and the image quality may not get poor. 
       FIG. 7  shows a charging power source  900   a  which is a first modification of the above-described embodiment. The charging power source  900   a  according to the first modification has a voltage dropping circuit  20   a  different from the voltage dropping circuit  20  of the charging power source  900  shown in  FIG. 5 . It is noted that the charging power source  900   a  other than the voltage dropping circuit  20   a  has the same configuration as the charging power source  900  shown in  FIG. 5  and the same reference numbers are assigned, but detail description is omitted. Further, in  FIG. 7 , electrical components other than the voltage dropping circuit  20   a  are appropriately omitted. 
     The voltage dropping circuit  20   a  of the charging power source  900   a  according to the first modification shown in  FIG. 7  includes a voltage adjusting circuit  42  configured to adjust the voltage to be applied to the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C in addition to the resistor  32  and the switching element  34 . The voltage adjusting circuit  42  has a transistor Q 11 , resistors R 11 , R 12  and R 13 , and a condenser C 11 . A collector of the transistor Q 11  is connected, via the resistor R 11 , to a fifth point P 5 , which is located on the second point P 2  side with respect to the resistor  32  on the second branch line L 2 . A base of the transistor Q 11  is connected, via the resistor R 12 , to a terminal T 2  of the voltage adjusting circuit  42 . To the terminal T 2 , the PWM signal Sp 2  serving as a voltage controlling signal is supplied from the controller  800 . 
     The controller  800  causes the switching element  34  to be in the connecting state and sets the duty ratio of the PWM signal Sp 2  to be a first value which is a relatively large value, in the first charging control. With this control, the output voltage CHG output by the voltage outputting circuit  60  is applied, as it is, to the wire electrode  623  of each of the chargers  620 Y,  620 M and  620 C. Further, the controller  800  causes the switching element  34  to be in the disconnecting state and sets the duty ratio of the PWM signal Sp 2  to a second value, which is smaller than the first value, in the second charging control. Then, the output voltage CHG output by the voltage outputting circuit  60  is dropped to the dropped output voltage dCHG by the resistor  32  of the voltage dropping circuit  20 . The dropped output voltage dCHG is further dropped to the second dropped output voltage sdCHG by the voltage adjusting circuit  42 , and the second dropped output voltage sdCHG is applied to the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C. It is noted that, by adjusting the duty ratio of the PWM signal Sp 2 , the voltage applied to the wire electrodes  623  of the chargers  620 Y,  620 M and  620 C can be adjusted. 
     As described above, according to the first modification, since the voltage dropping circuit  20   a  of the charging power source  900   a  includes the voltage adjusting circuit  42 , the amplitude of the voltage applied to the chargers  620 Y,  620 M and  620 C can be restricted when only the K color is used for image formation. 
     It is noted that the PWM signal Sp 2  of which duty ratio is set to the first value is an example of a first voltage controlling signal, and the PWM signal Sp 2  of which duty ratio is set to the second value is an example of a second voltage controlling signal. As an alternative configuration, a pulse signal of which value switches between an H (high) level and an L (low) level may be applied to the terminal T 2  of the voltage adjusting circuit  42  instead of the PWM signal Sp 2 . In such an alternative configuration, the pulse signal may be switched to the H level in the first charging control, while switched to the L level in the second charging control. 
       FIG. 8  shows a charging power source  900   b  according to a second modification of the above-described embodiment. In the second modification, a configuration of a voltage dropping circuit  20   b  is different from the voltage dropping circuit  20  shown in  FIG. 5 . The other parts of the charging power source  900   b  are the same as those of the charging power source  900  shown in  FIG. 5 , the same reference numbers are assigned, while the detail description of the charging power source  900   b  is omitted. It is also noted that, in  FIG. 8 , some parts of the charging power source  900   b  other than the voltage dropping circuit  20   b  will be appropriately omitted. 
     The voltage dropping circuit  20   b  of the charging power source  900   b  according to the second modification shown in  FIG. 8  includes a photocoupler  44  and a light emission adjusting circuit  46 . A phototransistor of the photocoupler  44  is arranged on the second branch line Lb 2 , and the light emission adjusting circuit  46  is connected to a light emission diode of the photocoupler  44 . The light emission adjusting circuit  46  includes a reference power supply Vcc, a transistor Q 2 , resistors R 21 , R 22  and R 23  and a capacitor C 21 . A collector of the transistor Q 21  is connected to the reference power supply Vcc via the resistor R 21 . A base of the transistor Q 21  is connected to a terminal T 3  of the light emission adjusting circuit  46  via the resistor R 22 . To a terminal T 3 , the PWM signal Sp 3  serving as the voltage controlling signal is supplied from the controller  800 . 
     The controller  800  sets the duty ratio of the PWM signal Sp 3  to 100% in the first charging control. With this setting, a relatively large amount of voltage is applied to the light emitting diode of the photocoupler  44 , and the output voltage CHG output by the voltage outputting circuit  60  is applied, at it is, to the wire electrode  623  of each of the chargers  620 Y,  620 M and  620 C. In the second charging control, the controller  800  sets the duty ratio of the PWM signal Sp 3  to a value less than 100%. With this setting, the output voltage CHG output by the voltage outputting circuit  60  is dropped to the dropped output voltage dCHG by the photocoupler  44  of the voltage dropping circuit  20   b , and the dropped output voltage dCHG is applied to the wire electrode  623  of each of the chargers  620 Y,  620 M and  620 C. By adjusting the duty ratio of the PWM signal Sp 3 , the voltage applied to the wire electrode  623  of each of the chargers  620 Y,  620 M and  620 C can be adjusted. 
     As described above, according to the second modification, since the voltage dropping circuit  20   b  of the charging power source  900   b  includes the photocoupler  44  and the voltage adjusting circuit  46 , the amplitude of the voltage applied to the chargers  620 Y,  620 M and  620 C can be restricted when only the K color is used for image formation. 
     It is noted that the PWM signal Sp 3  of which duty ratio is set to 100% is an example of a first voltage controlling signal, and the PWM signal Sp 3  of which duty ratio is set to a value less than 100% is an example of a second voltage controlling signal. As an alternative configuration, a pulse signal of which value switches between an H (high) level and an L (low) level may be applied to the terminal T 3  of the voltage adjusting circuit  46  instead of the PWM signal Sp 3 . In such an alternative configuration, the pulse signal may be switched to the H level in the first charging control, while switched to the L level in the second charging control. 
       FIG. 9  shows a block diagram of a printer  10   c  according to a modified embodiment of the disclosures. It is noted that the printer  10   c  shown in  FIG. 9  is different from the printer  10  shown in  FIG. 4  in that the printer  10   c  has a switching mechanism  150 . Since the other parts of the printer  10   c  shown in  FIG. 9  are the same as those of the printer  10  shown in  FIG. 4 , the same reference numerals are assigned, while the detail description of the printer  10   c  is omitted. 
     In the modified embodiment shown in  FIG. 9 , each of the developing units  630  for respective colors is configured to be movable between a contact position and a spaced position as shown in  FIGS. 10A-10C . At the contact position the developing rollers  632  of the developing units  630  contact the corresponding photosensitive bodies  610 . At the space position the developing rollers  632  of the developing units  630  is spaced from the corresponding photosensitive bodies  610 . The switching mechanism  150  is configured to move each of the developing units  630  between the contact position and the spaced position. 
     As shown in  FIGS. 10A-10C , each of the developing units  630  is provided with a protrusion  638 . The switching mechanism  150  has a translation cam  152  which extends in a front-rear direction (i.e., in the X axis direction) across the developing units  630  of the respective colors. The translation cam  152  has a push-up part  154 K, a push-up part  154 Y, a push-up part  154 M and a push-up part  154 C corresponding to the a protrusion  638 K, a protrusion  638 Y, a protrusion  638 M and a protrusion  638 C provided to the developing units  630  for respective colors. 
     As shown in  FIG. 10A , when none of the push-up parts  154  of the translation cam  152  engages with the protrusion  638  of the corresponding developing unit  630 , the developing units  630  corresponding to all the colors are located at the contact positions. As shown in  FIG. 10B , when the push-up parts  154  corresponding to the Y, M and C colors engage with the protrusions of the corresponding developing unit  630  while the push-up part  154 K does not engage with the protrusion  638 K of the developing unit  630 K, the developing unit  630 K is located at the contact position while the developing units  630 Y,  630 M and  630 C are located at the spaced positions. Further, as shown in  FIG. 10C , when the push-up parts  154  corresponding to all the colors could engage with the protrusions of the corresponding developing unit  630 , all the developing units  630  corresponding to all the colors are located at the spaced positions. 
     When the color printing is executed (i.e., all the four colors of K, Y, M and C colors are used for image formation), the controller  800  controls the switching mechanism  150  so that all the developing units  630  are located at the contact positions as shown in  FIG. 10A . With this control, the toner can be supplied from all the developing units  630  to the respective photosensitive bodies  610 . When the monochrome printing is executed (i.e., only the K color is used for image formation), the controller  800  controls the switching mechanism  150  so that only the developing unit  630 K is located at the contact position, while the developing units  630 Y,  630 M and  630 C are located at the spaced positions as shown in  FIG. 10B . With this control, it is less likely that the toner is adhered on the photosensitive bodies  610 Y,  610 M and  610 C inadvertently. Further, when the printer  10   c  is in a third mode in which none of the color printing and monochrome printing is executed, the controller  800  controls the switching mechanism  150  so that all the developing units  630  are located at the spaced positions as shown in  FIG. 10C . 
     It is noted that, the modified embodiment shown in  FIGS. 9 and 10A-10C  is configured such that the controller  800  controls the switching mechanism  150  to switch the states of the switching element  34  into the connecting state or the disconnecting states. As shown in  FIGS. 11-13 , the translation cam  152  of the switching mechanism  150  is formed with a groove  153 . The groove  153  includes two parallel parts  142  and  146  which extends in parallel with a moving direction of the translation cam  152  (i.e., in the X axis direction), and a curved part  144 , which is arranged between the two parallel parts  142  and  146  and curves upward to approach the switching element  34 . On the translation cam  152  side with respect to the switching element  34 , an interfering member  158  is arranged. The interfering member  158  has a connection part  157 , which engages with the groove  153  of the translation cam  152  (see  FIGS. 11-13 ). 
     When the translation cam  152  is located at a position shown in  FIG. 11 , the connection part  157  of the interfering member  158  is located in the parallel part  146 . In this state, an end part  159  on the switching element  34  side of the interfering member  158  does not interfere with the switching element  34 . Therefore, the switching element  34  is in the connecting state. As the translation cam  152  moves rightward in  FIG. 11  (i.e., a negative direction along the X axis) and located at a position shown in  FIG. 12 , the connecting part  157  of the interfering member  158  is located in the curved part  144 . In this state, the interfering member  158  moves toward the switching element  34 , and the end part  159  of the interfering member  159  interferes with the switching element  34 , thereby the switching element  34  being in the disconnecting state ( FIG. 12 ). As the translation cam  152  is further moved rightward in  FIG. 11  (i.e., in the negative direction along the X axis) and located at a position shown in  FIG. 13 , the connecting part  157  is located in the parallel part  142 . In this state, the end part  159  of the interfering member  158  does not interfere with the switching element  34 , and the switching element  34  is in the connecting state. 
     As described above, according to the modified embodiment shown in  FIGS. 9-13 , the connecting/disconnecting states of the switching element  34  can be switched with use of the switching mechanism which switches the positions of the developing units  630 . According to this configuration, the number of parts can be reduced and downsizing of the apparatus can be achieved. Further, it is ensured that switching of positions of the developing units  630  and switching of the connecting/disconnecting states of the switching element  34  can be carried out in an associated manner. 
     The technique disclosed in this specification should not be limited to the configurations described above. Rather, the configurations could be modified in various ways without departing from the gist of the disclosures. Some examples of such variations will be described below. 
     The configuration of the printer  10  according to the above-described embodiment is only an example, and could be modified in various ways. For example, the printer  10  according to the embodiment is configured to form an image using toner of K, Y, M and C colors. However, the number and/or colors to be used may not be limited to the configuration above. 
     Further, the image forming apparatus may not be limited to a stand-alone printer, but could be apparatuses such as a copier, a facsimile machine and a multi-function peripheral which includes a printer function. 
     The image forming apparatus may not be limited to a configuration of forming an image using toner having a positive polarity, but could be a configuration using toner having a negative polarity. In the latter case, the polarity of each voltage is opposite to what is described in the above-embodiments. 
     In the above-described embodiment, the chargers  620  are of the scorotron type having the grid electrodes  625  as examples. It is noted that the type of the chargers may not be limited to the scorotron type, but could be of corotron type which does not include a grid electrode. Alternately, the chargers could be of a roller type or a brush type, which is configured to charge the photosensitive bodies  610  by contacting the photosensitive bodies and applying voltages thereto. 
     In the above-described embodiment, the grid voltage GRID is adjusted with the grid voltage applying circuit  71 . However, such a circuit for adjusting the grid voltage GRID could be omitted. Further, circuits for detecting the grid currents Ig could also be omitted. 
     In the above-described embodiment, the processes executed by the controller  800  may be modified to be executed by one or more CPU&#39;s and/or one or more ASIC&#39;s  805 . In such a case, the execution subject of such processes is an example of a controller. It is noted that the controller  800  is a collective name including hardware used to control the printer  10  (e.g., the CPU  801 ) and does not necessarily mean a single piece of hardware of the printer  10 . 
     In the charge controlling process shown in  FIG. 6 , some of the process (steps) may be modified, omitted and/or exchanged. For example, in the charge controlling process described above, the controller executes the error notifying process (S 270 ) when it is determined that the least grid current Ig among the grid currents Ig 1 -Ig 4  is equal to or less than the second threshold value (S 260 : YES). However, in such a case, instead of executing the error notifying process, the least grid current Ig may be raised to be greater than the second threshold value by increasing the duty ratio of the PWM signal Sp 1  supplied to the voltage outputting circuit  60  to increase the output voltage CHG. According to such a control, even when the second charging control is executed, the grid currents Ig in the chargers  620 Y,  620 M and  620 C may be greater than the second threshold value. Thus, according to such a control, the absolute value of the charged potential of one of the photosensitive bodies  610 Y,  610 M and  610 C may not be excessively small, thereby the toner may not adhere to the photosensitive body  610  and the image quality may not get poor. 
     When the printer  10  has the voltage dropping circuit  20  as shown in  FIG. 7  or  FIG. 8 , modifications as follows may be made. In the charge controlling process described above, the controller executes the error notifying process (S 270 ) when it is determined that the least grid current Ig among the grid currents Ig 1 -Ig 4  is equal to or less than the second threshold value (S 260 : YES). However, in such a case, instead of executing the error notifying process, the duty ratio of the PWM signal Sp 2  or the PWM signal Sp 3  supplied to the voltage adjusting circuit  42  or the light emission adjusting circuit  46  to increase the dropped output voltage dCHG to be applied to the wire electrodes  623 Y,  623 M and  623 C, thereby the least grid current Ig is raised to be greater than the second threshold value. According to such a control, even when the second charging control is executed, the grid currents Ig in the chargers  620 Y,  620 M and  620 C may be greater than the second threshold value. Thus, according to such a control, the absolute value of the charged potential of one of the photosensitive bodies  610 Y,  610 M and  610 C may not be excessively small, thereby the toner may not adhere to the photosensitive body  610  and the image quality may not get poor. 
     In the charge controlling process according to the embodiment, control of the voltage outputting circuit  60  is executed based on the gird current Ig. As a modification, control of the voltage outputting circuit  60  is executed based on another index value such as the grid voltage GRID instead of the grid current Ig. 
     Further, in the above-described embodiment, a message encouraging the user to clean the wire electrode  623  of the charger  620  with the wire cleaner  627  is displayed on the display  820  in the error notifying process S 270 . In a modification, instead of, or in addition to displaying a message, another notifying methods such as sound and/or illumination may be used to notify information regarding contamination of the wire electrodes  623 .