Abstract:
In an image-forming device, each of the N chargers charges the opposed photoconductor with a discharge. The voltage applying unit applies voltages to the N chargers, individually. The abnormal discharge detecting unit detects an occurrence of an abnormal discharge at least one of the N chargers. The voltage detecting unit detects first voltages applied to the N chargers before the occurrence of the abnormal discharge is detected, and second voltages applied to the N chargers after the occurrence of the abnormal discharge is detected. The calculating unit calculates a difference between the first voltage and the second voltage for each of the N chargers. The identifying unit identifies one charger as a charger at which the abnormal discharge is occurring. The difference between the first voltage and the second voltage applied to the one charger is the greatest among the differences.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2010-195028 filed Aug. 31, 2010. The entire content of this application is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an electrophotographic image-forming device including a plurality of chargers that charges a plurality of photoconductors, respectively. 
     BACKGROUND 
     An electrophotographic image-forming device capable of performing a multicolor printing includes a plurality of photoconductors and a plurality of chargers opposed to the plurality of photoconductors, respectively. When voltages are applied to the plurality of chargers, corona discharges occur at the plurality of chargers to charge the plurality of photoconductors. When a wire of the charger becomes contaminated with airborne (accumulated) dust particles or other contaminants around the charger, an abnormal discharge. Therefore, an image-forming device that, when detecting the occurrence of the abnormal discharge, acquires the voltages applied to the plurality of chargers, sequentially changes the voltages applied to the plurality of chargers, and identifies, based on the voltage changing result, the charger at which the abnormal discharge is occurring, is proposed. 
     SUMMARY 
     However, since the above image-forming device sequentially changes the voltages applied to the plurality of chargers, much time is taken for identifying the charger at which the abnormal discharge is occurring. 
     In view of the foregoing, it is an object of the invention to provide an image-forming device capable of identifying the charger at which the abnormal discharge is occurring immediately after detecting the occurrence of the abnormal discharge, without providing an abnormal discharge detecting unit for each charger. 
     In order to attain the above and other objects, the invention provides an image-forming device including N photoconductors, N chargers, a voltage applying unit, an abnormal discharge detecting unit, a voltage detecting unit, a calculating unit, and an identifying unit. The N is equal to or greater than 2. The N chargers are opposed to the N photoconductors, respectively. Each of the N chargers charges the opposed photoconductor with a discharge. The voltage applying unit applies voltages to the N chargers, individually. The abnormal discharge detecting unit detects an occurrence of an abnormal discharge at least one of the N chargers. The voltage detecting unit detects first voltages applied to the N chargers before the occurrence of the abnormal discharge is detected, and second voltages applied to the N chargers after the occurrence of the abnormal discharge is detected. The calculating unit calculates a difference between the first voltage and the second voltage for each of the N chargers. The identifying unit identifies one charger as a charger at which the abnormal discharge is occurring. The difference between the first voltage and the second voltage applied to the one charger is the greatest among the differences. 
     Another aspect of the present invention provides an image-forming device including N photoconductors, N chargers, a voltage applying unit, an abnormal discharge detecting unit, a voltage detecting unit, a calculating unit, an identifying unit. The N is equal to or greater than 3. The N chargers are opposed to the N photoconductors, respectively. Each of the N chargers charges the opposed photoconductor with a discharge. The voltage applying unit applies voltages to the N chargers, individually. The abnormal discharge detecting unit detects an occurrence of an abnormal discharge at least one of the N chargers. The voltage detecting unit detects voltages applied to the N chargers at an interval. The calculating unit that calculates a difference between a voltage applied to one charger and a voltage applied to another charger that are detected immediately before or after the occurrence of the abnormal discharge is detected. The identifying unit identifies two chargers as a charger at which the abnormal discharge is occurring. The difference between the voltages applied to the two chargers is the greatest among the differences. 
     Another aspect of the present invention provides an image-forming device including N photoconductors, N chargers, a voltage applying unit, an abnormal discharge detecting unit, an identifying unit. The N is equal to or greater than 2. The N chargers are opposed to the N photoconductors, respectively. Each of the N chargers charges the opposed photoconductor with a discharge. The voltage applying unit applies voltages to the N chargers, individually. The abnormal discharge detecting unit detects an occurrence of an abnormal discharge at least one of the N chargers. The voltage detecting unit detects voltages applied to the N chargers at an interval. The identifying unit identifies at least one charger as a charger at which the abnormal discharge is occurring. The voltage applied to the at least one charger is outside a range into which the voltage detected when the discharge is occurring falls. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a block diagram showing an electrical configuration of an image-forming device according to a preferred embodiment of the present invention; 
         FIG. 2  is a block diagram showing a configuration of a high-voltage power supply unit according to the preferred embodiment; 
         FIG. 3  is a flowchart showing a main routine of a CPU according to the preferred embodiment; 
         FIG. 4  is an explanation diagram showing a principle of an identification process according to the preferred embodiment; 
         FIG. 5  is a flowchart showing the identification process according to the preferred embodiment; 
         FIG. 6  is an explanation diagram showing a principle of an identification process according to a first variation; 
         FIG. 7  is a flowchart showing the identification process according to the first variation; 
         FIG. 8  is an explanation diagram showing a principle of an identification process according to a second variation; and 
         FIG. 9  is a flowchart showing the discharge-channel identification process according to the second variation. 
     
    
    
     DETAILED DESCRIPTION 
     [Entire Configuration of Image Forming Device] 
     As shown in  FIG. 1 , an image forming device  1  according to the present embodiment has four photoconductors  62 Y,  62 M,  62 C, and  62 K corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. 
     The image forming device  1  is so-called a color laser printer that conveys a recording medium such as a paper by means of a belt (not illustrated) to form a color image on the recording medium that sequentially passes through the opposing parts between the belt and the photoconductors  62 Y,  62 M,  62 C, and  62 K. A CPU  91  for controlling components in the image forming device  1  is connected to a ROM  92  and a RAM  93  to constitute a microcomputer. The CPU  91  is also connected to a display unit  100  provided on the front surface of the casing of the image forming device  1 . In the following explanation, the signs Y, M, C, and K representing the four colors are omitted except when necessary. 
     A charger  117  is a scorotron type charger having a wire at which a corona discharge occurs. The charger  117  is disposed opposite to the photoconductor  62  to uniformly charge the surface of the photoconductor  62  before an electrostatic latent image is formed on the photoconductor  62  by exposure. When a high voltage (e.g., 6000 V to 8000 V) is applied to the charger  117  by a high-voltage power supply unit  110 , the corona discharge occurs at the charger  117 . With the corona discharge, multiple ions are discharged to the photoconductor  62  from the charger  117  to the photoconductor  62  to charge the photoconductor  62 . 
     A GRID portion  118  is also disposed between the photoconductor  62  and the charger  117  to detect the amount of the corona discharge. The multiple ions are also discharged to the GRID portion  118 . Due to the multiple ions, a current flows into the GRID portion  118 . For example, when the corona discharge normally occurs at the charger  117 , a current of 275 μA flows into the GRID portion  118 . 
     The GRID portions  118 Y,  118 M,  118 C, and  118 K output the current generated due to the multiple ions toward connection points P 2 Y, P 2 M, P 2 C, and P 2 K. To each of the connection points P 2 , a resistor R 5  and a capacitor  123  are connected in parallel. 
     The capacitor  123  cuts a DC component of the current (voltage) at the connection point P 2 . Therefore, only a sharply increasing (an AC component) current, which is generated when an abnormal discharge, such as, an arc discharge occurs at the charger  117 , flows toward a discharge detection circuit  130  through a common connection point P 1 . The single discharge detection circuit  130  is connected in common to the chargers  117 Y,  117 M,  117 C, and  117 K. The discharge detection circuit  130  detects, based on the current, that the abnormal discharge occurs at any one of the chargers  117 Y,  117 M,  117 C, and  117 K. 
     The discharge detection circuit  130  includes resistors  131  and  134 , a capacitor  132 , and a transistor  133 . The resistor  131 , the capacitor  132 , and the transistor  133  are connected to the connection point P 1  in parallel. The resistor  131  adjusts the voltage to be applied to the connection point P 1 . The capacitor  132  decreases a peak value of the voltage to be applied to the connection point P 1 . In other words, the capacitor  132  absorbs the influence of the noise. Therefore, the voltage from which the influence of the noise has been absorbed is applied to the transistor  133 . 
     An emitter of the transistor  133  is connected to the ground, a collector of the transistor  133  is connected to a power supply (3.3V in the preferred embodiment) through the resistor  134 , and a base of the transistor  133  is connected to the connection point P 1 . The resistor  134  is a pull-up resistor. A connection point P 4  provided between the transistor  133  and the resistor  134  is further connected to a discharge detection signal input port  91   a  provided in the CPU  91 . 
     The CPU  91  determines, based on the voltage inputted into the discharge detection signal input port  91   a , whether or not the abnormal discharge is occurring. Specifically, when a voltage smaller than an on-voltage of the transistor  133  is applied to the base of the transistor  133 , the transistor  133  is turned OFF. When the transistor  133  is turned OFF, the voltage of the connection point P 4  becomes approximately 3.3 V. When the 3.3V (a high signal, hereinafter, referred to as “H”) is inputted into the discharge detection signal input port  91   a , the CPU  91  determines that the abnormal discharge is not occurring at any one of the chargers  117 Y,  117 M,  117 C, and  117 K. 
     On the other hand, a voltage equal to or greater than the on-voltage of the transistor  133  is applied to the base of the transistor  133 , the transistor  133  is turned ON. When the transistor  133  is turned ON, a current flows between the collector and emitter of the transistor  133 , thereby the voltage of the connection point P 4  becomes 0V. When the 0V (a low signal, hereinafter, referred to as “L”) is inputted into the discharge detection signal input port  91   a , the CPU  91  determines that an abnormal discharge is occurring at any one of the chargers  117 Y,  117 M,  117 C, and  117 K. Hereinafter, this process is referred to as an abnormal-discharge detecting process. 
     The terminal of the resistor R 5  on the opposite side to the connection point P 2  is connected to a resistor R 6 . A connection point P 3  provided between the resistors R 5  and R 6  is connected to A/D port  97  ( 97   a ,  97   b ,  97   c , and  97   d ) of the CPU  91 . The terminal of the resistor R 6  on the opposite side to the connection point P 3  is connected to the ground. Hereinafter, when it is not necessary to distinguish the first to fourth A/D ports  97   a ,  97   b ,  97   c , and  97   d  of the CPU  91  from each other, they are collectively referred to as “A/D port  97 .” 
     As shown in  FIG. 2 , the CPU  91  outputs, from a control information output port  98  ( 98   a ,  98   b ,  98   c , and  98   d ), a PWM control signal corresponding to the voltage inputted into the A/D port  97 . Specifically, the CPU  91  outputs the PWM control signal such that the voltage of the GRID portion  118  becomes constant. When the voltage of the GRID portion  118  becomes constant, the charge voltage of the photoconductor  62  becomes constant. Hereinafter, this process is referred to as a high-voltage application process. 
     For example, when the amount of the current flowing into the GRID portion  118  is small, that is, the voltage of the GRID portion  118  is low, it is considered that the voltage applied to the photoconductor  62  is low. Therefore, in such case, the CPU  91  increases the duty value of the PWM control signal to increase the voltage applied to the charger  117  from the high-voltage power supply unit  110 . On the other hand, when the amount of the current flowing into the GRID portion  118  is large, that is, the voltage of the GRID portion  118  is high, it is considered that the voltage applied to the photoconductor  62  is high. Therefore, in such case, the CPU  91  decreases the duty value of the PWM control signal to decrease the voltage applied to the charger  117  from the high-voltage power supply unit  110 . 
     In theory, the voltage applied to the charger  117  from the high-voltage power supply unit  110  is proportional to the duty value of the PWM control signal. Accordingly, by calculating the duty value of the PWM control signal, the voltage applied to the charger  117  from the high-voltage power supply unit  110  can be detected. 
     Next, the high-voltage power supply unit  110  is explained with reference to  FIG. 2 . The high-voltage power supply units  110 Y,  110 M,  110 C, and  110 K are provided to correspond to the chargers  117 Y,  117 M,  117 C, and  117 K. Since the high-voltage power supply units  110 Y,  110 M,  110 C, and  110 K have the same configuration, only one high-voltage power supply unit  110  is illustrated in  FIG. 2 . 
     The control information output port  98  is connected to a base of a transistor TR 1  of the high-voltage power supply unit  110  through a resistor R 1 . A connection point P 5  between the resistor R 1  and the transistor TR 1  is also connected to the ground through a capacitor C 1 . The resistor R 1  adjusts the voltage to be applied to the connection point P 5  from the control information output port  98 . The capacitor C 1  smoothes the voltage applied to the base of the transistor TR 1 . 
     A collector of the transistor TR 1  is connected to a power supply (3.3V in the preferred embodiment) through a resistor R 2 , and an emitter is connected to a resistor R 3 . A connection point P 6  provided between the transistor TR 1  and the resistor R 3  is also connected to the ground through a capacitor C 2 . The terminal of the resistor R 3  on the opposite side to the connection point P 6  is connected to a base of a transistor TR 2  through a coil L 1 . 
     When no voltage is applied to the base of the transistor TR 1 , the transistor TR 1  is turned OFF. When the transistor TR 1  is turned OFF, no voltage is applied to the base of the transistor TR 2 . Therefore, when no voltage is applied to the base of the transistor TR 1 , no current flows between the collector and emitter of the transistor TR 2 . 
     On the other hand, when a voltage is applied to the base of the transistor TR 1 , the transistor TR 1  is turned ON. When the transistor TR 1  is turned ON, a voltage is applied to the base of the transistor TR 2 . Therefore, when a voltage is applied to the base of the transistor TR 1 , a current flows between the collector and emitter of the transistor TR 2 . Note that the voltage output from the transistor TR 1  is smoothed by the capacitor C 2  and the resistor R 3 . 
     The collector of the transistor TR 2  is connected to a primary coil L 2  of a transformer T. When a current flows between the collector and emitter of the transistor TR 2 , the transformer T increases a voltage (e.g., 24 V) applied to the primary coil L 2  from the power supply to, e.g., 6000 V to 8000 V in cooperation with a secondary coil L 3 . Thus, the transformer T outputs high-voltage AC power according to the switching operation of the transistor TR 2 . 
     The secondary coil L 3  of the transformer T is connected to the charger  117  through a diode D 1  and a resistor R 4 . An AC power outputted from the secondary coil L 3  is rectified in the diode D 1 , then converted into a DC current by a capacitor C 3 , and subsequently supplied to the charger  117 . The resistor R 4  is a short-circuit protection resistor. 
     [Control Performed by CPU] 
     Next, the abnormal-discharge detecting process performed by the CPU  91  will be explained with reference to  FIG. 3 . The abnormal-discharge detecting process is started when a high-voltage application command is issued to the charger  117  when, for example, a warm-up or image formation process in the image forming device  1  is started. 
     As shown in  FIG. 3 , in S 1  (hereinafter, S represents “Step”), the CPU  91  starts the abovementioned high-voltage application process for the charger  117  in another routine. Subsequently, in S 2 , the CPU  91  acquires the PWM control signal (hereinafter referred also to as output level) outputted from each control information output port  98  or the voltage (hereinafter referred to as FB level) inputted into each A/D port  97 , and stores the acquired output level or FB level as a present value A(x) in the RAM  93 . The sign x is a channel number and takes values of 0, 1, 2, and 3 in correspondence with C, M, K, and Y, respectively. As described above, since there is a proportional relationship between the output level and the FB level, it makes no difference if the output level or the FB level is used in S 2 . 
     Subsequently, in S 3 , the CPU  91  determines whether or not the abnormal discharge is occurring, based on the discharge detection signal inputted into the discharge detection signal input port  91   a . When the discharge detection signal is H, it is considered that the abnormal discharge is not occurring. Therefore, when the discharge detection signal is H (S 3 : N), in S 4 , the CPU  91  updates a previous value B(x) to the present value A(x) (the output level or the FB level stored in S 2 ), and stores the updated previous value B(x) in RAM  93 . Then, the processing flow shifts to the abovementioned S 2 . By repeating a loop from S 2  to S 4 , the present value A(x) and the previous value B(x) stored in the RAM  93  for each channel number x are repeatedly updated. 
     On the other hand, when the discharge detection signal is L, it is considered that the abnormal discharge is occurring. Therefore, when the discharge detection signal is L (S 3 : Y), in S 5 , the CPU  91  identifies the abnormal channel (color) in which the abnormal discharge is occurring, based on both the present value A(x) and the previous value B(x) stored in the RAM  93  (identification process). The details of the identification process will be described later. Subsequently, in S 6 , the CPU  91  outputs the PWM control signal for stopping the application of the high-voltage to all the chargers  117 . The order of S 6  and S 5  may be interchanged. Subsequently, in S 7 , the CPU  91  displays the abnormal channel determined in S 5  on the display unit  100  and then ends the process. 
     [Identification Process of Discharge Channel] 
     Next, the principle of the identification process will be explained with reference to  FIG. 4 . In the preferred embodiment, as shown in  FIG. 4 , the output level is stored in the RAM  93  at regular time intervals. The FB level may be stored in the RAM  93  in place of the output level. Here, the output level significantly changes immediately before the abnormal discharge has occurred at the charger  117 . In the example of  FIG. 4 , a larger difference is observed between the previous value B(x) and the present value A(x) in channel  0  (cyan) than is observed in other channels, which indicates that the abnormal discharge has occurred in channel  0 . 
     Next, the identification process performed at S 5  in  FIG. 3  will be explained with reference to  FIG. 5 . Firstly, the channel number x is set to 0 in S 51 . In S 52 , it is determined whether or not the channel number x is less than 4. In the first time of S 52 , an affirmative determination is made (S 52 : Y) since the channel x has been set to 0 in S 51 , and the processing flow shifts to S 53 . In S 53 , a difference Y(x) (absolute value) between the previous value B(x) and present value A(x) is calculated. Subsequently, in S 54 , the channel number x is incremented by 1, and the processing flow shifts to the abovementioned S 52 . When the differences Y(x) of all of the channel numbers x (=0 to 3) are thus calculated (S 53 ), a negative determination is made in S 52  (S 52 : N), and the processing flow shifts to S 55 . 
     In S 55 , the channel number x is set to 0, a variation Zmax is set to a maximum value (an assumable maximum value of the difference Y(x)), and a variable ch is set to 0. In S 56 , it is determined whether or not the channel number x is less than 4. In the first time of S 56 , an affirmative determination is made (S 56 : Y) since the channel x has been set to 0 in S 55 , and the processing flow shifts to S 57 . In S 57 , it is determined whether or not the difference Y(x) (initially, Y(0)) is less than the variation Zmax. In the first time of S 57 , an affirmative determination is made (S 57 : Y) since the variation Zmax is initially set to the level maximum value in S 55 , and the processing flow shifts to S 58 . In S 58 , the variation Zmax is updated to the difference Y(x), and the variable ch is updated to the present channel number x. In S 59 , the channel number x is incremented by 1, and the processing shifts to the abovementioned S 56 . On the other hand, when it is determined in S 57  that the difference Y(x) is not less than the variation Zmax (S 57 : N), the processing flow shifts to S 59 . When the processing of S 56  to S 59  for all the channel numbers x (=0 to 3) is terminated (S 56 : N), the channel number x corresponding to the largest difference Y(x) has been stored as the variable ch. 
     When a negative determination is made in S 56  (S 56 : N), in S 60 , a channel corresponding to the channel number x stored as the variable ch is identified as the abnormal discharge channel, and the processing flow shifts to the abovementioned S 6  of  FIG. 3 . Then, in S 7  of  FIG. 3 , the charger  117  corresponding to the color of the channel that has been determined in S 60  of  FIG. 5  as the abnormal discharge channel is displayed on the display unit  100 . 
     As described above, in the preferred embodiment, even though the single discharge detection circuit  130  is provided in common for the respective colors in order to reduce manufacturing cost, the charger  117  at which the abnormal discharge is occurring can be quickly identified after detecting the occurrence of the abnormal discharge. 
     In addition, in the preferred embodiment, a channel having the largest difference Y(x) between the previous value B(x) and the present value A(x) is identified as the abnormal discharge channel. Therefore, it is not required to previously set a threshold value for determining the abnormal discharge channel. Thus, even when the output level of the normal discharge changes with age, the charger  117  at which the abnormal discharge is occurring can be precisely identified. 
     Further, in the preferred embodiment, the charger  117  in which the abnormal discharge is occurring is displayed on the display unit  100 . Therefore, a user has only to clean just the charger  117  displayed on the display unit  100 . 
     [Variations of Present Invention] 
     While the invention has been described in detail with reference to the embodiment thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
     For example, in  FIG. 3 , the present value A(x) may be acquired (S 2 ) immediately after the detection of the abnormal discharge (S 3 : Y). Further, in S 57  of  FIG. 5 , a configuration may be adopted in which the difference Y(x) is compared to a predetermined threshold, and all the channels having the difference Y(x) exceeding the threshold are identified as the discharge channels. In this case, it becomes possible to identify the chargers  117  at which the abnormal discharge is occurring even if the abnormal discharge is occurring in a plurality of the chargers  117  at the same time. 
     [Another Identification Processing of Discharge Channel ( 1 )] 
     As the processing of S 53  and S 57  of  FIG. 5 , various approaches may be adopted instead of using the difference Y(x). For example, when the abnormal discharge occurs at a charger  117 , the output level or the FB level acquired just around the time of the occurrence of the abnormal discharge falls outside predetermined thresholds (between upper and lower limit values) defining a range into which the output level or the FB level acquired in the operation time where the abnormal discharge is not occurring falls. 
     In the example of  FIG. 6 , the present value A(x) of the FB level of the channel  0  (cyan) falls below the threshold (lower limit value), while the present values A(x) of the FB level of other channels fall within the thresholds, which indicates that the abnormal discharge is occurring at the channel  0 . 
     Thus, in S 5 , a channel in which the abnormal discharge is occurring may be identified as follows.  FIG. 7  is a flowchart showing an identification process according to a first variation. First, in S 151 , the channel number x is set to 0, discharge channel flags [x] given for respective channels are all set to 0. Subsequently, in S 152 , it is determined whether or not the channel number x is less than 4. In the first time of S 152 , an affirmative determination is made as in the case of S 52  (S 152 : Y), and the processing flow shifts to S 153 . In S 153 , it is determined whether or not the present value A(x) falls above the upper limit value of the threshold or falls below the lower limit value thereof. In either case, that is, when the present value A(x) falls outside the range of the thresholds (S 153 : Y), in S 154  the discharge channel flag [x] of the present channel number x is set to 1, and the processing flow shifts to S 155 . On the other hand, when the present value A(x) falls within the range of the thresholds (S 153 :N), the processing flow shifts to S 155 . In S 155 , the channel number x is incremented by 1, and the processing flow shifts to the abovementioned S 152 . When the processing of S 153  and S 154  has been executed for all of the channel numbers x (=0 to 3) (S 152 : N), and the processing flow shifts to S 156 . 
     In S 156 , the channel number x is set to 0. Subsequently, in S 157 , it is determined whether or not the channel number x is less than 4. In the first time of S 157 , an affirmative determination is made as in the case of S 152  (S 157 : Y), and the processing flow shifts to S 158 . In S 158 , it is determined whether or not the discharge channel flag [x] of the present channel number x is set to 1. When the discharge channel flag [x] of the present channel number x is set to 1 (S 158 : Y), the channel x whose discharge channel flag [x] is set to 1 is identified as the channel in which the abnormal discharge is occurring in S 159 , and the processing flow shifts to S 160 . That is, it is determined that the abnormal discharge is occurring in the channel in which the present value A(x) falls outside the range of the thresholds. On the other hand, when the discharge channel flag [x] is not set to 1 (S 158 : N), the processing shifts to S 160 . In S 160 , the channel number x is incremented by 1, and the processing shifts to the abovementioned S 157 . 
     The processing of S 157  to S 160  is executed for all of the channel numbers x (=0 to 3) (S 157 : N), and the processing flow shifts to the abovementioned S 6  ( FIG. 3 ). Then, in S 7 , the charger  117  corresponding to the colors of the channels in which the abnormal discharge has been determined to occur in S 159  of  FIG. 7  are displayed on the display unit  100 . 
     In the preferred embodiment, the discharge channel can be identified only by the present value A(x), so that the processing can be simplified and, specifically, the processing of S 4  in the main routine illustrated in  FIG. 3  can be omitted. Further, in the present embodiment, it becomes possible to detect the abnormal discharge occurring in a plurality of the chargers  117  at the same time. 
     [Another Identification Processing of Discharge Channel ( 2 )] 
     When the abnormal discharge occurs in a given charger  117  (S 3 : Y), the output level or the FB level acquired just around the time of the occurrence of the abnormal discharge significantly differs from the output level or the FB level of the other chargers  117 . In the example of  FIG. 8 , the output level of the channel  1  (magenta (M)) significantly changes at the time of occurrence of the discharge, that is, significantly becomes different from those of the other channels (yellow (Y), cyan (C), and black (K)). Therefore, at the time point when the abnormal discharge has occurred in any one of the chargers  117  (S 3 : Y), the output level of each channel as illustrated in  FIG. 8  is displayed on the display unit  100 . Then, a user may identify the channel  1  (magenta) having relatively the most different value as the other discharge channels based on the displayed data. 
     Further, the CPU  91  may identify the channel having relatively the most different value from one another among the present values A(x) of magenta (M), yellow (Y), cyan (C), and black (K) stored in the RAM  93 . However, it may be difficult for the CPU  91  to make determination based on the present value A(x) of the output level stored in the RAM  93 . 
     In such case, in S 5 , a channel in which the abnormal discharge is occurring may be identified as follows.  FIG. 9  is a flowchart showing an identification process according to a second variation. First, in S 251 , the channel numbers i and j are set to 0, a variable “level Max” is set to a maximum value (an assumable value of the present values A[i] and A[j]), and variables “discharge channels  1  and  2 ” are set to 0. Subsequently, in S 252 , it is determined whether or not the channel number i is less than 4. In the first time of S 252 , an affirmative determination is made as in the case of S 52  (S 252 : Y), and the processing flow shifts to S 253 . In S 253 , it is determined whether or not the channel number j is less than 4. Also in the first time of S 253 , an affirmative determination is made (S 253 : Y), and the processing flow shifts to S 254 . 
     In S 254 , a value obtained by subtracting the present value A[j] from the present value A[i] is calculated, and it is determined whether or not the calculated value is less than the level Max. In the first time of S 254 , an affirmative determination is made in S 254  (S 254 : Y) since the level Max is initially set to the maximum value in S 251 , and the processing flow shifts to S 255 . In S 255 , the value obtained by subtracting the present value A[j] from the present value A[i] is set to the level Max, the discharge channel  1  is set to i, and the discharge channel  2  is set to j. In S 256 , the channel number j is incremented by 1, and the processing flow shifts to the abovementioned S 253 . When the processing of S 254  and S 255  is thus executed for all of the channel numbers j (=0 to 3) (S 253 : N), and the processing flow shifts to S 257 . 
     In S 257 , the channel number i is incremented by 1, the channel number j is set to 0, and the processing flow shifts to the abovementioned S 253 . When the processing of S 253  to S 257  is executed for all of the channel numbers i (=0 to 3) (S 252 : N), the value obtained by subtracting the present value A[j] from the present value A[i] has been calculated in S 254  for all combinations of i and j (in which the order matters). Then, a combination of i and j having the largest value, that is, the largest difference between the present value A[i] and present value A[j] is stored as the discharge channels  1  and  2 . 
     When a negative determination is made in S 252  (S 252 : N), the processing flow shifts to S 258 , where channels corresponding to the channel numbers i and j stored as the discharge channels  1  and  2  are identified as the discharge channels, and the processing flow shifts to the abovementioned S 6  ( FIG. 3 ). Then, in S 7 , chargers  117  corresponding to any of the channel colors that have been determined as the discharge channels in which the abnormal discharge is occurring are displayed on the display unit  100 . 
     The two channels stored as the discharge channels  1  and  2  in S 257  may be subjected to the processing using the flowchart of  FIG. 5 . However, the processing according to the flowchart of  FIG. 5  is performed for four channels. Therefore, after the number of the target channels are narrowed down to 2 by the execution of the processing according to the flowchart of  FIG. 9 , the processing according to the flowchart of  FIG. 7  may be performed, in order to identify the discharge channel more precisely. 
     Also in the second variation, the discharge channel can be identified only by the present value A(x), so that the processing can be simplified and, specifically, the processing of S 4  in the main routine illustrated in  FIG. 3  can be omitted. Although an image forming device of four colors is taken as the above-mentioned example, the present invention may be applied to an image forming device of two or three colors (in the case of the third embodiment, three or more colors). 
     Although the application voltage of the high-voltage power supply unit  110  is detected by the duty value of the PWM control signal, the application voltage may be detected using an analog signal.