Patent Publication Number: US-8983314-B2

Title: Image forming apparatus capable of detecting contact fusion, and relay control apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus such as a copier or a printer, which forms images on recording materials using an electrophotographic process, and a relay control apparatus, and in particular to a power-feed path of a fixing apparatus that thermally fixes unfixed toner formed and carried on recoding materials. 
     2. Description of the Related Art 
     Conventionally, for electrophotographic image forming apparatuses, methods that heat and fix a toner image formed on a recording sheet (heat fixing methods) have been commonly adopted, and in particular, a method that brings a toner image into direct contact with a rotary member having a heat source therein and fixes the toner image has been in widespread use. As the heat source, a halogen heater, a ceramic heater, an IH heating, and so on are known, but all of them require so large amount of power as hundreds of watts. 
     Moreover, with an increase in demand for power saving, reducing standby electricity of image forming apparatuses has become an important issue. Thus, there has been proposed an image forming apparatus that raises fixing temperature at high speed by an on-demand fixing technique using a ceramic heater, and thus hardly requires standby electricity. 
     On the other hand, in such a fixing apparatus that raises fixing temperature at high speed, the temperature of a fixing heater abruptly rises, and it is thus important to quickly interrupt electric current to the fixing heater when an abnormal condition occurs. Moreover, to reliably interrupt electric current to the fixing heater, it is necessary to stop supplying electrical power to both ends of the fixing heater. 
     To stop the supply of electrical power to the fixing heater, a mechanical relay is commonly used. The relay uses a contact, and hence if the relay is repeatedly turned on and off, the contact may be welded due to age deterioration. If the contact of the relay is welded, electric current is passed through the fixing heater even when the relay is turned off, and thus power feeding to the fixing hearer does not stop, which may result in abnormal heating. To cope with this, there has been proposed a method that a zero cross detection circuit for detecting the presence or absence of input voltage is provided in a stage subsequent to the relay, and when a zero cross signal is output despite the mechanical relay being instructed to turn off, it is determined that contact fusion of the relay occurs (for example, see Japanese Laid-Open Patent Publication (Kokai) No. 2002-296955). 
     The above described method makes it possible to detect contact fusion of the relay by disposing the zero cross circuit in the stage subsequent to the relay. 
     However, when relays are disposed at respective both ends of the fixing heater, the zero cross circuit can detect contact fusion only when contacts of both relays are welded, and the zero cross circuit cannot detect contact fusion occurring in either one of the relays. For this reason, the above described method is insufficient in terms of safety. 
     SUMMARY OF THE INVENTION 
     The present invention provides an image forming apparatus that is capable of, in a circuit configuration in which relays are connected to respective both ends of a fixing heater, individually detecting contact fusion occurring in the respective relays, and a relay control apparatus. 
     Accordingly, a first aspect of the present invention provides an image forming apparatus comprising a heater configured to be supplied with electrical power from a power source, a first relay and a second relay each configured to be connected between the power source and the heater, a voltage detection unit configured to detect presence or absence of an input voltage to the heater on paths from the first relay to the heater and from the second relay to the heater, and a relay control unit configured to output control signals for turning on and off respective ones of the first relay and the second relay, wherein the relay control unit determines that the second relay has failed when the input voltage is detected by the voltage detection unit in a state in which the first relay is on and the second relay is off, and the relay control unit determines that the first relay has failed when the input voltage is detected by the voltage detection unit in a state in which the first relay is off and the second relay is on. 
     Accordingly, a second aspect of the present invention provides a relay control apparatus included in an image forming apparatus comprising a heater configured to be supplied with electrical power from a power source, a first relay and a second relay configured to be each connected between the power source and the heater, and a voltage detection unit configured to detect presence or absence of an input voltage to the heater on paths from the first relay to the heater and from the second relay to the heater, comprising a first control unit configured to, before starting passage of electric current through the heater, output a control signal to turn on the first relay, and when the input voltage is not detected by the voltage detection unit, output a control signal to turn on the second relay, a first determination unit configured to determine that the second relay has failed in a case where the input voltage is detected by the voltage detection unit when the first relay is turned on by the first control unit, a second control unit configured to, before ending passage of electric current through the heater, output a control signal to turn off the first relay, and when the input voltage is not detected by the voltage detection unit, output a control signal to turn off the second relay, and a second determination unit configured to determine that the first relay has failed in a case where the input voltage is detected by the voltage detection unit when the first relay is turned off by the second control unit. 
     According to the present invention, in a circuit configuration in which the relays are connected to the respective both ends of the fixing heater, contact fusion occurring in the respective relays can be individually detected. Moreover, because contact fusion is detected for one of the relays when it is on, and for the other one of the relays when it is off, the time required to detect contact fusion of the two relays can be reduced. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing a general arrangement of a full-color printer which is an exemplary image forming apparatus according to an embodiment of the present invention. 
         FIG. 2  is a block diagram showing a general arrangement of a control unit in the printer in  FIG. 1 . 
         FIG. 3  is a diagram showing a general arrangement and a connecting relation of a heater power-feed circuit in  FIG. 2 . 
         FIG. 4A  is a flowchart I showing the flow of a process for detecting contact fusion of first and second relays, and  FIG. 4B  is a flowchart II showing the flow of the process for detecting contact fusion of the first and second relays. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail with reference to the drawings showing an embodiment thereof. 
       FIG. 1  is a cross-sectional view showing a general arrangement of a full-color printer which is an exemplary image forming apparatus according to an embodiment of the present invention. 
     Referring to  FIG. 1 , the full-color printer (hereafter referred to merely as the “printer”) has the following four image forming units: an image forming unit  1 Y for forming yellow-color images, an image forming unit  1 M for forming magenta-color images, an image forming unit  1 C for forming cyan-color images, and an image forming unit  1 Bk for forming black-color images. The image forming units  1 Y,  1 M,  1 C, and  1 Bk are arranged in a row at regular intervals. 
     In the image forming units  1 Y,  1 M,  1 C, and  1 Bk, drum-shaped electrophotographic photosensitive units (hereafter referred to as the “photosensitive drums”)  2   a ,  2   b ,  2   c , and  2   d  which are image carriers are disposed. Primary chargers  3   a ,  3   b ,  3   c , and  3   d , developing devices  4   a ,  4   b ,  4   c , and  4   d , and transfer rollers  5   a ,  5   b ,  5   c , and  5   d , which are transfer units, and drum cleaner units  6   a ,  6   b ,  6   c , and  6   d  are disposed around the respective the photosensitive drums  2   a ,  2   b ,  2   c , and  2   d . An exposure unit  7  is placed at a lower portion between the primary chargers  3   a  to  3   d  and the developing devices  4   a  to  4   d.    
     The developing devices  4   a  to  4   d  store yellow toner, cyan toner, magenta toner, and black toner, respectively. 
     The photosensitive drums  2   a  to  2   d  have photoconductive layers on drum bases which are negatively-charged OPC photosensitive units and made of aluminum, and rotatively driven by a drive unit (not shown) in directions indicated by arrows (clockwise as viewed in  FIG. 1 ) at a predetermined process speed. 
     The primary chargers  3   a  to  3   d , which are primary charging units, uniformly charge surfaces of the photosensitive drums  2   a  to  2   d  to a predetermined negative potential by charging biases applied from charging bias power sources (not shown). 
     The developing devices  4   a ,  4   b ,  4   c , and  4   d  have toner therein, and attach toners of the respective colors to electrostatic latent images formed on the photosensitive drums  2   a  to  2   d  to develop (visualize) them as toner images. 
     The transfer rollers  5   a  to  5   d , which are primary transfer units, are disposed so as to be brought into abutment with the respective photosensitive drums  2   a  to  2   d  in respective primary transfer areas  32   a  to  32   d  via an intermediate transfer belt  8 . 
     The drum cleaner units  6   a ,  6   b ,  6   c , and  6   d  each have a cleaning blade for removing transfer residual toner remaining on the photosensitive drums  2   a  to  2   d  after primary transfer from the photosensitive drums  2   a  to  2   d , and others. 
     The intermediate transfer belt  8  is disposed on an upper surface side of the photosensitive drums  2   a  to  2   d , and tightly stretched between a secondary transfer opposing roller  10  and a tension roller  11 . The secondary transfer opposing roller  10  is disposed so as to be brought into abutment with a secondary transfer roller  12  in a secondary transfer area  34  via the intermediate transfer belt  8 . The intermediate transfer belt  8  is made of a dielectric resin such as a polycarbonate, a polyethylene terephthalate resin film, or a polyvinylidene fluoride resin film. 
     Moreover, the intermediate transfer belt  8  has a primary transfer surface (lower flat surface)  8   b , which is formed on a side opposing the photosensitive drums  2   a  to  2   d , inclined with its secondary transfer roller  12  side down. Namely, the intermediate transfer belt  8  is movably opposed to upper surfaces of the photosensitive drums  2   a  to  2   d , and has the primary transfer surface  8   b , which is formed on the side opposing the photosensitive drums  2   a  to  2   d , with its secondary transfer area  34  side down. Specifically, the inclination angle is set at about 15 degrees. 
     Moreover, the intermediate transfer belt  8  is tightly stretched by the secondary transfer opposing roller  10 , which is disposed on the secondary transfer area  34  side and applies driving force to the intermediate transfer belt  8 , and the tension roller  11  which is opposed to the secondary transfer opposing roller  10  across the primary transfer parts  32   a  to  32   d  and applies tension to the intermediate transfer belt  8 . 
     The secondary transfer opposing roller  10  is disposed so as to be able to abut on the secondary transfer roller  12  in the secondary transfer area  34  via the intermediate transfer belt  8 . A belt cleaning unit  13 , which removes and collects transfer residual toner remaining on a surface of the intermediate transfer belt  8 , is disposed outside the intermediate transfer belt  8  and in the vicinity of the tension roller  11 . A fixing unit  16  is disposed in a vertical path configuration at a location downstream of the secondary transfer area  34  in a direction in which a transfer material (recording material) P is conveyed. 
     An exposure unit  7  is comprised of a laser emission unit, which emits light according to time-series electric digital pixel signals of given image information, a polygon lens, a reflex mirror, and so on. By exposing the photosensitive drums  2   a  to  2   d  to light, the exposure unit  7  forms electrostatic latent images of the respective colors according to image information on surfaces of the photosensitive drums  2   a  to  2   d  charged by the respective primary chargers  3   a  to  3   d.    
     Next, a description will be given of a one-sided image forming operation performed by the printer in  FIG. 1 . 
     Upon an image formation start signal being issued, the photosensitive drums  2   a  to  2   d  of the respective image forming units  1 Y to  1 Bk, which are rotatively driven at a predetermined process speed, are uniformly charged to negative polarity by the respective primary chargers  3   a  to  3   d . Then, the exposure unit  7  applies a color-separated image signal input from outside from a laser light emitting element, and thus forms electrostatic latent images of the respective colors on the respective photosensitive drums  2   a  to  2   d  via the polygon lens, the reflex mirror, and so on. 
     Then, the developing device  4   a  to which a developing bias of the same polarity as the charging polarity (negative polarity) of the photosensitive drum  2   a  is applied attaches yellow toner to the electrostatic latent image formed on the photosensitive drum  2   a , and thus visualizes the electrostatic latent image as a toner image. In the primary transfer area  32   a  between the photosensitive drum  2   a  and the transfer roller  5   a , the yellow toner image is primarily transferred onto the intermediate transfer belt  8  by the transfer roller  5   a  to which a primary transfer bias (opposite in polarity to the toner (positive polarity)) is applied. 
     The intermediate transfer belt  8  onto which the yellow toner image has been transferred is moved toward the image forming unit  1 M. Then, in the image forming unit  1 M as well, a magenta toner image formed on the photosensitive drum  2   b  in the same way as described above is superimposed on the yellow toner image on the intermediate transfer belt  8  in the primary transfer area  32   b . On this occasion, transfer residual toner remaining on the photosensitive drums  2   a  to  2   d  is scraped off and collected by the cleaning blades or the like provided in the drum cleaning units  6   a  to  6   d.    
     Thereafter, in the same way, cyan and black toner images formed on the photosensitive drums  2   c  and  2   d  of the image forming units  1 C and  1 Bk are sequentially superimposed on the yellow and magenta toner images transferred onto the intermediate transfer belt  8  in superimposed manner in the respective primary transfer areas  32   c  and  32   d . Thus, full-color toner images are formed on the intermediate transfer belt  8 . 
     Then, a leading end of the full-color toner images on the intermediate transfer belt  8  is moved to the secondary transfer area  34  between the secondary transfer opposing roller  10  and the secondary transfer roller  12 . In accordance with this timing, a transfer material P selectively fed from a sheet feed cassette  17  or a manual feed tray  20  via a conveying path  18  is conveyed to the secondary transfer part  34  by registration rollers  19 . 
     The full-color toner images are secondarily transferred onto the transfer material P, which has been conveyed to the secondary transfer part  34 , in a collective manner by the secondary transfer roller  12  to which a secondary transfer bias (opposite in polarity to the toner (positive polarity)) is applied. 
     The transfer material P bearing the full-color toner images is conveyed to the fixing unit  16 , which thermally fixes the full-color toner images on a surface of the transfer material P by heating and pressurizing the transfer material P. The transfer material P is then discharged onto a discharged sheet tray  22  by sheet discharging rollers  21 , which completes the sequential image forming operation. It should be noted that secondary transfer residual toner or the like remaining on the intermediate transfer belt  8  is removed and collected by the belt cleaning unit  13 . 
     Next, a description will be given of a double-sided image forming operation performed by the printer in  FIG. 1 . 
     The procedure for the double-sided image forming operation is the same as for the one-sided image forming operation before the point where the transfer material P is conveyed to the fixing unit  16 , and the full-color toner images are heated and pressurized to be thermally fixed on the surface of the transfer material P. After that, the rotation of the sheet discharging rollers  21  is stopped in a state in which a major portion of the transfer material P has been discharged onto the discharged sheet tray  22  on an upper side of the main body by the sheet discharging rollers  21 . On this occasion, a trailing end of the transfer material P has reached an invertible position  42 . 
     Subsequently, the sheet discharging rollers  21  are reversely rotated so as to feed the transfer material P, the conveyance of which has been stopped by stopping the rotation of the sheet discharging rollers  21 , into a double-sided path having double-sided rollers  40  and  41 . By reversely rotating the sheet discharging rollers  21 , the trailing end of the transfer material P which has been positioned at the invertible position  42 , becomes a leading end and reaches the double-sided rollers  40 . After that, the transfer material P is conveyed to the double-sided rollers  41  by the double-sided rollers  40 , and sequentially conveyed toward the registration rollers  19  by the double-sided rollers  40  and  41 . In the meantime, an image formation start signal is output, and the same operation as in the one-sided image forming operation described above is carried out. Specifically, the transfer material P is moved to the secondary transfer area  34  by the registration rollers  19  in accordance with the timing in which the leading end of the full-color toner images on the intermediate transfer belt  8  is moved to the secondary transfer area  34  between the secondary transfer opposing roller  10  and the secondary transfer roller  12 . 
     In the secondary transfer area  34 , the leading end of the toner images and the leading end of the transfer material P are matched together, and the toner images are transferred onto the transfer material P. After that, the toner images on the transfer material P are fixed by the fixing unit  16  as with the one-sided image forming operation. Then, the transfer material P is conveyed again by the sheet discharging rollers  21 , and eventually discharged onto the discharged sheet tray  22 , which completes the sequential image forming operation. 
       FIG. 2  is a block diagram showing a general arrangement of a control unit in the printer in  FIG. 1 . It should be noted that in  FIG. 2 , only parts relating to the present invention and main functional units are illustrated, and other component elements and functional units are omitted. 
     Referring to  FIG. 2 , the control unit  110  is a basic control unit that controls the entire printer, and has a CPU  171 , a ROM  174 , and a RAM  175 . The ROM  174  stores control programs and others. The RAM  175  is used as a work memory when the CPU  171  executes control programs. The CPU  171  is connected to the ROM  174  and the RAM  175  via an address bus and a data bus. 
     The CPU  171  is connected to sensors (not shown) for detecting various loads (not shown) such as motors and clutches and sheet positions, and a temperature detection circuit (also referred to herein as a “temperature detection unit”)  700  via an I/O port  173 . The fixing unit  16  and a heater power-feed circuit  500  that supplies electrical power of an AC power source  550  to a fixing heater (not shown) in the fixing unit  16  are connected to the I/O port  173 , and the CPU  171  controls them. Specifically, by executing control programs read out from the ROM  174 , the CPU  171  sequentially controls input and output via the I/O port  173 , and controls the temperature of the fixing heater in the fixing unit  16 . 
     A temperature detection signal output from a temperature sensor (not shown) in the fixing unit  16  is input to the temperature detection circuit  700  via the I/O port  173 . The temperature detection circuit  700  outputs control signals to the heater power-feed circuit  500 . 
     The CPU  171  is connected to a console  172  having a display unit (not shown), which produces screen displays, and a key input unit (not shown), and controls screens displayed on the console  172  and key inputs. By operating the key input unit, an operator instructs the CPU  171  to switch between image forming operation modes and screen displays. As a result, the CPU  171  displays operation mode settings according to printer conditions and key inputs. 
     The CPU  171  is also connected to an external I/F processing unit  200 , an image memory unit  300 , and an image forming unit  400 . It should be noted that the image forming units  1 Y,  1 M,  1 C, and  1 Bk are included in the image forming unit  400 . 
     The external I/f processing unit  200  sends and receives image data and processing data from external devices such as a PC. The image memory unit  300  carries out an image expansion process and a temporary image storage process. The image forming unit  400  has the image forming units  1 Y,  1 M,  1 C, and  1 Bk described above, and carries out a process in which it causes the exposure unit  7  to expose line image data, which has been transferred from the image memory unit  300 , to light. 
       FIG. 3  is a diagram showing a general arrangement and a connecting relation of the heater power-feed circuit  500  in  FIG. 2 . 
     The fixing unit  16  has a fixing heater  601 , which is a heat source for heating and fixing toner images, and a temperature sensor  602  such as a thermistor, which is disposed in the vicinity of the fixing heater  601 , for detecting the temperature of the fixing heater  601 . It should be noted that the fixing unit  16  has a pressurizing roller and others, description of which is omitted. 
     The temperature sensor  602  is connected to the control unit  110  and the temperature detection circuit  700 . A temperature detection signal  604  output from the temperature sensor  602  is input to the control unit  110  and the temperature detection circuit  700 . The fixing heater  601  has both ends thereof connected to the heater power-feed circuit  500 . 
     The heater power-feed circuit  500  has a first relay  501  and a second relay  502  for supplying/interrupting electrical power supplied from the AC power source  550  to both ends of the fixing heater  601 . The first relay  501  has one end thereof connected to one end of the fixing heater  601 , and the other end thereof connected to the AC power source  550 . The second relay  502  has one end thereof connected to the other end of the fixing heater  601  via a semiconductor SW  510 , and the other end thereof connected to the AC power source  550 . The first relay  501  and the second relay  502  are controlled to be on and off by control signals  503  and  504  output from the control unit  110 , which is a relay control unit. The control signals  503  and  504  output from the control unit  110  are input to a first AND circuit  702  and a second AND circuit  703 , respectively, via the I/O port  173  in  FIG. 2  described above. 
     A zero cross detection circuit  505  is connected in parallel to the fixing heater  601  and the AC power source  550  as illustrated in the figure. Upon being supplied with electrical power from the AC power source  550 , the zero cross detection circuit  505  outputs a zero cross signal  506  (a detection signal) according to zero cross timing of an alternating waveform to the control unit  110  (a voltage detection unit). The zero cross signal  506  output from the zero cross detection circuit  505  is input to the control unit  110  via the I/O port  173  in  FIG. 2  described above. 
     The semiconductor SW  510  is a semiconductor switch such as a triac (registered trademark), which is disposed on a path for supplying electrical power to the fixing heater  601 , and capable of turning on and off power feeding to the fixing heater  601  irrespective of whether the first relay  501  and the second relay  502  are turned on or off. The semiconductor SW  510  is controlled to be on or off in response to a control signal  512  output from the control unit  110 . 
     The control unit  110  outputs the control signal  512  in response to the temperature detection signal  604  from the temperature sensor  602 . By controlling the semiconductor SW  510  to be on or off, the temperature of the fixing heater  601  is controlled. 
     The first AND circuit  702  is a logic circuit that performs logical conjunction (AND) based on the control signal  503  output from the control unit  110  and a control signal  701  output from the temperature detection circuit  700 , and outputs a first AND signal  704  to the first relay  501 . On the other hand, the second AND circuit  703  is a logic circuit that performs logical conjunction (AND) based on the control signal  504  output from the control unit  110  and the control signal  701  output from the temperature detection circuit  700 , and outputs a second AND signal  705  to the second relay  502 . Thus, when a control signal for turning off the first relay  501  and the second relay  502  is output from one or both of the control unit  110  and the temperature detection circuit  700 , both relays are turned off. 
     The control unit  110  controls the temperature of the fixing heater  601  based on the temperature detection signal  604  from the temperature sensor  602 . When determining that the temperature of the fixing heater  601  indicated by the temperature detection signal  604  is not less than a threshold value Tmax 1 , the control unit  110  determines that the fixing heater  601  has increased from a proper temperature, and stops power feeding to the fixing heater  601 . Specifically, the control unit  110  outputs the control signal  512  for turning off the semiconductor SW  510  and outputs the control signals  503  and  504  for turning off the first relay  501  and the second relay  502 . 
     On the other hand, the temperature detection circuit  700  functions as a heater temperature abnormality detection unit, and is able to stop power feeding to the fixing heater  601  based on the temperature detection signal  604  from the temperature sensor  602 . Specifically, when determining that the temperature of the fixing heater  601  indicated by the temperature detection signal  604  is not less than a threshold value Tmax 2 , the temperature detection unit  700  determines that the fixing heater  601  is abnormally heating, and outputs the control signal  701  for stopping power feeding to the fixing heater  601 . 
     Because the temperature detection circuit  700  outputs the control signal  701  for turning off the first AND circuit  702  and the second AND circuit  703 , no signal for turning on the first relay  501  and the second relay  502  is output even if the control signals  503  and  504  for turning off the first relay  501  and the second relay  502  are input from the control unit  110 . As a result, the first relay  501  and the second relay  502  are turned off, and power feeding to the fixing heater  601  is stopped. 
     The above described threshold values Tmax 1  and Tmax 2  have the following relationship, Tmax 2 &gt;Tmax 1 . Thus, even when temperature cannot be controlled due to some abnormal condition such as runaway occurring in the CPU  171  in the control unit  110 , power feeding to the fixing heater  601  can be stopped by the temperature detection circuit  700 . As a result, the fixing heater  601  and its surrounding components can be protected, and abnormal fixing can be prevented. 
       FIGS. 4A and 4B  are flowcharts showing the flow of a process for detecting contact fusion of the first and second relays. 
     As shown  FIG. 4A , the control unit  110  determines in step S 201  whether or not to start passing electric current through the fixing heater  601 . When determining to start the passage of electric current, the control unit  110  outputs the control signal  503  for turning on the first AND circuit  702 , and turns on the first relay  501  in response to the first AND signal  704  output from the first AND circuit  702  (step S 202 ). After that, the control unit  110  stands by for a predetermined time period (for example, 100 ms) (step S 203 ). This aims at keeping contact connection stable because the first relay  501  is a mechanical relay. 
     Then, in step S 204 , the control unit  110  determines whether or not it has detected the zero cross signal  506  from the zero cross detection circuit  505 . When the control unit  110  has detected the zero cross signal  506  (YES in the step S 204 ), the control unit  110  proceeds to step S 205 . 
     In the step S 205 , the control unit  110  determines that electric current is being passed through the second relay  502  due to a failure such as contact fusion because the zero cross signal  506  is detected even though the control unit  110  has not output the control signal  504  for turning on the second relay  502 . The step S 205  is an exemplary first determination unit. Then, the control unit  110  stops the operation of the printer in  FIG. 1  (step S 217 ), and causes the display unit on the console  172  to display an error message (step S 218 ), followed by terminating the process. 
     On the other hand, when in the step S 204 , the control unit  110  has not detected the zero cross signal  506 , (NO in the step S 204 ), the control unit  110  proceeds to step S 206 . 
     In the step S 206 , the control unit  110  outputs the control signal  504  for turning on the first AND circuit  703 , and turns on the second relay  502  in response to the second AND signal  705  output from the second AND circuit  703 . The step S 206  is an exemplary first control unit. After turning on the second relay  502 , the control unit  110  stands by for a predetermined time period (for example, 100 ms) (step S 207 ). The reason for this is the same as in the step S 203  described above. 
     Then, in the step S 208 , the control unit  110  determines whether or not it has detected the zero cross signal  506  from the zero cross detection circuit  505 . When the control unit  110  has not detected the zero cross signal  506  (NO in the step S 208 ), the control unit  110  proceeds to step S 209 . 
     In the step S 209 , the control unit  110  determines that electric current is not being passed through the first relay  501  and the second relay  502  due to a failure such as poor conduction because electric current is not being passed through the first relay  501  and the second relay  502  even though the first relay  501  and the second relay  502  are turned on. This failure can be detected irrespective of whether poor conduction occurs in one of the first relay  501  and the second relay  502  or both the first relay  501  and the second relay  502 . The step S 209  is an exemplary third determination unit. When determining that a failure such as poor conduction has occurred, the control unit  110  carries out the processes in the step S 217  and the subsequent steps, followed by terminating the present process. 
     On the other hand, in the step S 208 , when detecting the zero cross signal  506 , the control unit  110  determines that the first relay  501  and the second relay  502  properly work, and starts passing electric current through the fixing heater  601  (step S 210 ), which enables an image forming operation to be carried out. In the step S 210 , the control unit  110  stars passing electric current through the fixing heater  601  by outputting the control signal  512  for turning on the semiconductor SW  510 . 
     Referring to  FIG. 4B , upon the image forming operation being completed, the control unit  110  determines whether or not to end the passage of electric current through the fixing heater  601  (step S 211 ). When determining to end the passage of electric current, the control unit  110  outputs the control signal  503  for turning off the first AND circuit  702 , and turns off the first relay  501  in response to the first AND signal  704  output from the first AND circuit  702  (step S 212 ). After that, the control unit  110  stands by for a predetermined time period (for example, 100 ms) (step S 213 ), and then determines whether or not it has detected the zero cross signal  506  (step S 214 ). When detecting the zero cross signal  506  (YES in the step S 214 ), the control unit  110  proceeds to step S 215 . 
     In the step S 215 , the control unit  110  determines that electric current is being passed through the first relay  501  due to a failure such as contact fusion because the zero cross signal  506  is detected even though the control unit  110  has not output the control signal  503  for turning on the first relay  501 . The step S 215  is an exemplary second determination unit. Then, the control unit  110  carries out the processes in the step S 217  and the subsequent steps, followed by terminating the present process. 
     On the other hand, when in the step S 214 , the control unit  110  has not detected the zero cross signal  506 , (NO in the step S 214 ), the control unit  110  proceeds to step S 216 . 
     In the step S 216 , the control unit  110  outputs the control signal  504  for turning off the second AND circuit  703 , and turns on the second relay  502  in response to the second AND signal  705  output from the second AND circuit  703 . The step S 216  is an exemplary second control unit. It should be noted that the above described process can be applied to a case where the locations of the first relay  501  and the second relay  502  are reversed. 
     After the passage of electric current is started in the step S 210 , the temperature detection circuit  700  stops power feeding to the fixing heater  601  based on the temperature detection signal  604  from the temperature sensor  602 . Specifically, when determining that the temperature of the fixing heater  601  detected by the temperature sensor  602  is not less than the threshold value Tmax 2 , the control unit  110  determines that the fixing heater  601  is abnormally heating, and outputs the control signal  701  for stopping power feeding to the fixing heater  601 . 
     After the passage of electric current is started in the step S 210 , the temperature detection circuit  700  also stops power feeding to the fixing heater  601  based on the temperature detection signal  604  from the temperature sensor  602 . Specifically, when determining that the temperature of the fixing heater  601  detected by the temperature sensor  602  is not less than the threshold value Tmax 1 , the control unit  110  determines that the fixing heater  601  has increased from a proper temperature, and outputs the control signals  503  and  504  for stopping power feeding to the fixing heater  601 . 
     According to the above described embodiment, before starting the passage of electric current through the fixing heater  601 , the control unit  110  outputs a control signal to turn on the first relay  501 , and when no input voltage has been detected by the zero cross detection circuit  505 , outputs a control signal to turn on the second relay  502 . In a case where an input voltage is detected by the zero cross detection circuit  505  when the first relay  501  is turned on, the control unit  110  determines that the second relay  502  has failed. Moreover, before ending the passage of electric current through the fixing heater  601 , the control unit  110  outputs a control signal to turn off the first relay  501 , and when no input voltage has been detected by the zero cross detection circuit  505 , outputs a control signal to turn off the second relay  502 . In a case where an input voltage is detected by the zero cross detection circuit  505  when the first relay  501  is turned off, the control unit  110  determines that the first relay  501  has failed. Thus, in the circuit configuration in which the relays are connected to the respective both ends of the fixing heater, contact fusion of the respective relays can be individually detected. 
     As described above, by staggering the operation timing of the two relays, contact fusion of each relay can be reliably detected. As a result, the time required for detecting contact fusion of the two relays can be reduced because the presence or absence of contact fusion is detected for one of the relays when it is on, and the presence or absence of contact fusion is detected for the other one of the relays when it is off. 
     OTHER EMBODIMENTS 
     Aspects of the present invention can also be realized by a computer of a system or apparatus (or devices such as a CPU or MPU) that reads out and executes a program recorded on a memory device to perform the functions of the above-described embodiment(s), and by a method, the steps of which are performed by a computer of a system or apparatus by, for example, reading out and executing a program recorded on a memory device to perform the functions of the above-described embodiment(s). For this purpose, the program is provided to the computer for example via a network or from a recording medium of various types serving as the memory device (e.g., computer-readable medium). 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2010-106404 filed May 6, 2010, which is hereby incorporated by reference herein in its entirety.