Patent Publication Number: US-2007098432-A1

Title: Image forming device

Description:
FIELD OF THE INVENTION  
      The present invention relates to an image forming device, and more particularly to cooling the developing system and optical system using the waste heat of the fixing unit.  
     BACKGROUND INFORMATION  
      In image forming devices such as copiers, printers, and facsimiles, in the fixing unit in which toner images are fixed to sheets, fixing rollers are heated by heaters so that when a sheet passes the fixing roller the toner adhering to the sheet is melted. Although part of the heat of the heated fixing roller is used as heat energy to melt the toner, most of the heat is dissipated to the surroundings from the fixing roller as waste heat. Therefore technology is proposed in which the waste heat of the fixing roller is converted into electrical energy using a thermoelectric conversion device based on the Seebeck effect.  
      When high temperature waste heat emitted from the fixing roller to the surroundings is transferred outside the fixing unit, the temperature within the image forming device rises. This can cause hardening of the toner housed within the developing unit, or deformation of the resin lens within the laser scanner unit for irradiating the photosensitive drum with laser light. To prevent this, a cooling fan is provided near the fixing unit within the image forming device to cool the fixing unit. When this cooling fan is driven the rise in temperature of the fixing unit is prevented, and the temperature rise within the image forming device is suppressed.  
      However, the cooling fans used in conventional image forming devices are driven by electricity supplied from the power source, the same as for other parts. Therefore, when the power supply of the image forming device is turned ON the fan starts at the same time, and when the power supply of the image forming device is turned OFF the fan stops at the same time. This point is now explained in detail.  
      As shown in  FIG. 8 , in a conventional image forming device, when the power supply is turned ON at time T 1  the heater is turned on and the fixing roller starts to be heated. Then the temperature of the fixing roller rises at a fixed rate, and when the temperature of the fixing roller reaches t 1  at time T 2  the temperature of the fixing roller is maintained at a constant temperature by controlling the heater. During this time, as the temperature of the fixing roller rises the temperature within the image forming device also rises. At the time T 2  when the temperature of the fixing roller reaches t 1 , the temperature within the image forming device reaches t 2 , and thereafter the temperature within the image forming device is also maintained constant.  
      Thereafter, at time T 3  when the power supply of the image forming device is turned OFF, the power to the heater is also turned off, and the temperature of the fixing roller gradually reduces from t 1 . On the other hand, when the power supply is turned OFF the cooling fan also stops. Therefore the temperature within the image forming device temporarily rises higher than the temperature within the image forming device t 2  while the power is ON due to the influence of the waste heat of the fixing device to t 3 . In this way, the temperature within the image forming device after the power is turned OFF is higher than the temperature while the power is ON, so the above-mentioned problems can occur.  
      To overcome this, a thermoelectric conversion device using the Seebeck effect is provided within the image forming device, and the electrical energy obtained by supplying waste heat from the fixing roller to the thermoelectric conversion device is used to drive the cooling fan. As shown in  FIG. 9 , at time T 4  when the temperature of the fixing roller has risen to t 4 , thermoelectric force is generated between the two electrodes of the thermoelectric conversion device, which starts to drive the cooling fan. In other words, the time in which the cooling fan starts to be driven is later by (T 4 -T 1 ) compared with the case where a thermoelectric conversion device is not provided.  
      As a result of this, when the power supply of the image forming device is turned ON the cooling fan does not also start at the same time to cool the fixing unit. Therefore the rate of rise of the temperature of the fixing roller increases. In other words, as shown in  FIG. 9 , the slope of the graph representing the change in temperature of the fixing roller is steeper. The time T 2  at which the temperature of the fixing roller reaches t 1  is shortened, and the time for the fixing roller to reach the temperature required for fixing is also shortened. Also, at this time the heat emitted from the fixing device per unit time also increases, so the rate at which the temperature of the interior of the image forming device also increases. In other words, as shown in  FIG. 9 , the slope of the graph representing the change in temperature within the image forming device is steeper, and the time T 2  at which the temperature of the interior of the image forming device reaches t 2  is reduced.  
      Thereafter, when the power supply and the heater are turned OFF, as shown in  FIG. 9 , at time T 5  when the temperature of the fixing roller has fallen to t 4  the cooling fan stops. In other words, the time at which the cooling fan stops is delayed by (T 5 -T 3 ) compared with the case where a thermoelectric conversion device is not provided. As a result of this after the power to the image forming device is turned OFF the cooling fan continues to operate, so the temporary rise in temperature of the interior of the image forming device due to the waste heat of the fixing roller is suppressed.  
      In this way, instead of having a common power source for the image forming device and the cooling fan so that at the same time that the power supply of the image forming device is turned ON the cooling fan also starts, waste heat from the fixing roller is supplied to a thermoelectric conversion device and the electrical energy obtained is used to drive the cooling fan. Then when the power supply is turned OFF the cooling fan continues to operate for a while, so that the temporary rise in temperature of the interior of the image forming device after the power is turned off is suppressed.  
      Here the thermoelectric conversion device using the Seebeck effect is configured to generate a thermoelectromotive force between two electrodes due to the temperature difference between the metal plates forming the front and rear surface. Therefore, as shown in  FIG. 9 , thermoelectromotive force is generated at the time T 4  when the temperature of the fixing roller exceeds t 4 , and at the time T 5  when the temperature of the fixing roller falls below t 4  generation of the thermoelectromotive force ceases. In other words, at the time when the thermoelectromotive force starts and the time when the thermoelectromotive force ceases, the temperature of the fixing roller is the same.  
      Therefore, the lower the threshold value of temperature difference at which thermoelectromotive force is generated in the thermoelectric device is set, the longer the cooling fan operates after the power supply to the image forming device is turned OFF, and the longer the temporary temperature rise within the image forming device is suppressed. On the other hand, the lower the threshold value of temperature difference at which thermoelectromotive force is generated in the thermoelectric device is set, the shorter the time after the power supply to the image forming device is turned ON until the cooling fan starts to operate, so the longer the time for the fixing roller to reach the temperature necessary for fixing.  
      Also, the higher the threshold value of temperature difference at which thermoelectromotive force is generated in the thermoelectric device is set, the longer the time after the power to the image forming device is turned ON until the cooling fan starts to operate, and the shorter the time for the fixing roller to reach the temperature necessary for fixing. On the other hand, the higher the threshold value of temperature difference at which thermoelectromotive force is generated in the thermoelectric device is set, the shorter the time that the cooling fan operates after the power supply to the image forming device is turned OFF. For example, as shown in  FIG. 9 , due to the influence of the waste heat of the fixing roller the temperature within the image forming device rises temporarily to t 5 , which is higher than when the power is ON.  
      In this way, the conventional configuration of an image forming device cannot simultaneously achieve both short time from turning the power supply to the image forming device ON until the fixing roller reaches the temperature necessary for fixing, and prevention of a temporary rise in temperature within the image forming device after the power supply to the image forming device is turned OFF.  
      With the foregoing points in view, it is an object of the present invention to shorten the time from turning the power supply to the image forming device ON until the fixing roller reaches the temperature necessary for fixing, as well as to prevent a temporary rise in temperature within the image forming device after the power to the image forming device is turned OFF.  
     SUMMARY OF THE INVENTION  
      To achieve the above object, the image forming device of the present invention includes an exposure unit in which laser light irradiates a photosensitive body to form an electrostatic latent image on the surface of the photosensitive body; a developing unit that develops by applying toner to the electrostatic latent image formed on the surface of the photosensitive body; a fixing unit that includes a fixing roller and a heater that heats the fixing roller, and that fixes toner onto a sheet that passes the fixing roller heated by the heater; a thermoelectric conversion device that carries out thermoelectric conversion using waste heat of the fixing unit; and a cooling fan that cools the fixing device, wherein the exposure unit, the developing unit, and the fixing unit are supplied with electrical power by a power source, the cooling fan is driven by electrical power generated by thermoelectric conversion by the thermoelectric conversion device, and the voltage directly output from the thermoelectric conversion device is higher when starting to drive the static cooling fan than when driving the operating cooling fan stops.  
      In this type of image forming device, a delay circuit is included between the thermoelectric conversion device and the cooling fan, that delays the voltage output from the thermoelectric conversion device to the cooling fan, and outputs the voltage to the cooling fan.  
      In this way, the voltage output from the thermoelectric conversion device to the cooling fan is delayed by the delay circuit, before being output to the cooling fan. Therefore, when the cooling fan is static, even if the thermoelectric conversion device has started generating a thermoelectromotive force and has output a voltage to the cooling fan, for a while electricity is not supplied to the cooling fan and the cooling fan remains static. On the other hand, when the cooling fan is operating, even if the thermoelectromotive force of the thermoelectric conversion device ceases and the voltage output by the thermoelectric conversion device to the cooling fan becomes zero, the power supply to the cooling fan continues for a while and the cooling fan continues to operate.  
      In this case, the delay circuit includes an input terminal to which the voltage output from the thermoelectric conversion device is input; a resistance that is connected at a first end to the input terminal; a condenser that is connected at a first end to a second end of the resistance to ground at a second end; and an output terminal that is connected to the node at which the resistance and the condenser are connected and that outputs the output voltage to the cooling fan.  
      In this way, when the cooling fan is static, when the thermoelectromotive force is generated in the thermoelectric conversion device and voltage starts to be output from the thermoelectric conversion device to the cooling fan a delay is generated by the integration action. Then, when the voltage at the output terminal of the delay circuit is sufficient to drive the cooling fan, electrical power is supplied to the cooling fan and the cooling fan starts to operate. Also, when the cooling fan is operating and the thermoelectromotive force of the thermoelectric conversion device ceases, voltage ceases to be output from the thermoelectric conversion device to the cooling fan. However, electric charge is accumulated in the condenser of the delay circuit, so a voltage capable of driving the cooling fan is output from the output terminal, and electrical power continues to be supplied to the cooling fan for a while after the thermoelectromotive force of the thermoelectric device has ceased.  
      Also, this type of image forming device includes a switching circuit, so that when the cooling fan is static, electrical power starts to be supplied from the thermoelectric conversion device to the cooling fan when the voltage output from the thermoelectric conversion device reaches a first predetermined voltage. Also, when the cooling fan is operating and the voltage output from the thermoelectric conversion device reaches a second predetermined voltage, supply of electricity from the thermoelectric conversion device to the cooling fan stops. Furthermore, the first predetermined voltage is higher than the second predetermined voltage.  
      In this way, the voltage output from the thermoelectric conversion device when electrical power starts to be supplied from the thermoelectric conversion device to the cooling fan is set higher than the voltage output from the thermoelectric conversion device when electrical power stops being supplied from the thermoelectric conversion device to the cooling fan. In other words, the thermoelectromotive force generated by the thermoelectric conversion device is set so that it is higher when it is starting to drive a static cooling fan than when it is stops driving an operating cooling fan.  
      In this case the switching circuit includes an input terminal at which the voltage output from the thermoelectric conversion device is input; a first resistance a first end of which is connected to ground; a second resistance a first end of which is connected to a second end of the first resistance and a second end of which is connected to the power supply; a condenser a first end of which is connected to ground and a second end of which is connected to a second end of the second resistance; a comparator the inverting input terminal of which is connected to the input terminal and the non-inverting input terminal of which is connected to the node to which the first resistance and the second resistance are connected; a switch a first end of which is connected to the input terminal; an output terminal connected to a second end of the switch and which outputs the output voltage to the cooling fan; and a third resistance connected between the non-inverting input terminal of the comparator and the output terminal.  
      Then, in the switching circuit, after the power supply to the image forming device is turned ON, when the voltage applied to the input terminal, that is the voltage applied to the inverting input terminal of the comparator, rises to the level of the voltage applied to the non-inverting input terminal of the comparator, it becomes low. Therefore the voltage is output from the output terminal with the switch ON. On the other hand, in the switching circuit when the output of the comparator is low, after the power to the image forming device is turned OFF, when the voltage applied to the input terminal, that is the voltage applied to the inverting input terminal of the comparator falls to the level of the voltage applied to the non-inverting input terminal of the comparator, the comparator output becomes high. Therefore the voltage output from the output terminal is zero with the switch OFF.  
      According to the present invention, the voltage output from the thermoelectric conversion device to the cooling fan is higher when a static cooling fan starts to be driven than when a driven cooling fan stops being driven. Therefore the time period that the cooling fan is operated after the power supply to the image forming device is turned OFF can be longer than with conventional art. Also the temporary rise in temperature within the image forming device after the power to the image forming device is turned OFF due to the waste heat of the fixing roller can be reliably prevented. Also, the time at which the cooling fan starts to be driven can be delayed relative to the time that thermoelectromotive force is generated in the thermoelectric conversion device. Therefore it is possible to shorten the time from turning the power supply to the image forming device ON until the fixing roller reaches the temperature required for fixing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is an outline figure showing the configuration of an image forming device according to the present invention;  
       FIG. 2  is a block diagram showing the operation of an image forming device according to the first preferred embodiment;  
       FIG. 3  is a figure showing the configuration of the delay circuit in an image forming device according to the first preferred embodiment;  
       FIG. 4  is a figure showing the relationship between operation of the cooling fan, temperature of the fixing roller, and temperature within the image forming device in an image forming device according to the first preferred embodiment;  
       FIG. 5  is a block diagram showing the operation of an image forming device according to the second preferred embodiment;  
       FIG. 6  is a figure showing the configuration of the switching circuit in an image forming device according to the second preferred embodiment;  
       FIG. 7  is a figure showing the relationship between operation of the cooling fan, temperature of the fixing roller, and temperature within the image forming device in an image forming device according to the second preferred embodiment;  
       FIG. 8  is a figure showing the relationship between operation of the cooling fan, temperature of the fixing roller, and temperature within the image forming device in an image forming device according to conventional art; and  
       FIG. 9  is a figure showing the relationship between operation of the cooling fan, temperature of the fixing roller, and temperature within the image forming device in an image forming device including a thermoelectric conversion device according to conventional art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The following is a detailed explanation of the preferred embodiments of the present invention, based upon the drawings.  
     First Preferred Embodiment  
      As shown in  FIG. 1  and  FIG. 2 , the image forming device according to a first preferred embodiment includes a photosensitive drum  10 ; a charging unit  20  that charges the entire surface of the photosensitive drum  10 ; an exposure unit  30  that irradiates the surface of the photosensitive drum  10  with laser light; a developing unit  40  that develops by applying toner  41  to the surface of the photosensitive drum  10 ; a transfer unit  50  that transfers the toner  41  adhering to the surface of the photosensitive drum  10  to sheets; a fixing unit  60  that fixes toner  41  that has been transferred onto a sheet onto the sheet; and a cooling unit  70  that generates electricity using waste heat from the fixing unit  60  and cools the exposure unit  30  and the developing unit  40 .  
      Also, as shown in  FIG. 2 , the image forming device includes a control unit  80  that controls the operation of all the devices within the image forming device, and a power supply unit  90  that supplies electrical power to the charging unit  20 , the exposure unit  30 , the developing unit  40 , the transfer unit  50 , and the fixing unit  60 . The image forming device carries out a charging process, an exposure process, a developing process, a transfer process, and a fixing process in sequence.  
      In an image forming device with this configuration, the charging unit  20  includes a charging roller  21  as shown in  FIG. 1 . In the charging process the surface of the photosensitive drum  10  is charged by the charging roller  21 . Also, the exposure unit  30  includes a laser scanner unit  31  as shown in  FIG. 1 . In the exposure process laser light is emitted from the laser scanner unit  31  to the photosensitive drum  10 , forming an electrostatic latent image on the part of the surface of the photosensitive drum  10  which the laser light strikes. The optical system of the laser scanner unit  31  of the exposure unit  30  uses a resin lens (not shown in the drawings).  
      Also, as shown in  FIG. 1 , the developing unit  40  includes a developing roller  42  and also contains toner  41  within the developing unit  40 . In the developing process the developing roller  42  conveys charged toner  41  and applies it to the electrostatic latent image formed on the surface of the photosensitive drum  10  to develop the image. Furthermore, the transfer unit  50  includes a transfer roller  51  as shown in  FIG. 1 . In the transfer process the toner  41  adhering to the surface of the photosensitive drum  10  is transferred to a sheet passing between the transfer roller  51  and the photosensitive drum  10 .  
      Also, as shown in  FIG. 1  and  FIG. 2 , the fixing unit  60  includes a fixing roller  61 , a fixing heater  62  that heats the fixing roller  61 , a pressure roller  63 , and a temperature sensor  64  that measures the temperature of the fixing roller  61 . The fixing heater  62  and the temperature sensor  64  are connected to the control unit  80 . The temperature of the fixing roller  61  is measured by the temperature sensor  64  and the temperature information measured by the temperature sensor  64  is input to the control unit  80 . Then the control unit  80  controls the temperature of the fixing heater  62  based upon this temperature information. Also, in the fixing process the fixing roller  61  is heated by the fixing heater  62 , so that the toner  41  on a sheet passing between the fixing roller  61  and the pressure roller  63  is melted and fixed onto the sheet.  
      When the fixing unit  60  is operating in this way, although part of the heat of the heated fixing roller  61  is used as heat energy to melt the toner  41  on a sheet, most of the heat is dissipated to the surroundings from the fixing roller  61  as waste heat. When high temperature waste heat emitted from the fixing roller  61  to the surroundings is transferred outside the fixing unit  60 , the temperature within the image forming device rises. This can cause hardening of the toner  41  housed within the developing unit  40 , or deformation of the resin lens within the laser scanner unit  31 .  
      To prevent this, as shown in  FIG. 1 , the image forming device according to this preferred embodiment includes a cooling unit  70  to prevent accumulation of heat within the image forming device and cool the exposure unit  30  and developing unit  40  by cooling the fixing unit  60 . The cooling unit  70  is provided close to the fixing unit  60 , as shown in  FIG. 2 , and includes a thermoelectric conversion device  71 , a cooling fan  72 , and a delay circuit  73  containing an integrating circuit. The thermoelectric conversion device  71  of the cooling unit  70  converts the waste heat of the fixing roller  61  into electrical energy, and supplies electrical power to the cooling fan  72  via the delay circuit  73 .  
      The thermoelectric conversion device  71  can be a device using the Seebeck effect. Specifically, the thermoelectric conversion device  71  includes a high temperature electrode near the fixing roller  61 , a low temperature electrode on the opposite side, and p-type semiconductor and n-type semiconductor therebetween. In this case the low temperature electrode is at the temperature of the interior of the image forming device. Also, when the fixing roller  61  is at a predetermined high temperature condition, a thermoelectromotive force is generated between the two electrodes due to the temperature difference between high temperature electrode and the low temperature electrode. The electrical power generated by thermoelectric conversion is used as the power to drive the cooling fan  72 .  
      In this way, in the image forming device of this preferred embodiment the cooling fan  72  is driven by electrical power obtained by converting waste heat from the fixing roller  61  into electrical energy in the thermoelectric conversion device  71 . On the other hand, the other components apart from the cooling fan  72  (for example, the fixing heater  62 , etc.) are supplied with electrical power by the power supply unit  90 . In other words, in the image forming device the power sources for the cooling fan and for components other than the cooling fan are different.  
      Also, in the cooling unit  70 , the delay circuit  73  provided between the thermoelectric conversion device  71  and the cooling fan  72  is configured as an integrating circuit as shown in  FIG. 3 . That is, the integrating circuit includes an input terminal V in  to which the output voltage from the thermoelectric conversion device  71  is input; a resistance R a first end of which is connected to the input terminal V in ; a condenser C a first end of which is connected to a second end of the resistance R and a second end of which is grounded; and an output terminal V out  connected to the node to which the resistance R and the condenser C are connected. The output voltage output from the output terminal V out  is supplied to the cooling fan  72 .  
      Then, when the voltage obtained by converting heat energy into electrical energy in the thermoelectric conversion device  71  is applied to the input terminal V in , the voltage required to drive the cooling fan appears on the output terminal V out , delayed only in accordance with the time constant RC.  
      —Operation of the Image Forming Device— 
      The following is an explanation of the operation of the image forming device, referring to FIGS.  2  to  4 .  
      As shown in  FIG. 2 , when the power supply to the image forming device is turned ON, the power supply unit  90  supplies electrical power to the charging unit  20 , the exposure unit  30 , the developing unit  40 , the transfer unit  50 , the fixing unit  60 , and the control unit  80 . In addition, the charging unit  20 , the exposure unit  30 , the developing unit  40 , the transfer unit  50 , and the fixing unit  60  are controlled by the output signals from the control unit  80 . Therefore, the above-mentioned charging process, exposure process, developing process, transfer process, and fixing process can each be carried out in sequence.  
      As shown in  FIG. 4 , at the time T 1  in which the power supply to the image forming device is turned ON, electrical power is supplied from the power supply unit  90  to the fixing unit  60 . Then the fixing heater  62  heats the fixing roller  61 , and the temperature of the fixing roller  61  gradually rises. During this time temperature information from the temperature sensor  64  is input to the control unit  80 , and the temperature of the fixing heater  62  is controlled by signals output by the control unit  80 . Then at the time T 4  when the temperature of the fixing roller  61  reaches t 4  thermoelectromotive force is generated in the thermoelectric conversion device  71 .  
      On the other hand, the delay circuit  73  configured as an integrating circuit as shown in  FIG. 3  is provided between the thermoelectric conversion device  71  and the cooling fan  72 . Therefore, although at time T 4  thermoelectromotive force is generated in the thermoelectric conversion device  71 , a delay is generated by the action of the integrating circuit. At time T 6  when the voltage of the output terminal V out  becomes sufficient to drive the cooling fan  72  the cooling fan  72  starts to be driven. On other words, even after a thermoelectromotive force is generated by the thermoelectric conversion device  71 , the cooling fan  72  remains static for a while. The time at which the cooling fan  72  starts to be driven is delayed by (T 6 -T 4 ) from the time at which a thermoelectromotive force is generated in the thermoelectric conversion device  71 .  
      Also, at time T 2  when the temperature of the fixing roller  61  rises to t 1 , the temperature of the fixing heater  62  is controlled by signals from the control unit  80 , and thereafter the temperature of the fixing roller  61  is maintained constant. Also, as the temperature of the fixing roller  61  rises, the temperature within the image forming device also rises. At the time T 2  when the fixing roller  61  reaches t 1 , the temperature within the image forming device reaches t 2 , and thereafter is maintained constant. In this way, the time at which the cooling fan  72  starts to be driven is delayed relative to the time at which a thermoelectromotive force is generated within the thermoelectric conversion device  71 . Therefore, the rate of temperature rise of the fixing roller  61  is higher than in a conventionally configured image forming device in which electrical power is supplied to the cooling fan  72  at the same time as a thermoelectromotive force is generated in the thermoelectric conversion device  71 . In other words, as shown in  FIG. 4 , the slope of the graph representing the change in temperature of the fixing roller  61  is steeper. The time for the temperature of the fixing roller  61  to reach t 1  is shortened, and the time for the fixing roller  61  to reach the temperature necessary for fixing is also reduced.  
      Thereafter, as shown in  FIG. 4 , when the power supply to the image forming device is turned OFF at time T 3 , the electrical power from the power supply unit  90  to the charging unit  20 , the exposure unit  30 , the developing unit  40 , the transfer unit  50 , the fixing unit  60 , and the control unit  80  is turned off. Therefore, electricity to the fixing heater  62  is also turned off, and the temperature of the fixing roller  61  gradually falls from t 1 . Then, at time T 5  when the temperature of the fixing roller  61  falls to t 4  the thermoelectromotive force in the thermoelectric conversion device becomes zero.  
      However, at the time T 5  when the thermoelectromotive force in the thermoelectric conversion device  71  becomes zero, electric charge is accumulated in the condenser C of the delay circuit  73  configured as an integrating circuit as shown in  FIG. 3 . A voltage capable of driving the cooling fan  72  is output from the output terminal V out , so electrical power continues to be supplied to the cooling fan  72  for a while after the thermoelectromotive force of the thermoelectric conversion device  71  has become zero. Then the voltage at the output terminal V out  reduces due to the electrical discharge of the condenser C, and at time T 7  the output voltage from the delay circuit  73  reduces to a level at which it cannot drive the cooling fan  72 , and electrical power supply to the cooling fan ceases. In other words, in the image forming device according to this preferred embodiment, the time for which the cooling fan  72  continues to be driven after the power supply to the image forming device is turned OFF is increased by (T 7 -T 5 ) compared with a conventional image forming unit.  
      In this way, by providing a delay circuit  73  configured as an integrating circuit as shown in  FIG. 3  between the thermoelectric conversion device  71  and the cooling fan  72 , it is possible to increase the length of time that the cooling fan  72  is driven after the power supply to the image forming device is turned OFF by (T 7 -T 5 ) compared with a conventional image forming device. Also, after turning the power supply to the image forming device OFF, it is possible to reliably prevent the temperature within the image forming device from temporarily rising higher than t 2  due to the effect of the waste heat from the fixing roller  61 . Also, by delaying the time at which the cooling fan  72  starts to be driven by (T 6 -T 4 ) after the thermoelectromotive force is generated in the thermoelectric conversion device  71  it is possible to shorten the time for the temperature of the fixing roller  61  to reach the temperature necessary for fixing. Therefore the time from turning the power supply to the image forming device ON until the fixing process can start can be shortened.  
     Second Preferred Embodiment  
      The second preferred embodiment of the present invention is a modification of the image forming device according to the first preferred embodiment. The following is an explanation of the points in which the image forming device according to the second preferred embodiment differ from the first preferred embodiment.  
      In the image forming device according to the present preferred embodiment, in the cooling unit  70  a switching circuit  74  including a comparator as shown in  FIG. 6  is provided between the thermoelectric conversion device  71  and the cooling fan  72 , as shown in  FIG. 5 .  
      The switching circuit  74  includes an input terminal V in  at which the voltage output from the thermoelectric conversion device  71  is input; a resistance R 1  a first end of which is connected to ground; a resistance R 2  a first end of which is connected to a second end of the resistance R 1  and a second end of which is connected to the power supply; a condenser C 1  a first end of which is connected to ground and a second end of which is connected to a second end of the resistance R 2 ; a comparator CP the inverting input terminal of which is connected to the input terminal V in  and the non-inverting input terminal of which is connected to the node to which the resistance R 1  and the resistance R 2  are connected; a switch SW a first end of which is connected to the input terminal V in  and a second end of which is connected to an output terminal V out ; and a resistance R 3  connected between the non-inverting input terminal of the comparator CP and the output terminal V out . The voltage output from the output terminal V out  is supplied to the cooling fan  72 .  
      Also, the switching circuit  74  controls turning ON and OFF the switch SW by the output of the comparator CP. Specifically, in the switching circuit  74 , when the voltage applied to the input terminal V in , that is the voltage applied to the inverting input terminal of the comparator CP, rises to the level of the voltage applied to the non-inverting input terminal of the comparator CP, it becomes low. Therefore the voltage is output from the output terminal V out  with the switch SW ON. On the other hand, when the output of the comparator is low, when the voltage applied to the input terminal V in , that is the voltage applied to the inverting input terminal of the comparator CP, falls to the level of the voltage applied to the non-inverting input terminal of the comparator CP, the comparator output becomes high. Therefore the voltage output from the output terminal is zero with the switch SW OFF.  
      In an image forming device configured in this way, as shown in  FIG. 7 , when the power supply to the image forming device is turned ON at time T 1 , the fixing roller  61  is heated by the fixing heater  62  and the temperature of the fixing roller  61  gradually rises. Also, in the switching circuit  74  shown in  FIG. 6 , the power supply voltage is applied to the node at which the resistance R 2  and the condenser C 1  are connected. The voltage applied to the non-inverting input terminal of the comparator CP V th  becomes V 1  slightly delayed from T 1  due to the influence of the condenser C 1 . In addition, the voltage applied to the input terminal V in  is proportional to the temperature difference between the high temperature electrode and low temperature electrode of the thermoelectric conversion device  71 , so the voltage increases as the temperature of the fixing roller  61  rises.  
      Then at time T 8  when the temperature of the fixing roller  61  has risen to t 5  which is higher than t 6 , the voltage applied to the input terminal V in  becomes V in , equal to the voltage applied to the non-inverting input terminal of the comparator CP V th . The switch SW of the switching circuit  74  shown in  FIG. 6  is turned ON and an output voltage is output from the output terminal V out , electrical power is supplied to the cooling fan  72  and the cooling fan  72  starts to be driven.  
      Furthermore, at time T 2  when the temperature of the fixing roller  61  rises to t 1 , the temperature of the fixing heater  62  is controlled by output signals from the control unit  80 , and thereafter the temperature of the fixing roller  61  is maintained constant. Therefore the voltage applied to the input terminal V in  becomes V 2 .  
      On the other hand, as shown in  FIG. 7 , when the power supply to the image forming device is turned OFF at time T 3 , electrical power to the fixing heater  62  is stopped and the temperature of the fixing roller  61  gradually reduces. Also, when the power supply to the image forming device is turned OFF electrical charge is accumulated in the condenser C 1  of the switching circuit  74  shown in  FIG. 6 . Therefore the voltage V th  applied to the non-inverting input terminal of the comparator CP gradually reduces with time. Then, at time T 9  when the temperature of the fixing roller  61  has dropped to t 6  which is lower than t 5 , the voltage applied to the input terminal V in  becomes equal to the voltage V th  applied to the non-inverting input terminal of the comparator CP, V 3 . The switch SW on the switching circuit  74  shown in  FIG. 6  is turned OFF, voltage ceases to be output from the output terminal V out , and the cooling fan  72  ceases to be driven.  
      In this way, by providing the switching circuit  74  between the thermoelectric conversion device  71  and the cooling fan  72 , it is possible to set the thermoelectromotive force of the thermoelectric conversion device  71  when the cooling fan  72  starts to be driven higher than the thermoelectromotive force of the thermoelectric conversion device  71  when the cooling fan  72  stops being driven. In other words, it is possible to set the temperature of the fixing roller  61  when the cooling fan  72  starts to be driven higher than the temperature of the fixing roller  61  when the cooling fan  72  stops being driven.  
      Therefore, according to the preferred embodiment, it is possible to delay the time at which the cooling fan  72  starts to be driven after the power supply is turned ON compared with an image forming device configured according to conventional art. Therefore it is possible to shorten the time for the temperature of the fixing roller  61  to reach the temperature required for fixing. Also, it is possible to lengthen the time that the cooling fan  72  is driven after the power supply is turned OFF compared with an image forming device configured according to conventional art. Therefore it is possible to reliably prevent the temporary rise in temperature within the image forming device due to the influence of waste heat from the fixing unit  60  after the power supply to the image forming device is turned OFF.  
     Other Embodiments  
      The following modifications may be made to the configuration of the image forming devices according to the first preferred embodiment and the second preferred embodiment. In the following examples of variations to the image forming devices, the voltage at which the cooling fan  72  can start to be driven is provided so that it is between the voltage that can drive the cooling fan  72  and the rated voltage of the cooling fan  72 . Therefore the delay circuit  73  configured as an integrating circuit as shown in  FIG. 3  and the switching circuit  74  as shown in  FIG. 6  can be omitted.  
      In an image forming device configured in this way, after the power supply to the image forming device is turned ON, as the temperature of the fixing roller  61  rises the thermoelectromotive force of the thermoelectric conversion device  71  also increases. Then, even when the thermoelectromotive force of the thermoelectric conversion device  71  reaches the voltage that can drive the cooling fan  72 , the cooling fan  72  is prevented from being driven until the voltage reaches that at which the cooling fan  72  can start to be driven. Therefore, the cooling fan  72  remains static. Thereafter, when the thermoelectromotive force of the thermoelectric conversion device  71  further increases to the voltage at which the cooling fan  72  can start to be driven, the cooling fan  72  starts to be driven.  
      Also, after the power supply to the image forming device has been turned OFF, the thermoelectromotive force of the thermoelectric conversion device  71  reduces as the temperature of the fixing roller  61  reduces. Then the thermoelectromotive force of the thermoelectric conversion device  71  drops to the voltage at which the cooling fan  72  can be driven, which is lower than the voltage at which the cooling fan  71  can start to be driven, and subsequently as the thermoelectromotive force of the thermoelectric conversion device  71  reduces the rate of revolution of the cooling fan  72  gradually decreases, and the cooling fan  72  stops being driven.  
      In this way, in the image forming device of this embodiment by setting in advance the voltage at which the cooling fan  72  can start to be driven, it is possible to delay the time at which the cooling fan  72  starts to be driven after the power supply is turned ON compared with an image forming device configured according to conventional art. Therefore it is possible to shorten the time for the temperature of the fixing roller  61  to reach the temperature required for fixing.  
      As explained above, the present invention is useful for image forming devices, in particular image forming devices that use the waste heat of the fixing unit to cool the developing system or the optical system.