Abstract:
An image forming apparatus has a fixing unit that fixes a developer transferred onto a print medium. The apparatus includes a temperature detector and a movement detector. The temperature detector has a thermistor that detects a temperature of or near a fixing member. The movement detector detects that the thermistor has moved relative to the fixing member. The movement detector includes a first, second, and third electrically conductive members. The first electrically conductive member and a second electrically conductive member are connected to each other via the thermistor. The third electrically conductive member cooperates with the first electrically conductive member to form a first switch, and with the second electrically conductive member to form a second switch. When the thermistor moves out of contact engagement with the fixing member, at least one of the first switch and the second switch is closed.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to an image forming apparatus and more particularly to an improvement of safety of a fixing unit.  
       DESCRIPTION OF RELATED ART  
       [0002]     A conventional color electrophotographic printer incorporates yellow, magenta, cyan, and black image-forming sections aligned in a line.  FIG. 38  shows one such electrophotographic printer. Respective image-forming sections include recording heads  3 Y,  3 M,  3 C, and  3 B in which point light sources as a recording element are aligned in a line and energized in accordance with print data to form electrostatic latent images of corresponding colors. Toner tanks  12 Y,  12 M,  12 C, and  12 B supply yellow, magenta, cyan, and black toners to the electrostatic latent images to form toner images of corresponding colors. A paper cassette  41  feeds a recording medium S on a page-by-page basis. Each page of recording medium S is attracted to a transfer belt  20  and transported through the image-forming sections in a direction substantially perpendicular to a direction in which the recording elements are aligned. When the recording medium passes through the respective image forming sections, the yellow, magenta, cyan, and black images are transferred onto the recording medium one over the other in registration.  
         [0003]     Thereafter, the recording medium S passes through a fixing unit  63 . The fixing unit  63  includes a fixing roller  64  having a heater therein and a resilient pressure roller  65  in pressure engagement with the fixing roller  64 . When the recording medium S passes through a nip formed between the fixing roller  64  and the pressure roller  65 , the toner images on the recording medium S are fused into a permanent color image. The temperature of the fixing unit  63  is controlled in accordance with temperatures detected by a thermistor  700  in pressure contact with the fixing roller  64  and a thermistor  701  in pressure contact with the pressure roller  65 .  FIG. 39  is an enlarged view of the conventional fixing unit  63 . A support member  700   a  has one end to which the thermistor  700  is mounted and another end secured to a permanent portion of a chassis of the fixing unit  63 . A support member  701   a  has one end to which the thermistor  701  is mounted and another end secured to a permanent portion of a chassis of the fixing unit  63 .  
         [0004]      FIG. 40  illustrates a circuit that detects the surface temperature of the fixing roller  64 . Voltage-dividing resistors  72  and  73  cooperate with the thermistor  700  to divide a 5-V supply voltage by appropriate proportions. The resistance of the thermistor  700  follows the change in the surface temperature of the fixing roller  64 , so that the voltage across the resistor  73  changes. An A/D converter  69  of a controller reads the change in the voltage across the resistor  73  to detect the surface temperature of the fixing roller  64 . A similar circuit that incorporates the thermistor  701  detects the surface temperature of the pressure roller  65 .  
         [0005]      FIG. 41  illustrates another conventional fixing unit  63  of the belt type. A description will be given of only a portion different from that in  FIG. 39 . Pressure rollers  95  and  65  are oppositely disposed. A fixing belt  97  entrained about a heat roller  96  having a heater element therein and a pressure roller  95 . The pressure roller  95  is in pressure contact with the pressure roller  65  with the fixing belt  97  sandwiched therebetween.  
         [0006]     However, with the conventional image-forming apparatus that incorporates the aforementioned fixing unit, printing is performed assuming that the detected temperatures are within the normal range unless an abnormal condition is detected, for example, the temperatures of the fixing roller, fixing belt and pressure roller detected by the corresponding thermistor elements become constant or slowly deviates from the normal value monotonically due to shorting or opening of the circuit. If opening or shorting of the circuit is detected shortly after a printing operation has initiated, the fixing unit may be prevented from being overheated. If a contact type thermistor ( FIG. 1B ) is not in pressure contact with, for example, the fixing roller, fixing belt, and pressure roller or if a non-contact type thermistor ( FIG. 1A ) is away from, for example, the fixing roller, fixing belt, and pressure roller by more than a predetermined distance, the detected temperature is not accurate and temperature control is performed in accordance with the detected temperature with a large error. The erroneously detected temperature is lower than an actual temperature, causing the fixing unit to be overheated.  
       SUMMARY OF THE INVENTION  
       [0007]     An object of the invention is to detect paper jam quickly and prevent overheat of a fixing unit.  
         [0008]     An image forming apparatus has a fixing unit that fixes a developer transferred onto a print medium. The apparatus includes:  
         [0009]     a temperature detector having a temperature detecting element that detects a temperature of a fixing member; and  
         [0010]     a movement detector that detects that the temperature detecting element has moved relative to the fixing member.  
         [0011]     The fixing member is a fixing roller.  
         [0012]     The fixing roller is a heat roller that is heated by a heat source.  
         [0013]     The fixing roller is a pressure roller that is in pressure contact with the heat roller.  
         [0014]     The fixing member is a fixing belt.  
         [0015]     The fixing belt is a heating belt that is heated by a heat source.  
         [0016]     The movement detector includes a first electrically conductive member and a second electrically conductive member connected to each other via the temperature detecting element and a third electrically conductive member disposed in the vicinity of the temperature detecting element. When the temperature detecting element moves, either the first electrically conductive member or the second electrically conductive member establishes electrical continuity with the third electrically conductive member.  
         [0017]     The movement detector includes a first electrically conductive member mounted to the temperature detecting element, a second electrically conductive member disposed in the vicinity of the temperature detecting element. When the temperature detecting element moves, the first electrically conductive member and the second electrically conductive member establish electrical continuity with each other.  
         [0018]     The movement detector includes a first electrically conductive member and a second electrically conductive member connected to each other via the temperature detecting element. When the temperature detecting element moves, the first electrically conductive member and the second electrically conductive member establish electrical continuity with each other.  
         [0019]     The first electrically conductive member has a first end and a second end and the second electrically conductive member has a third end and a fourth end. The first electrically conductive member and the second electrically conductive member are electrically connected to the temperature detecting element through the first end and third end, the second end and the fourth end being connected to a stationary portion of the image forming apparatus.  
         [0020]     The movement detector includes a first electrically conductive member and a second electrically conductive member connected to each other via the temperature detecting element, and a third electrically conductive member disposed in proximity to the temperature detecting element. When the movement detector moves, the first electrically conductive member and the second electrically conductive member establish electrically continuity through the third electrically conductive member.  
         [0021]     The first electrically conductive member has a projection in proximity to the second electrically conductive member. When the temperature detecting element moves, the projection establishes electrical continuity with the second electrically conductive member.  
         [0022]     The temperature detecting element outputs an electrical signal indicative of the temperature of the fixing member, and said temperature detector includes an electrical signal detecting section that detects the electrical signal. The movement detector determines based on the electrical signal detected by the electrical signal detecting section that the temperature detecting element has moved.  
         [0023]     The movement detector includes an electrical switch disposed in proximity to the temperature detecting element. When the temperature detecting element moves, the switch either opens or closes.  
         [0024]     The temperature detecting element is mounted to a resilient member.  
         [0025]     The temperature detecting element is disposed in contact with the fixing member and said movement detector detects that the temperature detecting element has moved out of contact with the fixing member.  
         [0026]     The temperature detecting element is disposed in proximity to the fixing member, and the movement detector detects that the temperature detecting element has moved relative to the fixing member.  
         [0027]     The movement detector includes a first electrically conductive member, a second electrically conductive member connected to each other via the temperature detecting element, and a third electrically conductive member. When no paper jam occurs, the first electrically conductive member remains in electrical contact with the third electrically conductive member. When paper jam occurs, the first electrically conductive member moves out of contact with the third electrically conductive member.  
         [0028]     An image forming apparatus includes a fixing member, a temperature detector, and a movement detector. The fixing member heats a developer transferred onto a print medium to fix the developer. The temperature detector has a temperature detecting element detects a temperature of a fixing member. The movement detector detects a position of the temperature detecting element.  
         [0029]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limiting the present invention, and wherein:  
         [0031]      FIG. 1A  illustrates a contact type thermistor;  
         [0032]      FIG. 1B  illustrates a non-contact thermistor;  
         [0033]      FIG. 2  illustrates the configuration of a first embodiment;  
         [0034]      FIG. 3  is an electrically equivalent circuit of the first embodiment;  
         [0035]      FIG. 4  illustrates a fixing unit when paper jam like an accordion occurs;  
         [0036]      FIG. 5  illustrates analog waveforms before and after the occurrence of paper jam;  
         [0037]      FIG. 6  illustrates a fixing unit and a contact type thermistor that are employed in a second embodiment;  
         [0038]      FIG. 7  is an electrically equivalent circuit of the second embodiment;  
         [0039]      FIG. 8  illustrates the fixing unit in the second embodiment when paper jam like an accordion occurs;  
         [0040]      FIG. 9  illustrates analog voltage waveforms in the second embodiment before and after the occurrence of paper jam;  
         [0041]      FIG. 10  illustrates a fixing unit according to a third embodiment;  
         [0042]      FIG. 11  illustrates a thermistor according to the third embodiment;  
         [0043]      FIG. 12  is an electrically equivalent circuit of the third embodiment;  
         [0044]      FIG. 13  illustrates analog voltage waveforms in the third embodiment before and after the occurrence of paper jam;  
         [0045]      FIG. 14  illustrates a thermistor and a fixing unit according to a fourth embodiment;  
         [0046]      FIGS. 15A-15D  illustrate the details of the thermistor according to the fourth embodiment;  
         [0047]      FIG. 16  illustrates an electrically equivalent circuit of the fourth embodiment;  
         [0048]      FIG. 17  illustrates analog voltage waveforms in the fourth embodiment before and after the occurrence of paper jam;  
         [0049]      FIG. 18  illustrates a fixing unit and a contact type thermistor that are employed in a fifth embodiment;  
         [0050]      FIGS. 19A-19C  illustrate the configuration of the thermistor according to the fifth embodiment;  
         [0051]      FIG. 20  is an electrically equivalent circuit of the fifth embodiment;  
         [0052]      FIG. 21  illustrates analog voltage waveforms in the fifth embodiment before and after the occurrence of paper jam;  
         [0053]      FIG. 22  illustrates a sixth embodiment;  
         [0054]      FIG. 23  is an electrically equivalent circuit of the sixth embodiment;  
         [0055]      FIG. 24  illustrates a fixing unit according to the sixth embodiment;  
         [0056]      FIG. 25  illustrates analog voltage waveforms in the sixth embodiment before and after the occurrence of paper jam;  
         [0057]      FIGS. 26 and 27  illustrate the configuration of a seventh embodiment;  
         [0058]      FIG. 28  is an electrically equivalent circuit of the seventh embodiment;  
         [0059]      FIG. 29  illustrates analog voltage waveforms in the seventh embodiment before and after the occurrence of paper jam;  
         [0060]      FIG. 30  illustrates the configuration of an eighth embodiment;  
         [0061]      FIG. 31  is an electrically equivalent circuit of the eighth embodiment;  
         [0062]      FIG. 32  illustrates the fixing unit according to the eighth embodiment when paper jam-like an accordion;  
         [0063]      FIG. 33  illustrates analog voltage waveforms according to the eighth embodiment before and after the occurrence of paper jam;  
         [0064]      FIG. 34  illustrates the configuration of a ninth embodiment;  
         [0065]      FIG. 35  illustrates the fixing unit according to the ninth embodiment when paper jam like an accordion;  
         [0066]      FIG. 36  is an electrically equivalent circuit of the ninth embodiment;  
         [0067]      FIG. 37  illustrates analog voltage waveforms in the ninth embodiment before and after the occurrence of paper jam;  
         [0068]      FIG. 38  shows a conventional electrophotographic printer;  
         [0069]      FIG. 39  is an enlarged view of the conventional fixing unit;  
         [0070]      FIG. 40  illustrates a circuit that detects the surface temperature of a fixing roller of the conventional fixing unit; and  
         [0071]      FIG. 41  illustrates another conventional fixing unit of the belt type.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0072]     Embodiments of the invention will be described in detail with reference to the accompanying drawings.  
         [0000]     First Embodiment  
         [0073]     A fixing unit  63  in first to ninth embodiments is of the same configuration as the fixing unit  63  in  FIG. 38 , and is detachably mounted. The fixing unit  63  is a consumable item that can be replaced by a user. That is, when the accumulated number of printed pages exceeds a predetermined value, the user is prompted to replace the fixing unit  63  for a new, unused one.  
         [0074]      FIG. 1A  and  FIG. 1B  illustrate a contact type thermistor and a non-contact thermistor, respectively. Two types of thermistor can be used in the first embodiment. The first is a contact type ( FIG. 1A ) where signal lines connected to a temperature-sensing element  79  are electrically continuous with base plates  80   a  and  80   b  that support the temperature-sensing element  79 . The base plate  80   a  and  80  are made of a resilient material. The second is a non-contact type where signal lines connected to a temperature-sensing element  79  are electrically isolated from a base plate  76  that supports the temperature-sensing element  79 . The base plate  76  is made of a resilient material. Here, by way of the contact type thermistor  70 A, a description will be given of an example where the surface temperature of a fixing roller  64  that serves as a heat roller is detected. For this type of thermistor, the electrically conductive base plates  80   a  and  80   b  are used as both signal lines and reinforcing or supporting plates. A protection sheet  77  is formed of an insulating material to protect the temperature-sensing element  79 .  
         [0075]      FIG. 2  illustrates the configuration of the first embodiment. This configuration differs from the conventional art in that the base plates  80   a  and  80   b  that support the temperature-sensing element  79  are positioned midway between closely disposed conductors  75   a  and  75   b . When the conductors  75   a  and  75   b  are connected to 0 V and no paper jam has occurred, the base plates  80   a  and  80   b  are not in contact with any one of the conductors  75   a  and  75   b.    
         [0076]      FIG. 3  is an electrically equivalent circuit of the first embodiment that employs the thermistor  70 A, illustrating the temperature-sensing element  79 , conductors  75   a  and  75   b , base plates  80   a  and  80   b , and voltage-dividing resistors  72  and  73 . The base plate  80   a  and  80   b  are used as both signal lines and reinforcing plates.  
         [0077]     Switches  87  and  88  represent an electrically equivalent circuit of the contacts between the base plates  80   a  and  80   b  and the conductors  75   a  and  75   b . When no paper jam has occurred, the switch  87  or  88  is open. When paper jam like an accordion as shown in  FIG. 4  occurs near an entrance of the fixing unit  63  during printing, the jammed paper pushes the temperature-sensing element  79  and the base plates  80   a  and  80   b . This causes the temperature-sensing element  79  to move out of contact with the fixing roller  64 . Then, the base plate  80   a  or  80   b  moves into contact with the conductor  75   a  or  75   b  to close the switch  87  or  88 .  
         [0078]     When the paper jam has not occurred, the switches  87  and  88  are open and the voltage (Vt) detected by an A/D converter  69  in the controller is given by
 
 V ( t )=5* R   73 /( R   72 + R ( t ) +R   73 )
 
 where R 72  is the resistance of the resistor  72 , R 73  is the resistance of the resistor  73 , R(t) is the resistance of the temperature-sensing element  79  that reflects the surface temperature of the fixing roller  64 , and the numeral 5 denotes the supply voltage in volts for temperature detection. 
 
         [0079]      FIG. 5  illustrates analog waveforms before and after the occurrence of paper jam. A high voltage is input to the A/D converter  69  when no paper jam occurs. When the switch  87  or  88  is closed due to the occurrence of paper jam, the voltage V(t) falls to 0 V.  
         [0080]     Experiment was conducted to determine an input voltage to the A/D converter  69  when the fixing unit  63  operates normally, and an input voltage when the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 . Then, if an input voltage to the A/D converter  69  is lower than the normal value by more than a predetermined value, then it is determined that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 . The controller receives the output of the A/D converter  69  and generates an alarm signal.  
         [0081]     Once it is detected that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 , the operation of the fixing unit  63  will not performed any further unless the jammed paper is removed and the thermistor  70 A returns to its normal position. If the base plate(s) of the thermistor  70 A has deformed permanently, at least one of the base plates  80   a  and  80   b  remains in contact with one of the conductors  75   a  and  75   b . Thus, the input voltage to the A/D converter is 0 V even after the jammed paper has been removed.  
         [0000]     Second Embodiment  
         [0082]      FIG. 6  illustrates a fixing unit  63  and a contact type thermistor  70 A that are employed in a second embodiment. The configuration of the second embodiment differs from the prior art in that base plates  80   a  and  80   b  that support a temperature-sensing element  79  are positioned midway between conductors  75   a  and  75   b  closely positioned. The base plate  80   a  and  80  are made of a resilient material. The second embodiment may employ either of the contact type thermistor in  FIG. 1A  and the non-contact type thermistor in  FIG. 1B . The second embodiment will be described with respect to a case in which the non-contact type thermistor in  FIG. 1B  is employed. Two lines are electrically isolated from a base plate  76  by means of an insulator  78 , and are led out from the temperature-sensing element  79 . This type of thermistor has an electrically conductive base plate  76  that is used as both a signal line and a reinforcing plate. The base plate  76  is made of a resilient material.  
         [0083]      FIG. 7  illustrates an electrically equivalent circuit that includes voltage-dividing resistors  72  and  73 , temperature-sensing element  79 , conductors  75   a  and  75   b , and base plates  80   a  and  80   b . A switch  89  represents the contacts between the base plates  80   a  and  80   b  of the thermistor  70 A and the conductors  75   a  and  75   b . The resistor  74  has one end connected to an input port of an A/D converter  69  in the controller and the conductors  75   a  and  75   b , another end connected to the 5-V power supply for the controller.  
         [0084]     When no paper jam has occurred, there are a gap between the base plate  76  and the conductor  75   a  and a gap between the base late  76  and the conductor  75   b , so that the switch  89  is not closed. Therefore, the base plate  76  is not electrically continuous to the conductors  75   a  and  75   b . In other words, when the switch  89  is not closed to the conductor  75   a  or  75   b , the input port of the A/D converter  69  is at an “H” level, which is substantially equal to the supply voltage (e.g., 5 V) of the controller.  
         [0085]     If paper jam like an accordion as shown in  FIG. 8  occurs near the entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 A and the base plate  76 . This causes the temperature-sensing element  79  of the thermistor  70 A to move out of contact with a fixing roller  64  and the base plate  76  moves into contact with the conductor  75   b  to close the switch  89 .  
         [0086]      FIG. 9  illustrates analog voltage waveforms before and after the occurrence of paper jam. The voltage at the input of the A/D converter  69  is at the “H” level before paper jam, and at an “L” level after the occurrence of paper jam.  
         [0087]     When no paper jam has occurred, the common terminal of the switch  89  is positioned midway between the conductors  75   a  and  75   b  and the input of the A/D converter is at nearly 5 V, so that the “H” level is detected. When the switch  89  is closed to the conductor  75   b , the voltage at the input of the A/D converter  69  falls to 0 V, so that the “L” level is detected. Thus, the A/D converter  69  detects that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 , and generates an alarm signal. If an input voltage to the A/D converter  69  is lower than the normal value by more than a predetermined value, then it is determined that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 .  
         [0088]     When it is detected that the temperature-sensing element  79  has moved out of contact with the fixing roller  64 , the operation of the fixing unit  63  will not be performed any further unless the jammed paper S is removed and the thermistor  70 A returns to its normal position. That is, the operation of the fixing unit  63  will not be performed any further until the base plate  76  is positioned midway between the conductors  75   a  and  75   b  again. If the base plate  76  has deformed permanently, the detection signals of the controller or the A/D converter  69  continues to indicate that the thermistor  70 A has moved out of contact engagement with the fixing roller  64 . Thus, the operation of the fixing unit  63  will not be performed any further.  
         [0000]     Third Embodiment  
         [0089]      FIG. 10  illustrates a fixing unit  63  according to a third embodiment. The third embodiment differs from the conventional art in that base plates  80   a  and  80   b  that support a temperature-sensing element  79  of a thermistor  70 A is positioned midway between conductor  75   a  and  75   b . The base plate  80   a  and  80  are made of a resilient material. The third embodiment can employ either of two types of thermistor as shown in  FIG. 1A  (contact type) and  FIG. 1B  (non-contact type). Here, the third embodiment will be described with respect to the contact type in  FIG. 1A .  
         [0090]      FIG. 11  illustrates the thermistor  70 A according to the third embodiment. As shown in  FIG. 11 , the electrically conductive base plates  80   a  and  80   b  support the temperature-sensing element  79  and are used as both signal lines and a reinforcing plate. When the temperature-sensing element  79  of the thermistor  70 A moves out of contact with a fixing roller  64 , the base plates  80   a  and  80   b  move into contact with the conductors  75   a  and  75   b  at substantially the same time. As a result, there is electrical continuity between the base plates  80   a  and  80   b.    
         [0091]      FIG. 12  is an electrically equivalent circuit that includes voltage-dividing resistors  72  and  73 , temperature-sensing element  79 , base plates  80   a  and  80   b , and conductors  75   a  and  75   b . Switches  91   a  and  91   b  represent the contacts between the base plates  80   a  and  80   b  of the thermistor  70 A and the conductors  75   a  and  75   b , respectively. The conductors  75   a  and  75   b  have one ends connected to the resistors  72  and  73 , respectively. When no paper jam occurs, the base plates  80   a  and  80   b  are not in contact with the conductors  75   a  and  75   b.    
         [0092]     When no paper jam occurs, the switch  91  is open. As shown in  FIG. 10 , if paper jam like an accordion occurs near the entrance of the fixing unit  63  during printing, the jammed paper pushes the thermistor  70 A and the base plates  80   a  and  80   b . This causes the temperature-sensing element  79  of the thermistor  70 A to move out of contact with the fixing roller  64 . Thus, the base plates  80   a  and  80   b  move into contact with the conductor  75   a  or  75   b , closing the switches  91   a  and  91   b  in  FIG. 12 .  
         [0093]      FIG. 13  illustrates analog voltage waveforms before and after the occurrence of paper jam. Before paper jam occurs, the supply voltage is divided by the temperature-sensing element  79  and the voltage-dividing resistors  72  and  73 . Thus, the analog voltage before the occurrence of paper jam is the voltage across the resistor  73 , the voltage being divided by the temperature sensing element  79  and the voltage dividing resistors  72  and  73 . The analog voltage after the occurrence of paper jam is the voltage across the resistor  73 , the voltage being divided by the voltage dividing resistors  72  and  73 .  
         [0094]     When no paper jam occurs, the switches  91   a  and  91   b  are open. The voltage (Vt) detected in the A/D converter  69  in the controller is given by
 
 V ( t )=5* R   73 /( R   72   +R ( t ) +R   73 )
 
 where R 72  is the resistance of the voltage-dividing resistor R 72 , R 73  is the resistance of the voltage-dividing resistor R 73 , R(t) is the resistance of the temperature-sensing element  79  that reflects the surface temperature of the fixing roller  64 , and the supply voltage for temperature detection is 5 V. 
 
         [0095]     When no paper jam occurs, the switches  91   a  and  91   b  are closed and the resistance R(t) of the temperature-sensing element  79  that reflects the surface temperature t of the fixing roller  64  is short-circuited. Thus, the voltage (Vt) detected in the A/D converter  69  in the controller is given by
 
 V ( t )=5* R   73 /( R   72   +R   73 )
 
         [0096]     As described above, when the voltage V(t) changes, the controller determines that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 , and generates an alarm signal.  
         [0097]      FIG. 13  illustrates analog voltage waveforms before and after the occurrence of paper jam. The voltage is at an “L” level before the occurrence paper jam, and at an “H” level after the occurrence of paper jam. As described above, when no paper jam occurs, the voltage detected by the A/D converter  69  reflects the surface temperature of the fixing roller  64 . When paper jam occurs, the voltage detected by the A/D converter  69  is a fixed voltage that is divided by the voltage-dividing resistors R 72  and R 73 . If an input voltage to the A/D converter  69  is lower than the normal value by more than a predetermined value, then it is determined that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 .  
         [0098]     Once paper jam occurs, the operation of the fixing unit  63  will not performed any further unless the jammed paper S is removed and the thermistor  70 A has returned to its normal position. That is, the operation of the fixing unit  63  will not be performed any further until the base plates  8   a  and  80   b  are positioned midway between the conductors  75   a  and  75   b  and therefore the thermistor  70 A is again in contact with the fixing roller  64 . If the base plates  80   a  and  80   b  of the thermistor  70 A have deformed permanently, the base plates  80   a  and  80   b  remain in contact with either of the conductors  75   a  and  75   b . Thus, the detection signal of the A/D converter  69  continues to indicate that the temperature-sensing element  79  of the thermistor  70 A has moved out of contact with the fixing roller  64 , and the operation of the fixing unit  63  will not be performed any further.  
         [0099]     The first and second embodiments require the wiring materials that connect the switch ( FIG. 3 ) to 0 V. The second embodiment requires the resistor  74  ( FIG. 7 ) that detects a change in voltage. On the contrary, the third embodiment eliminates the need for the switch and wiring materials resistor  74 . The non-contact type thermistor may also be used in the third embodiment.  
         [0000]     Fourth Embodiment  
         [0100]      FIG. 14  illustrates a thermistor  70 C and a fixing unit  63  according to a fourth embodiment.  FIGS. 15A-15D  illustrate the details of the thermistor  70 C according to the fourth embodiment.  FIGS. 15A and 15B  are side views as seen in a direction shown by arrow B in  FIG. 15D .  FIG. 15C  is a front view as seen in a direction shown by arrow A in  FIG. 15D .  FIG. 15D  is a perspective view of the thermistor  70 C.  
         [0101]     The fourth embodiment differs from the conventional art in that base plates  80   a  and  80   b  that support the thermistor  70 C are positioned midway between stoppers  90   a  and  90   b  that are closely positioned. The base plate  80   a  and  80  are made of a resilient material. The fourth embodiment employs the thermistor in  FIGS. 15A-15D . The thermistor according to the fourth embodiment is similar to that of  FIG. 1A  but differs in that projections  80   d  and  80   e  extend in parallel with each other from, for example, the base plate  80   c , and the base plate  80   a  extends midway between the projections  80   d  and  80   e . When no paper jam occurs, the projections  80   d  and  80   e  are not in contact with the base plate  80   a.    
         [0102]     For a case where the thermistor illustrated in  FIGS. 15A-15D  is used, a description will be given of the operation of detecting that the temperature-sensing element  79  of the thermistor  70 C has moved out of contact a fixing roller  64 .  
         [0103]     When paper jam like an accordion as shown in  FIG. 14  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 C and the base plates  80   a  and  80   c . This causes the temperature-sensing element  79  of the thermistor  70 C to move out of contact with the fixing roller  64 . Further, the projection  80   d  or  80   e  abuts stopper  90   a  or  90   b , so that the projection  80   d  or  80   e  will deform to move into contact with the base plate  80   a . When the jammed paper S pushes the base plates  80   a  and  80   c , the base plates  80   a  and  80   c  deform as shown in  FIG. 15A , so that the projection  80   e  and the base plate  80   a  abut the stopper  90   b  to make good electrical contact between the  80   a  and  80   e.    
         [0104]      FIG. 16  illustrates an electrically equivalent circuit that includes voltage-dividing resistors  72  and  73 , the temperature-sensing element  79 , the base plates  80   a  and  80   c , and the projections  80   e  and  80   d . A switch  91  represents the contacts between the projections  80   e  and  80   d  and the base plate  80   a.    
         [0105]     When no paper jam occurs, the switch  91  is open. When paper jam like an accordion as shown in  FIG. 14  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 C and the base plates  80   a  and  80   c . This causes the temperature-sensing element  79  of the thermistor  70 C to move out of contact with the fixing roller  64 . Further, the base plate  80   a  or the base plate  80   c  moves into contact with the projections  80   e  and  80   d , so that the switch  91  in  FIG. 16  is closed.  
         [0106]     When the paper jam S has not occurred, the switch  91  is open and the voltage (Vt) detected by the A/D converter  69  of the controller is given by
 
 V ( t )=5* R   73 /( R   72   +R ( t ) +R   73 )
 
 where R 72  is the resistance of the voltage-dividing resistor  72 , R 73  is the resistance of the voltage-dividing resistor R 73 , R(t) is the resistance of the thermistor  70 C that reflects the surface temperature of the fixing roller  64 , and numeral 5 denotes the supply voltage in volts for temperature detection. 
 
         [0107]     When paper jam has occurred, the switch  91  is closed and the voltage across the thermistor  70 C (i.e., temperature-sensing element  79 ) is zero volts. Thus, the voltage V(t) is given by
 
 V ( t )=5* R   73 /( R   72   +R   73 ).
 
         [0108]      FIG. 17  illustrates analog voltage waveforms before and after the occurrence of paper jam. Before paper jam occurs, the supply voltage is divided by the temperature-sensing element  79  and the voltage-dividing resistors  72  and  73 . Thus, the analog voltage before the occurrence of paper jam is the voltage across the resistor  73 , the supply voltage being divided by the voltage dividing resistors  72  and  73  and the temperature sensing element  79 . The analog voltage after the occurrence of paper jam is the voltage across the resistor  72 , the supply voltage being divided by the voltage dividing resistors  72  and  73 .  
         [0109]     If an input voltage to the A/D converter  69  is lower than the normal value by more than a predetermined value, the controller determines that the temperature-sensing element  79  of the thermistor  70 C has moved out of contact with the fixing roller  64 , and generates an alarm signal.  
         [0110]     Once paper jam occurs, the operation of the fixing unit  63  will not performed any further unless the jammed paper S is removed and the thermistor  70 C returns to its normal position where the thermistor  70 C is in contact with the fixing roller  64 . If the base plate of the thermistor  70 C has deformed permanently, the detection signal from the A/D converter  69  continues to indicate that the temperature-sensing element  79  of the thermistor  70 C is out of contact with the fixing roller  64 . Thus, the operation of the fixing unit  63  will not be performed any further.  
         [0111]     The first and second embodiments require the wiring materials that connect the switch to a potential of 0 V. The second embodiment requires the resistor  74  for detecting a change in voltage. On the contrary, the fourth embodiment eliminates the need for these wiring materials and the resistor. The non-contact type thermistor may also be used in the fourth embodiment.  
         [0000]     Fifth Embodiment  
         [0112]      FIG. 18  illustrates a fixing unit  63  and a contact type thermistor  70 D that are employed in a fifth embodiment. The configuration of the fifth embodiment has the feature that a base plate  84  is fixed to a temperature-sensing element  79  and a base plate  83  is movable into and out of contact with the base plate  84 . The base plate  83  and  84  are made of a resilient material.  
         [0113]      FIGS. 19A-19C  illustrate the configuration of the thermistor  70 D. The thermistor  70 D includes the base plates  83  and  84  that are movable into and out of contact engagement with each other. The thermistor  70 D further includes a base plate  85  made of a resilient material. The base plates  84  and  85  are electrically connected to each other via a temperature-sensing element  79 . When paper jam does not occurred, the base plate  83  remains in electrical contact engagement with the base plate  84 . When paper jam occurs, the base plates  83  and  84  deform such that the base plate  83  abuts a stopper  90   b  and the base plate  84  moves out of contact with the base plate  83 .  
         [0114]     With respect to a case where the thermistor illustrated in  FIGS. 19A-19C  is used, a description will be given of the operation of detecting that the temperature-sensing element  79  of the thermistor  70 D has moved out of contact with a fixing roller  64 .  
         [0115]     When paper jam like an accordion as shown in  FIG. 18  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 D and the base plates  80   a  and  80   b . This causes the temperature-sensing element  79  of the thermistor  70 D to move out of contact with the fixing roller  64 . The stopper  90   b  interferes with the base plate  83  causing the base plate  84  to move out of contact with the base plate  83 .  
         [0116]      FIG. 20  is an electrically equivalent circuit that includes the voltage-dividing resistors  72  and  73 , temperature-sensing element  79 , and base plates  83  and  84 . A switch  92  represents the contact between the base plate  83  and the base plate  84 .  
         [0117]     When no paper jam occurs, the switch  92  remains closed. When paper jam like an accordion as shown in  FIG. 18  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 D and the base plate  84 . This causes the temperature-sensing element  79  of the thermistor  70 D to move out of contact with the fixing roller  64 . Moreover, the base plate  84  moves out of contact with the base plate  83  to open the switch  92 . When the switch  92  opens, the 5-V power supply is shut off so that the voltage across the resistor  73  falls to 0 V.  
         [0118]     When no paper jam occurs, the switch  92  remains closed and the voltage (Vt) detected by the A/D converter  69  of the controller is given by
 
 V ( t )=5* R   73 /( R   72   +R ( t )+ R   73 )
 
 where R 72  is the resistance of the resistor  72 , R 73  is the resistance of the resistor R 73 , R(t) is the resistance of the temperature-sensing element  79  that reflects the surface temperature of the fixing roller  64 , and numeral 5 denotes the supply voltage in volts for temperature detection. 
 
         [0119]     When no paper jam has occurred, the switch  92  opens and the voltage V(t) is at 0 V. As described above, in response to the change in the voltage input to the A/D converter  69 , the controller detects that the temperature-sensing element  79  of the thermistor  70 D has moved out of contact with the fixing roller  64 , and generates an alarm signal.  
         [0120]      FIG. 21  illustrates analog voltage waveforms before and after the occurrence of paper jam. The voltage input to an A/D converter  69  reflects the surface temperature of the fixing roller  64  when no paper jam occurs, and falls to 0 V when paper jam occurs. If an input voltage to the A/D converter  69  is lower than the normal value by more than a predetermined value, the controller determines that the temperature-sensing element  79  of the thermistor  70 D has moved out of contact with the fixing roller  64 .  
         [0121]     When paper jam occurs, the operation of the fixing unit  63  will not be performed any further unless the jammed paper is removed and the thermistor  70 D returns to its normal position. When the base plate of the thermistor  70 D has deformed permanently, even though the jammed paper is removed, the detection signal of the controller or the A/D converter  69  continues to indicate that the temperature-sensing element  79  is out of contact with the fixing roller  64 . Thus, the operation of the fixing unit  63  will not be performed any further.  
         [0122]     The fifth embodiment eliminates the need for the wiring materials that were required to connect the switch to 0 V in the first and second embodiments, and the resistor  74  for detecting the change in voltage, which were required in the second embodiment. Because a part of the thermistor serves as an electrical switch, the number of parts required is reduced, providing a simplified configuration. The non-contact type thermistor may also be used in the fifth embodiment.  
         [0000]     Sixth Embodiment  
         [0123]      FIG. 22  illustrates a sixth embodiment. The sixth embodiment differs from the conventional art in that a base plate  76  that supports a non-contact type thermistor  70 B is positioned midway between conductors  75   a  and  75   b . The base plate  76  is made of a resilient material. The sixth embodiment may employ either of a contact type thermistor in  FIG. 1A  and a non-contact type thermistor in  FIG. 1B . Here, the sixth embodiment will be described with respect to a case in which the thermistor in  FIG. 1B  is employed. Two leads are electrically isolated from the base plate  76  by means of an insulator  78 , and are led out from the temperature-sensing element  79 . This type of thermistor has an electrically conductive base plate  76  that is used as both a signal line and a reinforcing plate. The base plate  76  is connected to a potential of 0 V.  
         [0124]     When no paper jam has occurred, the base plate  76  is midway between the conductors  75   a  and  75   b  such that the base plate  76  is not in contact with the conductors  75   a  and  75   b . When paper jam occurs, the base plate  76  moves into contact with, for example, the conductor  75   b.    
         [0125]     With respect to a case where the thermistor illustrated in  FIG. 1B  is used, a description will be given of the operation of detecting that the temperature-sensing element  79  of the thermistor  70 B has moved out of contact with a fixing roller  64 .  
         [0126]     When paper jam like an accordion as shown in  FIG. 24  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 B and the base plate  76 . This causes the temperature-sensing element  79  of the thermistor  70 B to move out of contact with the fixing roller  64 . The base plate  76  moves into contact with either the conductor  75   a  or the conductor  75   b.    
         [0127]      FIG. 23  is an electrically equivalent circuit that includes the voltage-dividing resistors  72  and  7 , temperature-sensing element  79 , conductors  75   a  and  75   b , base plate  76 , and resistor  74 . A switch  89  represents the contacts between the base plate  76  and the conductors  75   a  and  75   b . The resistor  74  has one end connected to the conductors  75   a  and  75   b  and the input port of the A/D converter  69  in the controller, and another end connected to a 5-V power supply. When no paper jam occurs, the common electrode of the switch  89  is positioned midway between the conductors  75   a  and  75   b , so that an “H” level appears at the input of the A/D converter  69 . When paper jam occurs, the base plate  76  of the thermistor  70 B goes into electrical contact with either the conductor  75   a  or the conductor  75   a  or the conductor  75   b.    
         [0128]     A description will be given of the operation of detecting that the temperature-sensing element  79  of the non-contact type thermistor  70 B has moved out of contact with the fixing roller  64 .  
         [0129]     When paper jam like an accordion as shown in  FIG. 24  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 B and the base plate  76 . This causes the temperature-sensing element  79  of the thermistor  70 B to move out of contact with the fixing roller  64 . As shown in  FIG. 8 , the base plate  76  moves into contact with the conductor  75   b , causing the switch  89  in  FIG. 23  to switch to the conductor  75   b.    
         [0130]      FIG. 25  illustrates analog voltage waveforms before and after the occurrence of paper jam. The analog voltage is an “H” level before paper jam occurs and an “L” level (i.e., substantially 0 V) after paper jam occurs. Thus, the voltage input to A/D converter  69  reflects the surface temperature of the fixing roller  64  when no paper jam occurs, and falls to 0 V when paper jam occurs. If an input voltage to the A/D converter  69  is lower than the normal value by more than a predetermined value, the controller determines that the thermistor  70 B has moved out of contact with the fixing roller  64 .  
         [0131]     Once paper jam occurs, the operation of the fixing unit  63  will not be performed any further unless the jammed paper S is removed and the thermistor  70 B returns to its normal position. If the base plate  76  of the thermistor  70 B has deformed permanently, the output of the A/D converter  69  continues to indicate that the thermistor  70 B is out of contact with the fixing roller  64  even though the jammed paper S is removed. Thus, the operation of the fixing unit  63  will not be performed any further. The sixth embodiment has been described with respect to the same configuration as the second embodiment except that a non-contact type thermistor is used instead of the contact type thermistor.  
         [0000]     Seventh Embodiment  
         [0132]      FIGS. 26 and 27  illustrate the configuration of a seventh embodiment. The seventh embodiment differs from the conventional art in that a switch  93  is employed. Referring to  FIG. 26 , when a non-contact type thermistor  70 B moves a predetermined distance in such a direction as to be away from a fixing roller  64 , the switch  93  is driven by an electrically conductive base plate  76  of the thermistor  70 B to close as shown in  FIG. 27 . The base plate  76  is made of a resilient material.—The seventh embodiment may employ either of the type in  FIG. 1A  and the type in  FIG. 1B . Here, the seventh embodiment will be described with respect to a case in which the non-contact type thermistor of the type in  FIG. 1B . The thermistor of  FIG. 1B  includes two signal lines isolated by an insulator  78  from a base plate  76  that supports a temperature-sensing element  79 .  
         [0133]      FIG. 28  is an electrically equivalent circuit that includes the voltage-dividing resistors  72  and  73 , temperature-sensing element  79 , base plate  76 , and resistor  74 . The resistor  74  has one end connected to the input port of a controller and another end connected to a 5-V power supply for the controller. When no paper jam occurs, the switch  93  is open so that the voltage at the input port of the A/D converter  69  is at an “H” level. When paper jam occurs, the switch  93  is closed so that the voltage at the input pot of the A/D converter  69  is at an “L” level, which is substantially 0 V. As shown in  FIG. 28 , the switch  93  has one end connected to the resistor  74  and the A/D converter, and another end connected to a potential of 0 V.  
         [0134]     A description will be given of the operation of detecting that a non-contact type thermistor  70 B has moved out of contact with the fixing roller  64 .  
         [0135]     When paper jam like an accordion as shown in  FIG. 27  occurs near an entrance of a fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 B and the base plate  76 . This causes the temperature-sensing element  79  of the thermistor  70 B to move out of contact with the fixing roller  64 . The base plate  76  pushes the switch  93  in  FIG. 28  to close the switch  93 .  
         [0136]     As described above, when the input voltage to the controller falls to 0 V, the controller determines that the temperature-sensing element  79  of the thermistor  70 B has moved out of contact with the fixing roller  64 , and generates an alarm signal.  
         [0137]      FIG. 29  illustrates analog voltage waveforms before and after the occurrence of paper jam. The analog voltage is at an “H” level before paper jam occurs and at an “L” level (substantially 0 V) after paper jam has occurred. Thus, once paper jam occurs, the voltage input to A/D converter  69  no longer reflects the surface temperature of the fixing roller  64 .  
         [0138]     Once paper jam occurs, the operation of the fixing unit  63  will not be performed any further unless the jammed paper S is removed and the thermistor  70 B returns to its normal position. If the baseplate  76  of the thermistor  70 B has deformed permanently, the output of the A/D converter  69  in the controller continues to indicate that the thermistor  70 B is out of contact with the fixing roller  64  even though the jammed paper is removed. Thus, the operation of the fixing unit  63  will not be performed any further. The seventh embodiment is of the same configuration as the second embodiment except that a non-contact type thermistor is used instead of the contact type thermistor.  
         [0000]     Eighth Embodiment  
         [0139]      FIG. 30  illustrates the configuration of an eighth embodiment. The eighth embodiment differs from the conventional art in that an electrically conductive base plate  76  is midway between conductors  94   a  and  94   b . The base plate  76  is made of a resilient material. The eighth embodiment may employ either of the type in  FIG. 1A  and the type in  FIG. 1B . Here, the eighth embodiment will be described with respect to a case in which a thermistor of the type in  FIG. 1B . The thermistor  70 B of  FIG. 1B  includes two signal lines isolated by an insulator  78  from the base plate  76  that supports a temperature-sensing element  79 . The base plate  76  is connected to a potential of 0 V.  
         [0140]     With respect to a case where the thermistor  70 B illustrated in  FIG. 1B  is used, a description will be given of the operation of detecting that the thermistor  70 B has moved out of contact with a pressure roller  65 .  
         [0141]      FIG. 31  is an electrically equivalent circuit that includes the voltage-dividing resistors  72  and  73 , temperature-sensing element  79 , conductors  94   a  and  94   b , base plate  76 , and resistor  74 . The resistor  74  has one end connected to the input port of a controller and another end connected to a 5-V power supply for the controller. The base plate  76  and the conductors  94   a  and  94   b  form a switch  89 .  
         [0142]     When no paper jam occurs, the common electrode of the switch  89  is midway between the conductors  94   a  and  94   b  and the voltage at the input port of the A/D converter  69  in the controller is at an “H” level. When paper jam occurs, the switch  89  is closed so that the voltage at the input of the A/D converter  69  is at an “L” level. As shown in  FIG. 31 , the switch  89  has one end connected to the resistor  74  and the controller, and another end connected to a potential of 0 V.  
         [0143]     The resistor  74  has one end connected to a 5-V power supply and another end connected to the conductors  94   a  and  94   b  and the input port of the A/D converter  69  in the controller. The resistor  74  has one end connected to a 5-V power supply and another end connected to the conductors  94   a  and  94   b  and the input of the A/D converter  69  in the controller. When no paper jam occurs so that the switch  89  is switched to neither the conductor  94   a  nor the conductor  94   b , the voltage at the input port of the A/D converter  69  is at an “H” level.  
         [0144]     A description will be given of the operation of detecting that the non-contact type thermistor  71  has moved out of contact with the pressure roller  65  by a predetermined distance. The thermistor  71  is the same type as the thermistor  70 B in  FIG. 1B .  
         [0145]     When paper jam like an accordion as shown in  FIG. 32  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  71  and the base plate  76 . This causes the temperature-sensing element  79  of the thermistor  71  to move out of contact with the pressure roller  65 . Thus, the base plate  76  moves into contact with the conductor  94   b  in  FIG. 31  to switch the switch  89  to the conductor  94   b.    
         [0146]     As described above, when the input voltage to the A/D converter  69  changes, the controller determines that the temperature-sensing element  79  of the thermistor  71  has moved out of contact with the pressure roller  65 , and generates an alarm signal.  
         [0147]      FIG. 33  illustrates analog voltage waveforms before and after the occurrence of paper jam. When no paper jam occurs, the voltage at the input port of the controller is at an “H” level, which is substantially the same as 5-V supply voltage for the controller. When paper jam occurs, the voltage at the input of the A/D converter  69  in the controller is at an “L” level, which is 0 V. When paper jam occurs, the voltage at the input of the A/D converter  69  does not reflect the correct surface temperature of the pressure roller  65 .  
         [0148]     Once paper jam occurs, the operation of the fixing unit  63  will not be performed any further unless the jammed paper is removed and the thermistor  71  returns to its normal position. When the base plate of the thermistor  71  has deformed permanently, even though the jammed paper S is removed, the detection signal of the controller or the A/D converter  69  continues to indicate that temperature-sensing element  79  is out of contact with the pressure roller  65 . Thus, the operation of the fixing unit  63  will not be performed any further.  
         [0000]     Ninth Embodiment  
         [0149]      FIGS. 34 and 35  illustrate the configuration of a ninth embodiment. The ninth embodiment differs from the conventional art in that a base plate  76  that supports a non-contact type thermistor  70 B is positioned midway between conductors  75   a  and  75   b . The base plate  76  is made of a resilient material.  
         [0150]     The ninth embodiment may employ either of the type in  FIG. 1A  and the type in  FIG. 1B . Here, the ninth embodiment will be described with respect to a case in which the thermistor of the type in  FIG. 1B  is used. The thermistor of  FIG. 1B  includes two signal lines isolated by an insulator  78  from the electrically conductive base plate  76  that supports a temperature-sensing element  79 . The base plate  76  is connected to a potential of 0 V.  
         [0151]     When no paper jam occurs, the base plate  76  is midway between the conductors  75   a  and  75   b  as shown in  FIG. 34  such that the base plate  76  is not in contact with the conductors  75   a  and  75   b . When paper jam occurs, the base plate  76  moves into contact with, for example, the conductor  75   b  as shown in  FIG. 35 .  
         [0152]     With respect to a case where the thermistor  70 B illustrated in  FIG. 1B  is used, a description will be given of the operation of detecting that the thermistor  70 B has moved out of contact with a fixing belt  97  that serves as a heating belt.  
         [0153]     When paper jam like an accordion as shown in  FIG. 35  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 B and the base plate  76 . This causes the temperature-sensing element  79  of the thermistor  70 B to move out of contact with the fixing belt  97 . The base plate  76  moves into contact with either the conductor  75   a  or the conductor  75   b.    
         [0154]      FIG. 36  is an electrically equivalent circuit including the voltage-dividing resistors  72  and  73 , temperature-sensing element  79 , conductors  75   a  and  75   b , base plate  76 , and resistor  74 . A switch  89  represents the contacts between the base plate  76  and the conductors  75   a  and  75   b . The resistor  74  has one end connected to a 5-V power supply and another end connected to the input port of the controller. When no paper jam occurs, the common electrode of the switch  89  is positioned midway between the conductors  75   a  and  75   b . Therefore, the switch  89  is open, so that an “H” level appears at the input of the controller. When paper jam occurs, the base plate  76  of the thermistor  70 B moves into electrical contact with, for example, the conductor  75   b.    
         [0155]     The operation of detecting will be described in which the non-contact type thermistor  71  has moved out of contact with a pressure roller  65  by a predetermined distance.  
         [0156]     When paper jam like an accordion as shown in  FIG. 35  occurs near an entrance of the fixing unit  63  during printing, the jammed paper S pushes the thermistor  70 B and the base plate  76 , so that the thermistor  70 B moves out of contact with the pressure roller  65 . The base plate  76  moves into contact with the conductor  75   b.    
         [0157]     As described above, when the input to the A/D converter  69  of the controller changes, the controller detects that the thermistor  70 B has moved out of contact with the fixing belt  97 , and generates an alarm signal.  
         [0158]      FIG. 37  illustrates analog voltage waveforms before and after the occurrence of paper jam. The controller detects these waveforms. When no paper jam occurs, the voltage at the input of the A/D converter  69  in the controller is at an “H” level, which is substantially the same as 5-V supply voltage for the controller. When paper jam occurs, the voltage at the input pot of the A/D converter  69  in the controller is at an “L” level, which is 0 V. The voltage at the input of the A/D converter  69  reflects the correct surface temperature of the fixing belt  97  regardless of whether paper jam occurs.  
         [0159]     Once paper jam occurs, the operation of the fixing unit  63  will not be performed any further unless the jammed paper S is removed and the thermistor  70 B returns to its normal position. If the base plate of the thermistor  70 B has deformed permanently, the detection signals of the controller or the A/D converter  69  continues to indicate that the thermistor  70 B is out of contact with the fixing belt  97  even though the jammed paper is removed. Thus, the operation of the fixing unit  63  will not be performed any further. The ninth embodiment has been described with respect to the non-contact type thermistor. The embodiment may also be implemented by the use of a contact type thermistor.  
         [0160]     Although the present invention has been described with respect to a color printer, the invention may be applied to other apparatus provided that a developer image is fused by heat into a permanent image. While most of the embodiments have been described with respect to a contact type thermistor, the constructions of these embodiments may be used in combination with a non-contact type thermistor instead of a non-contact type thermistor. Although the embodiments have been described with respect to a case in which a thermistor is normally in contact with the fixing roller (i.e., heat roller), the thermistor may also be provided in contact with the pressure roller.