Abstract:
A fixing device includes a fixing roller, a heating member, a pressure roller, a first temperature detecting element that detects a temperature of a contact region of the fixing roller that comes into contact with a paper, and a second temperature detecting element that detects a temperature of a noncontact region, and in which energization control of the heating member is carried out by the first one, and energization control is stopped when a temperature detected by the second reaches a prescribed temperature, the device including a conduction shutoff portion that, shuts off the energization where the temperature of the contact region has reached an upper limit, wherein, in which a center of the paper in an axial direction of the fixing roller has been aligned with a reference position of the fixing roller, the paper is transported, and the first one and the shutoff portion are placed symmetrically.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2011-026192 filed in Japan on Feb. 9, 2011, the entire contents of which are herein incorporated by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to fixing devices that fix toner images formed on paper, and to image forming apparatuses provided with these fixing devices. 
       RELATED ART 
       [0003]    This type of fixing device, which is applied in image forming apparatuses that use methods such as electrography, electrostatic recording, and magnetography or the like, sandwiches a paper (such as plain paper, electrostatic recording paper, and photographic paper or the like) on which a toner image has been formed in a nip region between a fixing roller (also referred to as a heating roller) and a pressure roller, thereby applying heat and pressure to fix the toner image onto the paper. 
         [0004]    In these fixing devices, the paper center for example is aligned with the center of a nip region between the fixing roller and the pressure roller for the paper to pass through the nip region. In this case, the heat quantity near the center of the fixing roller is taken up by the paper such that the temperature near the center of the fixing roller drops, and furthermore there are few opportunities for the areas near the ends of the fixing roller to contact the paper such that there is a tendency for the temperatures near the ends of the fixing roller to become higher than the temperature near the center, and only the areas near the ends of the fixing roller tend to overheat. 
         [0005]    For this reason, technologies have been proposed in which multiple heaters are arranged inside the fixing roller, and the area near the center of the fixing roller and the areas near the ends are heated by respective heaters, thereby reducing the temperature difference between the areas near the center and near the ends of the fixing roller. However, providing multiple heaters and controlling these heaters separately involves increasing the number of components and making the control thereof more complex. 
         [0006]    Accordingly, in a technology described in JP 2004-013058A, a single lamp heater is provided inside the fixing roller and the fixing roller is heated by this single lamp heater only, and a temperature near the center and a temperature near an end of the fixing roller are detected by a first and second thermistor respectively, and the temperature of the fixing roller is controlled by carrying out energization control of the lamp heater based on the temperature near the center of the fixing roller detected by the first thermistor. Furthermore, in the technology described in JP 2004-013058A, in a case where the temperature near the end portion of the fixing roller detected by the second thermistor has reached a heat resistance temperature of a shaft bearing (made of resin) of the fixing roller, overheating of the fixing roller is prevented by stopping the energization of the lamp heater. 
         [0007]    However, in JP 2004-013058A, even though energization of the lamp heater is controlled and stopped based on the temperature detected by the first and second thermistors, no consideration is given to a countermeasure for when this heater control becomes erratic, and it does not suitably nor reliably prevent the heater control from becoming erratic. 
         [0008]    Furthermore, although the energization of the lamp heater is stopped when the temperature near the end of the fixing roller detected by the second thermistor reaches the heat resistance temperature of the shaft bearing (made of resin) of the fixing roller, thereby preventing overheating of the fixing roller, since the heat resistance temperatures vary greatly depending on the type of resin thereof, setting the heat resistance temperature of the resin as the temperature of when to stop the energization of the lamp heater cannot be considered favorable to image forming operations in an image forming apparatus. 
         [0009]    Accordingly, the present invention has been devised in consideration of these issues, and it is an object thereof to provide a fixing device in which it is possible to suitably prevent the heater control for heating the fixing roller from becoming erratic, and in which a temperature that does not hinder the image forming operations of an image forming apparatus is set as the temperature of when the energization of the heater is stopped and an image forming apparatus provided with this fixing device. 
       SUMMARY OF THE INVENTION 
       [0010]    In order to address these issues, a fixing device according to the present invention including a fixing roller, a single heating member that heats the fixing roller, a pressure roller that is pressed against the fixing roller, a first temperature detecting element that detects a temperature of a contact region of the fixing roller that comes into contact with a paper transported sandwiched between the fixing roller and the pressure roller, and a second temperature detecting element that detects a temperature of a noncontact region of the fixing roller that is outside the contact region, wherein energization control of the heating member is carried out based on a temperature detected by the first temperature detecting element, and energization control of the heating member is stopped when a temperature detected by the second temperature detecting element reaches a prescribed temperature, 
         [0011]    the fixing device further including a conduction shutoff portion that, in a case where the temperature of the contact region of the fixing roller has reached an upper limit temperature that is higher than the prescribed temperature, senses the upper limit temperature and shuts off the energization of the heating member, 
         [0012]    wherein, in a state in which a center position of the paper in an axial direction of the fixing roller has been aligned with a reference position of the fixing roller, the paper is transported sandwiched between the fixing roller and the pressure roller, and the first temperature detecting element and the conduction shutoff portion are arranged in a symmetrical manner with respect to the reference position. 
         [0013]    In the fixing device according to the present invention, the fixing roller is heated by a single heating member, and energization control of the heating member is carried out based on temperatures by the first and second temperature detecting elements, but a countermeasure is necessary for when control of the heating member becomes erratic. Accordingly, in the fixing device according to the present invention, a center position of the paper in the axial direction of the fixing roller is aligned with a reference position of the fixing roller, and the first temperature detecting element and the conduction shutoff portion are arranged in a symmetrical manner with respect to the reference position. Consequently, the position where the temperature of the fixing roller is detected by the first temperature detecting element and the position where the temperature of the fixing roller is sensed by the conduction shutoff portion have a symmetrical relationship with respect to the position at which the center of the paper passes. And since the temperature of the contact region of the fixing roller contacting the paper is detected by the first temperature detecting element, the temperature of the contact region of the fixing roller contacting the paper is sensed also by the conduction shutoff portion. For this reason, accompanying fixing processing of the paper, the temperature of the fixing roller detected by the first temperature detecting element and the temperature of the fixing roller sensed by the conduction shutoff portion always fluctuate in an equivalent manner and are substantially same values. Thus, when control of the heating member becomes erratic and the temperature of the fixing roller detected by the first temperature detecting element rises to the upper limit temperature, the temperature of the fixing roller sensed by the conduction shutoff portion also rises to the upper limit temperature, and the upper limit temperature is sensed immediately by the conduction shutoff portion such that energization of the heating member is stopped with a suitable timing. 
         [0014]    Furthermore, in the fixing device according to the present invention, it is preferable that the conduction shutoff portion is a thermostat inserted to a power line of the heating member. 
         [0015]    Furthermore, in the fixing device according to the present invention, it is preferable that a softening temperature or a melting temperature of a shaft bearing of the fixing roller is equal to or less than the upper limit temperature and higher than the prescribed temperature. 
         [0016]    In this case, even though the fixing roller reaches the prescribed temperature, operation of the fixing device is possible without softening or melting of the shaft bearings of the fixing roller. Furthermore, when the temperature of the fixing roller exceeds the prescribed temperature and continues to rise toward the upper limit temperature, the shaft bearings of the fixing roller soften or melt such that the fixing roller approaches or contacts the conduction shutoff portion and the upper limit temperature is reliably sensed by the conduction shutoff portion such that energization of the heating member is reliably stopped. 
         [0017]    Next, an image forming apparatus according to the present invention includes a fixing device that includes a fixing roller, a single heating member that heats the fixing roller, a pressure roller that is pressed against the fixing roller, a first temperature detecting element that detects a temperature of a contact region of the fixing roller that comes into contact with a paper transported sandwiched between the fixing roller and the pressure roller, and a second temperature detecting element that detects a temperature of a noncontact region of the fixing roller that is outside the contact region, and in which energization control of the heating member is carried out based on a temperature detected by the first temperature detecting element, and energization control of the heating member is stopped when a temperature detected by the second temperature detecting element reaches a prescribed temperature; and a paper feeding cassette that accommodates a specific number of sheets of paper to be in a full state, wherein a toner image is formed on paper supplied from the paper feeding cassette and heat and pressure are applied to the paper by the fixing device such that the toner image is fixed to the paper, and wherein a temperature higher than the temperature reached at the noncontact region of the fixing roller when, while energization control of the heating member is maintained, the toner images on the specific number of sheets of paper supplied consecutively from the paper feeding cassette are fixed continuously is set as the prescribed temperature of when energization control of the heating member is to be stopped. 
         [0018]    In the image forming apparatus according to the present invention, a temperature higher than the temperature reached at the noncontact region of the fixing roller when, while energization control of the heating member is maintained, the toner images on all the papers supplied consecutively from the paper feeding cassette in a full state are fixed continuously is set as the prescribed temperature of when energization control of the heating member is to be stopped. For this reason, the temperature detected by the second temperature detecting element (temperature of the noncontact region) does not reach the prescribed temperature until the fixing of all the papers supplied from the paper feeding cassette in a full state is completed, and thus fixing of the papers can continue without any stopping of the energization control of the heating member. Furthermore, even though energization control of the heating member is stopped after the fixing of all the papers supplied from the paper feeding cassette in a full state has been completed, at this time there is a necessity to refill papers into the paper feeding cassette and the temperature of the fixing roller drops during this refilling operation, and therefore fixing of papers can recommence immediately after this refilling operation has been completed. 
         [0019]    Furthermore, in the image forming apparatus according to the present invention, after the energization control of the heating member has been stopped, a transport operation of paper by the fixing roller and the pressure roller may continues. 
         [0020]    In this case, the fixing roller and the pressure roller continue to rotate, and therefore a flow of air is produced around the rollers, and the temperatures of the rollers drop swiftly due to this flow of air. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a cross-sectional view showing principal portions of an image forming apparatus in which one embodiment of a fixing device according to the present invention has been applied. 
           [0022]      FIG. 2  is a cross-sectional view that schematically shows the fixing device according to the present embodiment. 
           [0023]      FIG. 3  is a front view that schematically shows the fixing device of  FIG. 2 . 
           [0024]      FIG. 4A  is a diagram showing positions of a thermostat and first and second thermistors, which are arranged along a fixing roller in the fixing device. 
           [0025]      FIG. 4B  is a graph showing a surface temperature distribution of the fixing roller in a state corresponding to the positions of the fixing roller shown in  FIG. 4A . 
           [0026]      FIG. 5  is a block diagram showing a configuration of a control system of a heater lamp of the fixing roller. 
           [0027]      FIG. 6  is a graph showing an operational state of the fixing device obtained when the prescribed temperature of the fixing roller of when the energization control of the heater lamp is to be stopped is set appropriately and when all the recording papers are consecutively supplied from the paper feeding cassette in a full state and toner images on all the recording papers are fixed continuously. 
           [0028]      FIG. 7  is a graph showing an operational state of the fixing device obtained when the prescribed temperature of the fixing roller of when the energization control of the heater lamp is to be stopped is set low and when all the recording papers are consecutively supplied from the paper feeding cassette in a full state and toner images on all the recording papers are fixed continuously. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0029]    Hereinafter, embodiments of the present invention are described in detail with reference to the accompanying drawings. 
         [0030]      FIG. 1  is a cross-sectional view showing principal portions of an image forming apparatus in which one embodiment of a fixing device according to the present invention has been applied. An image forming apparatus  1  forms a toner image onto a recording paper using an electrographic method and is provided with components such as a paper feeding cassette  2 , a photosensitive drum  3 , a transfer belt  4 , and a fixing device  5 . 
         [0031]    The photosensitive drum  3  has a photosensitive layer on its surface and is rotationally driven in the direction of arrow Al at a fixed rotational velocity. Accompanying rotation of the photosensitive drum  3 , the surface of the photosensitive drum  3  is charged uniformly to a predetermined electric potential by a charging device (not shown in drawings), an electrostatic latent image is formed on the surface of the photosensitive drum  3  by an exposure device (not shown in drawings), and the electrostatic latent image on the surface of the photosensitive drum  3  is developed into a toner image by a development device (not shown in drawings). 
         [0032]    The transfer belt  4  is driven and orbitally moves in the direction of arrow B 1  at the same velocity as the surface velocity of the photosensitive drum  3  and presses against the photosensitive drum  3  such that a nip region forms therebetween. A recording paper that is transported in from the paper feeding cassette  2  below is introduced to the nip region, and a toner image on the surface of the photosensitive drum  3  is transferred onto the recording paper while the recording paper is transported in the nip region. A high voltage transfer bias (a high voltage that has opposite polarity (+) to the charge polarity (−) of the toner) is applied to the transfer belt  4  to achieve transfer of the toner image. 
         [0033]    After the toner image is transferred at the nip region between the photosensitive drum  3  and the transfer belt  4 , the recording paper is again transported upward and introduced to the fixing device  5 . The fixing device  5  is provided with a fixing roller (also referred to as a heating roller)  11  and a pressure roller  12 . The fixing roller  11  and the pressure roller  12  press against each other to form a nip region therebetween, and the fixing roller  11  is rotationally driven, and the pressure roller  12  rotates idly. The paper that is transported in is sandwiched in the nip region between the rollers  11  and  12  so as to be subjected to heat and pressure, thereby fixing the toner image onto the paper. 
         [0034]    Furthermore, a paper pickup roller  6  is provided at an end portion of the paper feeding cassette  2 , and recording papers are withdrawn sheet by sheet from the paper feeding cassette  2  by this paper pickup roller  6  to be transported to a paper transport path S. 
         [0035]    Arranged along the paper transport path S are components such as paper registration rollers  7 , the photosensitive drum  3 , the transfer belt  4 , and the fixing device  5 . The paper registration rollers  7  transports the recording papers, after temporarily stopping the recording paper that has been transported in, and then, aligning the leading edge of the recording paper, well timed with the rotations of the photosensitive drum  3  and the transfer belt  4  so that the toner image on the photosensitive drum  3  is transferred to the recording paper at the nip region between the photosensitive drum  3  and the transfer belt  4 . The recording paper passes through the nip region between the photosensitive drum  3  and the transfer belt  4 , then is transported upward after further passing through the nip region between the fixing roller  11  and the pressure roller  12 , and is then discharged to a discharge tray or the like (not shown in drawing). 
         [0036]    Next, detailed description is given of the fixing device  5  in the image forming apparatus  1 .  FIG. 2  is a cross-sectional view that schematically shows the fixing device  5 , and  FIG. 3  is a front view that schematically shows the fixing device  5 . 
         [0037]    In the fixing device  5 , each of the two end portions of a rotating shaft  11   a  of the fixing roller  11  is supported by a shaft bearing  13  made of a synthetic resin, and each of the two end portions of a rotating shaft  12   a  of the pressure roller  12  is supported by a shaft bearing  14  made of a synthetic resin. Furthermore, the synthetic resin shaft bearings  13  of the fixing roller  11  and the synthetic resin shaft bearings  14  of the pressure roller  12  are supported so as to readily connect to and disconnect from each other, and the synthetic resin shaft bearings  14  of the pressure roller  12  are biased by a spring (not shown in drawings) toward the synthetic resin shaft bearings  13  of the fixing roller  11 , such that the fixing roller  11  and the pressure roller  12  press against each other and a nip region is formed between the fixing roller  11  and the pressure roller  12 . 
         [0038]    Furthermore, a gear  15  having a same axis as the rotating shaft  11   a  is secured to one end portion of the rotating shaft  11   a  of the fixing roller  11 , and a drive gear (not shown in drawings) on the image forming apparatus  1  side meshes with the gear  15  such that the rotation of the drive gear is transmitted to the gear  15 , thereby causing the fixing roller  11  to rotate, and causing the pressure roller  12  that presses against the fixing roller  11  to be idly rotated. 
         [0039]    The fixing roller  11  is a roller having a three-layer structure in which an elastic layer is provided on an outer surface of the hollow tubular rotating shaft  11   a  for example and a mold release layer is formed on an outer surface of the elastic layer. In a same manner as the fixing roller  11 , the pressure roller  12  is also a roller having a three-layer structure in which an elastic layer is provided on an outer surface of the hollow tubular rotating shaft  12   a  and a mold release layer is formed on an outer surface of this elastic layer. 
         [0040]    A single heater lamp (halogen lamp)  16 , which is a heating member that heats the roller  11 , is provided inside the rotating shaft  11   a  of the fixing roller  11 . The heater lamp  16  is a rod shaped component that extends in a lengthwise direction of the fixing roller  11  and heats substantially all of the fixing roller  11 . And the heat of the fixing roller  11  is transmitted to the pressure roller  12  that presses against the fixing roller  11 , thereby also heating the pressure roller  12 . Both the fixing roller  11  and the pressure roller  12  are hollow tubular rollers having a three-layer structure, and the thicknesses of the peripheral walls of these are made thin, thereby keeping low the heat quantity of the peripheral walls. For this reason, the peripheral wall of the fixing roller  11  is readily heated by the heater lamp  16 , and the pressure roller  12  is also readily heated by the fixing roller  11  such that the surface temperature of the fixing roller  11  rises to a fixing temperature in a short time, and the standby time from when the heater lamp  16  is turned on until fixing processing commences is made short. 
         [0041]    Further still, a thermostat  17  and a first thermistor  21  are provided near the center of the fixing roller  11 , and a second thermistor  22  is provided near one end portion of the fixing roller  11 . The thermostat  17  senses the surface temperature of the fixing roller  11 , and switches from on to off when the fixing roller  11  becomes overheated. Furthermore, the first and second thermistors  21  and  22  detect the surface temperature of the fixing roller  11 . 
         [0042]    Next, detailed description is given regarding the thermostat  17  and the first and second thermistors  21  and  22 .  FIG. 4A  is a diagram showing positions of the thermostat and the first and second thermistors, which are arranged along the fixing roller in the fixing device.  FIG. 4B  is a graph showing a surface temperature distribution of the fixing roller in a state corresponding to the positions of the fixing roller shown in  FIG. 4A . 
         [0043]    Here, in the fixing device  5 , the recording paper passes through the nip region with a center of the recording paper in the axial direction of the fixing roller  11  being matched to a center (reference position of the fixing roller  11 ) G of the nip region between the fixing roller  11  and the pressure roller  12  regardless of the size of the recording paper. Accordingly, when the region where the smallest sized recording paper used in the image forming apparatus  1  comes into contact with the rollers  11  and  12  is given as smallest contact region Es, then the center of the smallest contact region Es is matched to the center G of the nip region. Furthermore, when the region where the largest sized recording paper used in the image forming apparatus  1  comes into contact with the rollers  11  and  12  is given as largest contact region Ed, then the center of the largest contact region Ed is matched to the center G of the nip region. The heater lamp  16  is configured such that its length is slightly longer than the width of the largest contact region Ed and so as to be capable of reliably heating the largest contact region Ed of the rollers  11  and  12 . 
         [0044]    In this manner, regardless of its size, the recording paper always passes through the center G of the nip region between the rollers  11  and  12 . For this reason, the heat quantity near the center of the fixing roller  11  is taken up by the recording paper and there is a tendency for the surface temperature to drop near the center of the fixing roller  11 . Furthermore, there is a tendency that the probability of the recording paper contacting near the ends of the fixing roller  11  is lowered, and the probability that the heat quantity near the ends of the largest contact region Ed is taken up by the recording paper is also lowered, and also that the surface temperature near the ends of the largest contact region Ed is higher than the surface temperature near the center of the fixing roller  11 . As a result, the surface temperature distribution of the fixing roller  11  becomes as shown in the graph of  FIG. 4B . 
         [0045]    The first thermistor  21  is arranged in the smallest contact region Es and is in contact with the surface of the fixing roller  11  to detect the surface temperature near the center of the fixing roller  11 . Furthermore, the second thermistor  22  is arranged in a noncontact region D (near an end portion of the heater lamp  16 ) away from the largest contact region Ed and is in contact with the surface of the fixing roller  11  to detect the surface temperature of one end portion of the fixing roller  11  that does not contact even a largest sized recording paper. 
         [0046]    Furthermore, the thermostat  17  is arranged in a position that is symmetrical to the first thermistor  21  with respect to the center G of the nip region, and is configured to be slightly apart from the surface of the fixing roller  11  such that it is in a noncontact state with the surface of the fixing roller  11 . Here, the when the distance from the center G of the nip region to the position of the first thermistor  21  that is apart on one end side of the fixing roller  11  is given as H 1 , the distance from the center G of the nip region to the position of the thermostat  17  that is apart on the other end side of the fixing roller  11  is also H 1 . Since this positional relationship is set and the length of the heater lamp  16  is sufficiently longer than the smallest contact region Es such that the smallest contact region Es of the fixing roller  11  is uniformly heated by the heater lamp  16 , the temperature of the fixing roller  11  detected by the first thermistor  21  and the temperature of the fixing roller  11  sensed by the thermostat  17  always fluctuate in an equivalent manner and are substantially same values. 
         [0047]    Next, detailed description is given of the energization control of the heater lamp  16  based on the detected temperatures of the first and second thermistors  21  and  22 .  FIG. 5  is a block diagram showing a configuration of a control system of the heater lamp  16  of the fixing roller  11 . 
         [0048]    In  FIG. 5 , a heater power portion  23  is a portion that supplies power pulse signals to the heater lamp  16  and performs PWM control of the power pulse signals, thereby causing the heater lamp  16  to generate heat and varying the quantity of heat generated by the heater lamp  16 . A control portion  24  controls the heater power portion  23  to adjust the pulse width (duty ratio) of the power pulse signals. For example, the control portion  24  compares the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21  and a preset fixing temperature F 1  (for example, F 1 =165° C.), and when the surface temperature of the smallest contact region Es of the fixing roller  11  is lower than the fixing temperature F 1 , widens the pulse width of the power pulse signal to cause an increase in the quantity of heat generated by the heater lamp  16 , thereby raising the surface temperature of the smallest contact region Es of the fixing roller  11 . Furthermore, when the surface temperature of the smallest contact region Es of the fixing roller  11  is higher than the fixing temperature F 1 , the control portion  24  narrows the pulse width of the power pulse signal to cause a decrease in the quantity of heat generated by the heater lamp  16 , thereby reducing the surface temperature of the smallest contact region Es of the fixing roller  11 . In this way, control is performed so that the surface temperature of the smallest contact region Es of the fixing roller  11  is matched to the prescribed temperature F 1 . 
         [0049]    Furthermore, the control portion  24  compares the surface temperature of the noncontact region D of the fixing roller  11  detected by the second thermistor  22  and a preset prescribed temperature F 2  (for example, F 2 =230° C. and F 2 &gt;F 1 ), and when the surface temperature of the noncontact region D of the fixing roller  11  is lower than the prescribed temperature F 2 , continues control of the heater lamp  16  based on the temperature detected by the first thermistor  21 , and when the surface temperature of the noncontact region D of the fixing roller  11  reaches the prescribed temperature F 2 , stops the output of power pulse signals from the heater power portion  23 , thereby stopping the heat generation of the heater lamp  16 . In this way, the temperature of the fixing roller  11  is prevented from rising abnormally. 
         [0050]    Further still, after stopping the output of power pulse signals from the heater power portion  23  and when the temperature detected by the second thermistor  22  drops by a fixed temperature width, the control portion  24  recommences output of the power pulse signals from the heater power portion  23 , thereby causing the heater lamp  16  to generate heat so as to recommence the fixing operations of the fixing device  5 . 
         [0051]    That is, when the surface temperature of the smallest contact region Es of the fixing roller  11  is adjusted to the fixing temperature F 1  based on the temperature detected by the first thermistor  21  and the temperature detected by the second thermistor  22  reaches the prescribed temperature F 2 , energization of the heater lamp  16  is shut off, thereby preventing the temperature of the fixing roller  11  from rising abnormally. 
         [0052]    By carrying out energization control and energization shutoff control of the heater lamp  16  in this manner based on the temperatures detected by the first and second thermistors  21  and  22 , the surface temperature near the center of the fixing roller  11  is controlled, and overheating of the fixing roller  11  is prevented. 
         [0053]    However, in a case where a malfunction occurs in the first and second thermistors  21  and  22  or the like, the control of the heater lamp  16  becomes erratic such that the fixing roller  11  overheats and peripheral components malfunction or melt, and therefore a countermeasure is necessary for when the control of the heater lamp  16  becomes erratic. 
         [0054]    Consequently, in the present embodiment, a power line  25  is inserted to the thermostat  17  from the heater power portion  23  to the heater lamp  16  as shown in  FIG. 5 . The thermostat  17  senses the surface temperature of the smallest contact region Es of the fixing roller  11  (this matches the temperature of the fixing roller  11  detected by the first thermistor  21 ) and maintains an on state while the sensed surface temperature is less than a preset upper limit temperature F 3  (F 3 &gt;F 2 &gt;F 1 ) so as to allow the power line  25  to conduct and enabling energization control of the heater lamp  16 , and when the sensed surface temperature equals or exceeds the upper limit temperature F 3 , switches from on to off so as to shut off the power line  25  causing energization control of the heater lamp  16  to stop, thereby preventing control of the heater lamp  16  from becoming erratic. 
         [0055]    Specifically, in a state in which control of the heater lamp  16  by the control portion  24  has become erratic, even if the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21  becomes higher than the upper limit temperature F 3 , or even if the surface temperature of the noncontact region D of the fixing roller  11  detected by the second thermistor  22  becomes equal to or higher than the prescribed temperature F 2 , the power pulse signals from the heater power portion  23  continue to be outputted, and therefore the heater lamp  16  continues to generate heat and the temperature of the fixing roller  11  continues to rise. At this time, the thermostat  17  senses the surface temperature of the smallest contact region Es of the fixing roller  11 , and when the sensed temperature reaches the upper limit temperature F 3 , switches from on to off such that the power line  25  is shut off and energization control of the heater lamp  16  is stopped. 
         [0056]    As described earlier, the temperature detected by the first thermistor  21  and the temperature sensed by the thermostat  17  always fluctuate in an equivalent manner and are substantially same values, and therefore when the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21  rises to the upper limit temperature F 3  due to control of the heater lamp  16  becoming erratic, the temperature sensed by the thermostat  17  also rises to the upper limit temperature F 3 . In this case, the upper limit temperature F 3  is sensed immediately by the thermostat  17  and the thermostat  17  switches from on to off, and therefore the energization of the heater lamp  16  is stopped with a suitable timing. 
         [0057]    Furthermore, the upper limit temperature F 3  near the center of the fixing roller  11  for the thermostat  17  to switch from on to off is set equal to or above the softening temperature or the melting temperature of each of the synthetic resin shaft bearings  13  (the softening temperature or the melting temperature of each of the synthetic resin shaft bearings  13  is not greater than the upper limit temperature F 3 ). For this reason, even if the temperature of the thermostat  17  does not rise sufficiently due to heat dissipation from near the center of the fixing roller  11  when the surface temperature near the center of the fixing roller  11  rises to the upper limit temperature F 3  so that the thermostat  17  does not switch from on to off, when the temperatures of the end portions of the fixing roller  11  rise to the upper limit temperature F 3 , the synthetic resin shaft bearings  13  soften or melt due to thermal conduction from the end portions of the fixing roller  11  such that the synthetic resin shaft bearings  13  deform being unable to withstand the contact pressure from the pressure roller  12  to the fixing roller  11 , and the fixing roller  11  displaces to an opposite side from the pressure roller  12 . And since the thermostat  17  is arranged in a position on an opposite side from the pressure roller  12  with respect to the fixing roller  11 , it contacts the fixing roller  11 , which has displaced to the opposite side from the pressure roller  12 . As a result, the surface temperature of the fixing roller  11  is directly sensed by the thermostat  17  and the sensed temperature of the thermostat  17  reliably rises to the upper limit temperature F 3  such that the thermostat  17  switches from on to off, and the energization of the heater lamp  16  is reliably stopped. 
         [0058]    Note however that the softening temperature or the melting temperature of the synthetic resin shaft bearings  13  that support the end portions of the rotating shaft  11   a  of the fixing roller  11  are set higher than the prescribed temperature F 2  of the end portions of the fixing roller  11  of when the energization control of the heater lamp  16  is stopped. For this reason, even if the temperature of the end portions of the fixing roller  11  reaches the prescribed temperature F 2 , the end portions of the rotating shaft  11   a  of the fixing roller  11  are supported by the synthetic resin shaft bearings  13  and operation of the fixing device  5  is possible. 
         [0059]    Since the first thermistor  21  and the thermostat  17  are arranged symmetrically with respect to the center G of the nip region in this manner, the temperature detected by the first thermistor  21  and the temperature sensed by the thermostat  17  always fluctuate in an equivalent manner, and when the temperature detected by the first thermistor  21  rises to the upper limit temperature F 3 , the temperature sensed by the thermostat  17  also rises to the upper limit temperature F 3  such that the thermostat  17  switches from on to off, and energization of the heater lamp  16  is stopped with a suitable timing. 
         [0060]    On the other hand, in the present embodiment, when the temperature detected by the second thermistor  22  reaches the prescribed temperature F 2  (for example, F 2 =230° C.), heat generation by the heater lamp  16  is caused to stop, but when the heat generation by the heater lamp  16  is stopped inadvertently, this results in the fixing operations of the fixing device  5  being interrupted, and therefore the prescribed temperature F 2  is set to a temperature that does not hinder the fixing operations of the fixing device  5 . 
         [0061]    Specifically, as the prescribed temperature F 2 , a temperature is set that is higher than the temperature reached at the noncontact region D of the fixing roller  11  when, in a state in which heat generation by the heater lamp  16  is continuous, all the toner images on all the recording papers supplied consecutively from the paper feeding cassette  2  in a full state have been fixed continuously by the fixing device  5 . In this way, it becomes possible to continuously fix all the recording papers that are accommodated in the paper feeding cassette  2 . 
         [0062]    For example, if the paper feeding cassette  2  is in a full state when 250 sheets of the largest sized recording paper are accommodated in the paper feeding cassette  2 , then a temperature is set as the prescribed temperature F 2  (for example, F 2 =230° C.) that is higher than the temperature (for example, 220° C.) reached at the noncontact region D of the fixing roller  11  when, in a state in which heat generation by the heater lamp  16  is continuous, 250 sheets of recording paper are consecutively supplied from the paper feeding cassette  2  and toner images are transferred and formed on all the recording papers such that the toner images on all the recording papers are fixed continuously by the fixing device  5 . 
         [0063]      FIG. 6  is a graph showing an operational state of the fixing device  5  obtained when 250 sheets of the largest sized recording papers are consecutively supplied from the paper feeding cassette  2  and toner images on all the recording papers are fixed continuously, and shows a characteristic f 1  of the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21 , a characteristic f 2  of the surface temperature of the noncontact region D of the fixing roller  11  detected by the second thermistor  22 , a power pulse signal p that is supplied from the heater power portion  23  to the heater lamp  16 , and a drive signal w of the drive motor that drives the fixing roller  11  on the image forming apparatus  1  side. 
         [0064]    In the graph in  FIG. 6 , paper feeding by the paper feeding cassette  2  and fixing by the fixing device  5  commence from a time t 1 , and paper feeding and fixing are completed at a time t 2 , and in the period from the time t 1  until the time t 2 , the power pulse signal p is continuously outputted from the heater power portion  23  to the heater lamp  16 , and the pulse width of the power pulse signal p is controlled such that the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21  is maintained substantially constant, and the surface temperature of the noncontact region D of the fixing roller  11  detected by the second thermistor  22  rises gradually to 220° C. 
         [0065]    Accordingly, the temperature detected by the second thermistor  22  does not reach the prescribed temperature F 2  until the fixing of all the recording papers supplied from the paper feeding cassette  2  in a full state is completed, and thus fixing of the recording papers can continue without any stopping of the energization control of the heater lamp  16 . Thus, it can be said that the prescribed temperature F 2  of when to stop the energization control of the heater lamp  16  is set to a temperature that is favorable to image forming operations of the image forming apparatus  1 . 
         [0066]    Furthermore, even though the temperature detected by the second thermistor  22  reaches the prescribed temperature F 2  and energization control of the heater lamp  16  is stopped after the fixing of all the recording papers supplied from the paper feeding cassette  2  in a full state has been completed, at this time there is a necessity to refill recording papers into the paper feeding cassette  2  and the surface temperature of the fixing roller  11  drops during this refilling operation, and therefore fixing of recording papers can recommence immediately after this refilling operation has been completed. 
         [0067]    Further still, it is preferable that even if the energization control of the heater lamp  16  has been stopped, the drive signal w of the drive motor is caused to continue for keeping the fixing roller  11  driving such that the fixing roller  11  and the pressure roller  12  continue to rotate. In this case, a flow of air is produced around the rollers  11  and  12 , and the surface temperatures of the rollers  11  and  12  drop swiftly due to this flow of air such that the fixing of recording papers can recommence reliably. 
         [0068]      FIG. 7  shows the characteristic f 1  of the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21 , the characteristic f 2  of the surface temperature of the noncontact region D of the fixing roller  11  detected by the second thermistor  22 , the power pulse signal p that is supplied from the heater power portion  23  to the heater lamp  16 , and the drive signal w of the drive motor that drives the fixing roller  11  on the image forming apparatus  1  side, which are obtained under equivalent conditions to the graph in  FIG. 6 , that is, conditions in which 250 sheets of the largest sized recording paper are supplied consecutively from the paper feeding cassette  2  and the toner images on all the recording papers are fixed continuously by the fixing device  5  and when the prescribed temperature F 2  is set to the above-mentioned temperature that is reached (for example, 220° C.). 
         [0069]    As is evident from the graph in  FIG. 7 , in a case where the prescribed temperature F 2  is set to the above-mentioned temperature that is reached, in the interval until the fixing of all the recording papers supplied from the paper feeding cassette  2  in a full state is completed, the temperature detected by the second thermistor  22  repetitively reaches the prescribed temperature F 2  multiple times, and each time this happens, the energization control of the heater lamp  16  is stopped. For this reason, the surface temperature of the smallest contact region Es of the fixing roller  11  detected by the first thermistor  21  becomes unstable, and the recording papers cannot be fixed continuously. That is, the image forming apparatus  1  or the fixing device  5  loses the capability of being able to continuously fix all the recording papers of the paper feeding cassette  2  in a full state. 
         [0070]    The present invention can be embodied and practiced in other different forms without departing from the spirit and essential characteristics thereof. Therefore, the above-described working examples are considered in all respects as illustrative and not restrictive. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All variations and modifications falling within the equivalency range of the appended claims are intended to be embraced therein.