Patent Publication Number: US-11644774-B2

Title: Image heating device and heater for use in image heating device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/204,800, filed on Mar. 17, 2021, which is a continuation of U.S. patent application Ser. No. 16/805,490, filed Feb. 28, 2020 and issued as U.S. Pat. No. 10,983,463, issued Apr. 20, 2021, which is a continuation of U.S. patent application Ser. No. 16/540,600, filed Aug. 14, 2019 and issued as U.S. Pat. No. 10,671,001, issued Jun. 2, 2020, which is a continuation of U.S. patent application Ser. No. 15/758,204, filed Mar. 7, 2018 and issued as U.S. Pat. No. 10,429,781, issued Oct. 1, 2019, which is a National Stage application of International Patent Application No. PCT/JP2016/003724, filed Aug. 12, 2016, which claims the benefit of Japanese Patent Application No. 2015-179567, filed Sep. 11, 2015, all of which are hereby incorporated by reference herein in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an image heating device such as a fixer mounted in an image forming apparatus for electrophotographic recording such as a copier and a printer or a gloss providing device which re-heats a toner image fixed to a recording material to improve the gloss level of the toner image. The present invention further relates to a heater for use in the image heating device. 
     BACKGROUND ART 
     An image heating device includes a tubular film, a heater in contact with an inner surface of the film, and a roller forming a nip part together with the heater through the film. When an image forming apparatus having the image heating device is used to continuously print on small-sized sheets, a phenomenon may occur that the temperature of a region through which paper does not pass in a longitudinal direction in the nip part gradually increases (rise of temperature in a non-paper-passing part). An excessively increased temperature of the non-paper-passing part may damage parts within the device. In a case where printing is performed on larger-sized paper when rise of temperature in the non-paper-passing part occurs, hot offset of toner may be caused on a film in a region corresponding to a non-paper-passing part for small-sized paper. 
     One of schemes for suppressing such a rise of temperature in a non-paper-passing part, an apparatus has been proposed which includes a plurality of groups (heating blocks) of longitudinal heating resisters in a heater, wherein the heating distribution of the heater is changed in accordance with the size of a recording material (PLT1). 
     CITATION LIST 
     Patent Literature 
     
         
         [PTL 1] 
         Japanese Patent Laid-Open No. 2014-59508 
       
    
     SUMMARY OF INVENTION 
     In consideration of occurrence of a failure in such an apparatus, it may be configured so as to monitor a temperature of each heating block. Even when one of the plurality of heating blocks is uncontrollable and abnormal heating occurs, power supply may be stopped quickly based on a result of the temperature monitoring of each heating block. 
     However, as the number of heating blocks increases, the number of temperature sensors each for monitoring a temperature also increases. Providing many temperature sensors within a region of a substrate of the heater may increase the size of the heater. 
     Solution to Problem 
     An aspect of the present invention provides a heater for use in an image heating device, the heater including a substrate, a first heating block provided on the substrate and configured to generate heat from electric power supplied thereto, a second heating block provided at a position different from the position of the first heating block in a longitudinal direction of the substrate and configured to separately control the first heating block, a first temperature sensor provided at a position corresponding to the first heating block, a second temperature sensor provided at a position corresponding to the second heating block, a first conductive pattern electrically coupled to the first temperature sensor, a second conductive pattern electrically coupled to the second temperature sensor, and a common conductive pattern electrically coupled to the first and second temperature sensors. 
     Another aspect of the present invention provides a heater usable in an image heating device, the heater including a substrate, a heat generating member provided on one surface of the substrate and configured to generate heat from electric power supplied thereto, a temperature sensor provided on another surface on the opposite side of the one surface of the substrate and configured to detect a temperature of the heater, and an electrode in contact with an electric contact for supplying electric power to the heat generating member, wherein the electrode is placed within a region having the heat generating member in a longitudinal direction of the heater on the one surface of the substrate. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
     Advantageous Effects of Invention 
     According to the present invention, an increase of the size of a heater can be suppressed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a cross section view of an image forming apparatus. 
         FIG.  2    is a cross section view of an image heating device. 
         FIG.  3 A  illustrates a configuration of a heater according to a first exemplary embodiment. 
         FIG.  3 B  illustrates the configuration of the heater according to the first exemplary embodiment. 
         FIG.  3 C  illustrates the configuration of the heater according to the first exemplary embodiment. 
         FIG.  4    illustrates a heater control circuit according to the first exemplary embodiment. 
         FIG.  5    is a heater control flowchart according to the first exemplary embodiment. 
         FIG.  6 A  illustrates a configuration of a heater according to a second exemplary embodiment. 
         FIG.  6 B  illustrates a configuration of the heater according to the second exemplary embodiment. 
         FIG.  7    illustrates a heater control circuit according to the second exemplary embodiment. 
         FIG.  8    is a heater control flowchart according to the second exemplary embodiment. 
         FIG.  9 A  illustrates a variation example of the heater. 
         FIG.  9 B  illustrates the variation example of the heater. 
         FIG.  10 A  illustrates a variation example of the heater. 
         FIG.  10 B  illustrates another variation example of the heater. 
         FIG.  11 A  illustrates a conduction control pattern of a heater. 
         FIG.  11 B  illustrates another conduction control pattern of a heater. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG.  1    is a cross section view of a laser printer (image forming apparatus)  100  applying an electrophotographic recording technology. In response to occurrence of a print signal, a scanner unit  21  emits laser light modulated based on image information so that a photosensitive member  19  electrostatically charged to a predetermined polarity by a charging roller  16  can be scanned. Thus, an electrostatic latent image is formed on the photosensitive member  19 . Toner is supplied from a developing unit  17  to the electrostatic latent image so that a toner image according to the image information is formed on the photosensitive member  19 . On the other hand, recording materials (recording paper) P stacked in a paper feed cassette  11  are fed one by one by a pickup roller  12  and are conveyed by a roller  13  toward a resist roller  14 . Each of the recording materials P is conveyed from the resist roller  14  to a transfer position simultaneously with a time when the toner image on the photosensitive member  19  reaches the transfer position formed by the photosensitive member  19  and a transfer roller  20 . During a process in which the recording material P passes through the transfer position, the toner image on the photosensitive member  19  is transferred to the recording material P. After that, the recording material P is heated by an image heating device (fixing device)  200  so that the toner image is heated and is fixed to the recording material P. The recording material P bearing the fixed toner image is output to a tray in an upper part of the laser printer  100  through rollers  26  and  27 . A cleaner  18  cleans the photosensitive member  19 . A motor  30  drives an image heating device  200  and so on. Electric power is supplied from a control circuit  400  connected to a commercial alternating current (AC) power supply  401  to the image heating device  200 . The photosensitive member  19 , charging roller  16 , scanner unit  21 , developing unit  17 , and transfer roller  20  are components of an image forming unit configured to form an unfixed image to a recording material P. A cartridge  15  is a replaceable unit. The laser printer  100  further includes a light source  22 , a polygon mirror  23 , and a reflection mirror  24 . 
     The laser printer  100  according to this exemplary embodiment supports a plurality of sizes of recording material. Letter paper (about 216 mm×279 mm) and Legal paper (about 216 mm×356 mm) can be set in the paper feed cassette  11 . In addition, A4 paper (210 mm×297 mm), Executive paper (about 184 mm×267 mm), JIS B5 paper (182 mm×257 mm), and A5 paper (148 mm×210 mm) can be set therein. 
     The printer in this embodiment is a laser printer fundamentally configured to feed paper vertically (or paper can be conveyed such that the long side of the paper can be in parallel with the conveying direction). The present configuration is also applicable to a printer which feed paper horizontally. Letter paper and Legal paper are largest (widest) among regular recording materials (based on widths of recording materials on catalogs) supported by the apparatus and have a width of about 216 mm. In the following description of this exemplary embodiment, a recording material P having a paper width smaller than a maximum size supported by the apparatus will be called small-sized paper. 
       FIG.  2    is a cross section view of the image heating device  200 . The image heating device  200  has a tubular film  202 , a heater  300  in contact with an inner surface of the film  202 , and a pressure roller (nip part forming member)  208  forming a fixing nip part N together with the heater  300  through the film  202 . The film  202  has a base layer made of a heat-resistant resin such as polyimide or metal such as stainless. The film  202  may have an elastic layer of heat-resistant rubber. The pressure roller  208  has a cored bar  209  made of iron, aluminum, or the like, and an elastic layer  210  made of silicone rubber. The heater  300  is held by a holding member  201  of heat-resistant resin such as liquid crystal polymer. The holding member  201  has a guide function for guiding rotation of the film  202 . The pressure roller  208  rotates in the direction as indicated by the arrow illustrated in  FIG.  2    by receiving motive power from the motor  30 . Rotation of the pressure roller  208  is followed by rotation of the film  202 . The recording material P bearing an unfixed toner image is pinched and is conveyed by the fixing nip part N to be heated and be fixed. The apparatus  200  as described above has the tubular film  202  and the heater  300  in contact with an inner surface of the film  202 , and an image formed on the recording material is heated by the heater  300  through the film  202 . 
     The heater  300  has a ceramic substrate  305 , and a heating resister (heat generating member) (see  FIGS.  3 A to  3 C ) provided on the substrate  305  for generating heat which supplies electric power. A surface protection layer  308  of glass for providing a sliding property to the film  202  is provided on a surface (first surface) close to the fixing nip part N of the substrate  305 . A surface protection layer  307  of glass for insulating a heating resister is provided on the opposite surface (second surface) of the plane close to the fixing nip part N of the substrate  305 . The second surface has an electrode (representatively indicated by E 4 ) exposed, and when an electric contact (representatively indicated by C 4 ) for feeding power touches the electrode, the heating resister is coupled electrically to the AC power supply  401 . Details of the heater  300  will be described below. 
     A protection element  212  such as a thermo switch and a temperature fuse is configured to block electric power to be supplied to the heater  300  in response to abnormal heating of the heater  300 . The protection element  212  may be abutted against the heater  300  or may be placed in a gap of the heater  300 . A metallic stay  204  for applying pressure of a sprint, not illustrated, to the holding member  201  plays a role of reinforcing the holding member  201  and heater  300 . 
       FIGS.  3 A and  3 B  illustrate a configuration of the heater  300  according to the first exemplary embodiment.  FIG.  3 A  illustrates a cross section view of the heater  300  near a conveyance reference position X on the recording material P illustrated in  FIG.  3 B .  FIG.  3 B  is a plan view of layers of the heater  300 .  FIG.  3 C  is a plan view of the holding member configured to hold the heater  300 . 
     The printer according to this embodiment is a center reference printer configured to convey a recording material by placing the center of the recording material in the width direction (orthogonal to the conveying direction) at the conveyance reference position X. 
     Next, details of the configuration of the heater  300  will be described. A back surface layer  1  of the heater  300  which is a heater surface on the opposite side of the heater surface in contact with the film  202  has thereon a plurality of heating blocks each having a group of a first electric conductor  301 , a second electric conductor  303 , and a heating resister (heat generating member)  302  in the longitudinal direction of the heater  300 . The heater  300  of this exemplary embodiment has a total of seven heating blocks HB 1  to HB 7 . Assuming one of the seven heating blocks as a first heating block and another heating block as a second heating block, the heater  300  has a following configuration. That is, the heater  300  has a substrate and the first heating block provided on the substrate for generating heat by receiving power supply. The heater  300  further has the second heating block which is provided at a position different from the position of the first heating block in the longitudinal direction of the substrate and is controlled independently from the first heating block. The independent control over the heating blocks will be described below. 
     Each of the heating blocks has a first electric conductor  301  and a second electric conductor  303 . The first electric conductors  301  are provided along the longitudinal direction of the substrate, and the second electric conductors  303  are provided along the longitudinal direction of the substrate at positions different from the positions of the first electric conductors  301  in the short-side direction of the substrate. Each of the heating blocks further has a heating resister  302  provided between the first electric conductor  301  and the second electric conductor  303  for generating heat from electric power supplied through the first electric conductor  301  and the second electric conductor  303 . 
     The heating resisters  302  in the heating blocks may be divided into heating resisters  302   a  and heating resisters  302   b  at mutually symmetrical positions about the center of the substrate in the short-side direction of the heater  300 . The first electric conductors  301  may be divided into electric conductors  301   a  connected to the heating resisters  302   a  and electric conductors  301   b  connected to the heating resisters  302   b . Because the heating resisters  302   a  and the heating resisters  302   b  are placed at mutually symmetrical positions about the center of the substrate, the substrate is not easily broken even when the heater generates heat and a heat stress occurs in the substrate. 
     Because the heater  300  has the seven heating blocks HB 1  to HB 7 , the heating resisters  302   a  include seven heating resisters  302   a - 1  to  302   a - 7 . In the same manner, the heating resisters  302   b  include seven of  302   b - 1  to  302   b - 7 . The second electric conductors  303  include seven electric conductors  303 - 1  to  303 - 7 . The heating resisters  302   a - 1  to  302   a - 7  are placed on an upstream side in the conveying direction of the recording materials P within the substrate  305 , and the heating resisters  302   b - 1  to  302   b - 7  are placed on a downstream side in the conveying direction of the recording materials P within the substrate  305 . 
     A back surface layer  2  of the heater  300  has thereon an insulative surface protection layer  307  (of glass in this exemplary embodiment) which covers the heating resisters  302 , the first electric conductors  301 , and the second electric conductors  303 . In this case, the surface protection layer  307  does not cover electrodes E 1  to E 7 , and E 8 - 1  and E 8 - 2  in contact with electric contacts C 1  to C 7 , and C 8 - 1  and C 8 - 2  for feeding power. The electrodes E 1  to E 7  supply electric power to the heating blocks HB 1  to HB 7  through the second electric conductors  303 - 1  to  303 - 7 , respectively. The electrodes E 8 - 1  and E 8 - 2  feed electric power to the heating blocks HB 1  to HB 7  through the first electric conductors  301   a  and  301   b.    
     Because the resistance values of the electric conductors are not equal to zero, the resistance has an influence on the heating distribution in the longitudinal direction of the heater  300 . Accordingly, the electrodes E 8 - 1  and E 8 - 2  are separated on both ends in the longitudinal direction of the heater  300  so as to prevent nonuniformity of the heating distribution even when influenced by electric resistances of the first electric conductors  301   a  and  301   b  and second electric conductor  303 - 1  to  303 - 7 . 
     As illustrated in  FIG.  2   , a safety element  212  and the electric contacts C 1  to C 7 , C 8 - 1 , and C 8 - 2  are placed between the stay  204  and the holding member  201 . As illustrated in  FIG.  3 C , the holding member  201  has holes HC 1  to HC 7 , HC 8 - 1 , and HC 8 - 2  through which the electric contact C 1  to C 7 , C 8 - 1 , and C 8 - 2  connected to the electrodes E 1  to E 7 , E 8 - 1 , and E 8 - 2  extend. The holding member  201  further has a hole H 212  through which the heat-sensitive part of the protection element  212  extends. The electric contacts C 1  to C 7 , C 8 - 1 , and C 8 - 2  are electrically coupled to the corresponding electrodes by urging of a spring, welding or other scheme. The protection element  212  is also urged by the spring, and the heat-sensitive part is in contact with the surface protection layer  307 . The electric contacts are connected to a control circuit  400  in the heater  300 , which will be described below, through a cable or a conductive member such as a thin metal plate provided between the stay  204  and the holding member  201 . 
     Providing the electrodes on the back surface of the heater  300  can eliminate the necessity for providing a region for wiring which electrically connects the second electric conductors  303 - 1  to  303 - 7  on the substrate  305 , which thus can reduce the width in the short-side direction of the substrate  305 . Therefore, an increase of the size of the heater can be prevented. As illustrated in  FIG.  3 B , the electrodes E 2  to E 6  are provided within a region having the heating resisters in the longitudinal direction of the substrate. 
     The heater  300  of this embodiment separately controls the plurality of heating blocks so that various heating distributions can be formed, which will be described below. For example, a heating distribution in accordance with the size of a recording material can be defined. Furthermore, the heating resisters  302  may be formed from a material having a PTC (Positive Temperature Coefficient). The use of a material having a PTC can suppress a temperature rise of the non-paper-passing part even in a case where an end of the recording material is not matched with a boundary of the heating blocks. 
     A sliding surface layer  1  closer to a sliding surface (in contact with the film) of the heater  300  has thereon a plurality of thermistors (temperature sensors) T 1 - 1  to T 1 - 4 , and T 2 - 4  to T 2 - 7  configured to sense temperatures of the heating blocks HB 1  to HB 7 . The thermistors may be made of a material having a positively or negatively large TCR (Temperature Coefficient of Resistance). According to this embodiment, the thermistors are formed by printing a material having an NTC (Negative Temperature Coefficient) thinly on the substrate. One or more thermistors provided for each of the heating blocks HB 1  to HB 7  can sense temperatures of all of the heating blocks. 
     Assuming that one of the thermistors T 1 - 1  to T 1 - 4  is a first temperature sensor and another one of the thermistors T 1 - 1  to T 1 - 4  is a second temperature sensor, the heater  300  has the following configuration. That is, the heater  300  has the first temperature sensor at a position corresponding to the first heating block and the second temperature sensor at a position corresponding to the second heating block. 
     The thermistors T 1 - 1  to T 1 - 4  are electrically coupled to the conductive patterns ET 1 - 1  to ET 1 - 4 , respectively, on the substrate  305 . Assuming that a conductive pattern to be connected to the first temperature sensor of the conductive patterns ET 1 - 1  to ET 1 - 4  is a first conductive pattern and a conductive pattern connected to the second temperature sensor is a second conductive pattern, the heater  300  has the following configuration. That is, the heater  300  has the first conductive pattern electrically coupled to the first temperature sensor and the second conductive pattern electrically coupled to the second temperature sensor. The heater  300  further has a common conductive pattern EG 1  electrically coupled to the first and second temperature sensors. Hereinafter, a group of the thermistors T 1 - 1  to T 1 - 4 , the conductive patterns ET 1 - 1  to ET 1 - 4 , and the common conductive pattern EG 1  will be called a thermistor group TG 1 . 
     The heater  300  further has a thermistor group TG 2  of the thermistors T 2 - 4  to T 2 - 7 , the conductive patterns ET 2 - 4  to ET 2 - 7 , and a common conductive pattern EG 2 . The thermistor groups TG 1  and TG 2  are provided on a substrate surface on the opposite side of the substrate surface having the first and second heating blocks of the substrate  305 . 
     According to this example, at least one corresponding thermistor is provided for each of the heating blocks HB 1  to HB 7 . However, providing one corresponding thermistor for at least two heating blocks may also improve the reliability of the apparatus. However, as in this embodiment, at least one corresponding thermistor may be provided for all of the heating blocks. 
     By using the common conductive patterns EG 1  and EG 2  as in this embodiment to handle the first and second temperature sensors as one group, the following effect may be provided. That is, the cost for conductive patterns can be reduced and an increase of the size of the heater can be prevented, compared to a case where two conductive patterns are connected to each of the thermistors T 1 - 1  to T 1 - 4  without using a common conductive pattern. 
     In order to acquire a sliding property of the film  202 , a surface (sliding surface layer  2 ) close to the fixing nip part N of the substrate  305  is coated by an insulative surface protection layer  308  (of glass in this embodiment). The surface protection layer  308  covers the thermistors T 1 - 1  to T 1 - 4  and T 2 - 4  to T 2 - 7 , the conductive patterns ET 1 - 1  to ET 1 - 4  and ET 2 - 4  to ET 2 - 7 , and the common conductive patterns EG 1  and EG 2 . However, in order to acquire connection to the electric contacts, a part of the conductive patterns ET 1 - 1  to ET 1 - 4  and ET 2 - 4  to ET 2 - 7  and a part of the common conductive patterns EG 1  and EG 2  are exposed at both ends of the heater  300  as illustrated in  FIG.  3 B . 
       FIG.  4    is a circuit diagram of the control circuit  400  in the heater  300 . A commercial AC power supply  401  is connected to the laser printer  100 . Power control over the heater  300  is executed by conduction/non-conduction of triacs  411  to  414 . The triacs  411  to  414  operate in accordance with FUSER 1  to FUSER 4  signals from the CPU  420 . A driving circuit for the triacs  411  to  414  is not illustrated in  FIG.  4   . 
     It may be understood from  FIGS.  3 A to  3 C  and  FIG.  4    that the seven heating blocks HB 1  to HB 7  are divided into four groups (group 1: HB 4 , group 2: HB 3  and HB 5 , group 3: HB 2  and HB 6 , and group 4: HB 1  and HB 7 ). The control circuit  400  in the heater  300  has a circuit configuration capable of controlling the four groups independently from each other. The triac  411 , the triac  412 , the triac  413 , and the triac  414  can control the group 1, the group 2, the group 3, and the group 4, respectively. 
     A zero-crossing detecting unit  421  is a circuit configured to detect zero-crossing of the AC power supply  401  and outputs a ZEROX signal to the CPU  420 . The ZEROX signal is usable as a reference signal for controlling phases of the triacs  411  to  414 , for example. 
     Next, a method for detecting a temperature of the heater  300  will be described. The thermistor group TG 1  will be described first. The CPU  420  receives signals (Th 1 - 1  to Th 1 - 4 ) acquired by dividing voltage Vcc by a resistance value of the thermistors (T 1 - 1  to T 1 - 4 ) and the resistance value of the resistances ( 451  to  454 ). For example, the signal Th 1 - 1  is a signal acquired by dividing voltage Vcc by a resistance value of the thermistor T 1 - 1  and a resistance value of the resistance  451 . Because thermistor T 1 - 1  has a resistance value according to the temperature, when the temperature of the heating block HB 1  changes, the level of the signal Th 1 - 1  to be input to the CPU also changes. The CPU  420  converts the input signal Th 1 - 1  to a temperature according to the level. Because the same processing is performed on the signals Th 1 - 2  to Th 1 - 4  corresponding to the other thermistors T 1 - 2  to T 1 - 4  in the thermistor group TG 1 , any repetitive description will be omitted. 
     Next, the thermistor group TG 2  will be described. In the thermistor group TG 2 , like the thermistor group TG 1 , the CPU  420  receives signals (Th 2 - 4  to Th 2 - 7 ) acquired by dividing voltage Vcc by resistance values of the thermistors (T 2 - 4  to T 2 - 7 ) and resistance values of resistances ( 464  to  467 ). Because the same method for converting to a temperature is applied by the CPU  420  as that for the thermistor group TG 1 , any repetitive description will be omitted. 
     Next, power control over the heater  300  (temperature control over the heater) will be described. During fixing processing, the heating blocks HB 1  to HB 7  are controlled such that the temperatures sensed by the thermistors (T 1 - 1  to T 1 - 4 ) in the thermistor group TG 1  can be maintained at a set temperature (control target temperature). More specifically, the electric power to be supplied to the group 1 (heating block HB 4 ) is controlled by controlling the driving of the triac  411  such that the temperature sensed by the thermistor T 1 - 4  can be maintained at a set temperature. The electric power to be supplied to the group 2 (heating blocks HB 3  and HB 5 ) is controlled by controlling the driving of the triac  412  such that the temperature sensed by the thermistor T 1 - 3  can be maintained at a set temperature. The electric power to be supplied to the group 3 (heating blocks HB 2  and HB 6 ) is controlled by controlling the driving of the triac  413  such that the temperature sensed by the thermistor T 1 - 2  can be maintained at a set temperature. The electric power to be supplied to the group 4 (heating blocks HB 1  and HB 7 ) is controlled by controlling the driving of the triac  414  such that the temperature sensed by the thermistor T 1 - 1  can be maintained at a set temperature. The thermistors in the thermistor group TG 1  are used for executing control for maintaining the heating blocks at a predetermined temperature. 
     The CPU  420  calculates amounts of power supply by performing PI control, for example, based on the set temperatures (control target temperature) for the heating blocks and the temperatures sensed by the thermistors (T 1 - 1  to T 1 - 4 ) within the thermistor group TG 1 . Furthermore, the amounts of power supply are converted to control times for the corresponding phase angle (phase control) or a wave number (wave number control), and the triacs  411  to  414  are controlled based on the control times. The set temperature for the groups in the apparatus of this embodiment is 250° C. for fixing plain paper having a maximum size. The set temperature for the group 1 is 250° C. and the set temperature for the other groups is lower than 250° C. for fixing plain paper having a smaller size. The set temperatures for the groups may be defined in accordance with information such as a size, a type, and a surface property of a recording material. 
     A relay  430  and a relay  440  are mounted as units for shutting down electric power to the heater  300  when the temperature of the heater  300  excessively rises due to a failure in the apparatus, for example. Next, circuit operations of the relay  430  and relay  440  will be described. 
     When an RLON signal output from the CPU  420  is changed to a High state, the transistor  433  is changed to an ON state, and conduction is brought from the direct current power supply (voltage Vcc) to a secondary coil of the relay  430 . The primary side contact of the relay  430  is changed to an ON state. When the RLON signal is changed to a Low state, the transistor  433  is changed to an OFF state. Electric current fed from the power supply (voltage Vcc) to the secondary coil of the relay  430  is blocked, and the primary side contact of the relay  430  is changed to an OFF state. Also, when the RLON signal is changed to a High state, the transistor  443  is changed to an ON state. Conduction is brought from the power supply (voltage Vcc) to the secondary coil of the relay  440 , and the primary side contact of the relay  440  is changed to an ON state. When the RLON signal is changed to a Low state, the transistor  443  is changed to an OFF state. The electric current fed from the power supply (voltage Vcc) to the secondary coil of the relay  440  is blocked, and the primary side contact of the relay  440  is changed to an OFF state. 
     Next, operations of a protection circuit employing the relay  430  and relay  440  (or hardware circuit not through the CPU  420 ) will be described. When a level of one of the signals Th 1 - 1  to Th 1 - 4  exceeds a predetermined value set within a comparing unit  431 , the comparing unit  431  causes a latch unit  432  to operate, and the latch unit  432  latches an RLOFF 1  signal to a Low state. When the RLOFF 1  signal is changed to a Low state, the transistor  433  is maintained at an OFF state even though the CPU  420  changes the RLON signal to a High state. Thus, the relay  430  can be kept at an OFF state (or a safe state). The latch unit  432  in a non-latching mode outputs the RLOFF 1  signal for an open state. 
     Also, when a level of one of the signals Th 2 - 4  to Th 2 - 7  exceeds the predetermined value set within a comparing unit  441 , the comparing unit  441  is caused to operate a latch unit  442 , and the latch unit  442  latches an RLOFF 2  signal to a Low state. When the RLOFF 2  signal is changed to a Low state, the relay  440  can keep the OFF state (or safe state) because the transistor  443  is kept at an OFF state even though the CPU  420  changes the RLON signal to a High state. The latch unit  442  in the non-latching state outputs the RLOFF signal for an open state. Both of the predetermined value set within the comparing unit  431  and the predetermined value set within the comparing unit  441  are equivalent to 300° C. 
     Next, protection operations of a circuit employing the two thermistor groups TG 1  and TG 2  will be described. As illustrated in  FIGS.  3 A to  3 C  and  FIG.  4   , one thermistor of the thermistor group TG 1  and one thermistor of the thermistor group TG 2  are provided for each of the four groups (groups 1 to 4). At least one thermistor is provided for each of the heating blocks HB 1  to HB 7 . More specifically, for the group 1 (HB 4 ), the thermistor T 1 - 4  in the thermistor group TG 1  and the thermistor T 2 - 4  in thermistor group TG 2  are placed correspondingly. For the group 2 (HB 3  and HB 5 ), the thermistor T 1 - 3  in the thermistor group TG 1  and the thermistor T 2 - 5  in the thermistor group TG 2  are placed correspondingly. For the group 3 (HB 2  and HB 6 ), the thermistor T 1 - 2  in the thermistor group TG 1  and the thermistor T 2 - 6  in the thermistor group TG 2  are placed correspondingly. For the group 4 (HB 1  and HB 7 ), the thermistor T 1 - 1  in the thermistor group TG 1  and the thermistor T 2 - 7  in the thermistor group TG 2  are placed correspondingly. For each of the heating blocks HB 1  to HB 7 , at least one thermistor of the eight thermistors is placed correspondingly. This layout of the thermistors can improve the reliability of the protection operations performed by the circuit when the apparatus fails. This will be described below. 
     For example, a case is assumed in which one of the thermistors T 1 - 1  to T 1 - 4  in the thermistor group TG 1  fails. Even when a group including a heating block corresponding to the failed thermistor is uncontrollable due to the failed thermistor, the group having the heating block having the failed thermistor also includes the thermistor (one of T 2 - 4  to T 2 - 7 ) in the thermistor group TG 2 . Thus, the protection circuit works through the thermistor in the thermistor group TG 2  (which stops the power supply). 
     Next, advantages of the configuration in which at least one thermistor of the eight thermistors is arranged correspondingly for one of the heating blocks HB 1  to HB 7 . 
     For example, a case is assumed in which the thermistor T 2 - 5  corresponding to the group 2 is placed at a position corresponding to the heating block HB 3  in the same group 2 as that of the heating block HB 5  rather than the position corresponding to the heating block HB 5 . In this case, the thermistor T 1 - 3  in the thermistor group TG 1  and the thermistor T 2 - 5  in the thermistor group TG 2  are placed at a position corresponding to the heating block HB 3 , and no thermistor is placed at a position corresponding to the heating block HB 5 . Also in this configuration, the temperature of the group 2 can be monitored. However, when the electrode E 3  and the electric contact C 3  in this configuration have a contact failure, there is a possibility that the heating block HB 3  may not be heated but the heating block HB 5  in the same group 2 as that of the heating block HB 3  may be heated. Even when the heating block HB 5  of the group 2 generates heat abnormally, the two thermistors T 1 - 3  and T 2 - 5  corresponding to the group 2 cannot monitor it, and the protection circuit does not work. 
     On the other hand, according to this embodiment, the thermistor T 1 - 3  in the thermistor group TG 1  is placed at a position corresponding to the heating block HB 3 , and the thermistor T 2 - 5  in the thermistor group TG 2  is placed at a position corresponding to the heating block HB 5 . Therefore, even when the electrode E 3  and the electric contact C 3  have a contact failure and the heating block HB 5  in the group 2 only generates heat, the temperature may be monitored by the thermistor T 2 - 5 , and the protection circuit can be operated. As described above, because at least one thermistor of the eight thermistors is placed correspondingly for one of the heating blocks HB 1  to HB 7 , the reliability of the apparatus may be improved. 
       FIG.  5    is a flowchart illustrating a control sequence of the control circuit  400  in the CPU  420 . If a print request occurs in S 100 , the relay  430  and relay  440  are changed to an ON state in S 101 . 
     In S 102 , the triac  414  is PI controlled such that the temperature (signal Th 1 - 1 ) sensed by the thermistor T 1 - 1  can reach a control target temperature to control electric power to be supplied to the heating blocks HB 1  and HB 7 . 
     In S 103 , the triac  413  is PI controlled such that the temperature (signal Th 1 - 2 ) sensed by the thermistor T 1 - 2  can reach a control target temperature to control electric power to be supplied to the heating blocks HB 2  and HB 6 . 
     In S 104 , the triac  412  is PI controlled such that the temperature (signal Th 1 - 3 ) sensed by the thermistor T 1 - 3  can reach a control target temperature to control the electric power to be supplied to the heating blocks HB 3  and HB 5 . 
     In S 105 , the triac  411  is PI controlled such that the temperature (signal Th 1 - 4 ) sensed by the thermistor T 1 - 4  can reach a control target temperature to control the electric power to be supplied to the heating block HB 4 . 
     As described above, the control target temperature for each of the heating blocks is set based on information regarding the size of a given recording material. In the apparatus according to this embodiment, the control target temperature for the heating block HB 4  including the conveyance reference X is set to one temperature irrespective of the size of recording materials, and control target temperatures for the other heating blocks are changed based on the size of recording materials. As the size of recording materials decreases, the control target temperature to be set for the other heating blocks than the heating block HB 4  is reduced. 
     In S 106 , whether the temperature rise in the non-paper-passing part is equal to or lower than a predetermined threshold temperature (tolerance temperature) Tmax is determined. According to this embodiment, Tmax is set higher than a control target temperature of 250° C. for the heating block HB 4  and set to 280° C. being a lower temperature than a predetermined value of 300° C. set for the comparing unit  431  and the comparing unit  441 . The positional relationship between the thermistor in the thermistor group TG 1  is different from the reference X and the positional relationship between the thermistors in the thermistor group TG 2  and the reference X. The thermistors in the thermistor group TG 2  are placed on an outer side in the longitudinal direction of the heater  300  about the conveyance reference position X within each of the heating blocks, compared with the thermistors in the thermistor group TG 1 . As illustrated in  FIG.  3 B , the relationship may be easy to understood by comparing the distance from the reference X to the thermistor T 1 - 4  corresponding to the heating block HB 4  and the distance from the reference X to the thermistor T 2 - 4  corresponding to the heating block HB 4 . Because of this arrangement, a temperature rise in a non-paper-passing part occurring within one heating block if any can be detected by the thermistors in the thermistor group TG 2 . 
     When it is determined in S 106  that the temperatures sensed by the thermistors T 2 - 4  to T 2 - 7  are equal to or lower than the threshold temperature Tmax, the processing moves to S 108 . The control in S 102  to S 106  is repeated until the end of a print JOB is detected in S 108 . 
     If it is determined in S 106  that the temperatures of the thermistors T 2 - 4  to T 2 - 7  are higher than the threshold temperature Tmax, the processing speed for image formation by the image forming apparatus  100  is reduced in S 107 , and the control target temperatures for the thermistors T 1 - 1  to T 1 - 4  are reduced so that fix processing can then be performed. The reduced processing speed of image formation can provide a fixing property even at a lower temperature compared with processing at full speed. Therefore, the temperature rise in the non-paper-passing part can be suppressed. 
     The processing above is repeated, and if the end of the print JOB is detected in S 108 , the relay  430  and the relay  440  are turned off in S 109 . Then, the control sequence for the image formation ends in S 110 . 
     Second Exemplary Embodiment 
     Next, a second exemplary embodiment will be described in which the heater  300  and the control circuit  400  for the heater according to the first exemplary embodiment are changed to a heater  600  and a control circuit  700 . Like numbers refer to like parts in the descriptions of the first and second exemplary embodiments, and any repetitive description will be omitted. The heater  600  according to the second exemplary embodiment is different from the heater  300  in configuration of the sliding surface layer  1 . The control circuit  700  has the heating blocks HB 1  to HB 7  all of which are controlled independently. 
       FIGS.  6 A and  6 B  illustrate a configuration of the heater  600  according to the second exemplary embodiment. Because the configuration except for the sliding surface layer  1  is the same as that of the heater  300 , any repetitive description will be omitted. 
     The sliding surface layer  1  of the heater  600  has thereon thermistors T 3 - 1   a  to T 3 - 4   a , T 3 - 1   b  to T 3 - 3   b , T 4 - 4   a  to T 4 - 7   a , T 4 - 5   b  to T 4 - 7   b , and T 5  configured to detecting temperatures of the heating blocks HB 1  to HB 7 . Because two or more thermistors are associated with all of the heating blocks HB 1  to HB 7 , the temperatures of all of the heating blocks can be detected even when one of the thermistors fails. 
     The thermistor group TG 3  has seven thermistors T 3 - 1   a  to T 3 - 4   a  and T 3 - 1   b  to T 3 - 3   b , conductive patterns ET 3 - 1   a  to ET 3 - 4   a  and ET 3 - 3   b , ET 3 - 12   b , a common conductive pattern EG 3 . 
     Also, the thermistor group TG 4  has seven thermistors T 4 - 4   a  to T 4 - 7   a  and thermistors T 4 - 5   b  to T 4 - 7   b , conductive patterns ET 4 - 4   a  to ET 4 - 7   a , ET 4 - 5   b , and ET 4 - 67   b , and a common conductive pattern EG 4 . 
     First, the thermistor group TG 3  will be described. The thermistor T 3 - 1   b  and the thermistor T 3 - 2   b  are configured to detect temperatures of the heating blocks HB 1  and HB 2 , and the two thermistors are connected in parallel between the conductive pattern ET 3 - 12   b  and the common conductive pattern EG 3 . Also when the temperature of one of the heating blocks HB 1  and HB 2  increases, one of resistance values of the thermistor T 3 - 1   b  and thermistor T 3 - 2   b  largely decreases. Thus, the temperatures of both of the heating blocks HB 1  and HB 2  can be detected by one conductive pattern ET 3 - 12   b  configured to detect resistance values of the thermistors. Therefore, the cost for forming the wiring of a conductive pattern can be reduced, compared to a case where conductive patterns are connected and are wired to the thermistor T 3 - 1   b  and the thermistor T 3 - 2   b . The width in the short-side direction of the substrate  305  can be reduced. Also, the thermistor T 4 - 6   b  and the thermistor T 4 - 7   b  can be connected in parallel. 
     The common conductive patterns EG 3  and EG 4  are connected on the substrate  305  through a conductive pattern EG 34  for disconnection detection as illustrated in  FIG.  7   . Performing such a disconnection detection can increase the security level upon occurrence of a disconnection failure. 
     The two thermistors T 3 - 3   a  and T 3 - 3   b  are provided for one heating block HB 3 , and a temperature-detectable configuration is provided by the conductive patterns ET 3 - 3   a  and ET 3 - 3   b  configured to detect resistance values and the common conductive pattern EG 3 . 
     In a range of the a heating block HB 3 , the thermistor T 3 - 3   b  placed at a position spaced from the conveyance reference position X is configured to detect the temperature of an edge, and the thermistor T 3 - 3   a  placed at a position close to the conveyance reference position X is configured for temperature adjustment. A plurality of thermistors may be provided for one heating block as required. 
     Because the configuration and operations of thermistor group TG 4  are the same as those of the thermistor group TG 3 , any repetitive description will be omitted. 
     A thermistor T 5  is a single thermistor provided between the conductive patterns ET 5  and EG 5  for detection of resistance values. A single thermistor may be combined with a thermistor group as required. 
       FIG.  7    is a circuit diagram of the control circuit  700  for the heater  600  according to the second exemplary embodiment. The electric power control over the heater  600  is executed by conduction/non-conduction of a triac  711  to a triac  717 . The triacs  711  to  717  operate in accordance with FUSER 1  to FUSER 7  signals from the CPU  420 . The control circuit  700  for the heater  600  has a circuit configuration in which seven triacs  711  to  717  are used to independently control seven heating blocks HB 1  to HB 7 . 
     Next, how the temperature of the heater  600  is detected will be described. The CPU  420  receives signals (Th 3 - 1   a  to Th 3 - 4   a , Th 3 - 3   b , Th 3 - 12   b ) acquired by dividing voltage Vcc by resistance values of the thermistor T 3 - 1   a  to T 3 - 4   a , T 3 - 1   b , and T 3 - 2   b  in the thermistor group TG 3  and resistance values of resistances  751  to  756 . The CPU  420  further receives signals acquired by dividing the voltage Vcc by resistance values of thermistors T 4 - 4   a  to T 4 - 7   a , T 4 - 5   b  to T 4 - 7   b  in a thermistor group TG 4  and resistance values of resistances  771  to  776 . These signals are indicated by Th 4 - 4   a  to Th 4 - 7   a , Th 4 - 5   b , and Th 4 - 67   b  in  FIG.  7   . The CPU further receives a signal (Th 5 ) acquired by dividing the voltage Vcc by a resistance value of a thermistor T 5  and a resistance value of a resistance  761 . The CPU  420  converts the received signals to temperatures based on their levels. 
     The CPU  420  calculates amounts of power supply by performing PI control, for example, based on set temperatures (control target temperatures) for the heating blocks and the temperatures sensed by the thermistors. The amounts of calculated power supply are converted to control times for the corresponding phase angle (phase control) or a wave number (wave number control), and the triacs  711  to  717  are controlled based on the control times. 
     Next, operations of the protection circuit employing the relay  430  and relay  440  will be described. Based on the Th 3 - 1   a  to Th 3 - 4   a  signals of the thermistor group TG 3  and Th 4 - 5   b  and Th 4 - 67   b  signals of the thermistor group TG 4 , if one of the sensed temperatures exceeds the respectively set predetermined values, the comparing unit  431  causes the latch unit  432  to operate. 
     Also, based on Th 4 - 4   a  to Th 4 - 7   a  signals of the thermistor group TG 4  and Th 3 - 3   b  and Th 3 - 12   b  signals of the thermistor group TG 3 , if one of the sensed temperatures exceeds the respectively set predetermined values, the comparing unit  441  causes the latch unit  442  to operate. 
     Next, a disconnection detection circuit  780  will be described. The disconnection detection circuit  780  is a circuit usable for improving the security in a case where the common conductive pattern EG 3  and EG 4  are disconnected. 
     Circuit operations of the disconnection detection circuit  780  will be described. When the common conductive patterns EG 3  and EG 4  are disconnected, the pull-up to the power supply voltage Vcc by a resistance  781  and a resistance  782  changes the disconnection detection signal ThSafe to a High state. The resistance  781  and resistance  782  are provided in consideration of a failure due to a short circuit of the resistances. When the disconnection detection signal ThSafe is changed to a High state, the latch unit  432  and latch unit  442  are caused to operate. 
     Next, effects of the disconnection detection circuit  780  and conductive pattern EG 34  will be described. First, a case will be described in which the common conductive pattern EG 3  and the common conductive pattern EG 4  are connected to a GND, as in the configuration of the first exemplary embodiment, without both of the conductive pattern EG 34  and the disconnection detection circuit  780 . In this case, when the common conductive pattern EG 3  is disconnected, all of the thermistors of the thermistor group TG 3  are disabled. Thus, the protection circuit does not work which is configured to terminate power supply to the heating blocks HB 1  to HB 3 . Also, when the common conductive pattern EG 4  is disconnected, all of the thermistors of the thermistor group TG 4  are disabled. Thus, the protection circuit does not work which is configured to terminate the heating blocks HB 5  to HB 7 . 
     Next, a case will be described in which the common conductive patterns EG 3  and EG 4  are connected to a GND, as in the configuration of the first exemplary embodiment, without the disconnection detection circuit  780 , though the conductive pattern EG 34  is provided which connected the common conductive patterns EG 3  and EG 4 . In this case, because of the effect of the conductive pattern EG 34 , one of the common conductive patterns EG 3  and EG 4  is connected to a GND through the conductive pattern EG 34  even when the other one is disconnected. Thus, the temperature detection can be performed by the thermistor groups TG 3  and TG 4 . However, a connector, not illustrated, configured to connect the conductive patterns (ET 3 - 1   a  to ET 3 - 4   a , and ET 3 - 12   b , ET 3 - 3   b , and EG 3 ) of the thermistor group TG 3  and the control circuit  700  is disconnected, all of the thermistors of the thermistor group TG 3  are disabled. Thus, the protection circuit does not work which terminates the power supply to the heating blocks HB 1  to HB 3 . Also, a connector configured to connect the conductive patterns (ET 4 - 4   a  to ET 4 - 7   a , and ET 4 - 67   b , ET 4 - 5   b , and EG 4 ) of the thermistor group TG 4  and the control circuit  700  is disconnected, all of the thermistors of the thermistor group TG 4  are disabled. Thus, the protection circuit does not work which terminates power supply to the heating blocks HB 5  to HB 7 . 
     On the other hand, the apparatus of this embodiment has the conductive pattern EG 34  and the disconnection detection circuit  780 . Thus, failure states of both cases where the common conductive patterns EG 3  and EG 4  are disconnected and where the connector connecting the thermistor groups TG 3  and TG 4  and the control circuit  700  is disconnected can be detected. 
       FIG.  8    is a flowchart illustrating a control sequence over the control circuit  700  to be performed by the CPU  420 . Like numbers refer to like components in  FIG.  5    and  FIG.  8   , and any repetitive description will be omitted. 
     In S 201 , the triac  711  is PI-controlled such that the temperature (signal Th 3 - 1   a ) sensed by the thermistor T 3 - 1   a  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 1 . 
     In S 202 , the triac  712  is PI-controlled such that the temperature (signal Th 3 - 2   a ) sensed by the thermistor T 3 - 2   a  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 2 . 
     In S 203 , the triac  713  is PI-controlled such that the temperature (signal Th 3 - 3   a ) sensed by the thermistor T 3 - 3   a  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 3 . 
     In S 204 , the triac  714  is PI-controlled such that the temperature (signal Th 5 ) sensed by the thermistor T 5  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 4 . 
     In S 205 , the triac  715  is PI-controlled such that the temperature (signal Th 4 - 5   a ) sensed by the thermistor T 4 - 5   a  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 5 . 
     In S 206 , the triac  716  is PI-controlled such that the temperature (signal Th 4 - 6   a ) sensed by the thermistor T 4 - 6   a  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 6 . 
     In S 207 , the triac  717  is PI-controlled such that the temperature (signal Th 4 - 7   a ) sensed by the thermistor T 4 - 7   a  can reach a predetermined target temperature to control the electric power to be supplied to the heating block HB 7 . 
     In S 208 , whether the temperature rise in the non-paper-passing part is equal to or lower than a predetermined threshold temperature (tolerance temperature) Tmax is determined. 
     When it is determined in S 208  that the temperatures sensed the thermistors T 3 - 4   a , T 4 - 4   a , T 3 - 3   b , and T 4 - 5   b  are equal to or lower than the threshold temperature Tmax, the processing moves to S 108 . Then, the control in S 201  to S 208  is repeated until the end of the print JOB is detected in S 108 . 
     Third Exemplary Embodiment 
     A heater  800  in  FIGS.  9 A and  9 B  has a heating resister  802  closely to a fixing nip part N and a thermistor group TG 6  on the opposite side of the fixing nip part N. Like numbers refer to like parts in the descriptions of the first and third exemplary embodiments, and any description will be omitted. 
       FIG.  9 A  is a cross section view of a center area (near a conveyance reference position X) of the heater  800 . A back surface layer  1  has a conductive pattern only, and a chip thermistor T 6 - 2  is bonded thereon. The heater  800  further has electrodes  810  and  811  for the chip thermistor T 6 - 2 . The chip thermistor T 6 - 2  is connected to a conductive pattern EG 6  and a conductive pattern ET 6 - 2  through the electrode  810  and electrode  811 . Placing the thermistor group TG 6  on the opposite side of the fixing nip part N as in the heater  800  can eliminate the necessity of flatness of a sliding surface layer thereof so that the thick chip thermistor T 6 - 2  can be mounted. 
     The thermistor group TG 6  provided in the back surface layer  1  of the heater  800  has three chip thermistors T 6 - 1  to T 6 - 3 , conductive patterns ET 6 - 1  to ET 6 - 3  configured to detect resistance values of the thermistors, and a common conductive pattern EG 6 . 
     A sliding surface layer  1  of the heater  800  has three heating blocks HB 1  to HB 3 . The heating resister  802  is divided into three of  802 - 1  to  802 - 3  and receives power supply through the first electric conductor  801  and the three second electric conductors  803 - 1  to  803 - 3 . The second electric conductors  803 - 1  to  803 - 3  are connected to electrodes E 1  to E 3 , and the first electric conductor  801  is connected to an electrode E 8 . A switch element such as a triac is provided for each of the electrodes E 1  to E 3  where the electrode E 8  is provided as a common electrode so that the three heating blocks HB 1  to HB 3  can be controlled independently from each other. A sliding surface layer  2  of the heater  800  has a protective layer  808  of glass having a sliding property and an insulative property. 
     In the heater  800 , the first electric conductor  801  and the second electric conductor  803  may be connected by wiring on both ends of the heater in a short-side direction for power supply to the heating blocks HB 1  to HB 3 . Because of the necessity, when the number of heating blocks increases in particular, the area for wiring the first electric conductor  801  and the second electric conductor  803  may increase, which thus increases the size of the heater. 
     The electrodes E 2  to E 6  may be provided within a heating region, as in the heater  300  according to the first exemplary embodiment and the heater  600  according to the second exemplary embodiment so that the area required for wiring the first electric conductor  301  and the second electric conductor  303  is not required. Thus, the size of the heater does not increase while the number of heating blocks can be increased. In the configuration having the electrodes E 2  to E 6  in a heating region, the electrode E 2  to the electrode E 6  may be required to be provided on the opposite side of the fixing nip part N for connecting electric contacts C 2  to C 6 . For that, the heating blocks (HB 1  to HB 7 ) may be provided on the opposite side of the fixing nip part N, the thermistor groups (TG 1 , TG 2 , TG 3 , and TG 4 ) may be formed closely to the fixing nip part N. 
     When a lower number of heating blocks are provided, the thermistor group TG 6  having a plurality of chip thermistors may be placed on the opposite side of the fixing nip part N, as in the heater  800  according to this exemplary embodiment. 
     Fourth Exemplary Embodiment 
     A heater according to a fourth exemplary embodiment illustrated in  FIGS.  10 A and  10 B  is different from the heaters according to the first exemplary embodiment and the second exemplary embodiment in shape of heating resisters. Heating resisters  902   a  and  902   b  in a heater  900  illustrated in  FIG.  10 A  are continuous (or not divided) in a longitudinal direction. 
       FIG.  10 A  is a plan view of a back surface layer  1  of the heater  900 . Because an electric conductor  303  is divided into seven in the longitudinal direction, the heating resistors  902   a  and  902   b  are controlled in temperature independently in a region of heating blocks HB 1  to HB 7 . Because the heating resistors  902   a  and  902   b  are not divided, the heater  900  generates heat continuously in the longitudinal direction even in a gap region in which the electric conductor  303  is divided. Thus, no region exists in which the heating value is equal to 0 (zero), and the heater can thus generate heat uniformly in the longitudinal direction. 
     A heater  1000  illustrated in  FIG.  10 B  has heating resisters  1002   a  and  1002   b  further divided into a plurality of heating resisters which are connected in parallel. 
       FIG.  10 B  is a plan view of a back surface layer  1  of the heater  1000 . The heating resister  1002   a  is divided into a plurality of heating resisters which are connected in parallel between a connected electric conductor  303  and an electric conductor  301   a . Also, the heating resister  1002   b  is divided into a plurality of heating resisters which are connected in parallel between the electric conductor  303  and the electric conductor  301   a.    
     The heating resisters acquired by dividing the heating resisters  1002   a  and  1002   b  are tilted in the longitudinal direction and the short-side direction of the heater  1000  and overlap with each other in the longitudinal direction of the heater  1000 . This can reduce the influence of the gaps between the plurality of divided heating resisters and can thus improve the uniformity of the heating distribution in the longitudinal direction of the heater  1000 . In the heater  1000 , because the divided heating resisters at the edges of adjacent heating blocks overlap with each other in the longitudinal direction, a more uniform heating distribution can be provided in the longitudinal direction of the heater  1000  even in gaps between the heating blocks. The heating resisters at the edges of adjacent heating blocks may be, for example, a heating resister at the right end of the heating block HB 1  and a heating resister at the left end of the heating block HB 2 . 
     Uniformity of heating distributions of the heating resisters  1002   a  and  1002   b  may be acquired by adjusting the width, length, interval, slope and so on of the divided heating resisters. Adoption of the configuration of the heater  900  or heater  1000  can inhibit unevenness in temperature in gaps between a plurality of heating blocks. 
     Fifth Exemplary Embodiment 
       FIGS.  11 A and  11 B  illustrate waveforms of electric current fed to the heating blocks in the control circuit  400  according to the first exemplary embodiment.  FIG.  11 A  illustrates a driving pattern (or a table of waveforms of electric current to be fed to the heating block HB 4 ) for the triac  411 , which are defined for each duty ratio of electric power to be supplied to the heater  300 . Also,  FIG.  11 B  illustrates driving patterns (or tables of waveforms of electric current to be fed to the heating blocks HB 1  to HB 3 , and HB 5  to HB 7 ) for triacs  412  to  414 . 
     The CPU  420  calculates a level (duty ratio) of electric power to be supplied to the heater for each one control period and then selects a waveform according to the duty ratio for each heating block to which the electric power is to be supplied. In a control method according to this exemplary embodiment, four half-waves are defined as one control period to set a conduction control pattern for each triac and thus control electric power to be supplied to the heater  300 . 
     An example of the conduction control pattern for the triac  411  will be described where the duty ratio is equal to 25%. According to a conduction control pattern A for the triac  411  illustrated in  FIG.  11 A , a first half-wave to a second half-waves are controlled by a 90° phase angle to supply 50% electric power, and power supply is turned off in a third half-wave to a fourth half-wave. Thus, an average of 25% electric power is supplied to the heating block HB 4  of the heater  300 . In the conduction control pattern A, phase control is performed in the first half-wave to the second half-wave. 
     In a conduction control pattern for the triacs  412  to  414  illustrated in  FIG.  11 B , a third half-wave to a fourth half-wave are controlled with 90° phase angle to supply 50% electric power, and the power supply is turned off in the first half-wave to the second half-wave. Thus, an average of 25% electric power is supplied to the heating blocks HB 1  to HB 3 , and HB 5  to HB 7  of the heater  300 . A conduction control pattern B performs phase control in the third half-wave to the fourth half-wave. 
     Because the heating block HB 4  of the heater  300  has a lower resistance value than those of the other heating blocks, the amount of change in electric current during the phase control is larger, compared with the other heating blocks. According to this embodiment, the period (first half-wave to second half-wave) for feeding electric current of the phase control to the heating block HB 4  is different from the period (third half-wave to fourth half-wave) for feeding electric current of phase control to the other heating blocks HB 1  to HB 3 , and HB 5  to HB 7 . Thus, the fluctuation of the electric current under the phase control fed to the entire heater  300  can be suppressed. The same is true for other duty ratios than 25%. 
     As illustrated in  FIGS.  11 A and  11 B , the control periods for a plurality of triacs may be synchronized for control (which is called synchronized control over a plurality of triacs) so that harmonic current in the image heating device  200  can be reduced.  FIGS.  11 A and  11 B  illustrate an exemplary synchronized control, and synchronized control over a plurality of triacs may be performed to reduce flicker, for example. 
     The same method is applicable to the triacs  711  to  717  in the control circuit  700  to execute synchronized control over a plurality of triacs. 
     The synchronized control over a plurality of triacs can advantageously reduce harmonic current and flicker and can further satisfy standards against harmonic current and flicker even when a total resistance value of the heater  300  is set lower. When a lower resistance value can be set for the heater  300 , maximum electric power which can be supplied from the AC power supply  401  to the heater  300  can be increased. 
     In the plurality of exemplary embodiments as described above, a center reference printer is used in which a recording material is conveyed by placing the center of the recording material in the width direction at a conveyance reference position X. However, the present invention is also applicable to a one-side reference printer in which one end in the longitudinal direction of a heater is defined as a conveyance reference position, and a recording material is conveyed by placing one end in the width direction of the recording material at the conveyance reference position. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     REFERENCE SIGNS LIST 
     
         
           200  Image heating device 
           300  Heater 
           301  First electric conductor 
           302  Heating resister 
           303  Second electric conductor 
           305  Substrate 
         E 1  to E 7 , E 8 - 1 , E 8 - 2  Electrodes 
         HB 1  to HB 7  Heating blocks