Patent Publication Number: US-2020278631-A1

Title: Heater and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/624,990, filed on Jun. 16, 2017, which claims priority from Japanese Patent Application No. 2016-122177 filed on Jun. 20, 2016, and Japanese Patent Application No. 2017-097533 filed on May 16, 2017, the contents of each of which are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     Embodiments of the present invention relate to a heater and an image forming apparatus. 
     BACKGROUND 
     An image forming apparatus has a fixing device. The fixing device thermally fixes toner on a sheet. The fixing device has a fixing roller or fixing belt and a heat source. 
     When the size of a sheet being passed therethrough is switched to a larger size, for example, a temperature distribution on the fixing roller or fixing belt is not immediately eliminated. After the sheet size is switched, the fixing is performed in a state in which unevenness of the temperature distribution is large. 
     When a small sheet is consecutively passed therethrough, a state in which the fixing roller or the fixing belt is heated to a high temperature continues in a portion that is not in contact with the sheet. 
     Such unevenness in temperature distribution degrades a fixing quality. Particularly, in the case of color printing, unevenness in color formation and in gloss may occur in the fixed image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing a configuration example of an image forming apparatus according to a first embodiment. 
         FIG. 2  is an enlarged schematic cross-sectional view showing a portion of an image forming unit according to the first embodiment. 
         FIG. 3  is a schematic cross-sectional view showing a configuration example of a main part of a fixing device including a heater according to the first embodiment. 
         FIG. 4  is a schematic cross-sectional view in a longitudinal direction showing a configuration example of a heater according to the first embodiment. 
         FIG. 5  is a schematic plan view showing a configuration example of a heater according to the first embodiment. 
         FIG. 6  is a block diagram showing a configuration example of a control system of an image forming apparatus according to the first embodiment. 
         FIG. 7  is a flowchart showing an operation example of an image forming apparatus according to the first embodiment at the time of printing. 
         FIG. 8  is a flowchart showing an operation example of fixing temperature control of an image forming apparatus according to the first embodiment. 
         FIG. 9  is a schematic plan view showing a control operation example of a heater according to the first embodiment. 
         FIG. 10  is a schematic plan view showing a control operation example of a heater of a comparative example. 
         FIG. 11  is a block diagram showing a configuration example of a control system of an image forming apparatus according to a second embodiment. 
         FIG. 12  is a schematic cross-sectional view in a longitudinal direction showing a configuration example of a heater according to the second embodiment. 
         FIG. 13  is a schematic plan view showing a configuration example of a heater according to the second embodiment. 
         FIG. 14  is a flowchart showing an operation example of fixing temperature control of an image forming apparatus according to the second embodiment. 
         FIG. 15  is a schematic cross-sectional view showing a configuration example of a main part of a heater according to a third embodiment. 
         FIG. 16  is a schematic plan view showing a configuration example of a heater according to a fourth embodiment. 
         FIG. 17  is a schematic plan view showing a configuration example of a heater according to a fifth embodiment. 
         FIG. 18  is a schematic cross-sectional view showing a configuration example of a heater according to a sixth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a heater has a base member, a heat generating resistor, a wiring and a switch. Three or more electrodes are disposed on the base member to be spaced apart from each other in a longitudinal direction of the base member. The heat generating resistor electrically interconnects electrodes which form an electrode pair and face each other in the longitudinal direction among the electrodes. The wiring connects the electrodes forming the electrode pair to have different polarities from each other. The switch is connected to the wiring and configured to select an electrode to which a voltage is to be applied. 
     Hereinafter, a heater and an image forming apparatus of embodiments will be described with reference to the drawings. In each of the drawings, the same components are designated by the same reference characters. 
     First Embodiment 
       FIG. 1  is a schematic cross-sectional view showing a configuration example of an image forming apparatus according to a first embodiment. In  FIG. 1 , a dimension and a shape of each member are exaggerated or simplified for easier viewing (the same applies to subsequent drawings). 
     An image forming apparatus  10  according to the first embodiment shown in  FIG. 1  is, for example, a multi-function peripheral (MFP) which is a multi-function copier, a printer, a copier, or the like. In the following description, an MFP will be described as an example. 
     A document table  12  including transparent glass is provided on an upper portion of a main body  11  of the image forming apparatus  10 . An automatic document feeder (ADF)  13  is provided on the document table  12 . A manipulation panel  14  is provided on the upper portion of the main body  11 . The manipulation panel  14  includes an operation panel  14   a  having various keys and a touch panel type display  14   b.    
     A scanner  15  serving as a reading device is provided under the ADF  13 . The scanner  15  reads a document sent by the ADF  13  or a document placed on the document table  12 . The scanner  15  generates image data of the document. The scanner  15  has an image sensor  16 , for example. The image sensor  16  may be a contact type image sensor. The image sensor  16  is disposed in a main scanning direction (a depth direction in  FIG. 1 ). 
     The image sensor  16  is configured to move along the document table  12  when it reads an image of the document placed on the document table  12 . The image sensor  16  is configured to read one page of the document image line by line. 
     When the image sensor  16  reads the image of the document sent by the ADF  13 , the image sensor  16  is configured to read the received document at a fixed position shown in  FIG. 1 . 
     The main body  11  of the image forming apparatus  10  has a printer  17  at a center portion in a height direction. The main body  11  has a plurality of paper feeding cassettes  18  which accommodate a sheet P (paper) of any of various sizes at a lower portion thereof. 
     The paper feeding cassettes  18  are configured to accommodate the sheet P of any of various sizes on a central reference. A central axis of the width of the sheet P of any of various sizes in a direction perpendicular to a conveying direction is aligned at a normal position. 
     Hereinafter, a direction perpendicular to the conveying direction of the sheet P along a conveying surface of the sheet P in the image forming apparatus  10  is referred to as “a conveyance perpendicular direction.” 
     The printer  17  has a photosensitive drum and an exposure unit  19 . 
     The printer  17  is configured to form an image on the sheet P according to image data read by the scanner  15  or according to image data created by a personal computer or the like. The printer  17  is a tandem type color printer, for example. 
     The printer  17  has image forming units  20 Y,  20 M,  20 C, and  20 K of respective colors yellow (Y), magenta (M), cyan (C), and black (K). The image forming units  20 Y,  20 M,  20 C, and  20 K are disposed at a lower side of an intermediate transfer belt  21 . The image forming units  20 Y,  20 M,  20 C, and  20 K are disposed in parallel from upstream to downstream in a moving direction (a direction from the left side to the right side in the drawing) of the lower side of the intermediate transfer belt  21 . 
     The exposure unit  19  has exposure units  19 Y,  19 M,  19 C, and  19 K corresponding to the image forming units  20 Y,  20 M,  20 C, and  20 K, respectively. 
     The exposure unit  19  may be an exposure unit using laser scanning or an exposure unit using a solid scanning device such as a light emitting diode (LED). When the laser scanning is used as the exposure unit, a deflector may be commonly used between a plurality of exposure units when each of the exposure units  19 Y,  19 M,  19 C, and  19 K has a different laser light source. 
       FIG. 2  is a schematic view showing an enlarged cross section of the image forming unit  20 K among the image forming units  20 Y,  20 M,  20 C, and  20 K. 
     Configurations of the respective image forming units  20 Y,  20 M,  20 C, and  20 K are different merely in toner. Hereinafter, components common to the image forming units  20 Y,  20 M,  20 C, and  20 K will be described with reference to an example of the image forming unit  20 K. 
     As shown in  FIG. 2 , the image forming unit  20 K has a photosensitive drum  22 K serving as an image carrier. A charger  23 K, a developer  24 K, a primary transfer roller  25 K, a cleaner  26 K, a blade  27 K, and the like are disposed around the photosensitive drum  22 K in a rotation direction t. 
     The charger  23 K of the image forming unit  20 K uniformly charges a surface of the photosensitive drum  22 K. 
     The exposure unit  19 K is configured to irradiate the surface of the photosensitive drum  22 K with light modulated according to the image data. The exposure unit  19 K is configured to form an electrostatic latent image on the photosensitive drum  22 K. 
     The developer  24 K is configured to supply black toner to the photosensitive drum  22 K by a developing roller  24   a  to which a developing bias is applied. The developer  24 K is configured to develop the electrostatic latent image on the photosensitive drum  22 K. 
     The cleaner  26 K has the blade  27 K that comes into contact with the photosensitive drum  22 K. The blade  27 K is configured to remove residual toner on the surface of the photosensitive drum  22 K. 
     As shown in  FIG. 1 , a toner cartridge  28  is disposed above the image forming units  20 Y,  20 M,  20 C, and  20 K. 
     The toner cartridge  28  is configured to supply the toner to each of the developers  24 Y,  24 M,  24 C, and  24 K. The toner cartridge  28  includes toner cartridges  28 Y,  28 M,  28 C, and  28 K which respectively contain toners of yellow (Y), magenta (M), cyan (C), and black (K). 
     The intermediate transfer belt  21  is configured to move circularly. The intermediate transfer belt  21  is stretched over a drive roller  31  and a plurality of driven rollers  32  (refer to  FIG. 1 ). The intermediate transfer belt  21  comes into contact with the photosensitive drums  22 Y,  22 M,  22 C, and  22 K from an upper side as in the drawing. 
     For example, as shown in  FIG. 2 , in the intermediate transfer belt  21 , the primary transfer roller  25 K is disposed at an inner side of the intermediate transfer belt  21  at a position facing the photosensitive drum  22 K. 
     When a primary transfer voltage is applied, the primary transfer roller  25 K primarily transfers a toner image on the photosensitive drum  22 K to the intermediate transfer belt  21 . 
     As shown in  FIG. 1 , a secondary transfer roller  33  faces the drive roller  31  with the intermediate transfer belt  21  interposed therebetween. 
     A secondary transfer voltage is applied to the secondary transfer roller  33  when the sheet P passes through a secondary transfer position between the drive roller  31  and the secondary transfer roller  33 . When the secondary transfer voltage is applied, the secondary transfer roller  33  secondarily transfers the toner image on the intermediate transfer belt  21  to the sheet P. 
     A belt cleaner  34  is disposed near the driven rollers  32  on the left side in the drawing. The belt cleaner  34  removes a residual transfer toner on the intermediate transfer belt  21  from the intermediate transfer belt  21 . 
     As shown in  FIG. 1 , a paper feeding roller  35  is provided between the paper feeding cassettes  18  and the secondary transfer roller  33 . The paper feeding roller  35  is configured to convey the sheet P taken out from the inside of the paper feeding cassettes  18 . 
     A fixing device  36  is disposed downstream (an upper side in the drawing) of the secondary transfer roller  33  in the conveying direction of the sheet P. 
     A conveyance roller  37  is disposed downstream of the fixing device  36  (an upper left side in the drawing) in the conveying direction of the sheet P. The conveyance roller  37  is configured to discharge the sheet P to a sheet discharger  38 . 
     A reverse conveying path  39  is disposed downstream of the fixing device  36  (the right side in the drawing) in the conveying direction of the sheet P. The reverse conveying path  39  is configured to reverse the sheet P and guides it toward the secondary transfer roller  33 . The reverse conveying path  39  is used when double-sided printing is performed. 
       FIG. 3  is a schematic cross-sectional view showing a configuration example of a main part of the fixing device  36 . 
     As shown in  FIG. 3 , the fixing device  36  has a fixing belt  363 , a press roller  366 , belt conveying rollers  364 , a tension roller  365 , and a heating member  361  (a heater). 
     Although not shown in  FIG. 3 , the fixing device  36  further has a temperature detector  362  (refer to  FIG. 6 ) as will be described below. 
     The fixing belt  363  is an endless belt. The fixing belt  363  has an elastic layer on its surface. The fixing belt  363  faces the press roller  366 . 
     The press roller  366  has an elastic layer on the surface thereof. The press roller  366  is rotatably supported in a rotation direction t as in the drawing. 
     The fixing belt  363  and the press roller  366  have a width (a width in the conveyance perpendicular direction of the sheet P) larger than a maximum sheet width that can be passed in the image forming apparatus  10 . 
     The fixing belt  363  is pressed against a surface of the press roller  366 . A contact portion between the fixing belt  363  and the press roller  366  form a fixing nip. The fixing nip is slightly longer than the maximum sheet width that can be passed through. The width of the fixing nip in a conveying direction A of the sheet P is set to a predetermined width. The predetermined width is a width to which an amount of heat for thermally fixing the toner image that has been transferred to the sheet P can be supplied. 
     The tension roller  365  and two belt conveying rollers  364  are disposed inside the fixing belt  363 . The fixing belt  363  is stretched from the inside by the tension roller  365  and the two belt conveying rollers  364 . 
     The tension roller  365  presses an inside of the fixing belt  363  at a position at which a contact portion of the fixing belt  363  and the press roller  366  is sandwiched therebetween. The tension roller  365  provides tension to the fixing belt  363 . 
     The belt conveying rollers  364  are configured to be rotationally driven counterclockwise as in the drawing (refer to the arrow s) by a driving motor (not shown in the figure). When the belt conveying rollers  364  rotate, the fixing belt  363  is rotationally driven counterclockwise as represented by an arrow B as in the drawing. 
     The heating member  361  is configured to heat the sheet P via the fixing belt  363  at the fixing nip. 
     A main part of the heating member  361  is formed in a plate shape. The heating member  361  has a heat generating portion on one surface in a plate thickness direction. The heat generating portion of the heating member  361  is in contact with an inner side of the fixing belt  363  on a back side of the fixing nip. The heating member  361  is disposed such that the longitudinal direction thereof follows a longitudinal direction of the fixing nip. 
     The heating member  361  is pressed toward the press roller  366  by a pressing member (not shown in the figure). Since the heating member  361  presses the fixing belt  363  to the press roller  366 , the width of the fixing nip in the conveying direction is maintained at a predetermined width. 
     Heat generated in the heat generating portion of the heating member  361  is thermally conducted in a thickness direction of the fixing belt  363 . The heat thermally conducted to the fixing belt  363  is also thermally conducted to the press roller  366  at a portion of the fixing nip. When the sheet P passes through the fixing nip, the sheet P is heated at the fixing nip. 
     The heating member  361  heats the fixing nip by thermal conduction via the fixing belt  363 . Therefore, in the fixing device  36 , temperature responsiveness of the fixing nip is excellent compared to the case of a heating method using radiation such as a halogen lamp. 
     A configuration example of the heating member  361  will be described in detail. 
       FIG. 4  is a schematic cross-sectional view in a longitudinal direction showing a configuration example of the heating member  361  according to the first embodiment.  FIG. 5  is a schematic plan view showing a configuration example of the heating member  361  according to the first embodiment. However, in  FIG. 5 , a surface protective layer  361   d  to be described below is omitted for easier viewing in the drawing. 
     As shown in  FIG. 4 , the heating member  361  has a base member  361   a , an electrode  361   b , a heat generating resistor  361   c , and the surface protective layer  361   d . As shown in  FIG. 5 , the heating member  361  further has wirings  361   f  and  361   g  and a switch  361   e.    
     As shown in  FIG. 4 , the base member  361   a  includes a ceramic substrate, for example. A glaze layer (not shown in the figure) is laminated on one surface in a plate thickness direction of the ceramic substrate (a surface of an upper side in the drawing). 
     A heat sink (not shown in the figure) configured to dissipate extra heat of the heat generating portion may be attached to the other surface in the plate thickness direction of the ceramic substrate. The heat sink may be made of an aluminum alloy. When the heat sink is attached to the ceramic substrate, the heat sink also functions to prevent a warp of the ceramic substrate. 
     The electrode  361   b  applies a voltage to the heat generating resistor  361   c  to be described below. When paper conveyance is on the central reference, five or more electrodes  361   b  are provided on the glaze layer (not shown in the figure) of the base member  361   a.    
     In the configuration examples shown in  FIGS. 4 and 5 , the electrode  361   b  is constituted with seven electrodes, including an electrode E 0  (a central electrode) and electrodes E 1 L, E 2 L, E 3 L, E 1 R, E 2 R, and E 3 R. 
     As shown in  FIG. 5 , the electrode E 0  is formed in a linear shape and extends in a short direction perpendicular to a longitudinal direction of the base member  361   a . The line width of the electrode E 0  is We. It is preferable that the line width We be thin. The line width We may be in a range of 0.5 to 3.0 mm, for example. 
     The electrode E 0  is disposed at a center portion in the longitudinal direction of the base member  361   a . In the fixing device  36 , the longitudinal direction of the base member  361   a  is aligned with the conveyance perpendicular direction. The position of the electrode E 0  in the longitudinal direction of the base member  361   a  is aligned with a center of the conveying path in the conveyance perpendicular direction of the fixing device  36 . 
     Each electrode  361   b  is formed of a metal having a high degree of conductivity. Each electrode  361   b  may be formed of a metal such as aluminum or copper, for example. 
     As shown in  FIG. 4 , on the left side of the electrode E 0  in the drawing, the electrodes E 1 L, E 2 L, and E 3 L are disposed in that order at a pitch P 0  from the center toward a left end as in the drawing in the conveyance perpendicular direction. 
     On the right side of the electrode E 0  in the drawing, the electrodes E 1 R, E 2 R, and E 3 R are disposed in that order at the pitch P 0  from the center toward a right end shown in the drawing in the conveyance perpendicular direction. 
     As described above, the seven electrodes  361   b  in the embodiment are disposed to be line-symmetrical with respect to a center line of the line width of the electrode E 0  as the axis of symmetry. 
     As shown in  FIG. 5 , a shape of the electrodes E 1 L, E 2 L, E 3 L E 1 R, E 2 R, and E 3 R in a plan view is the same as that of the electrode E 0 . All the electrodes E 1 L, E 2 L, E 3 L, E 1 R, E 2 R, and E 3 R are disposed in parallel with the electrode E 0 . The position of the electrodes E 1 L, E 2 L, E 3 L, E 1 R, E 2 R, and E 3 R in a short direction of the base member  361   a  may be different from that of the electrode E 0 . However, the electrodes E 0 , E 1 L, E 2 L, E 3 L, E 1 R, E 2 R, and E 3 R extend in the longitudinal direction of the base member  361   a  and are formed at positions and with lengths that can traverse rectangular regions having a width Wh in the short direction. 
     The wiring  361   f  electrically connects each end of the electrodes E 2 L, E 0 , and E 2 R in the longitudinal direction thereof (the short direction of the base member  361   a ) to a fixing power source  150   a  to be described below. 
     The wiring  361   g  electrically connects each end of the electrodes E 3 L, E 1 L, E 1 R, and E 3 R in the longitudinal direction thereof (the short direction of the base member  361   a ) to the fixing power source  150   a  to be described below. 
     The fixing power source  150   a  is an alternating current (AC) power source. In the fixing power source  150   a , an AC voltage oscillating at an amplitude V and a frequency f is applied between a terminal T 1  (a first terminal) and a terminal T 2  (a second terminal). The terminals T 1  and T 2  have polarities opposite to each other. 
     The fixing power source  150   a  may be disposed at any position in the image forming apparatus  10 . In the embodiment, the fixing power source  150   a  is provided as a part of a fixing control circuit  150  to be described below as an example. 
     The electrodes E 0 , E 2 L, and E 2 R are wired to the terminal T 1  of the fixing power source  150   a  by the wiring  361   f . The electrodes E 1 L, E 1 R, E 3 L and E 3 R are wired to the terminal T 2  of the fixing power source  150   a  by the wiring  361   g.    
     The switch  361   e  is provided between the fixing power source  150   a  and the electrode  361   b  on the wirings  361   f  and  361   g . The switch  361   e  selects the electrode  361   b  to which a voltage is applied by the fixing power source  150   a.    
     The switch  361   e  includes switches S 2 L, S 2 R, S 1 , S 3 L, and S 3 R. 
     The electrode E 0  is always electrically connected to the terminal T 1  by the wiring  361   f.    
     The electrodes E 2 L and E 2 R are connected to the terminal T 1  respectively via the switches S 2 L and S 2 R. The switch S 2 L (S 2 R) can turn on (ON) or turn off (OFF) the electrical connection between the electrode E 2 L (E 2 R) and the terminal T 1 . 
     The electrodes E 1 L and E 1 R are connected to the terminal T 2  via the switch S 1 . The switch S 1  can turn on or turn off the electrical connection between the electrodes E 1 L and E 1 R and the terminal T 2 . 
     The electrodes E 3 L and E 3 R are connected to the terminal T 2  respectively via the switches S 3 L and S 3 R. The switch S 3 L (S 3 R) can turn on or turn off the electrical connection between the electrode E 3 L (E 3 R) and the terminal T 2 . 
     A switching operation of each switch  361   e  is individually controlled by the fixing control circuit  150  to be described below. 
     Specific examples of each switch  361   e  include a switching device, a field-effect transistor (FET), a triac, a switching integrated circuit (IC), and the like. 
     Each switch  361   e  may be provided at any position of the wirings  361   f  and  361   g  as long as the above-described electrical connection is possible. 
     Each switch  361   e  may be integrated with a member disposed in the fixing device  36  such as the base member  361   a , for example. Each switch  361   e  may be provided on the wirings  361   f  and  361   g  extending to the outside of the fixing device  36 , for example. Each switch  361   e  may be disposed inside the image forming apparatus  10  in which the fixing power source  150   a  is disposed. 
     According to such wirings, when each switch  361   e  is turned on, the electrodes E 3 L, E 2 L, E 1 L, E 0 , E 1 R, E 2 R, and E 3 R are electrically connected respectively to the terminals T 2 , T 1 , T 2 , T 1 , T 2 , T 1 , and T 2 . Electrode pairs facing each other in the longitudinal direction of the base member  361   a  (the electrodes E 3 L and E 2 L, the electrodes E 2 L and E 1 L, and the like, for example) are connected to the terminals having polarities opposite to each other. 
     As shown in  FIG. 4 , the heat generating resistor  361   c  is laminated on a surface of the base member  361   a . The heat generating resistor  361   c  covers each electrode  361   b . In the embodiment, the layer thickness of the heat generating resistor  361   c  is constant. 
     As shown in  FIG. 5 , the heat generating resistor  361   c  has a strip shape extending in the longitudinal direction of the base member  361   a  in a plan view. The width of the heat generating resistor  361   c  in the short direction is Wh in a plan view. 
     In a plan view, each electrode  361   b  traverses the heat generating resistor  361   c  in the short direction. Each electrode  361   b  is covered with the heat generating resistor  361   c  within a range of the width Wh. 
     The heat generating resistor  361   c  electrically interconnects the electrode pair facing each other in the longitudinal direction of the base member  361   a  among each of the electrodes. 
     As shown in  FIG. 4 , the electrode pair formed with the electrode E 0  and the electrode E 1  are electrically interconnected by the heat generating resistor  361   c  having a length of substantially P 0  and the width Wh, for example. This also applies to the other electrode pairs facing each other in the longitudinal direction of the base member  361   a.    
     The heat generating resistor  361   c  is formed of a material that generates Joule heat when an AC voltage is applied. As a material of the heat generating resistor  361   c , a well-known material used to form a thermal head may be used. The heat generating resistor  361   c  may be formed of TaSiO 2  or the like, for example. 
     The surface protective layer  361   d  protects a surface of the heating member  361  which slides on an inner peripheral surface of the fixing belt  363 . As shown in  FIG. 5 , the surface protective layer  361   d  is laminated on the base member  361   a  to cover at least a surface of the heat generating resistor  361   c.    
     The surface protective layer  361   d  may be formed of Si 3 N 4  or the like as an example. The material of the surface protective layer  361   d  is not limited to Si 3 N 4 . 
     The laminate of the heat generating resistor  361   c  and the surface protective layer  361   d  constitutes a heat generating portion of the heating member  361 . 
     With such a configuration, when a voltage is applied to the electrode pair facing each other in the longitudinal direction of the heating member  361  by the fixing power source  150   a , an AC current flows between the electrode pair to which the voltage is applied. The heat generating resistor  361   c  between the electrode pair to which the voltage is applied generates Joule heat. 
     As shown in  FIG. 4 , the heating member  361  has six heat generating regions R 23 L, R 12 L, R 01 L, R 01 R, R 12 R, and R 23 R formed with the heat generating resistor  361   c  sandwiched by the respective electrode pairs. Here, the heat generating region R 23 L is a region of the heat generating resistor  361   c  sandwiched by the electrode pair of the electrodes E 2 L and E 3 L. 
     The size of each heat generating region is determined by a disposition interval of each electrode pair. In the embodiment, since the electrode  361   b  is arranged at a regular pitch P 0 , the sizes of the respective heat generating regions are equal to each other. 
     Here, the pitch P 0  is determined according to the sheet width of the sheet P that can be passed in the image forming apparatus  10 . 
     For example, it is assumed that the sizes that can be passed in the image forming apparatus  10  are a postcard size (100 mm×148 mm), a compact disk (CD) jacket size (121 mm×121 mm), an A5R size (148 mm×210 mm), a B5R size (182 mm×257 mm), an A4R size (210 mm×297 mm), a B5 size (257 mm×182 mm), an A4 size (297 mm×210 mm), and an A3R size (297 mm×420 mm). In this case, the sheet widths of each sheet P are 100 mm, 121 mm, 148 mm, 182 mm, 210 mm, 257 mm, 297 mm, and 297 mm, respectively. 
     For example, it is assumed that the arrangement pitch P 0  of the respective electrodes  361   b  is 54.5 mm. At this time, a disposition interval L 1  of the electrodes E 1 L and E 1 R is 109 mm, a disposition interval L 2  of the electrodes E 2 L and E 2 R is 218 mm, and a disposition interval L 3  of the electrodes E 3 L and E 3 R is 327 mm. 
     In this case, when heat is generated at the heat generating regions ROIL and R 01 R, it is possible to fix the sheet P of the postcard size. When heat is generated at the heat generating regions R 12 L, R 01 L, R 01 R, and R 12 R, it is possible to fix the sheet P of the CD jacket size, the A5R size, the B5R size, and A4R size. When heat is generated at the heat generating regions R 23 L, R 12 L, R 01 L, R 01 R, R 12 R, and R 23 R, it is possible to fix the sheet P of the B5 size, the A4 size, and A3R size. 
     The width of the heat generating region needed for fixing is set to have a margin in consideration of conveyance accuracy of the sheet P, skewing, and heat escape via a non-heated portion with respect to the sheet width. However, when an image forming width on the sheet P is smaller than the sheet width, the width of the heat generating region needed for fixing may be set to have the same margin as that of the image forming width. 
     Next, a configuration of a control system of the image forming apparatus  10  will be described. 
       FIG. 6  is a block diagram showing a configuration example of a control system  50  of the image forming apparatus  10  according to the first embodiment. 
     However, members distinguished by the suffixes Y, M, C, and K are represented by characters from which the subscripts are deleted in  FIG. 6  for easier viewing. For example, a photosensitive drum  22  represents the photosensitive drums  22 Y,  22 M,  22 C, and  22 K. The same applies to a charger  23 , a developer  24 , a primary transfer roller  25 , and an exposure unit  19 . 
     As shown in  FIG. 6 , the control system  50  has a central processor (CPU)  100 , a read-only memory (ROM)  120 , a random-access memory (RAM)  121 , an interface (I/F)  122 , an input/output control circuit  123 , a paper feeding/conveyance control circuit  130 , an image forming control circuit  140 , and a fixing control circuit  150  (a fixing controller). 
     The CPU  100  controls the entire image forming apparatus  10 . The CPU  100  realizes a processing function to form an image by executing a program stored in the ROM  120  or the RAM  121 . 
     The ROM  120  stores control programs, control data, and the like, which manage a basic operation of the image forming process. The RAM  121  is a working memory. 
     The ROM  120  (or the RAM  121 ) stores, for example, a control program which controls the image forming unit  20 , the fixing device  36 , and the like, and various control data used by the control program. As a specific example of the control data in the embodiment, a corresponding correlation between the size of the sheet P and the switch which selects the heat generating region of the heating member  361  to be heated is an exemplary example. 
     A fixing temperature control program which controls a temperature of the fixing device  36  includes a heating width determination logic and a heating control logic. The heating width determination logic detects the size of the sheet P on which the toner image is formed and determines a required heating width. The heating control logic selects a heat generating region corresponding to the required heating width and controls heating by the heating member  361 . 
     The I/F  122  communicates with various devices such as a user terminal, a facsimile, or the like. 
     The input/output control circuit  123  controls the operation panel  14   a  and the display  14   b.    
     The paper feeding/conveyance control circuit  130  controls a driving system included in the main body  11 . For example, a motor group  130   a  which drives a paper feeding roller  35  (refer to  FIG. 1 ) and a resist roller  41  (refer to  FIG. 1 ) of the conveying path is included in the driving system. The paper feeding/conveyance control circuit  130  controls a driving system such as the motor group  130   a  according to a control signal from the CPU  100  and detection results of various sensors  130   b  near the paper feeding cassettes  18  (refer to  FIG. 1 ) or according to the conveying path. 
     The image forming control circuit  140  controls the photosensitive drum  22 , the charger  23 , the exposure unit  19 , the developer  24 , the primary transfer roller  25 , and the secondary transfer roller  33  according to the control signal from the CPU  100 . 
     The fixing control circuit  150  controls a driving motor  360 , the heating member  361 , and the temperature detector  362  which are in the fixing device  36  according to the control signal from the CPU  100 . 
     The temperature detector  362  directly or indirectly detects the temperature of the fixing belt  363  (refer to  FIG. 3 ) which forms the fixing nip. A thermistor is one specific example of the temperature detector  362 . 
     The temperature detector  362  may be disposed at an inner side or an outer side of the fixing belt  363 . The temperature detector  362  may be disposed in the heat generating portion of the heating member  361  in contact with the fixing belt  363 . 
     The number of temperature detectors  362  disposed is not limited. When one temperature detector  362  is disposed, it is disposed in a range corresponding to the width L 1  (refer to  FIG. 4 ) of the heating member  361 , for example. 
     When a plurality of temperature detectors  362  are disposed, for example, one temperature detector  362  may be disposed in each of the heat generating regions of the heating member  361 . The temperature distribution in the longitudinal direction of the heat generating portion of the heating member  361  is substantially symmetrical with respect to the electrode E 0 . Therefore, one temperature detector  362  may be disposed in each of the ranges corresponding to the heat generating regions R 01 L, R 12 L, and R 23 L, or in each of the ranges corresponding to the heat generating regions R 01 R, R 12 R, and R 23 R. 
     In the embodiment, the control program and the control data of the fixing device  36  are configured to be stored in the storage of the image forming apparatus  10  and are executed by the CPU  100 ; however a configuration in which an arithmetic processor and a storage are separately provided for exclusive use of the fixing device  36  is also acceptable. 
     Next, the following description will be based on an operation of the image forming apparatus  10  at the time of printing. 
       FIG. 7  is a flowchart showing an operation example of the image forming apparatus  10  according to the first embodiment at the time of printing. 
     The image forming apparatus  10  executes ACT  1  to ACT  14  shown in  FIG. 7  in accordance with the flow shown in  FIG. 7  to print an image on the sheet P. 
     In ACT  1 , the image forming apparatus  10  reads image data. Reading of the image data may be performed by an operator operating the scanner  15  to make the scanner  15  read the document. Alternatively, the image data may be read through a communication line connected to the image forming apparatus  10  via the I/F  122 . 
     After the image data is read, ACT  2  is executed. 
     In ACT  2 , the CPU  100  determines the paper size to be printed. The CPU  100  determines the paper size of the sheet P to be used for printing according to a setting by the operation panel  14   a , a document size detected by the scanner  15 , or a control signal from an external device. 
     As a result, a sheet width of the sheet P passing through the fixing device  36  is determined. 
     Therefore, ACT  2  ends. 
     When ACT  2  ends, ACT  3  is performed. In ACT  3 , the CPU  100  selects a heat generating region corresponding to the paper size. 
     The CPU  100  selects the heat generating region to be heated according to the correlation between the sheet width and the heat generating region. 
     In the ROM  120  of the embodiment, the correspondence between the paper size and the heat generating region to be selected is stored as follows, for example. In the case of the postcard size, the heat generating regions R 01 L and R 01 R are selected. In the case of the CD jacket size, the A5R size, the B5R size, and the A4R size, the heat generating regions R 12 L, R 01 L, R 01 R, and R 12 R are selected. In the case of the B5 size, the A4 size, and the A3R size, the heat generating regions R 23 L, R 12 L, R 01 L, R 01 R, R 12 R and R 23 R are selected. 
     In ACT  2 , when the sheet P to be used for printing is determined as the postcard size, for example, the CPU  100  selects the heat generating regions R 01 L and R 01 R to be heated. 
     Therefore, ACT  3  ends. 
     After ACT  3 , ACT  4  is performed. In ACT  4 , the CPU  100  sends a control signal to start the fixing temperature control (referred to as a fixing temperature control start signal) to the fixing control circuit  150 . The CPU  100  sends information on the selected heat generating region to the fixing control circuit  150  together with the fixing temperature control start signal. 
     The fixing temperature control by the fixing control circuit  150  is continuously performed until the CPU  100  sends a control signal for ending the fixing temperature control to the fixing control circuit  150 . 
     When ACT  4  ends, ACT  5  is performed. In ACT  5 , the CPU  100  sends the paper feeding/conveyance control circuit  130  a control signal for feeding the sheet P used for printing from the paper feeding cassettes  18 . 
     The paper feeding/conveyance control circuit  130  performs the control of feeding the sheet P used for printing from the paper feeding cassettes  18  according to the control signal from the CPU  100 . In addition, the paper feeding/conveyance control circuit  130  drives the paper feeding roller  35 . The paper feeding roller  35  stops in a state in which a leading end of the sheet P is in contact with the resist roller  41 . 
     Therefore, ACT  5  ends. 
     After ACT  5 , ACT  6  is performed. Before describing ACT  6 , the fixing temperature control by the fixing control circuit  150  will be described. The fixing temperature control is performed in parallel with each operation after ACT  6 . 
       FIG. 8  is a flowchart showing an operation example of the fixing temperature control of the image forming apparatus  10  according to the first embodiment.  FIG. 9  is a schematic plan view showing a control operation example of the heating member  361  according to the first embodiment. 
     The fixing control circuit  150  executes ACT  21  to ACT  29  shown in  FIG. 8  in accordance with the flow shown in  FIG. 8 . 
     First, ACT  21  is performed. In ACT  21 , the fixing control circuit  150  turns on the switch  361   e  connected to the electrode pair sandwiching the selected heat generating region according to the information of the heat generating region selected by the CPU  100  sent from the CPU  100 . 
     When it is selected to heat the heat generating regions R 01 L and R 01 R, for example, the fixing control circuit  150  turns on the switch S 1  among the switch  361   e  and turns off the other switch. 
     As shown in  FIG. 9 , the electrode E 0  is electrically conducted to the terminal T 1 . When the switch S 1  is turned on, the electrodes E 1 L and E 1 R are electrically conducted to the terminal T 2 . The electrodes E 2 L and E 2 R and the electrodes E 3 L and E 3 R (not shown in the figure) are not electrically conductive with the terminals T 1  and T 2 , respectively. 
     A voltage corresponding to the potential difference between the terminals T 1  and T 2  is applied between the electrodes E 0  and E 1 L and between the electrodes E 0  and E 1 R. A current flows between the electrodes E 0  and E 1 L and between the electrodes E 0  and E 1 R in opposite directions to each other. This current flows in substantially a uniform amount in the width direction of the heat generating resistor  361   c  having the width Wh and the length P 0 . 
     Due to the Joule heat generated by this current, the heat generating resistor  361   c  of the heat generating regions ROIL and R 01 R starts to generate heat. 
     In the embodiment, the heat generating resistor  361   c  is laminated on the electrode E 0  which is a boundary of the heat generating regions. Since the heat generating resistor  361   c  on the electrode E 0  has low current density compared to the side of the electrode E 0 , an amount of heat generation thereof is reduced. However, the line width of the electrode E 0  can be an extent of 0.5 mm or more and 3.0 mm or less. As a result, the heat generating resistor  361   c  on the electrode E 0  immediately rises in temperature by thermal conduction from the surroundings. 
     On the other hand, no voltage is applied between the electrode pairs other than the electrodes E 0  and E 1 L and the electrodes E 0  and E 1 R. Heat generation does not occur in the heat generating regions R 12 L, R 12 R, R 23 L, and R 23 R. 
     The heat generating resistor  361   c  generates heat substantially uniformly in a rectangular range, whose center is the electrode E 0 , having the width Wh and the length L 1  (=2·P 0 ). 
     Similarly, when the heat generating regions R 01 L, R 01 R, R 12 L, and R 12 R are selected to be heated, the fixing control circuit  150  turns on the switches S 1 , S 2 L, and S 2 R among the switch  361   e  and turns off the switches S 3 L and S 3 R. The heat generating resistor  361   c  generates heat substantially uniformly in a rectangular range, whose center is the electrode E 0 , having a width Wh and a length L 2  (=4·P 0 ). 
     When the heat generating regions R 01 L, R 01 R, R 12 L, R 12 R, R 23 L, and R 23 R are selected to be heated, the fixing control circuit  150  turns all of the switch  361   e  on. The heat generating resistor  361   c  generates heat substantially uniformly in a rectangular range, whose center is the electrode E 0 , having a width Wh and a length L 3  (=6·P 0 ). 
     After ACT  21 , ACT  22  is performed. In ACT  22 , the fixing control circuit  150  determines whether or not a control signal (hereinafter referred to as a fixing temperature control end signal) for ending the fixing temperature control sent by the CPU  100  has been received. 
     When the fixing control circuit  150  receives the fixing temperature control end signal (ACT  22 : YES), ACT  29  is performed. 
     When the fixing control circuit  150  has not received the fixing temperature control end signal (ACT  22 : NO), ACT  23  is performed. 
     In ACT  23 , the fixing control circuit  150  detects the temperature of the fixing belt  363  which forms the fixing nip using the temperature detector  362 . 
     When the temperature detector  362  indirectly detects the temperature of the fixing belt  363 , the fixing control circuit  150  converts the detected temperature into the temperature of the fixing nip of the fixing belt  363 . For example, a correlation between the detected temperature by the temperature detector  362  and the temperature of the fixing nip is stored in the ROM  120  in advance as a conversion table or the like. 
     When a plurality of temperature detectors  362  are disposed, in ACT  23 , the fixing control circuit  150  executes the following operations according to the temperature information by the temperature detectors  362  which detect the temperature of the selected heat generating region. 
     Therefore, ACT  23  ends. 
     After ACT  23 , ACT  24  is performed. In ACT  24 , the fixing control circuit  150  determines whether or not the temperature detected in ACT  23  (hereinafter referred to as a detected temperature) falls within a predetermined temperature range. When the fixing control circuit  150  obtains the detected temperature according to the detection signals of the plurality of temperature detectors  362 , the determination is carried out according to the lowest detected temperature in the range corresponding to the selected heat generating region. 
     The predetermined temperature range is determined in advance as a temperature range to fix the toner image on the sheet P and is stored inside the fixing control circuit  150  or the ROM  120 . When the fixing temperature is 150° C., for example, the predetermined temperature range may be a temperature range of 150° C.±10° C. 
     When the detected temperature falls within the predetermined temperature range (ACT  24 : YES), ACT  25  is performed. 
     When the detected temperature is outside the predetermined temperature range (ACT  24 : NO), ACT  26  is performed. 
     In ACT  25 , the fixing control circuit  150  turns on a conveyance permission signal. The conveyance permission signal is used for conveyance control of the image forming apparatus  10  by the CPU  100  as will be described below. 
     After ACT  25  ends, ACT  22  is performed. 
     In ACT  26 , it is determined whether or not the detected temperature exceeds an upper limit value (a temperature upper limit) of the predetermined temperature range. 
     When it is determined that the detected temperature exceeds the upper limit value of the predetermined temperature range (ACT  26 : YES), ACT  27  is performed. 
     When it is determined that the detected temperature is equal to or less than the upper limit value of the predetermined temperature range (ACT  26 : NO), ACT  28  is performed. 
     In ACT  27 , power supply to the heat generating region, that has started to apply voltage in ACT  22 , is turned off. Thereafter, ACT  22  is performed. 
     In ACT  28 , power supply to the heat generating region that has started to apply the voltage in ACT  22  is turned on. Thereafter, ACT  22  is performed. 
     In this manner, the fixing control circuit  150  performs the ON/OFF control of the power supply for the heat generating region by repeating the operation from ACT  22  until the fixing temperature control end signal is detected in ACT  22 . As a result, the temperature of the selected heat generating region is controlled to be within the predetermined temperature range. 
     In ACT  29 , the fixing control circuit  150  turns off all the switches  361   e . Therefore, the fixing temperature control ends. 
     Here, the process returns to the description of ACT  6  shown in  FIG. 7 . 
     In ACT  6 , the CPU  100  determines whether or not the conveyance permission signal is turned on by the fixing control circuit  150 . 
     When the conveyance permission signal is turned on (ACT  6 : YES), the CPU  100  executes an image forming control program which controls the image forming unit  20 . Therefore, the image forming control is started. Specifically, first, ACT  7  and ACT  10  are performed in parallel. 
     When the conveyance permission signal is turned off (ACT  6 : NO), ACT  6  is performed. 
     As described above, in the embodiment, the image forming control is not started until the fixing nip reaches the fixing temperature and the conveyance permission signal is turned on. 
     When the image forming process is started, the CPU  100  processes the read image data (ACT  7 ). Thereafter, the CPU  100  sends image data and a control signal of starting image formation to the image forming control circuit  140 . 
     The image forming control circuit  140  performs control of writing an electrostatic latent image on the surface of the photosensitive drum  22  (ACT  8 ) and developing the electrostatic latent image in the developer  24  (ACT  9 ). The developed toner image is conveyed to the secondary transfer position by the intermediate transfer belt  21 . 
     Thereafter, the image forming control circuit  140  performs control of ACT  11  to be described below. 
     On the other hand, in ACT  10 , the CPU  100  performs control of conveying the sheet P to a transfer portion. The transfer portion is a position in which the intermediate transfer belt  21  and the secondary transfer roller  33  are in contact with each other. The CPU  100  sends the paper feeding/conveyance control circuit  130  a control signal of starting conveyance of the sheet P toward the transfer portion. The paper feeding/conveyance control circuit  130  drives the resist roller  41 . 
     The leading end of the sheet P reaches the secondary transfer position in accordance with the timing at which the developed toner image moves to the secondary transfer position by the intermediate transfer belt  21 . 
     ACT  11  is started at the timing at which the toner image and the sheet P reach the transfer portion. 
     In ACT  11 , the image forming control circuit  140  performs control of transferring the toner image on the intermediate transfer belt  21  to the sheet P. The image forming control circuit  140  applies the secondary transfer voltage to the secondary transfer roller  33 . The application of the secondary transfer voltage is performed until the entire sheet P has passed through the secondary transfer position. The sheet P to which the toner image is transferred is conveyed to the fixing device  36 . 
     Therefore, ACT  11  ends. 
     When the sheet P is conveyed to the fixing device  36 , ACT  12  is performed. In ACT  12 , the toner image on the sheet P is fixed by the fixing device  36 . 
     As shown in  FIG. 3 , the sheet P enters between the fixing belt  363  and the press roller  366  of the fixing device  36 . At the fixing nip, the heat generating region corresponding to the paper size of the sheet P is temperature-controlled to be in the predetermined temperature range. The sheet P is heated and pressurized while passing through the fixing nip so that the toner image is fixed on the sheet P. 
     The fixing temperature control by the fixing control circuit  150  is continued even while the sheet P passes through the fixing device  36 . As a result, the fixing belt  363  can maintain the predetermined temperature range at the fixing nip corresponding to the selected heat generating region. 
     On the other hand, since the fixing belt  363  is not heated in the heat generating region through which the sheet P does not pass, heating of the region to which the toner image need not be fixed is reduced. 
     When the entire sheet P passes through the fixing nip, passage of the sheet P is detected by a sensor  130   b  (not shown in the figure). ACT  12  ends. 
     When the passage of the sheet P in the fixing device  36  is detected by the sensor  130   b  (not shown in the figure), ACT  13  shown in  FIG. 7  is performed. In ACT  13 , the CPU  100  determines whether or not to end the printing. The CPU  100  compares the set number of copies with the number of copies already printed and determines to end the printing when printing for the set number of copies is completed. 
     When the CPU  100  determines that the printing is completed (ACT  13 : YES), ACT  14  is performed. 
     When the CPU  100  determines that the printing is continued (ACT  13 : NO), ACT  1  is performed. That is, when image data to be printed still remains, the process is returned to ACT  1  and the same process is repeated until all the printing is completed. 
     In ACT  14 , the CPU  100  performs control to end the fixing temperature control. Specifically, the CPU  100  sends the fixing temperature control end signal to the fixing control circuit  150 . 
     When the fixing control circuit  150  receives the fixing temperature control end signal, ACT  29  branched off from ACT  22  in  FIG. 8  is executed. As a result, power supply to all of the heat generating regions is turned off to end the fixing temperature control. 
     When ACT  14  ends, printing by the image forming apparatus  10  ends. 
     In the image forming apparatus  10  of the embodiment, the heating member  361  of the fixing device  36  has the plurality of heat generating regions. The plurality of heat generating regions are heated only in a range in which the generating regions overlap the sheet width of the sheet P to be printed. Therefore, power supply to the heat generating resistor  361   c  of the heat generating region through which the sheet P does not pass is stopped in accordance with the size of the sheet width of the sheet P. Therefore, energy saving is possible when the sheet P with a small sheet width is printed. 
     In the heating member  361 , the electrodes  361   b  are facing each other in the longitudinal direction of the base member  361   a . The electrode pair of the electrodes  361   b  facing each other in the longitudinal direction of the base member  361   a  are connected to different polarities of the fixing power source  150   a  from each other. As a result, the heat generating resistor  361   c  can be disposed continuously on each of the electrodes  361   b . In the heating member  361 , no gap is required between adjacent heat generating regions. Since a substantially rectangular heat generating region is continuously formed by the plurality of heat generating regions, unevenness in temperature distribution of the fixing region is reduced. 
     The unevenness of the temperature distribution in the fixing region will be described with reference to a comparative example shown in  FIG. 10 . 
       FIG. 10  is a schematic plan view showing an example of a control operation of a heater of the comparative example. 
     In a heating member  96  serving as a heater of the present comparative example, in order to heat a heat generating region r 01  corresponding to the heat generating regions R 01 L and R 01 R, electrodes e 01  and e 02  on the base member  361   a  are disposed to face in the short direction of the base member  361   a . A heat generating resistor HO made of the same material as the heat generating resistor  361   c  is disposed between the electrodes e 01  and e 02 . The widths of the electrodes e 01  and e 02  and the heat generating resistor HO in the longitudinal direction of the base member  361   a  are all D 1 . 
     In the heating member  96 , in order to heat a heat generating region r 12 L (r 12 R) corresponding to the heat generating region R 12 L (R 12 R), electrodes e 11 L and e 12 L (e 11 R and e 12 R) are disposed to face in the short direction of the base member  361   a . A heat generating resistor H 1 L (H 1 R) made of the same material as the heat generating resistor  361   c  is disposed between the electrodes e 11 L and e 12 L (e 11 R and e 12 R). The widths of the electrodes e 11 L and e 12 L (e 11 R and e 12 R) and the heat generating resistor H 1 L (H 1 R) in the longitudinal direction of the base member  361   a  are all D 2 L (D 2 R). 
     The electrodes e 01 , e 11 L, and e 11 R are electrically connected to a terminal T 1 . 
     The electrodes e 02 , e 12 L, and e 12 R are electrically connected to a terminal T 2  respectively via switches S 1 , S 2 L, and S 2 R. 
     In the electrode disposition of the present comparative example, the heat generating region r 01  generates heat by turning on the switch Si and turning off the switches S 2 L and S 2 R as in the embodiment. At this time, the electrode e 02  has the same potential as the terminal T 2 , whereas the electrodes e 12 L and e 12 R have the same potential as the terminal T 1 . That is, a potential difference between the terminals T 1  and T 2  is generated between the electrode e 12 L (e 12 R) and the electrode e 02 . Therefore, it is necessary to separate the electrode e 12 L (e 12 R) and the electrode e 02  from each other by an insulation distance d. When the potential difference between the terminals T 1  and T 2  is AC 100 V, the insulation distance d is approximately 1.5 mm, for example. When the potential difference between the terminals T 1  and T 2  is AC 200 V, the insulation distance d is approximately 3.2 mm, for example. 
     A gap having a width d in which there is no heat generating resistor is necessary between the heat generating region r 12 L (r 12 R) and the heat generating region r 01 . 
     In the comparative example, when a sheet P having a sheet width larger than D 1  is to be passed through, at least the switches S 1 , S 2 L, and S 2 R need to be turned on. In this case, the heat generating regions r 12 L, r 01 , and r 12 R generate heat. However, since the region of the width d between the heat generating region r 12 L (r 12 R) and the heat generating region r 01 , the region of the width d between the heat generating region r 12 L (r 12 R) and the heat generating region r 01  is a low temperature region. 
     In such an electrode disposition in the comparative example, fixing unevenness due to the low temperature region between adjacent heat generating regions occurs at the time of fixing the sheet P which requires a plurality of heat generating regions to be heated at the same time. 
     On the other hand, in the electrode disposition of the heating member  361  in the embodiment, the fixing unevenness due to such unevenness of the temperature distribution can be prevented. 
     Second Embodiment 
     A heater according to a second embodiment and a fixing device using the same will be described. 
       FIG. 11  is a block diagram showing a configuration example of a control system of an image forming apparatus according to the second embodiment.  FIG. 12  is a schematic cross-sectional view in a longitudinal direction showing a configuration example of a heating member  461  according to the second embodiment.  FIG. 13  is a schematic plan view showing a configuration example of the heating member  461  according to the second embodiment. However, a surface protective layer  361   d  is omitted in  FIG. 13  for easier viewing. 
     As shown in  FIG. 1 , an image forming apparatus  40  according to the second embodiment has a fixing device  46  instead of the fixing device  36  of the image forming apparatus  10  of the above-described first embodiment. As shown in  FIG. 11 , the image forming apparatus  40  has a control system  51  instead of the control system  50  of the image forming apparatus  10  of the above-described first embodiment. The control system  51  has a fixing control circuit  151  (a fixing controller) instead of the fixing control circuit  150  of the control system  50 . 
     As shown in  FIG. 3 , the fixing device  46  has the heating member  461  (a heater) instead of the heating member  361  of the fixing device  36  according to the above-described first embodiment. 
     In the fixing device  46 , a plurality of temperature detectors  362  (not shown in  FIG. 3 , see  FIG. 11 ) may be disposed at any position of an inner side and outer side of a fixing belt  363  and a heat generation portion of the heating member  461  in contact with the fixing belt  363 . Each of the plurality of temperature detectors  362  is disposed in a position from which a temperature of the range corresponding to one of the heat generating regions of the heating member  461  to be described below is detectable. However, in the embodiment, since a voltage applied to heat generating regions R 0 L and R 0 R is controlled by a voltage adjuster V 1  which will be described below, the temperature detector  362  is provided only in the range corresponding to either of the heat generating regions R 0 L and R 0 R. 
     A detection output of each temperature detector  362  is sent to the fixing control circuit  151 . 
     Hereinafter, differences from the above-described first embodiment will be mainly described. 
     As shown in  FIG. 12 , the heating member  461  has a heat generating resistor  461   c  instead of the heat generating resistor  361   c  of the heating member  361  of the above-described first embodiment. However, in the heating member  461 , the disposition (disposition position) in which the electrodes E 1 L, E 1 R, E 2 L, E 2 R, E 3 L, and E 3 R are positioned in the longitudinal direction (in a lateral direction as shown in  FIG. 12 ) of the base member  361   a  is different from that of the disposition according to the above-described first embodiment. 
     The fixing device  46  is an example of the case in which the disposition of the respective electrodes  361   b  of the heating member  461  is at an irregular interval. 
     A disposition interval between the electrode E 0  and the electrode E 1 L (E 1 R) in the longitudinal direction of the base member  361   a  is P 1 . Similarly, a disposition interval between the electrode E 1 L (E 1 R) and the electrode E 2 L (E 2 R) is P 2 . Similarly, a disposition interval between the electrode E 2 L (E 2 R) and the electrode E 3 L (E 3 R) is P 3 . 
     In the embodiment, P 1 =77.7 mm, P 2 =32.6 mm, P 3 =45.7 mm are satisfied as an example. 
     In such a numerical example, a disposition interval L 1  of the electrodes E 1 L and E 1 R is 155.4 mm, a disposition interval L 2  of the electrodes E 2 L and E 2 R is 220.6 mm, and a disposition interval L 3  of the electrodes E 3 L and E 3 R is 312 mm. 
     In this numerical example, L 1 , L 2 , and L 3  are 105% the length of sheet widths 148 mm, 210 mm, and 297 mm, respectively. 
     In this case, when the heat generating regions R 01 L and R 01 R are heated, it is possible to fix sheets P of a postcard size, a CD jacket size, and an A5R size. When the heat generating regions R 12 L, R 01 L, R 01 R, and R 12 R are heated, it is possible to fix the sheets P of a B5R size and an A4R size. When the heat generating regions R 23 L, R 12 L, R 01 L, R 01 R, R 12 R, and R 23 R are heated, it is possible to fix the sheets P of a B5 size, an A4 size, and an A3R size. 
     In this numerical example, the width of the heat generating region needed for fixing is set to have a margin of 5% with respect to the sheet width of the A5R size, the A4R size, and the A4 size (the A3R size), respectively. When the papers of the A5R size, the A4R size, and the A4 size (the A3R size) are passed through, this numerical example is a setting for heating with a smaller heat generation rate even while including a necessary margin as compared with the case in which paper with a larger size is passed through. 
     Hereinafter, the electrode disposition will be described using the numerical example described above unless otherwise specified. 
     As shown in  FIG. 13 , the heat generating resistor  461   c  is configured similar to the heat generating resistor  361   c  according to the above-described first embodiment except that the width in the short direction of the base member  361   a  is different. Hereinafter, when there is no risk of misunderstanding, the width of the heat generating resistor  461   c  in the short direction of the base member  361   a  is referred to simply as a width of the heat generating resistor  461   c.    
     The width of the heat generating resistor  461   c  is set to have an electric resistance by which a necessary heat generation rate can be obtained in each heat generating region when a voltage is applied. 
     In the above numerical example, P 1 &gt;P 3 &gt;P 2 . When electric resistivity p of the heat generating resistor  461   c  is constant, a layer thickness T (refer to  FIG. 12 ) is constant, and a voltage v applied to each of the heat generating regions R 01 L (R 01 R), R 12 L (R 12 R), and R 23 L (R 23 R) is constant, electric resistance r in each heat generating region is proportional to the length L of each heat generating region and is inversely proportional to the width W of the heat generating resistor  461   c  in each heat generating region. 
     For example, a heat generation rate q per unit length (hereinafter referred to as a “heat generation rate per unit length”) in the longitudinal direction of the heat generating region is expressed as in the following equation (1). 
         q=v   2 /( r·L )= v   2   ·T·W/ρ   (1)
 
     For example, it is assumed that the width of the heat generating resistor  461   c  in the heat generating region R 01 L (R 01 R) is W 1 , the width of the heat generating resistor  461   c  in the heat generating region R 12 L (R 12 R) is W 2 , and the width of the heat generating resistor  461   c  in the heat generating region R 23 L (R 23 R) is W 3 . 
     In this case, when v, T, and p are constant, the heat generation rate per unit length in each of the heat generating regions is proportional to the widths W 1 , W 2 , and W 3  of the heat generating resistor  461   c  in each of the heat generating regions. When W 1 =W 2 =W 3 , the heat generation rate per unit length becomes constant. 
     The heat generation rate per unit length may be set such that the temperature of the fixing nip reaches the required fixing temperature according to heat transfer efficiency from the heating member  461  to the fixing belt  363  in the fixing device  46 . In  FIG. 13 , an example in the case of W 1 &gt;W 3 &gt;W 2  is shown, but this is merely an example. The sizes of W 1 , W 2 , and W 3  can be appropriately set. 
     For example, the case that W 1 &gt;W 2 &gt;W 3  may be adopted. In this case, the heat generation rate per unit length decreases in the order of the heat generating regions R 01 L (R 01 R), R 02 L (R 02 R), and R 03 L (R 03 R). According to such a setting, since the heating capacity of the heating member  461  is the highest in the heat generating region R 01 L (R 01 R), a decrease in temperature distribution at a center portion of the fixing belt  363  after small-sized paper is consecutively passed through is easily reduced, for example. 
     For example, W 1 &lt;W 2 &lt;W 3  may be adopted. In this case, the heat generation rate per unit length increases in the order of the heat generating regions R 01 L (R 01 R), R 02 L (R 02 R), and R 03 L (R 03 R). According to such a setting, since the heating capacity of the heating member  461  is the highest in the heat generating region R 03 L (R 03 R), a decrease in temperature at both ends of the fixing belt  363  in a conveyance perpendicular direction is easily reduced, for example. 
     As shown in  FIG. 13 , the heating member  461  further has a voltage adjuster  461   h . As shown in  FIG. 11 , the voltage adjuster  461   h  is connected to the fixing control circuit  151  to communicate therewith. 
     As schematically shown in  FIG. 13 , the voltage adjuster  461   h  includes voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R. 
     The voltage adjuster V 1  is provided to adjust a voltage applied from a terminal T 2  to the electrodes E 1 L and E 1 R. The voltage adjuster V 2 L (V 2 R) is provided to adjust a voltage applied from a terminal T 1  to the electrode E 2 L (E 2 R). The voltage adjuster V 3 L (V 3 R) is provided to adjust a voltage applied from the terminal T 2  to the electrode E 3 L (E 3 R). 
     The voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R change the voltages applied to the electrodes E 1 L, E 1 R, E 2 L, E 2 R, E 3 L, and E 3 R according to the control signal from the fixing control circuit  151 . 
     The voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R may change the voltages applied to the electrodes E 1 L, E 1 R, E 2 L, E 2 R, E 3 L, and E 3 R continuously or in stages. The value being changed in stages may be an appropriate number of two or more. 
     The configuration of the voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R is not limited as long as the voltage adjusters can change voltages according to the control signal from the fixing control circuit  151 . As the voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R, a configuration in which electric resistance is changed to cause a voltage drop may be employed, for example. As the voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R, an electronic volume, a digital volume, or the like may be used, for example. As the voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R, a transformer circuit may be used, for example. 
     The voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R are not limited to a single electronic device. The voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R may be configured with an electric circuit including a plurality of electric devices. 
     The voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R are connected to appropriate positions on wirings  361   f  and  361   g  depending on the respective configurations. In  FIG. 13 , a disposition example in which the voltage adjusters have a variable resistor type configuration is schematically shown as an example. For example, the voltage adjuster V 1  is connected in series to the wiring path of the wiring  361   g  between the fixing power source  150   a  and the electrode E 1 L (E 1 R). The voltage adjuster V 2 L (V 2 R) is connected in series with the wiring path of the wiring  361   f  between the fixing power source  150   a  and the electrode E 2 L (E 2 R). The voltage adjuster V 3 L (V 3 R) is connected in series with the wiring path of the wiring  361   g  between the fixing power source  150   a  and the electrode E 3 L (E 3 R). For example, when the voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R are configured to include a transformer circuit, the voltage adjusters may be wired to different positions from those in  FIG. 13 . 
     The voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R change the voltages applied to the electrodes E 1 L, E 1 R, E 2 L, E 2 R, E 3 L, and E 3 R according to the control signal from the fixing control circuit  151 . 
     In addition to the control function of the fixing control circuit  150  according to the above-described first embodiment, the fixing control circuit  151  has a function of controlling the applied voltages to the electrodes E 1 L (E 1 R), E 2 L (E 2 R), and E 3 L (E 3 R) by controlling the voltage adjusters V 1 , V 2 L, V 2 R, V 3 L, and V 3 R. 
     A specific control function of the fixing control circuit  151  will be described in the operation description below. 
     Next, the following description will be based on an operation of the image forming apparatus  40  at the time of printing. 
       FIG. 14  is a flowchart showing an operation example of a fixing temperature control of the image forming apparatus according to the second embodiment. 
     The image forming apparatus  40  of the embodiment having the heating member  461  of the embodiment in the fixing device  46  can print on the sheet P according to the flow shown in  FIG. 7  as in the image forming apparatus  10  of the above-described first embodiment. 
     However, in ACT  3  in  FIG. 7 , the CPU  100  selects the heat generating region to be heated according to the correlation between the sheet width and the heat generating region of the embodiment. 
     In the above-described numerical example of the disposition interval of the electrodes, the correspondence between the paper size and the heat generating region to be selected is stored in the ROM  120  as follows, for example. For the postcard size, the CD jacket size, and the A5R size, the heat generating regions R 01 L and R 01 R are selected. For the B5R size and the A4R size, the heat generating regions R 12 L, R 01 L, R 01 R, and R 12 R are selected. For the B5 size, the A4 size, and the A3R size, the heat generating regions R 23 L, R 12 L, R 01 L, R 01 R, R 12 R, and R 23 R are selected. 
     In addition, in the image forming apparatus  40  of the embodiment, the fixing temperature control is performed in accordance with the flow shown in  FIG. 14 . Hereinafter, the fixing temperature control of the embodiment will be described focusing on differences from the above-described first embodiment. 
     The fixing control circuit  151  executes ACT  31  to ACT  41  shown in  FIG. 14  in accordance with the flow shown in  FIG. 14 . 
     First, ACT  31  is performed. ACT  31  performs the same operation as in ACT  21  of the above-described first embodiment. In the embodiment, a default value determined in advance is used as the voltage setting value of the voltage adjuster  461   h  in ACT  31 . The variation width of voltage from the default value is set so that at least the voltage to be applied to the electrode pair can increase. 
     After ACT  31 , ACT  32  is performed. In ACT  32 , the same operation as in ACT  32  of the above-described first embodiment is performed by the fixing control circuit  151 . 
     However, when the fixing control circuit  151  receives a fixing temperature control end signal (ACT  32 : YES), ACT  41  is performed. 
     When the fixing control circuit  151  has not received the fixing temperature control end signal (ACT  32 : NO), ACT  33  is performed. 
     In ACT  33 , the fixing control circuit  151  acquires temperature information of the fixing belt  363  forming the fixing nip, for each heat generating region, from each temperature detector  362 . 
     When the temperature detector  362  indirectly detects the temperature of the fixing belt  363 , the fixing control circuit  151  converts the detected temperature into the temperature of the fixing nip of the fixing belt  363 . The correlation between the temperature detected by the temperature detector  362  and the temperature of the fixing nip is stored in the ROM  120  in advance as a conversion table or the like, for example. 
     However, the fixing control circuit  151  executes the following operation according to the temperature information by the temperature detector  362  which detects the temperature of the selected heat generating region. 
     Therefore, ACT  33  ends. 
     After ACT  33 , ACT  34  is performed. In ACT  34 , the fixing control circuit  151  determines whether or not the temperature of each heat generating region detected in ACT  33  (hereinafter referred to as a detected temperature) falls within an allowable temperature difference range. 
     For example, the fixing control circuit  151  calculates a temperature difference between the highest temperature in the detected temperatures and each detected temperature. The fixing control circuit  151  compares the calculated temperature difference with the allowable temperature difference range stored in advance in the ROM  120  and determines the temperature difference for each heat generating region. 
     When all of the temperature differences fall within the allowable temperature difference range (ACT  34 : YES), ACT  36  is performed. 
     When any one of the respective temperature differences does not fall within the allowable temperature difference range (ACT  34 : NO), ACT  35  is performed. 
     In ACT  35 , a voltage adjustment is performed by the fixing control circuit  151 . The fixing control circuit  151  changes the voltage to be applied to the electrode pair corresponding to the heat generating region in which the temperature detected is outside the allowable temperature difference range. Specifically, the fixing control circuit  151  sends the voltage adjuster  461   h  a control signal for changing the voltage setting value according to the correspondence table of the temperature difference and the voltage change value stored in advance in the ROM  120 . 
     When the temperature difference does not fall within the allowable temperature difference range due to the excessively low temperature of the heat generating regions R 01 L and R 01 R, for example, the applied voltage between the electrodes E 0  and E 1 L (E 1 R) is further increased by the control of the fixing control circuit  151 . 
     At this time, when there is no need to change the voltage to be applied to the other electrode pair, the voltages by the voltage adjusters V 2 L, V 2 R, V 3 L, and V 3 R are changed as needed to maintain the voltage to be applied to the other electrode pair. When it is necessary to change the voltage to be applied to the other electrode pair, the applied voltage of the other electrode pair is also changed. 
     Therefore, ACT  35  ends. After ACT  35 , ACT  36  is performed. 
     In this manner, when power supply is started after the voltage to be applied to the electrode pair is changed in ACT  35 , the heat generation rate of the heat generating region between the electrode pair in which the applied voltage is changed is changed. For example, when the voltages applied to the heat generating regions R 01 L and R 01 R increase, the heat generation rate in the heat generating regions R 01 L and R 01 R increases. 
     In ACT  34 , the fixing control circuit  151  determines whether or not the detected temperature detected in ACT  33  is within the predetermined temperature range. The fixing control circuit  151  determines it according to the lowest detected temperature (referred to as a lower limit detected temperature) in a range corresponding to the selected heat generating region. 
     The predetermined temperature range is the same as the temperature range in ACT  24  according to the above-described first embodiment. 
     When the lower limit detected temperature falls within the predetermined temperature range (ACT  36 : YES), ACT  37  is performed. 
     When the lower limit detected temperature is outside the predetermined temperature range (ACT  36 : NO), ACT  38  is performed. 
     The same operations as in ACTs  26 ,  27 ,  28 , and  29  of the above-described first embodiment are performed in ACTs  38 ,  39 ,  40 , and  41  respectively by the fixing control circuit  151 . However, when ACT  39  and ACT  40  end, ACT  32  is performed. 
     As in the image forming apparatus  10  of the above-described first embodiment, the image forming apparatus  40  of the embodiment having the heating member  461  of the embodiment can save energy and reduce unevenness of the temperature distribution of the fixing region. 
     In the embodiment, the disposition interval of the electrodes is not limited to a regular interval. When paper with a specific size such as a size with a high frequency of use is passed, for example, it is possible to set heating with an even lower heat generation rate even while including a necessary margin as compared with a case in which paper with an even larger size is passed through. As a result, the average amount of power usage of the image forming apparatus  40  is more easily reduced. 
     In addition, in the image forming apparatus  40  of the present invention, electric resistance of each heat generating region is appropriately set by changing the width of the heat generating resistor  461   c . Therefore, the heat generation rate per unit length can be changed for each heat generating region. 
     In the heat generating region in which a temperature decline may easily occur due to passage of thick paper such as a postcard, for example, by increasing the heat generation rate per unit length in advance, the temperature decline of the fixing belt  363  is less likely to occur even when a many sheets of paper are consecutively passed therethrough. As a result, it is possible to reduce an influence on the image quality such as uneven gloss due to the temperature decline. 
     In the heat generating region in which the temperature decline of the fixing belt  363  may easily occur due to an influence such as a temperature distribution inside the image forming apparatus  40 , for example, by increasing the heat generation rate per unit length in advance, the temperature decline of the fixing belt  363  is less likely to occur even when many sheets of paper are consecutively passed therethrough. As a result, it is possible to reduce an influence on the image quality such as uneven gloss due to the temperature decline. 
     Furthermore, since the voltage adjuster  461   h  is provided in the image forming apparatus  40  of the embodiment, the voltage applied to each electrode in the heating member  461  can be changed. 
     The voltage applied to each electrode can be changed for each heat generating region by setting a default value in advance. Therefore, the heat generation rate per unit length in each heat generating region can be changed also by a level of the applied voltage. 
     When the heat generation rate per unit length is different according to the electric resistance in each heat generating region, for example, the heat generation rate per unit length in each heat generating region can be equalized using the default value. 
     It is also possible to set the default value such that the difference of the heat generation rate per unit length is further increased according to the electric resistance in each heat generating region. 
     As described above, in the image forming apparatus  40  of the embodiment, flexibility in setting the heat generation rate per unit length in each heat generating region is increased using a combination of the electric resistance and the applied voltage in each heat generating region. 
     Furthermore, in the image forming apparatus  40  of the embodiment, when the temperature difference between the heat generating region increases during the fixing operation, the voltage to be applied to the electrode pair corresponding to the heat generating region in which the temperature difference has increased is automatically changed to reduce the temperature difference by the fixing control circuit  151 . 
     Therefore, even when printing is performed for mixed sheets P with various sizes and thicknesses, the temperature distribution in the conveyance perpendicular direction of the heat generating region used for fixing is stabilized. As a result, occurrence of fixing unevenness, gloss unevenness, or the like due to a temperature variation in each heat generating range is reduced. 
     Third Embodiment 
     A heater according to a third embodiment and a fixing device using the same will be described. 
       FIG. 15  is a schematic cross-sectional view showing a configuration example of a main part of the heater according to the third embodiment. 
     As shown in  FIG. 15 , a fixing device  56  has a heating film  563  and a heating member  561  serving as the heater of the embodiment instead of the fixing belt  363  and the heating member  361  of the fixing device  36  according to the above-described first embodiment. In the fixing device  56 , the belt conveying rollers  364  and the tension roller  365  of the fixing device  36  according to the above-described first embodiment are removed. 
     The fixing device  56  further has a heater holder  562 . 
     Hereinafter, differences from the above-described first embodiment will be mainly described. 
     The heating film  563  is a tubular member that rotates in conjunction with rotation of the press roller  366 . The heating film  563  is slidable with the heating member  561  to be described below on an inner circumferential surface. The heating film  563  is configured of a resin film having heat resistance against heat generated by the heating member  561 , for example. 
     An elastic layer may be formed on an outer circumferential surface of the heating film  563  so that a fixing nip with an appropriate width is formed between the elastic layer and the press roller  366 . 
     The heating member  561  is configured similar to the heating member  361  of the above-described first embodiment except that it has a projected surface  561   a  which comes into contact with the inner circumferential surface of the heating film  563 . The projected surface  561   a  enables smooth sliding of the heating film  563  by reducing sliding resistance with the heating film  563 . That is, a friction coefficient between the inner circumferential surface of the heating film  563  and the projected surface  561   a  is smaller than a friction coefficient between the outer circumferential surface of the heating film  563  and a surface of the press roller  366 . 
     Furthermore, the friction coefficient between the inner circumferential surface of the heating film  563  and the projected surface  561   a  is smaller than a friction coefficient with respect to the surface of the sheet P, on which the toner image is formed, entering between the heating film  563  and the press roller  366 . 
     The heating member  561  is held by the heater holder  562 . The heater holder  562  presses the projected surface  561   a  of the heating member  561  against the inner circumferential surface of the heating film  563 . The heating film  563  against which the projected surface  561   a  is pressed is in contact with the opposing press roller  366  and forms the fixing nip between the press roller  366  and the heating film  563 . 
     In addition, at the position near the heating member  561 , the heater holder  562  guides the inner circumferential surface of the heating film  563  in contact with the heating member  561  in substantially arc shape. 
     The heater holder  562  is configured of a resin material which has heat resistance against heat generated by the heating member  561  and is slidable with the inner circumferential surface of the heating film  563 , for example. A friction coefficient between the inner circumferential surface of the heating film  563  and the heater holder  562  is smaller than the friction coefficient between the outer circumferential surface of the heating film  563  and the surface of the press roller  366 . 
     In the fixing device  56  having such a configuration, as the press roller  366  rotates clockwise as in the drawing, the heating film  563  rotates counterclockwise as in the drawing. On the inner circumferential surface of the heating film  563 , heat from the heating member  561  is thermally conducted via the projected surface  561   a . When the sheet P enters the fixing nip, the heating film  563  is driven to rotate by the press roller  366  with the sheet P interposed therebetween and continues to rotate counterclockwise as in the drawing. 
     The fixing device  56  differs from the fixing device  36  in terms of the driving method of the heating film  563  instead of the fixing belt  363  and in terms of the projected surface  561   a  being formed on the surface of the heating member  561 . However, the fixing device  56  is the same as the fixing device  36  of the above-described first embodiment in that the fixing can be performed by switching the heat generating region according to the paper size of the sheet P. 
     The fixing device  56  can be used instead of the fixing device  36  of the image forming apparatus  10  of the above-described first embodiment. 
     The image forming apparatus  10  having the heating member  561  of the embodiment instead of the fixing device  36  can save energy and reduce unevenness of the temperature distribution of the fixing region as described in the above-described first embodiment. 
     Fourth Embodiment 
     A heater according to a fourth embodiment and a fixing device using the same will be described. 
       FIG. 16  is a schematic plan view showing a configuration example of the heater according to the fourth embodiment. However, a surface protective layer  361   d  is omitted in  FIG. 16  for easier viewing. 
     As shown in  FIG. 1 , an image forming apparatus  60  according to the fourth embodiment has a fixing device  66  instead of the fixing device  36  of the image forming apparatus  10  of the above-described first embodiment. As shown in  FIG. 6 , the image forming apparatus  60  has a control system  52  instead of the control system  50  of the image forming apparatus  10  of the above-described first embodiment. The control system  52  has a fixing control circuit  152  (a fixing controller) instead of the fixing control circuit  150  of the control system  50 . 
     As shown in  FIG. 3 , the fixing device  66  has a heating member  661  (a heater) instead of the heating member  361  of the fixing device  36  according to the above-described first embodiment. 
     Hereinafter, differences from the above-described first embodiment will be mainly described. 
     The heating member  661  is configured by changing the disposition of the electrodes E 3 L, E 2 L, E 1 L, E 0  E 1 R, E 2 R, and E 3 R in the heating member  361  of the above-described first embodiment. The disposition of electrodes E 3 L, E 2 L, E 1 L, E 0  E 1 R, E 2 R, and E 3 R in the embodiment is similar to the disposition of the respective electrodes in the heating member  461  according to the second embodiment. Heat generating regions R 23 L, R 12 L, R 01 L, R 01 R, R 12 R, and R 23 R same as those according to the second embodiment are formed between each electrode pair. The width of a heat generating resistor  361   c  in each heat generating region of the embodiment is constant as described in the above-described first embodiment. 
     The fixing control circuit  152  has the same configuration as the fixing control circuit  150  of the above-described first embodiment except that selection of heat generation of the heat generating region is performed as in the fixing control circuit  151  according to the second embodiment according to the disposition of each heat generating region. 
     The image forming apparatus  60  of the embodiment prints an image on a sheet P as described in the above-described first embodiment except that the operation of ACT  3  (refer to  FIG. 7 ) according to the above-described first embodiment is performed as described in the second embodiment. 
     In the embodiment, since the width of the heat generating resistor  361   c  has a constant value Wh in each heat generating region, a heat generation rate per unit length is constant in each heat generating region. 
     The image forming apparatus  60  of the embodiment having the heating member  661  of the embodiment can save energy and reduce unevenness in temperature distribution of the fixing region as in the image forming apparatus  10  of the above-described first embodiment. 
     In the embodiment, a disposition interval of the electrode is not limited to a regular interval. When paper of a specific size such as a size with a high frequency of use is passed through, for example, it is possible to set heating with an even lower heat generation rate even while including a necessary margin as compared with a case in which paper with an even larger size is passed through. As a result, the average amount of power usage of the image forming apparatus  60  is easily reduced. 
     Furthermore, according to the heating member  661  of the embodiment, heat generation is performed such that the heat generation rate per unit length is equal by applying a constant voltage to each electrode pair as described in the above-described first embodiment. Therefore, the temperature distribution among a plurality of selected heat generating regions is easily uniform. 
     Fifth Embodiment 
     A heater according to a fifth embodiment and a fixing device using the same will be described. 
       FIG. 17  is a schematic plan view showing a configuration example of the heater according to the fifth embodiment. However, a surface protective layer  361   d  is omitted for ease of viewing in  FIG. 17 . 
     As shown in  FIG. 1 , an image forming apparatus  70  according to the fifth embodiment has a fixing device  76  and a paper feeding cassette  78  instead of the fixing device  36  and the paper feeding cassette  18  of the image forming apparatus  10  of the above-described first embodiment. As shown in  FIG. 6 , the image forming apparatus  70  has a control system  53  instead of the control system  50  of the image forming apparatus  10  of the above-described first embodiment. The control system  53  has a fixing control circuit  153  (a fixing controller) instead of the fixing control circuit  150  of the control system  50 . 
     As shown in  FIG. 3 , the fixing device  76  has a heating member  761  (a heater) instead of the heating member  361  of the fixing device  36  according to the above-described first embodiment. 
     The paper feeding cassette  78  accommodates the sheet P of any of various sizes on one side reference. For example, in the sheet P of any of various sizes, the sheet end on the left side with respect to the conveying direction is aligned at a normal position. 
     The image forming apparatus  70  is different in that the sheet P is conveyed on one side reference, whereas the image forming apparatus  10  of the above-described first embodiment conveys the sheet P on the central reference. 
     Hereinafter, differences from the above-described first embodiment will be mainly described. 
     As shown in  FIG. 17 , the heating member  761  has an electrode  361   b  as in the heating member  361  of the above-described first embodiment. However, while the electrode  361   b  of the configuration example of the heating member  361  includes the electrodes E 3 L, E 2 L, E 1 L, E 0 , E 1 R, E 2 R, and E 3 R, the electrode  361   b  of the heating member  761  includes an electrode E 1  (a first end electrode), electrodes E 2  and E 3 , and an electrode E 4  (a second end electrode). However, the number of the electrodes  361   b  in the embodiment is an example. The number of the electrodes  361   b  in the embodiment is not limited as long as it is three or more. 
     The electrodes E 1  and E 4  are configured and disposed similar to the electrodes E 3 L and E 3 R in the heating member  461  of the above-described first embodiment. For example, a disposition interval of the electrodes E 1  and E 4  in the longitudinal direction of the heating member  661  is L 3  (=2·(P 1 +P 2 +P 3 )) similar to the disposition interval of the electrodes E 3 L and E 3 R according to the second embodiment. 
     The electrode E 2  is disposed between the electrodes E 1  and E 4  at a position whose disposition interval with respect to the electrode E 1  is L 2 . Here, L 2  is L 1  (=2·P 1 ) as in the disposition interval of the electrodes E 2 L and E 2 R according to the second embodiment. 
     The electrode E 3  is disposed between the electrodes E 1  and E 4  at a position whose disposition interval with respect to the electrode E 1  is L 1 . Here, L 1  is L 2  (=2·(P 1 +P 2 )) as in the disposition interval of the electrodes E 1 L and E 1 R according to the second embodiment. 
     With such a configuration, a heat generating region R 12  having the length L 1  is formed between the electrodes E 1  and E 2 . A heat generating region R 23  having a length L 2 -L 1  is formed between the electrodes E 2  and E 3 . A heat generating region R 34  having a length L 3 -L 2  is formed between the electrodes E 3  and E 4 . The width of the heat generating resistor  361   c  in each heat generating region of the embodiment is a constant value Wh as described in the above-described first embodiment. 
     The heating member  761  has a switch  361   e  as in the heating member  361  of the above-described first embodiment. However, while the switch  361   e  of the configuration example of the heating member  361  includes switches S 2 L, S 2 R, S 1 , S 3 L, and S 3 R, the switch  361   e  of the heating member  761  includes switches S 2 , S 3 , and S 4 . However, the number of switches  361   e  in the embodiment is an example. The number of the switches  361   e  in the embodiment can be appropriately provided according to the number of the electrodes  361   b.    
     The electrode E 1  is always electrically connected to a terminal T 1  by a wiring  361   f.    
     The electrode E 2  is connected to a terminal T 2  via the switch S 2 . The switch S 2  can turn on or off the electric connection between the electrode E 2  and the terminal T 2 . 
     The electrode E 3  is connected to the terminal T 1  via the switch S 3 . The switch S 3  can turn on or off the electric connection between the electrode E 3  and the terminal T 1 . 
     The electrode E 4  is connected to the terminal T 2  via the switch S 4 . The switch S 4  can turn on or off the electric connection between the electrode E 4  and the terminal T 2 . 
     A switching operation of each switch  361   e  is individually controlled by the fixing control circuit  153  to be described below. 
     The fixing control circuit  153  has the same configuration as the fixing control circuit  150  of the above-described first embodiment except that selected control of heat generation of the heat generating region is different according to the disposition of each heat generating region. 
     The image forming apparatus  70  of the embodiment prints an image on the sheet P as described in the above-described first embodiment except that the operation of ACT  3  (refer to  FIG. 7 ) according to the above-described first embodiment is different. 
     Specifically, in a ROM  120  in the embodiment, a correspondence between a paper size and the heat generating region to be selected is stored as follows. In the case of a postcard size, the heat generating region R 12  is selected. In the case of a CD jacket size, an A5R size, a B5R size, and an A4R size, the heat generating regions R 12  and R 23  are selected. In the case of a B5 size, an A4 size, and an A3R size, the heat generating regions R 12 , R 23 , and R 34  are selected. 
     In ACT  3  in the embodiment, the CPU  100  selects the heat generating region corresponding to the paper size according to the above information stored in the ROM  120 . 
     As in the image forming apparatus  10  of the above-described first embodiment, the image forming apparatus  70  of the embodiment having the heating member  761  of the embodiment can save energy and reduce unevenness in temperature distribution of the fixing region. 
     In the embodiment, when the sheet P is conveyed on one side reference, for example, when paper with a specific size such as a size with a high frequency of use is passed through, it is possible to set heating with a minimum heat generation rate including a necessary margin. As a result, the average amount of power usage of the image forming apparatus  70  is more easily reduced 
     Furthermore, according to the heating member  761  of the embodiment, when the sheet P of the same size as described in the above-described first embodiment is fixed, it is possible to reduce the number of the electrodes  361   b  and the switches  361   e.    
     Sixth Embodiment 
     A heater according to a sixth embodiment and a fixing device using the same will be described. 
       FIG. 18  is a schematic cross-sectional view showing a configuration example of the heater according to the sixth embodiment. 
     As shown in  FIG. 1 , an image forming apparatus  80  according to the sixth embodiment has a fixing device  86  instead of the fixing device  36  of the image forming apparatus  10  of the above-described first embodiment. 
     As shown in  FIG. 3 , the fixing device  86  has a heating member  861  (a heater) instead of the heating member  361  of the fixing device  36  according to the above-described first embodiment. 
     Hereinafter, differences from the above-described first embodiment will be mainly described. 
     As shown in  FIG. 18 , the heating member  861  has a heat generating resistor  861   c  instead of the heat generating resistor  361   c  in the heating member  361  of the above-described first embodiment and is configured to further include an insulating layer  861   j.    
     The insulating layer  861   j  is disposed in a region other than the electrode  361   b  on a surface of a base member  361   a  on which the electrode  361   b  is disposed. The insulating layer  861   j  is disposed at least in a portion sandwiched between the electrodes  361   b  in a longitudinal direction of the base member  361   a  and in a range overlapping the heat generating region. 
     The layer thickness of the insulating layer  861   j  is not limited. The insulating layer  861   j  can be used to change the thickness of a heat generating resistor  861   c  to be described below in the longitudinal direction. In this case, the layer thickness of the insulating layer  861   j  is appropriately set so that a necessary thickness can be formed for the heat generating resistor  861   c.    
     In the configuration example shown in  FIG. 18 , the insulating layer  861   j  is disposed to fill the space between the respective electrodes. The layer thickness of the insulating layer  861   j  in the configuration example shown in  FIG. 18  is the same as the thickness of the electrode  361   b . Therefore, a layered portion with a constant layer thickness including the electrode  361   b  and the insulating layer  861   j  is formed on the surface of the base member  361   a.    
     The material of the insulating layer  861   j  is not particularly limited as long as it has a withstand voltage against the voltage applied to each electrode. As a material of the insulating layer  861   j , a metal oxide, a resin, a ceramic material, or the like may be used, for example. 
     A method of manufacturing the insulating layer  861   j  is not particularly limited. As a method of manufacturing the insulating layer  861   j , an appropriate method of forming a film may be used depending on the material, for example. 
     The heat generating resistor  861   c  is formed of the same material as the heat generating resistor  361   c  according to the above-described first embodiment and is formed in the same shape as the heat generating resistor  361   c  in a plan view. The heat generating resistor  861   c  is formed in a layer shape covering each electrode  361   b  and the insulating layer  861   j . In the configuration example shown in  FIG. 18 , the layer thickness of the heat generating resistor  861   c  is constant. 
     Such a heat generating resistor  861   c  is easily manufactured using, for example, an appropriate method of forming a film after forming the electrode  361   b  and the insulating layer  861   j  on the base member  361   a.    
     As in the configuration example shown in  FIG. 18 , when the thickness of the electrode  361   b  is equal to the layer thickness of the insulating layer  861   j , since the heat generating resistor  861   c  is formed on a smooth flat surface, the heat generating resistor  861   c  with uniform thickness can be easily manufactured. 
     In this case, since unevenness of a heat generation rate due to a variation in thickness of the heat generating resistor  861   c  is eliminated, unevenness in temperature distribution of the heat generating region is reduced. 
     Furthermore, in this case, since surface irregularities of the heat generating resistor  861   c  are reduced, the surface after a surface protective layer  361   d  is further laminated is also formed to be smooth. Since the flatness of a surface of the surface protective layer  361   d  is improved, a pressing force of the heating member  861  against a fixing belt  363  becomes uniform. As a result, since unevenness in thermal conduction due to non-uniformity in a pressed state is reduced, uniformity of the temperature distribution in the heat generating region is improved. 
     It is conceivable to increase the thickness of the electrode  361   b  to reduce electric resistance of the electrode  361   b.    
     In this case, when there is no insulating layer  861   j , there is a concern that variation in thickness of the heat generating resistor  861   c  near the electrode  361   b  increases. When such a variation in thickness is excessively large, a non-uniform temperature distribution may be formed. 
     Alternatively, when there is no insulating layer  861   j , the surface of the heat generating resistor  861   c  near the electrode  361   b  may rise. As a result, flatness of the surface of the heat generating resistor  861   c  may be degraded. 
     However, in the embodiment, even when the electrode  361   b  is formed to be thick, since the thickness of the insulating layer  861   j  can be changed according to a thickness thereof, the variation in thickness of the heat generating resistor  861   c  can be reduced and the flatness of the surface of the heat generating resistor  861   c  can be secured. 
     However, unlike the configuration example of  FIG. 18 , the heat generating resistor  861   c  may be laminated in a state in which the layer thickness of the insulating layer  861   j  is different from the thickness of the electrode  361   b . In this case, the thickness of the heat generating resistor  861   c  can be changed according to the change in thickness of the insulating layer  861   j.    
     In this case, even when the width of the heat generating resistor  861   c  is constant, the electric resistance in the heat generating region is changed due to the variation in thickness of the heat generating resistor  861   c.    
     As in the image forming apparatus  10  of the above-described first embodiment, the image forming apparatus  80  of the embodiment having the heating member  861  of the embodiment can save energy and reduce unevenness in temperature distribution of the fixing region. 
     According to at least one of the above-described embodiments, it is possible to provide a heater that can save energy and reduce unevenness in temperature distribution of the fixing region by including three or more electrodes disposed on the base member to be spaced apart from each other in a longitudinal direction of the base member, a heat generating resistor which electrically interconnects electrodes which form an electrode pair and face each other in the longitudinal direction among the electrodes, a wiring which connects the electrodes forming the electrode pair to have different polarities from each other, and a switch connected to the wiring and configured to select an electrode to which a voltage is to be applied. 
     In addition, in each of the above-described embodiments, the switch  361   e  may be integrally formed with the heating member (the heater)  361 ,  461 ,  561 ,  661 ,  761 , or  861 , or may be separately formed therefrom. Similarly, the fixing control circuit (the fixing controller)  150 ,  151 ,  152  or  153  may also be integrally formed with the heating member  361 ,  461 ,  561 ,  661 ,  761 , or  861 , or may be separately formed therefrom. Similarly, the voltage adjuster  461   h  may also be integrally formed with the heating member  461  or may be separately formed therefrom. 
     In the above-described second embodiment, an example in which the voltage to be applied to the electrode pair can be changed by the voltage adjuster  461   h  has been described. However, a configuration in which the voltage adjuster  461   h  is removed may be used when printing with a high degree of quality can be performed without changing the applied voltage. 
     In the above-described first embodiment and third to sixth embodiments, examples in which the fixing device does not have the voltage adjuster  461   h  have been described. However, also in the above-described first embodiment and third to sixth embodiments, configurations in which the applied voltage can be changed may be used by having the voltage adjuster  461   h  and the fixing control circuit  151  as described in the second embodiment. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.