JP2018014163A - Heater, fixing device and image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent destruction in a portion where an electrode and a heating resistor come into contact with each other due to temperature difference.SOLUTION: A heater includes: a heating resistor; and an electrode which is provided in contact with the heating resistor and supplies electric power to the heating resistor. When the temperature of the electrode rises, a resistance value of the electrode is decreased.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a heater used for information appliances, small appliances such as home appliances and manufacturing equipment, a heating device such as a printer on which the heater is mounted, and an image forming apparatus using the heating device.

  Conventionally, an image forming apparatus using an electrophotographic process is known. In such an image forming apparatus, the unfixed toner image formed on the transfer paper by the image forming means by the electrophotographic process is fixed on the transfer paper by the thermal fixing device. As the heat fixing device, for example, as described in Patent Document 1 or Patent Document 2, a heat roller type heat fixing device using a halogen heater as a heat source, or a film heating type heat fixing using a ceramic heater as a heat source. The device is known.

  In general, such a heater is supplied with electric power from an AC power supply via a switching element such as a triac.

  In a fixing device using a halogen heater as a heat source, the temperature of the fixing device is detected by a temperature detection element such as a thermistor temperature sensing element, and the switching element is on / off controlled by a sequence controller based on the detected temperature. . That is, power supply to the halogen heater is turned on / off, and the temperature is controlled so that the temperature of the fixing device becomes the target temperature.

  On the other hand, in a fixing device using a ceramic heater as a heat source, the power ratio supplied to the ceramic heater is calculated based on the temperature difference between the temperature detected by the temperature detection element and a preset target temperature by the sequence controller. The corresponding phase angle or wave number is determined. Then, the switching element is ON / OFF controlled at the determined phase or wave number, and the temperature of the fixing device is controlled.

  In addition, in order to prevent the heater from being damaged in the event of an abnormality, when the current detection means detects an overcurrent of the heater, some control is performed to cut off the energization of the heater by a control circuit.

  In Patent Document 3, when the ceramic heater becomes high temperature, the power control means for controlling power supply executes power supply control according to the value detected by the power detection means. Thus, there has been proposed an image forming apparatus in which the temperature rise of the ceramic heater of the fixing device is delayed and the heater current can be interrupted by a thermo switch or a thermal fuse before the ceramic heater is damaged.

Japanese Unexamined Patent Publication No. 64-77887 JP 2007-212868 A JP 2010-197585 A

  However, in recent years, many cases have been reported in which the heater is damaged at the boundary between the heating resistor and the electrode part for power supply, and the cause is a thermal shock caused by a temperature difference between the heating resistor and the electrode part. There is destruction.

  FIG. 11 is a diagram showing the configuration of such a conventional fixing heater.

  As shown in the figure, in the conventional fixing heater 201, an excessive temperature rise prevention element 500 such as a thermo switch or a thermal fuse is installed in the vicinity of the portion 36 where the heating resistor 33 of the fixing heater 201 has the highest temperature in the longitudinal position. Yes. The power from the power source 300 is supplied to the heating resistor 33 via the power control unit 400, the excessive temperature rise prevention element 500, and the electrode units 32a and 32b. For this reason, before the temperature of the heating resistor 33 rises to the temperature at which the excessive temperature rise prevention element 500 operates, the heat due to the temperature difference between the heating resistor 33 and the boundary portions 35a and 35b between the electrode portions 32a and 32b. There is a problem that impact destruction occurs and cannot serve as a safety device. Further, there is a problem that even if no breakdown occurs, there is little time margin until the excessive temperature rise prevention element 500 reacts.

  Furthermore, in recent years, the heat conduction of the heater substrate has been increased from the viewpoint of supplying heat to the fixing member, and it has been proposed to reduce the substrate thickness without changing the substrate material in order to achieve high heat conduction without increasing the cost. ing. However, a heater with a thin substrate thickness decreases the substrate strength, and the heater is easily damaged. It has also been found that a heater with a thin substrate thickness is weak against thermal shock breakdown at the boundary between the heating resistor and the electrode.

  The present invention has been made in order to solve the above-described problem, and by reducing the temperature difference between the electrode of the fixing heater and the heating resistor in the fixing device, the fixing heater can contact the electrode and the heating resistor. An object of the present invention is to provide an image forming apparatus capable of preventing damage due to a temperature difference.

    In order to achieve the above object, the heater of the present invention is provided in contact with the heating resistor and the heating resistor, and supplies power to the heating resistor whose resistance value decreases as the temperature rises. And an electrode.

  According to the present invention, it is possible to provide an image forming apparatus in which the temperature difference between the electrode of the fixing heater and the heating resistor is small, and damage due to the temperature difference at the portion where the electrode and the heating resistor are in contact can be prevented. .

1 is a diagram illustrating an overall configuration of an image forming apparatus according to an exemplary embodiment of the present invention. FIG. 2 is a diagram illustrating a configuration of an image forming unit including a heat fixing device of an image forming apparatus according to an embodiment of the present invention and a control system. FIG. 2 is a block diagram illustrating a configuration of a control unit that controls the image forming apparatus of the image forming apparatus according to the exemplary embodiment of the present invention. It is a figure which shows the structure of the fixing heater of embodiment of this invention. It is a graph which shows the resistance temperature characteristic of the electrode part of embodiment of this invention. It is a graph which shows the temperature transition of the fixing heater of a prior art example. It is a graph which shows the temperature transition of the fixing heater of embodiment of this invention. It is a figure which shows the structure of the fixing heater of 2nd Embodiment of this invention. It is a graph which shows the temperature transition of a heating resistor and an electrode part in 2nd Embodiment of this invention, and the temperature transition of a heating resistor and an electrode part in the structure of a prior art example. It is a graph which shows the temperature transition of the heat generating resistor of the prior art example at the time of normal use, and the heat generating resistor temperature transition of the structure of 2nd Embodiment of this invention. It is a figure which shows the structure of the conventional fixing heater.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<First Embodiment>
(Overall configuration of image forming apparatus)
First, the overall configuration of an image forming apparatus according to an embodiment of the present invention will be described.

  FIG. 1 is a diagram illustrating the overall configuration of the image forming apparatus.

  As shown in the figure, each part of the apparatus main body A1 of the image forming apparatus includes a scanner part B1, an image forming part C1, and a sheet deck D1.

  A scanner unit B1 for reading image information of a document is disposed at the upper part of the apparatus main body A1, and an image forming unit C1 (image forming means) is provided at the lower part of the apparatus main body A1, and further below the image forming part C1. A seat deck D1 is provided on the side.

  The scanner unit B1 includes a scanning light source 151, a platen glass 152, a document pressure plate 153 that can be opened and closed with respect to the apparatus main body A1, a mirror 154, a lens 155, and the like. A scanning light source 151 scans the lower part of the platen glass 152 to read image information on the document surface. The image information of the document read by the scanning light source 151 is processed by the image processing unit, converted into an electrical signal, and transmitted to the laser scanner 111 of the image forming unit C1. Here, the apparatus main body A1 functions as a copying machine when a processing signal of the image processing section is input to the laser scanner 111 of the image forming section C1, and functions as a printer when an output signal of an external apparatus (computer) is input. The apparatus main body A1 functions as a facsimile apparatus when it receives a signal from another facsimile apparatus or transmits a signal from the image processing unit to another facsimile apparatus.

  On the other hand, sheet cassettes 1a, 1b, 1c, and 1d are mounted below the image forming unit C1.

  Sheet-like recording media (paper sheets) housed in the sheet cassettes 1a, 1b, 1c, and 1d are fed out by the pickup roller 3 and separated one by one by the cooperative operation of the feed roller 4 and the retard roller 5. Be fed. Thereafter, the toner is conveyed to the registration roller 106 by the conveyance rollers 104 and 105, and is fed to the image forming unit C1 by the registration roller 106 in synchronization with an image forming operation described later.

  In addition to the sheet cassettes 1a, 1b, 1c, and 1d, the manual feed tray 6 is disposed on the side surface of the apparatus main body A1, and the recording medium on the manual feed tray 6 is transferred to the registration roller 106 by the manual feed roller 7. It is paid out.

  Further, the image forming unit C1 includes a photosensitive drum 112, an image writing optical system 113, a charging roller 116, a developing device 114, a transfer roller 115, and the like.

  A laser beam corresponding to image information emitted from the laser scanner 111 is scanned on the surface of the photosensitive drum 112 uniformly charged by the charging roller 116 by the image writing optical system 113 to form an electrostatic latent image. The electrostatic latent image is developed by the developing device 114 to form a developer image (toner image). The toner image on the photoconductive drum 112 is transferred to a recording medium fed from the registration roller 106 in synchronization with the rotation of the photoconductive drum 112 by a transfer unit where the transfer roller 115 is arranged. The transport unit 117 transports the sheet-like recording medium on which the toner image is formed to the heat fixing device 118 by the transport unit 117. After the toner image is fixed on the surface of the recording medium by heating and pressurizing the recording medium in the heat fixing device 118, the recording medium is disposed outside the image forming apparatus by the discharge rollers 119 and 120. It is discharged to the tray 122 and loaded.

(Configuration of image forming unit)
FIG. 2 is a diagram illustrating a configuration of an image forming unit including the heat fixing device 118 and a control system.

  The image forming apparatus includes a fixing film 202, a pressure roller 204 (pressure member), photo interrupters 22 and 23, paper detection sensors 24 and 25, a print control unit 100, a motor speed control device 101, and a motor M2 for driving the fixing device. It has. The photosensitive drum 112 is disposed in contact with the transfer roller 115. The transfer roller 115 forms a transfer portion R. The transport unit 117 transports the recording medium P on which the toner image is formed. The paper detection sensors 24 and 25 are provided with flag portions (not shown), respectively, and the arrival and passage of the recording medium P can be detected by the photo interrupters 22 and 23.

  The heat fixing device 118 is a tensionless type film heating type device. In other words, a cylindrical (endless) fixing film 202 is used, and at least a part of the circumference of the fixing film 202 is tension-free (a state where no tension is applied), and the fixing film 202 is driven to rotate by the pressure roller 204. Driven by force.

  The fixing film 202 is a single layer film of a metal such as polyimide or SUS having a thickness of about 40 to 300 μm, or a composite layer film in which the outer peripheral surface is coated with PFA or the like or covered with a PFA tube.

  The plastic stay 203a is a heat-resistant and heat-insulating stay having a substantially semicircular arc-shaped cross section as a heating element supporting member and film inner surface guide member, and is made of a heat-resistant molding material. The fixing heater 201 (heating element) is fitted into a groove formed along the length of the stay at a substantially central portion of the lower surface of the plastic stay 203a and fixedly supported by a heat resistant adhesive. The plastic stay 203a is backed up by a sheet metal stay 203b in order to suppress the pressure of the pressure roller 204 and the bending of the fixing heater 201.

  The cylindrical fixing film 202 is loosely fitted so as to be movable to an assembly including a plastic stay 203a, a fixing heater 201, and a sheet metal stay 203b.

  The pressure roller 204 is an elastic roller provided with an elastic layer formed by foaming heat-resistant rubber such as silicone rubber or fluorine rubber, or silicone rubber on the outer periphery of the metal core 204a. A release layer such as PFA, PTFE, or FEP may be formed thereon. Both end shaft portions of the cored bar 204a are rotatably held by bearings. An assembly including a plastic stay 203a, a fixing heater 201, and a sheet metal stay 203b with a fixing film 202 fitted thereon is arranged above the pressure roller 204 so that the fixing heater 201 faces downward.

  A pressing force of a total pressure of 4 to 20 kg is applied to the plastic stay 203a by an unillustrated urging means so that the fixing heater 201 is pressed against the elasticity of the pressure roller by sandwiching the fixing film 202 with the pressure roller 204. I'm allowed. A fixing nip N is formed between the fixing heater 201 and the pressure roller 204 with the fixing film 202 interposed therebetween. In this embodiment, the diameter of the pressure roller 204 is φ30, and the width of the nip portion is 8 mm.

  The pressure roller 204 is driven to rotate counterclockwise by a rotational driving force transmitted by a driving unit (not shown) via a power transmission system (not shown). When the pressure roller 204 is driven to rotate, a rotational force acts on the fixing film 202 by a frictional force between the pressure roller 204 and the outer surface of the fixing film 202 in the fixing nip portion N. As a result, the fixing film 202 is rotationally driven around the plastic stay 203 a in the clockwise direction while closely sliding on the fixing heater 201. The plastic stay 203a serves to facilitate the rotational movement of the fixing film 202. In order to reduce the sliding resistance between the fixing heater 201 and the fixing film 202 in the fixing nip portion N, a lubricant such as heat resistant grease is applied between them.

  Based on a print start signal or the like, rotation of the pressure roller 204 is started, and heat-up of the fixing heater 201 is started.

  Next, the rotation peripheral speed of the fixing film 202 by the rotation of the pressure roller 204 becomes steady, and the temperature of the fixing heater 201 rises to a predetermined level. In this state, the recording medium P carrying the toner image T between the fixing film 202 and the pressure roller 204 in the fixing nip N is introduced with the toner image carrying surface side of the fixing film 202 side. The recording medium P is brought into close contact with the fixing heater 201 via the fixing film 202 at the fixing nip portion N and moves and passes through the fixing nip portion N together with the fixing film 202. That is, the recording medium P is fixed by sandwiching the recording medium P and the fixing film 202 between the pressure roller 204 and the fixing heater 201.

  In the moving and passing process, the heat of the fixing heater 201 is applied to the recording medium P through the fixing film 202, and the toner image T is heated and fixed to the recording medium P. The recording medium P that has passed through the fixing nip N is separated from the surface of the fixing film 202 and conveyed.

  In the heat fixing device 118, when the fixing film 202 is rotationally driven, a tension acts only on the fixing nip portion N and the region where the outer surface portion on the upstream side of the plastic stay 203a and the fixing film 202 slide, and most other fixings. No tension is generated on the film 202. For this reason, the driving torque of the fixing film 202 is small, and the moving force of the fixing film 202 along the stay length is small.

  FIG. 3 is a block diagram illustrating a configuration of a control unit that controls the image forming apparatus.

  The print control unit 100 is connected to the CPU 63 via the integrated chipset 61 and the system bus 62. The CPU 63 performs various arithmetic processes based on programs stored in the ROM 64. The ROM 64 stores information such as a paper feed interval from the sheet deck D1 of the recording medium P to the image forming unit C1, a fixing temperature, a paper feed temperature, and the remaining number of print jobs.

  The SRAM 65 is used as a work area in arithmetic processing by the CPU 63. An operation panel 66 and a USB control board 67 are connected to the system bus 62. The USB control board 67 is connected to an external device 68 such as a computer and can receive image data from the external device 68.

  The integrated chipset 61 is connected to an image memory 69 made of SDRAM or the like. The image memory 69 temporarily stores image data read by the scanner unit B1, image data received from a computer, and the like. The

  The print controller 100 is connected to a power controller 400 and a thermistor 201 d provided in the fixing heater 201. The temperature of the fixing heater 201 is determined from the voltage value of the thermistor 201d, and the power input from the power source 300 is controlled by the power control unit 400 so that the fixing heater 201 has an appropriate temperature.

(Configuration of fixing heater)
The fixing heater 201 which is a characteristic part of the present invention will be described.

  FIG. 4 is a diagram illustrating a configuration of the fixing heater 201 of the present embodiment.

  The substrate 30 has a rectangular shape with a length of 400 mm (in the case of an A3 size fixing device), a width of 10 mm, and a thickness of 0.6 mm, and is made of a ceramic such as alumina or aluminum nitride.

  On the substrate 30, electrode portions 34a and 34b having a length of 30 mm and a width of 2 mm based on manganese (Mn), nickel (Ni), cobalt (Co) or the like are provided at both ends in the longitudinal direction. Between the electrode portions 34 a and 34 b (electrodes), a heating resistor 33 having a width of 2 mm based on silver and palladium is provided along the longitudinal direction of the substrate 30.

  The heating resistor 33 has a long shape, and is provided so that the electrode portions 34a and 34b are in contact with both ends of the long shape in the longitudinal direction. The electrode portions 34a and 34b are provided at positions outside the surface area of the fixing heater 201 outside the area through which the recording medium P passes.

  The heating resistor 33 has a total resistance of 24Ω, and is about 0.8Ω in a 10 mm section.

  FIG. 5 is a graph showing resistance temperature characteristics of the electrode portions 34a and 34b.

  As shown in the figure, the electrode portions 34a and 34b have NTC characteristics (negative resistance temperature characteristics) in which the resistance value decreases as the temperature rises. In the present embodiment, an electric resistance in the 10 mm section is 0.7Ω at 25 ° C. (point a in FIG. 5) and 0.015Ω at 200 ° C. (b in FIG. 5). At this time, the heating resistor 33 has a B constant (B constant: an indication of how much the electrical resistance of the material changes with temperature) is about 3000 (K). The electrode portions 34a and 34b having NTC characteristics are at the same temperature as the room temperature where the image forming apparatus is installed.

(Effects of this embodiment over the conventional example)
FIG. 6 is a graph showing the temperature transition of the fixing heater of the conventional example (configuration shown in FIG. 11).

  As shown in FIG. 6, when 2000 W of electric power is applied to the conventional heater, the temperature of the heating resistor 33 rises rapidly (A1). At this time, since the electrode portions 32a and 32b in contact with the heating resistor 33 do not actively generate heat (A2), the temperature rise is moderate as compared with the heating resistor 33. This is because the resistance is set low because the electrode portions 32a and 32b serve as electrodes for supplying power. Further, if power continues to be supplied, a temperature difference at the boundary surface between the heating resistor 33 and the electrode portions 32a and 32b opens, and eventually reaches the thermal shock breakdown temperature of the ceramic substrate 30 as the base material. The temperature leading to thermal shock destruction of the ceramic substrate 30 varies depending on the material, thickness, usage environment, and the like, but in this embodiment, the temperature difference at the boundary between the heating resistor 33 and the electrode portions 32a and 32b is 220. When the temperature reached C (A3), destruction occurred.

  FIG. 7 is a graph showing the temperature transition of the fixing heater of this embodiment.

  As shown in the figure, when power exceeding 2000 W is applied to the fixing heater of the present embodiment as in the conventional fixing heater, the electrode portions 34a and 34b have a high resistance value at the beginning. It generates heat positively and shows almost the same temperature transition as the heating resistor 33 (A1, B2). Therefore, it is possible to suppress a large temperature difference between the heating resistor 33 and the electrode portions 34a and 34b, and it is possible to extend the time for reaching the temperature at which the thermal shock destruction of the ceramic substrate 30 occurs.

  That is, the resistance values of the electrode portions 34a and 34b in the present embodiment before the power supply are substantially the same as those of the heating resistor 33.

  The B constant indicating the NTC characteristic of the electrode part in the present invention is a value in this embodiment, and the value varies depending on the material, thickness, and shape of the ceramic substrate. In the heater base material used in the heat fixing device, it is desirable to provide an initial resistance value so that the temperature difference at the interface where the heating resistor and the electrode portion are connected does not reach the temperature at which thermal shock destruction occurs.

  The fixing heater 201 used in the present embodiment is a surface heating type, and the side having the electrode portions 34a and 34b and the heating resistor 33 on the substrate is the front side, and the opposite side is the back side of the fixing heater. Further, the fixing heater 201 has at least one heating resistor, and includes an electrode portion for supplying electric power to each. Further, the electrode portions do not have to be at both ends of the heating resistor, and can be freely set depending on the role and arrangement of the heating resistor.

  The electrode portions 34a and 34b and the heating resistor 33 are overcoated with insulating glass as an electrical insulating layer. The overcoat layer secures insulation between the electrode portions 34a and 34b, the heat generating resistor 33, and the external conductive member (not shown), and has a corrosion resistance function for preventing a change in resistance value due to oxidation and the like. Has a role to prevent damage.

  The present invention is not limited to the surface heating type fixing heater. Even in the backside heating type in which the side having the electrode portions 34a and 34b and the heating resistor 33 is the back surface and the opposite side is the surface, the electrode portion that supplies power to the heating resistor over may have NTC characteristics.

Each set value in the present embodiment is a reference value and is not limited to this.
In the present embodiment, the material has a resistance-temperature characteristic in order to change the resistance value of the electrode portion, but the electrode portion is provided with a plurality of electrode portions having different resistance values according to the temperature of the heating resistor. It is good also as a structure switched and used.

Second Embodiment
Next, an image forming apparatus according to another embodiment of the present invention will be described. The configuration of the present embodiment is based on the first embodiment described above, and only the portions that are different from the first embodiment will be described below, and redundant descriptions will be omitted. Moreover, the same code | symbol is attached | subjected to the part same as 1st Embodiment, and the overlapping description is abbreviate | omitted.

  FIG. 8 is a diagram showing a configuration of the fixing heater 201 (heat generating element) of the present embodiment.

  In the present embodiment, an excessive temperature rise prevention element 500 (power cut-off means) that cuts off power when a temperature above a predetermined temperature such as a thermo switch or a thermal fuse is detected is installed at the center of the substrate 30. When an abnormality occurs in the power control unit 400 and a large amount of power flows from the power supply 300 to the electrodes 34a and 34b and the heating resistor 33, the overheating prevention element 500 is activated by the heat generation of the heating resistor 33, and is safe. The power supply is cut off.

  The excessive temperature rise prevention element 500 is provided in an area through which the recording medium P passes in the surface area of the fixing heater 201.

(Effect of this embodiment)
FIG. 9 is a graph showing the temperature transition of the heating resistor 33 and the electrode portions 34a and 34b and the temperature transition of the heating resistor 33 and the electrode portions 32a and 32b in the configuration of the conventional example in this embodiment.

  In the conventional example, the electrode unit has a resistance value per predetermined area of 0.016Ω, and the adjacent heating resistor has a resistance value per predetermined area of 0.8Ω.

  As shown by the temperature transition of FIG. 9, in the conventional example (configuration of FIG. 11), the temperature transition A2 of the electrode portion does not increase positively with respect to the temperature transition A1 of the heating resistor, and the heating resistor and electrode Since the temperature difference gradually spreads at the part, thermal shock destruction occurred at the time when the temperature difference reached 220 ° C. (A3). At this time, although the temperature of the heating resistor has reached 290 ° C., it is almost the same as the temperature 293 ° C. at which the thermoswitch operates, and the heater is damaged.

  On the other hand, in the fixing heater according to the second embodiment of the present invention, when the installation environment of the image forming apparatus is a room temperature of 25 ° C., the resistance value of the electrode portion per predetermined area is 0.7Ω, and the adjacent heating resistance The resistance value per predetermined area of the body is almost the same as 0.8Ω. For this reason, as shown in FIG. 9, the heating resistor temperature transition B1 and the electrode portion temperature transition B2 rise with substantially the same slope. Therefore, a temperature difference hardly occurs between the electrode part and the heating resistor part. If time elapses from the start of power supply and the temperature of the electrode part and the heating resistor part continues to rise, the resistance value of the electrode part decreases to about 0.015Ω due to thermal resistance characteristics, so the temperature rise of the electrode part is suppressed. It will come.

  Then, the temperature difference between the heating resistor and the electrode portion begins to open, but before the temperature difference between the heating resistor and the electrode portion reaches the thermal shock breakdown temperature (B3) of the ceramic heater substrate, the highest temperature portion of the heating resistor. Reaches the operating temperature of the thermoswitch 293 ° C. For this reason, it is possible to safely cut off the power supply before causing the heater to break.

  FIG. 10 is a graph showing a temperature transition A of a conventional heating resistor during normal use and a heating resistor temperature transition B of the configuration of the second embodiment of the present invention.

  As shown in the figure, in the configuration of the present embodiment, the resistance value per predetermined area of the electrode portions 34a and 34b at room temperature is 0.7Ω, and the resistance per predetermined area of the electrode portion 32 in the configuration of the conventional example. The resistance value is higher than 0.016Ω. Therefore, the current flowing into the heating resistor 33 is less than that of the conventional example, and the temperature rise of the heating resistor 33 is slower than that of the conventional example as shown in FIG. For this reason, the time from when the image forming apparatus receives the copy request to when the fixing apparatus is ready for fixing becomes longer than in the conventional example. However, when electric power is supplied to the electrode portions 34a and 34b and the heating resistor 34, the resistance values of the electrode portions 34a and 34b are reduced to about 0.015Ω due to the NTC characteristics, and therefore a delay time until the target temperature is reached. Can be suppressed to less than 1 second from the conventional example.

  Also in the temperature control during paper feeding, since the resistance values of the electrode portions 34a and 34b are the same as those in the conventional configuration, the control responsiveness is not affected at all.

  Each set value in the present embodiment is a reference value and is not limited to this.

  The B constant indicating the NTC characteristic of the electrode portion in the present embodiment is a value in the present embodiment, and the value varies depending on the material, thickness, and shape of the ceramic base material. In the heater base material used in the fixing device, it is desirable to provide an initial resistance value so that the temperature difference at the interface where the heating resistor and the electrode portion are connected does not reach the temperature at which thermal shock breakdown occurs. In this embodiment, alumina is used for the heater substrate, a substrate having a substrate thickness of 0.6 mm and a substrate width of 10 mm is used, and thermal shock destruction occurs at 220 ° C.

  By using the configuration of the present invention, when power control is abnormal, the temperature difference at the boundary between the heating resistor and electrode is prevented, and power is supplied safely with an overheat prevention element installed near the heating resistor. Can be cut off. In addition, the influence on the preparation time of the fixing device during normal use can be suppressed.

DESCRIPTION OF SYMBOLS 30 Heater board 32a, 32b Electrode part 33 Heating resistor 34a, 34b Electrode part 100 Print control part 201 Fixing heater 300 Power supply 400 Power control part 500 Over temperature rise prevention element

Claims (11)

A heating resistor;
An electrode for supplying electric power to the heating resistor, which is provided in contact with the heating resistor and whose resistance value decreases as the temperature rises;
A heater comprising:   The heater according to claim 1, wherein the electrode has a material whose resistance value decreases with an increase in temperature.   The heater according to claim 1, wherein the electrode includes a switching unit that is switched so that a resistance value becomes smaller as the temperature increases. The heater has a substrate,
The heater according to any one of claims 1 to 3, wherein the heating resistor and the electrode are provided on the substrate.   The heater according to any one of claims 1 to 4, wherein the heating resistor has a long shape, and the electrodes are provided at both ends of the long shape in the longitudinal direction.   The heater according to claim 4 or 5, wherein an overcoat layer is provided on the substrate leaving at least a part of the electrode. A fixing device for fixing a recording medium on which a toner image is formed,
A pressure member that pressurizes the recording medium;
A fixing device comprising: the heater according to claim 1 that heats the recording medium. A fixing film provided to be movable between the heater and the pressure member;
The fixing device according to claim 7, wherein the recording medium is fixed by sandwiching the recording medium and the fixing film between the pressure member and the heater.   The power supply device further includes a power cut-off means provided in a region through which the recording medium passes in a region on the surface of the heater, and cuts off the power supplied to the electrode when a predetermined temperature or higher is detected. The fixing device according to claim 7 or 8.   The fixing device according to claim 9, wherein the electrode is provided outside the region through which the recording medium passes. Image forming means for forming a toner image on a recording medium;
An image forming apparatus comprising: the fixing device according to claim 7, which fixes the recording medium on which a toner image is formed by the image forming unit.

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