JP2003307966A - Fixing apparatus and temperature sensor for fixation - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing apparatus which efficiently collects heat from a fixing roller and accurately detects temperature by a temperature sensor for fixation, so as to obtain good image quality. <P>SOLUTION: As for the fixing apparatus constituted of a heating means, the fixing roller heated by the heating means, and a pressure roller arranged in press contact with the fixing roller, so that a recording medium carrying an unfixed image can pass through a nip part formed by the pressure roller and the fixing roller, the fixing apparatus is provided with a temperature detecting means for detecting the temperature of the fixing roller by the temperature sensor for fixation having a thermosensitive surface which is curved so as to come into face-contact with the inner peripheral surface of the fixing roller, a support for fixedly supporting the temperature detecting means, and a temperature control means for controlling the power supply to the heating means based on the detection result of the temperature detecting means. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fixing device used in an image forming apparatus such as a printer or a copying machine using an electrophotographic system, and a fixing temperature sensor used in the fixing device.

[0002]

2. Description of the Related Art In an image forming apparatus such as a printer or a copying machine, an apparatus for fixing an unfixed toner image formed and carried on a recording paper in correspondence with desired image information by an appropriate electrophotographic operation process mechanism is known. Widely used.
As such a fixing device, a fixing unit (roller, endless belt member, etc.) in which recording paper and toner electrostatically carried on the recording paper, toners made of a magnetic material, colorant, etc. are pressed against each other and rotated. In addition, a device for fusing and fixing is used by applying heat and pressure while sandwiching and transporting at a pressure contact portion (nip portion) of a pressure means (roller, endless belt body, etc.).

This fixing device generates an eddy current in a conductive layer provided on a fixing roller by a magnetic field generated from an exciting coil (not shown) as a heating unit for fixing toner as shown in FIG. 9A, for example. There is a method of causing the fixing roller to generate heat by the Joule heat of the conductive layer. As the temperature detecting means for controlling the temperature of the fixing roller, the fixing roller is brought into contact with the outer peripheral surface of the fixing roller to detect the temperature of the fixing roller. A temperature sensor (Fig. 9 (b)) is used.

[0004]

However, the above-mentioned prior art has the following problems.

As shown in FIG. 9 (a), in the conventional fixing device, the heat-sensitive surface of the fixing temperature sensor is always attached to the outer peripheral surface of the fixing roller so as to slide. The fluororesin coating surface of the fixing roller, which is in contact with the heat-sensitive surface, is abraded, which is a factor of shortening the durable life of the fixing roller. Further, dust and small dust adhering to the outer peripheral surface of the fixing roller are sandwiched between the heat-sensitive surface of the fixing temperature sensor and the fixing roller, and the outer peripheral surface of the fixing roller is scratched by the dust and small dust and transferred onto the recording paper. This has been a factor that deteriorates the quality of the copied image. Further, although a cleaning mechanism (not shown) for removing the residual toner on the fixing roller is provided, the residual toner that cannot be completely removed by the cleaning mechanism may be attached and deposited on the heat-sensitive surface of the fixing temperature sensor. there were. For this reason, the fixing temperature sensor detects the temperature from the fixing roller via the toner, so that the temperature of the fixing roller cannot be accurately detected, which may cause fixing failure. .

As a method of solving such a problem, a method of mounting the fixing temperature sensor outside the region where the recording paper passes through the fixing roller, that is, near the end of the fixing roller, is considered. However, it is difficult to accurately control the temperature control of the entire fixing roller because the end portion of the fixing roller radiates a large amount of heat.

Further, in the structure of the conventional fixing temperature sensor, since the heat-sensitive surface is in line contact with the surface of the fixing roller, it is difficult to accurately detect the heat from the fixing roller due to the mounting error of the temperature sensor, and the temperature is There is a case where the detection accuracy deteriorates and a fixing failure occurs.

Therefore, in view of the above problems, the present invention brings the fixing temperature sensor into contact with the inner peripheral surface of the fixing roller to efficiently collect the heat from the fixing roller, detect the temperature with high accuracy, and detect the image quality. It is an object to provide a good fixing device.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and the invention of claim 1 comprises a heating means, a fixing roller heated by the heating means,
In a fixing device, which comprises a pressure roller arranged so as to be pressed against the fixing roller and pass a recording medium carrying an unfixed image in the nip portion therebetween, the fixing device is in surface contact with the inner peripheral surface of the fixing roller. A temperature detecting means for detecting the temperature of the fixing roller by a fixing temperature sensor having a curved heat-sensitive surface, a support for fixedly supporting the temperature detecting means, and a temperature sensor for detecting the temperature of the fixing roller. The fixing device is provided with a temperature control unit that controls energization to the heating unit.

According to a second aspect of the present invention, in the fixing device according to the first aspect, the fixing temperature sensor used in the temperature detecting means has a pair of curved metal plates and is disposed between the metal plates. A thermosensitive element that is provided and electrically connected, a holding body in which one end of the metal plate is fixedly held, a guide hole provided in the holding body so that the other end of the metal plate is movable, A fixing temperature sensor, characterized in that at least a surface of the metal plate which is in contact with the fixing roller side is covered with an insulating sheet.

According to a third aspect of the present invention, there is provided a fixing temperature sensor according to the second aspect, wherein a thin film thermistor is used as the heat sensitive element.

According to a fourth aspect of the present invention, there is provided the fixing temperature sensor according to the second or third aspect, wherein the insulating sheet is made of a polyimide resin or a fluorine resin. Is.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a perspective view showing an embodiment of a fixing temperature sensor of the present invention used as a temperature detecting means of a fixing device, and FIGS. 2 (a) and 2 (b) are respectively the fixing temperature sensor of FIG. 3 is a plan view and a sectional view taken along the line AA, and FIG.

As shown in FIG. 1, the fixing temperature sensor S of the present invention includes a holder 7, metal plates 3a and 3b whose one end is fixedly held by the holder 7, and metal plates 3a and 3b. A guide hole 6a provided in the holder 7 to bend and make the other end movable, a heat sensitive element 9 disposed between the metal plates 3a and 3b and electrically connected, and the metal plate. 3a,
Insulating sheet 11 covered so as to cover the curved portion of 3b
It consists of and. By bending the metal plates 3a and 3b, the curved metal plates 3a and 3b can be inscribed on the inner peripheral surface of the cylindrical fixing roller, which will be described later. The heat can be effectively transferred to the heat sensitive element 9. Further, since the other end of the metal plate is made movable, the metal plates 3a and 3b follow the eccentricity of the rotating fixing roller so that the heat sensitive surface is always in surface contact with the inner peripheral surface of the fixing roller. The temperature of the fixing roller surface can be accurately detected.

Next, the process of assembling the fixing temperature sensor will be described.

In FIG. 2, 3a and 3b are metal plates made of stainless steel or a plate-shaped metal such as Kovar or nickel alloy formed by means such as chemical etching or pressing.

One end of each of the metal plates 3a and 3b is fixed to a holder 7, and the holder 7 has the metal plates 3a and 3b.
A guide hole 6a for introducing the other end portion of b, a screw hole 6b, and a protruding portion 8 are provided as necessary. The holding body 7 is formed by resin molding by means such as transfer molding using an insulating resin material or the like, and the groove portion 13 is formed so that the terminal portion 3 connected to the metal plates 3a and 3b is exposed. . The external lead wire 12 has a groove 1
3, the core wire 12 'and the terminal portion 3 are electrically connected by means such as welding and pulled out. The protruding portion 8 formed on the holding body 7 is used as a detent for a fixing temperature sensor when mounted inside a fixing device to be described later, and is fitted into a hole provided in a supporting body on the fixing device side. The fixing temperature sensor can be supported and fixed at two points by screwing with the screw holes 6b. Therefore, the detection position of the fixing temperature sensor can be accurately positioned.
The protrusion 8 provided on the holder 7 is not limited to this, and a recess may be formed on the holder 7 instead of the protrusion 8 or both of them may be formed. Of course.

The heat-sensitive element 9 used in the fixing temperature sensor S of this embodiment is composed of a thin film thermistor having a lead portion 10, and is arranged at a substantially central portion between the metal plates 3a and 3b.
In addition, the heat sensitive portion of the thin film thermistor is electrically connected to the metal plates 3a and 3b by welding or the like so that the flat surface portion of the heat sensitive portion is in surface contact with the fixing roller. An insulating sheet 11 having abrasion resistance and heat resistance, such as a polyimide film, is attached to at least the side that contacts the fixing roller.

One end of each of the metal plates 3a and 3b is fixed to the holding body 7, and the other end is curved and inserted into a guide hole 6a formed in the holding body 7 as shown in FIG.
The other end of 3b is bent at a right angle so as not to protrude from the guide hole 6a. By doing so, when the metal plates 3a, 3b are pressed against the inner surface of the fixing roller via the insulating sheet 11, the metal plates 3a, 3b are pressed.
Since the other end of the fixing roller moves using the guide hole 6a as a guide, it can be brought into contact with the inner surface of the fixing roller with a constant pressure. Since the guide holes 6a prevent the other ends of the metal plates 3a and 3b from swaying to the left and right, the heat-sensitive element 9 accurately contacts the inner surface of the fixing roller, which enables accurate temperature detection.

The external lead wire 12 is inserted into the groove portion 13 formed in the holding body 7, and the terminal portion 3 connected to the metal plates 3a and 3b and the core wire 12 'of the external lead wire 12 are welded or soldered. It is electrically connected and fixed by such a method.
Further, after connecting the terminal portion 3 and the core wire 12 'of the external lead wire 12 to reinforce the connection portion, the groove portion 1
The external lead wire 12 may be fixed integrally with the holding body 7 by injecting and hardening a resin into the portion 3.

FIG. 3 is a side view showing an example of the heat sensitive element 9 comprising a thin film thermistor used in the fixing temperature sensor S of this embodiment. In the figure, the thermosensitive element 9 which is a thin film thermistor has a thermosensitive film 16 on the surface of the insulating substrate 14.
And the electrodes 17 are formed on the heat-sensitive film,
After the lead portion 10 is bonded to the electrode surface, the whole is further covered with a protective film 18.

The insulating substrate 14 used in this thin film thermistor has a thickness of 100 to 300 μm, a length of 1.0 × width of 0.
The heat sensitive film 16 is formed of an alumina substrate having a size of about 5 mm, and the heat sensitive film 16 made of a metal oxide such as manganese, cobalt or nickel is patterned and formed on one surface of the insulating substrate 14 by a known thin film forming technique such as sputtering. The electrodes 17 are formed on both end surfaces of the. And
The lead portion 10 is led out from the electrode 17. The lead portion 10 is formed of a lead frame formed of a metal plate such as stainless steel, kovar, or nickel alloy by means such as chemical etching or pressing.
The method of joining the 0 and the electrode 17 is generally a method in which a conductive paste is applied and dried, and then baked and joined.
It is also possible to use a method such as welding. Also,
The lead portion 10 may be a lead wire such as a copper-coated steel wire (CP wire) or a Dumet wire, and can be joined to the electrode 17 using a conductive paste by the method described above.

In order to reinforce the joint between the lead portion 10 and the electrode 17 and protect the heat sensitive film 16 from the outside air, the entire surface of the insulating substrate 14 on which the heat sensitive film is formed is made of an insulating material such as glass. A protective film 18 made of a material is coated.

The heat sensitive element 9 is not limited to the thin film thermistor shown in the above embodiment, and for example, a rectangular chip thermistor having electrodes formed on its side surface, a laminated chip thermistor, a bead thermistor, or the like is used. You may. A material having abrasion resistance and heat resistance is used for the insulating sheet 11, and a polyimide resin tape or a fluororesin tape having a thickness of 20 μm to 50 μm and having an adhesive applied to one surface thereof is used.

FIG. 4 shows a fixing temperature sensor S of the present invention.
It is a top view and a sectional view showing other examples. Figure 4
As shown in (a), the metal plates 3a and 3b are curved from the side held by the holder 7 to the guide hole 6a, and an insulating sheet is attached so as to substantially cover the curved metal plates 3a and 3b. It is worn. Other than that, the configuration is the same as that of the fixing temperature sensor (FIGS. 1 to 3). In this fixing temperature sensor S, like the fixing temperature sensor described above, the metal plates 3a and 3b can be inscribed on the inner peripheral surface of the fixing roller, and the heat-sensitive surfaces are in surface contact with each other to effectively apply heat from the fixing roller. It can be transferred to the heat sensitive element 9. Similarly, since the other end of the metal plate is made movable, the metal plates 3a and 3b follow the eccentricity of the rotating fixing roller so that the heat-sensitive surface is always on the inner peripheral surface of the fixing roller. Surface contact,
The temperature of the fixing roller surface can be accurately detected.

Next, a method for attaching the fixing temperature sensor of the present invention to the fixing device will be described.

FIG. 5 shows the mounting state of the fixing temperature sensor of the present invention mounted inside the fixing roller. As shown in the figure, in the fixing temperature sensor S of the present invention, the protruding portion 8 formed on the fixing temperature sensor S is fitted into the hole 57a provided in the support 57, and is screwed by the screw hole 6b. . Accordingly, the fixing temperature sensor S can be supported and fixed at two points, and the heat-sensitive portion of the heat-sensitive element 9 of the fixing temperature sensor S can be accurately brought into contact with the inner peripheral surface of the fixing roller 51.

Moreover, the fixing temperature sensor S is the metal plate 3
Since the portions a and 3b are curved, they can be brought into surface contact with the inner peripheral surface of the fixing roller 51, and the heat from the fixing roller 51 can be applied to the insulating sheet 11 and the metal plates 3a and 3a.
Heat can be efficiently collected by b and the heat-sensitive portion of the heat-sensitive element 9. Further, the other end of the metal plates 3a and 3b is provided with a guide hole
Since it can be moved by a, even if the fixing roller 51 is eccentric, the pressure contact force to the inner peripheral surface of the fixing roller 51 can be made the same, so that accurate temperature detection is possible.

FIG. 6 is a schematic sectional view illustrating an example in which the fixing temperature sensor of the present invention is used in a fixing device of an electromagnetic induction heating system.

Since a series of processes in image formation are well known, description thereof will be omitted.

The fixing roller 51 has an outer diameter of 40 mm and a thickness of 0.
A layer of PTFE or PFA, for example, is provided in a thickness of 10 to 50 μm on the core metal cylinder made of iron of 7 mm in order to enhance the releasability of the surface. As another material of the fixing roller 51, a material having a relatively high magnetic permeability μ and an appropriate resistivity ρ such as a magnetic material (magnetic metal) such as magnetic stainless steel may be used. Further, a conductive material such as a metal may be vapor-deposited on the non-magnetic material.

The pressure roller 52 has a layer of silicone rubber having a thickness of 5 mm on the outer periphery of an iron cored bar having an outer diameter of 30 mm and, like the fixing roller 51, has a surface releasability of, for example, P.
A layer of TFE or PFA is provided in a thickness of 10 to 50 μm.

The fixing roller 51 and the pressure roller 52 are rotatably supported, and the fixing roller 51 is rotationally driven by a driving means (not shown). The pressure roller is arranged in pressure contact with the surface of the fixing roller and is driven to rotate by the frictional force at the pressure contact portion (nip portion). Further, the pressure roller 52 is pressed by a mechanism (not shown) using a spring or the like in the rotation axis direction of the fixing roller 51.

The fixing temperature sensor S is fixed to a support member 57 provided in the fixing roller 51, and is arranged so as to contact the inner peripheral surface of the fixing roller 51. The detection signal of the fixing temperature sensor S is output by voltage, digitally converted by the A / D converter, and the digitally converted temperature data is arithmetically processed by the controller unit 70. The high frequency converter 60 increases or decreases the power supply to the exciting coil, which is the heating unit 55, according to the control signal calculated by the controller unit 70, thereby fixing the fixing roller 5.
The surface temperature of No. 1 is automatically controlled so as to reach a predetermined temperature.

The conveyance guide 59 is arranged at a position for guiding the recording paper P conveyed while carrying the unfixed toner t to the nip portion between the fixing roller 51 and the pressure roller 52.

The heating means 55 is composed of an exciting coil and a magnetic core (not shown) and is connected to the high frequency converter 60. A high frequency current is passed through the exciting coil to heat the conductive layer of the fixing roller by Joule heat.

In the fixing temperature sensor S of the present invention, the projecting portion 8 formed on the fixing temperature sensor S mounted inside the fixing roller is fitted into the hole 57a provided in the support 57, and the screw hole is formed. It is screwed with 6b. Since the fixing temperature sensor S of the present invention has a structure in which the metal plates 3a and 3b are curved, it can be brought into surface contact with the inner peripheral surface of the fixing roller 51, and the heat from the fixing roller 51 can be removed. The insulating sheet 11, the metal plates 3a and 3b, and the heat-sensitive portion of the heat-sensitive element 9 can efficiently collect heat. Also metal plate 3
Since the other ends of a and 3b can be moved by the guide hole 6a, even if the fixing roller 51 is eccentric, the pressure contact force to the inner peripheral surface of the fixing roller 51 can be made the same, so that accurate temperature detection is possible. .

Further, since the fixing temperature sensor S of the present invention is a method of detecting the temperature of the inner peripheral surface of the fixing roller 51 by pressing it against the inner peripheral surface of the fixing roller 51, the outer peripheral surface of the fixing roller is scratched. It is possible to obtain a high quality copied image without causing the occurrence.

Further, by mounting the fixing temperature sensor S inside the fixing roller, stable and accurate temperature detection can be performed without being influenced by the atmosphere around the fixing device.

FIG. 7 shows an example in which the fixing temperature sensor of the present invention is used in an electromagnetic induction heating type fixing device in which a belt member 53 is attached to a fixing roller in another embodiment.

In this fixing device, the induction heating means 5
5, the heating roller 55a heated by the electromagnetic induction, the fixing roller 51, the belt body 53 that rotates so as to stretch the heating roller 55a and the fixing roller 51, and the fixing roller 51 via the belt body 53. A pressure roller 52 that forms a fixing nip portion so as to be in pressure contact, and a heating roller 55a
The fixing temperature sensor S1 of the present invention disposed on the inner surface of the belt body 53 and the temperature sensor S2 for detecting the temperature of the belt body 53 near the entrance of the fixing nip portion of the belt body 53.

In this fixing device, the induction heating means 5
When the heating roller 55a and the belt body 53 are heated by electromagnetic induction by 5, the endless belt body 53 is continuously heated, and as a result, the entire belt body 53 is heated. Then, temperature control is performed so that the temperature of the belt body 53 near the entrance of the fixing roller becomes a predetermined temperature. Further, the temperature of the heating roller 55a detected by the fixing temperature sensor S1 and the detected temperature of the belt body 53 near the entrance of the fixing roller detected by the temperature sensor S2 are sent to a determination means (not shown), and from this temperature information, the belt body 53 is detected. The rotation state is determined.

In this embodiment, the structure of the temperature sensor S2 for detecting the inner surface temperature of the belt 53 is not specifically shown, but the fixing temperature sensor S of the present invention shown in FIGS. Is a suitable means, but a temperature sensor using a sponge can be used as a means for press-contacting the object to be detected.

Further, in the present embodiment, the method of heating the fixing roller by the induction heating type fixing device is disclosed. However, other than this, a fixing device using a well-known halogen lamp inside the fixing roller may be used.

FIG. 8 shows an example in which the fixing temperature sensor of the present invention is used in a heating film type fixing device in another embodiment.

In this fixing device, a ceramic heater, a heater support 57 for fixing the ceramic heater, and a cylindrical fixing film 5 externally attached to the heater support 57.
4, a pressure roller 52 forming a fixing nip portion so as to be pressed against a fixing nip portion so as to be pressed against a ceramic heater portion via a fixing film 54, and a heater support 57.
And a fixing temperature sensor S of the present invention arranged so as to be in pressure contact with the inner surface of the cylindrical fixing film 54.

The pressure roller 52 is rotationally driven in the direction of the arrow by a driving means (not shown). Rotational force acts on the fixing film 54 due to the pressure contact frictional force in the fixing nip portion,
The toner image carrying surface side of the recording paper P carrying the toner image is in close contact with the outer surface of the fixing film 54, and is conveyed while sandwiching the fixing nip portion together with the rotating fixing film 54.
In this process, the heat of the ceramic heater is applied to the recording paper P via the fixing film 54 to be pressure-fixed. The temperature control of the ceramic heater in this embodiment is performed by the fixing temperature sensor S of the present invention provided near the ceramic heater of the heater support 57.

Since the fixing temperature sensor S of the present invention used in this embodiment has a structure in which the metal plates 3a and 3b are curved, it can be brought into surface contact with the inner peripheral surface of the fixing roller 51. Therefore, the heat from the fixing roller 51 can be efficiently collected by the insulating sheet 11, the metal plates 3a and 3b, and the heat-sensitive portion of the heat-sensitive element 9. In addition, the metal plate 3a,
Since the other end of 3b can be moved by the guide hole 6a, even if the fixing roller 51 is eccentric,
Since the pressure contact force to the inner peripheral surface of can be made the same, accurate temperature detection is possible.

[0050]

According to the temperature sensor for fixing of the present invention, a pair of metal plates are curved and attached so as to be in close contact with the inner peripheral surface of a cylindrical fixing roller so that the inner surface of the fixing roller is brought into surface contact. The heat from the fixing roller can be efficiently collected by the insulating sheet, the metal plate, and the heat-sensitive portion of the heat-sensitive element, and accurate temperature detection can be performed. In addition, since the other end of the metal plate is movable through the guide hole, the metal plate can follow the eccentricity of the fixing roller and make contact with the inner peripheral surface of the fixing roller with a constant pressure contact force. It can detect and control the temperature.

Further, the fixing temperature sensor in the fixing device of the present invention is not in contact with the outer peripheral surface of the fixing roller, but is in contact with the inner peripheral surface of the fixing roller. Generation of scratches on the surface is eliminated and high quality copied images can be obtained.

Further, since the problem that the residual toner adheres to and accumulates on the heat-sensitive surface of the fixing temperature sensor can be solved, a high quality copied image can be obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing an embodiment of a fixing temperature sensor of the present invention used as a temperature detecting means of a fixing device.

2A and 2B are a plan view and an AA cross-sectional view of the fixing temperature sensor of FIG.

FIG. 3 is a side view of a heat-sensitive element used in the fixing temperature sensor of the present invention.

FIG. 4 is a plan view and a cross-sectional view showing another embodiment of the fixing temperature sensor of the present invention.

FIG. 5 is a view showing a mounting state of a fixing temperature sensor of the present invention mounted inside a fixing roller.

FIG. 6 is a schematic sectional view illustrating an example in which the fixing temperature sensor of the present invention is used in an electromagnetic induction heating type fixing device.

FIG. 7 is a diagram showing an example in which a fixing temperature sensor of the present invention is used in an electromagnetic induction heating type fixing device in which a belt body 53 is attached to a fixing roller.

FIG. 8 is a diagram showing an example in which the fixing temperature sensor of the present invention is used in a heating film type fixing device.

FIG. 9 is an example of a conventional fixing device and a fixing temperature sensor mounting method and a fixing temperature sensor.

[Explanation of symbols]

S fixing temperature sensor P recording paper t toner 3a, 3b Metal plate 6a Guide hole 6b screw hole 7 Holder 9 Thermal element 10 Lead part 11 Insulation sheet 12 External leader 51 fixing roller 52 Pressure roller 55 Heating means 57 Support 60 high frequency converter 70 Controller section

Claims (4)

[Claims] 1. A heating unit, a fixing roller heated by the heating unit, and a heating unit arranged to press the fixing roller and pass a recording medium carrying an unfixed image through a nip portion therebetween. In a fixing device consisting of a pressure roller,
A temperature detecting means for detecting the temperature of the fixing roller by a fixing temperature sensor having a heat-sensitive surface curved so as to make surface contact with the inner peripheral surface of the fixing roller; and a support body for fixing and supporting the temperature detecting means. And a temperature control unit that controls energization to the heating unit based on the detection result of the temperature detection unit. 2. The fixing device according to claim 1,
A fixing temperature sensor used for the temperature detecting means includes a pair of curved metal plates, a heat sensitive element electrically connected between the metal plates, and one end of the metal plate fixedly held. And a guide hole provided in the holder to make the other end of the metal plate movable, and at least a surface of the metal plate that is in contact with the fixing roller side is covered with an insulating sheet. Fixing temperature sensor. 3. The fixing temperature sensor according to claim 2, wherein a thin film thermistor is used for the heat sensitive element. 4. The fixing temperature sensor according to claim 2, wherein the insulating sheet is made of a polyimide resin or a fluorine resin.