JPH0619366A - Image forming device - Google Patents

Abstract

PURPOSE:To improve the safety of an image forming device having an electric heating element without complicating a device. CONSTITUTION:When ordinary printing operation is not performed in a fixing device, a relay(safety switch) 106 is turned off first, a triac 102 is turned on for a specified time, and the temperature of a halogen heater(electric heating element) 103 is detected 104. In the case temperature rise is found, it is judged as the fault of the relay 106, and in the case it is not found, the triac 102 is turned off and the realy is turned on. Then, the temperature of the heater 103 is detected again, and in the case the temperature rise is found, it is judged as the fault of the triac 102. When it is not found, the triac 102 is turned on and the temperature of the heater 103 is detected. In the case the termperatur rise is not found, it is judged as the fault of all or any of the relay 106, the triac 102 and a thermistor 104, and in the case it is found, operation is shifted to the ordinary operation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus for heating and fixing by an electric heating element, and more particularly to an image forming apparatus having a function of judging a failure of temperature detecting means and the like.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine, a halogen heater or the like has been used as an electric heating element. In order to control the temperature of the halogen heater to a predetermined temperature suitable for fixing, an electric heating element is used. The temperature is detected by a temperature sensor which is a temperature detecting means installed in the vicinity, and the power supply to the electric heating element is switched by a semiconductor element such as a triac which is an energization controlling means. Hereinafter, such a conventional device will be described with reference to FIG.

FIG. 13 shows a fixing device which is a constituent unit of a laser beam printer which is an example of an image forming apparatus,
The control circuit is shown. Reference numeral 401 denotes a fixing roller having a hollow structure, inside which a halogen heater 402 as an electric heating element is located. Therefore, by turning on the halogen heater 402, the surface of the fixing roller 401 is heated.
A pressure roller 403 has a role of pressing a print sheet on which toner has not been fixed against the fixing roller 401.

Reference numeral 404 is a thermistor which is a temperature detecting means and can detect the surface temperature of the fixing roller by bringing it into contact with the surface of the fixing roller. That is, the resistance value of the thermistor 404 tends to decrease as the temperature of the fixing roller surface rises, and the fixing roller surface temperature can be detected by detecting this resistance value change.

Reference numeral 405 is a resistor which is a voltage dividing means for converting the resistance value of the thermistor 404 into a voltage value. 406
Is a CPU as a control means having an A / D converter input
The port 1 output is controlled by inputting the thermistor voltage divided by the resistor 405 and referring to the table stored in the internal memory and showing the correlation between the voltage and the temperature.

That is, when the thermistor voltage is higher than the reference voltage (the surface temperature of the fixing roller is lower than the reference temperature), the output of port 1 is set to HIGH. When the thermistor voltage is lower than the reference voltage (the surface temperature of the fixing roller is higher than the reference temperature), the port 1 output is set to LOW.
And

[0007] On the other hand, 407 is a transistor, CP
Based on the output of the port 1 of U406, ON / OFF of the relay 408 which is a switching means is controlled.

That is, when the surface temperature of the fixing roller is lower than the reference temperature, the output of the port 1 of the CPU is set to HIGH, the transistor 407 is turned on, the contact of the relay 408 is closed, and the surface temperature of the fixing roller is the reference temperature. If it is higher than that, the output of the port 1 of the CPU is set to LOW, the transistor 407 is turned off, and the contact of the relay 408 is opened. When the contact of the relay 408 is closed, the halogen heater 402 is turned on and the fixing roller 401 is heated. When the contact of the relay 408 is opened, the halogen heater 402 is turned off and the surface temperature of the fixing roller 401 is lowered.

In this conventional example, the turning on and off of the halogen heater is controlled by the relay 408, but it is also possible to use a triac.

As described above, the temperature of the fixing roller can be controlled so that the surface temperature of the fixing roller is always within the reference temperature range by detecting the temperature with the thermistor which is in contact with the surface of the fixing roller.

However, since the temperature control depends on the thermistor as described above, when the thermistor is disconnected, not only the temperature cannot be adjusted to a normal temperature, but also abnormal heating of the fixing roller causes smoke and ignition of printer paper. There is sex.

Since the thermistor exhibits a high resistance value especially from low temperature to room temperature, the A / D converter input of the CPU 406 becomes almost the power supply voltage although it is divided by the resistor 405. Further, even when the thermistor is broken, the input of the A / D converter is pulled up by the resistor 405 and becomes the power supply voltage. In other words, from the low temperature to the room temperature, it was not possible to distinguish from the broken state of the thermistor.

Therefore, conventionally, the halogen heater is energized to heat the fixing roller, and the A / D is nevertheless used.
When the converter input voltage did not change, it was determined that the thermistor might be broken.

[0014]

However, the above-mentioned conventional example has the following problems. First, in order to shorten the image forming time, when the heat generating capacity of the electric heating element is increased and preheating is performed even during the time other than the image forming time, a plurality of safety means are required and the apparatus is There was a problem that it became complicated. In other words, if the electric heating element is power-controlled for a long time, switching elements such as triacs or safety switches such as relays
Since it is affected by heat cycle, AC line noise, rush current, etc., it is more likely to deteriorate than other parts used in the equipment, and failure of parts may lead to a fire accident. It was necessary to have multiple safety measures corresponding to the device.

Next, in the detection of disconnection of the thermistor in the above-mentioned conventional example, when the thermistor is disconnected, since the halogen heater is energized in an uncontrolled state, there is still a possibility of smoke and ignition, and the thermistor is also disconnected. Since it is necessary to heat the fixing roller until it can be discriminated from the broken state, there is a problem that it consumes a lot of time and power.

Further, when the temperature rise cannot be detected even though the halogen heater is energized for a certain period of time or longer, there is a problem that the thermistor is disconnected, the halogen heater is disconnected, or it cannot be separated. there were.

A first object of the present invention is to solve the above problems and improve the safety of an image forming apparatus having an electric heating element without complicating the apparatus.

A second object of the present invention is to solve the above problems and to provide an image forming apparatus capable of quickly detecting the disconnection of the thermistor without energizing the halogen heater.

[0019]

According to the first invention of the present application,
The first object is to provide an electric heating element that is provided in a fixing device and generates heat by applying a commercial power source, a temperature detecting means for detecting the temperature of the electric heating element, the electric heating element and the commercial power source. Switching means for switching the connection so as to energize or de-energize the electric heating element, and energization for controlling the energizing period within one cycle of the AC waveform of the commercial power source to energize or de-energize the electric heating element. Control means,
In an image forming apparatus provided with the switching means and a temperature control means for appropriately driving the energization control means based on a signal from the temperature detection means, the temperature control means is provided when a normal printing operation is not performed. , When the switching means and the energization control means are alternately driven for a predetermined period so that only one of the electric heating elements is energized, and when both are energized for a predetermined period of time This is achieved by setting the temperature detecting means to detect the temperature of the electric heating element when driven, and to determine the failure of the switching means, the energization controlling means and the temperature detecting means.

According to the second invention of the present application, the second object is to provide an electric heating element provided in the fixing device and a temperature detecting means which is arranged near the electric heating element and whose electric resistance changes depending on the temperature. A control means for judging the temperature of the electric heating element from the electric resistance value of the temperature detecting means; and for inputting the electric resistance value of the temperature detecting means to the control means as a predetermined voltage value. In an image forming apparatus having a voltage dividing unit connected to the temperature detecting unit, the control unit is configured to bypass the voltage dividing unit and apply a predetermined voltage to the temperature controlling unit, A self-heating means for self-heating is connected, and the control means causes the self-heating means to apply a predetermined voltage to the temperature detecting means, and changes the electric resistance of the temperature detecting means inputted at that time. It is accomplished by being configured to determine a failure of the temperature detection means.

[0021]

According to the first aspect of the present invention, when the normal printing operation is not performed, for example, first, only the switching means is driven by the control means so that the electric heating element is in the energized state. However, in this case, the energization control means is in a state of not energizing the electric heating element, so that the temperature of the electric heating element does not rise if the energization controlling means is functioning normally. Therefore, in this case, when the temperature rise of the electric heating element is detected, it is determined that the energization control unit has a failure. Further, when the temperature rise of the electric heating element cannot be detected, only the energization control means is driven by the control means so that the electric heating element is in the energized state. However, in this case, since the switching means is in a state in which the electric heating element is not energized, the temperature of the electric heating element does not rise if the switching means is functioning normally. Therefore, in this case, when the temperature rise of the electric heating element is detected, it is determined that the switching means is out of order. When the temperature rise of the electric heating element cannot be detected, these means are driven by the control means so that both the energization control means and the switching means bring the electric heating element into the energized state. In this case, the temperature rise of the electric heating element is detected as long as the energization control means, the switching means, and the temperature detection means are functioning normally. Therefore, when the temperature rise of the electric heating element is not detected, it is determined that all or any of these means is defective. Further, when the temperature rise is detected, it is determined that none of these means has a failure.

According to the second invention of the present application, in order to detect the failure of the temperature detecting means, the control means drives the self-heating means, bypasses the voltage dividing means, and applies a predetermined voltage to the temperature detecting means. To do. At this time, if the temperature control means is normal, the electric resistance of the temperature control means changes due to self-heating, the change is detected by the control means, and the temperature detection means is judged to be normal. On the other hand, if a change in electrical resistance is not detected despite application of a predetermined voltage to the temperature detecting means, it is determined that the temperature detecting means has a failure.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First to fifth embodiments of the present invention will be described with reference to the drawings.

<First Embodiment> First, a first embodiment of the present invention will be described with reference to FIGS. FIG. 1 is a block diagram showing a schematic configuration of the electric heating element control unit of the present embodiment. In FIG. 1, reference numeral 101 denotes a commercial power source input to the apparatus. The input commercial power source is supplied to an electric heating element of the heating and fixing apparatus and to a low voltage power source section 107 for generating a DC power source used in the apparatus. It is supplied and used for driving an actuator (not shown) and a semiconductor element (not shown) in the apparatus. Reference numeral 103 denotes an electric heating element, which is usually composed of a halogen heater. Reference numeral 102 denotes a switching element which is an energization control means for controlling the power supplied to the electric heating element 103, and a triac or the like is used. A safety switch 106 is a switching means, and a relay or the like is used. 104 is a temperature detecting element such as a thermistor. 10
Reference numeral 5 is a CPU which is a control means for controlling the triac 102 and the relay 106.

In the apparatus of this embodiment as described above, the CPU 105 controls the triac 102 according to the difference between the temperature detection information from the thermistor and the target temperature during the normal printing operation to supply the electric power to the electric heating element. Have control.

However, the above-mentioned triac, relay, etc. are more easily deteriorated than the other parts used in the apparatus, and failure of these parts leads to an accident such as a fire. Fault detection is performed.

The failure detection of this embodiment is set to be performed by the CPU at times other than the printing operation, as shown in FIG.
FIG. 3 shows a timing chart of each element at the time of failure detection, and FIG. 3 shows a schematic flowchart of failure detection.

When the apparatus is powered on as shown in step 301 of FIG. 3, the triac drive signal is turned on while the relay drive signal is kept off after a lapse of time T ₁ from the time when the power is turned on (see FIG. In step 3, steps 302 to 303). Then, as shown in FIG. 2, this state is maintained until time T _2, and the heat generation state of the electric heating element is detected during this period (step 30 in FIG. 3).
4). Here, the time from T ₁ to T _{2 is set} to the minimum time for the CPU to sufficiently recognize the abnormality of the element from the temperature rising speed of the electric heating element, and the temperature rising time that does not damage the heat fixing device. ing.

Further, the detection of the heat generation state is performed from the above T ₁ to T ₂
This is done by monitoring the voltage of the thermistor 104 during the period. That is, when the electric heating element generates heat despite the relay being in the OFF state, the routine proceeds to the relay failure processing routine (step 304 to step 305 in FIG. 3). Here, as the thermistor, an NTC type thermistor is used, and the higher the detection voltage is, the lower the temperature is. In FIG. 2, the thermistor voltage at times T ₁ and T ₂ shows a room temperature state, that is, a normal state.

On the other hand, if not detected abnormal heat until time T _2, the OFF state the triac drive signal after a time T ₂ has elapsed (step 306 in FIG. 3),
At time T ₃ , the relay drive signal is turned on (step 307 in FIG. 3).

Then, the output voltage of the thermistor 104 is monitored again to check whether or not the electric heating element has generated heat (step 308 in FIG. 3). That is, if heat generation is detected while the triac is off, it is determined that the triac has failed, and the routine proceeds to the triac failure processing routine (step 309 in FIG. 3).

The triac drive signal is turned on at a time T ₄ after the relay operation signal is turned on from a time T ₃ which is a time in consideration of the operation time of the relay and the bouncing time of the relay and a minimum time (see FIG. 3). In step 310), this state is maintained until time T _{5 as} shown in FIG. In this way, the output voltage of the thermistor 104 is monitored with respect to the input power with the relay and triac turned on, and it is checked whether or not the electric heating element is rising at a predetermined temperature rise rate (step 311 in FIG. 3). ). If the heat generation of the electric heating element is not detected, it is determined that there is an abnormality, and the routine proceeds to the relay triac thermistor failure detection routine (step 312 in FIG. 3).

On the other hand, when it is determined that there is no abnormality, the triac is turned off at time T ₅ (step 313 in FIG. 3), and normal operation control is performed (step 314 in FIG. 3). Here, the time from T ₄ to T _{5 is set} to the minimum time at which the CPU can sufficiently recognize the abnormality from the temperature rising rate of the electric heating element and the temperature rising time which does not damage the heat fixing device.

Next, referring to FIG. 3, steps 305 and 30 are performed.
A case where a failure is determined as indicated by reference numerals 9 and 312 will be described with reference to FIGS.

FIG. 4 shows a timing chart in the case of shifting to step 305 in FIG. 3, and the flow up to step 304 is as described above. That is, at time T ₁ , the CPU 105 outputs the triac drive signal O
N state. Then, when the heat generation of the electric heating element is detected by the output voltage of the thermistor 104 as shown in FIG. 4 despite the relay being in the OFF state, the relay failure processing routine of step 305 of FIG. 3 is performed. Migrated,
The relay drive signal is not turned on even at time T ₃ .
The failure mode of the relay at this time may be burn-in of the contacts, failure of the driver in the short mode, or the like.

Further, FIG. 5 shows step 30 in FIG.
It shows a timing chart when moving to 9.
The flow up to step 308 is as described above. That is, at the time T ₃ , the CPU 105 outputs the relay drive signal O
N Then, when heat generation of the electric heating element is detected by the output voltage of the thermistor 104 despite the triac being turned off, step 309 in FIG.
The process shifts to the triac failure processing routine indicated by and the relay drive signal is turned off at time T ₆ . The failure mode of the triac at this time may be a failure of the short mode of the triac, a failure of the short mode of the gate driver, or the like.

Further, FIG. 6 shows step 3 in FIG.
It is a timing chart when shifting to 12, and the flow up to step 311 is as described above. That is,
At time T ₄ , the CPU 105 turns on the triac drive signal. Then, turn on the triac and relay.
However, if the output voltage of the thermistor 104 cannot detect the heat generation of the electric heating element,
In step 312, the routine proceeds to the relay triac thermistor failure processing routine to turn off the triac drive signal at time T ₅ and turn off the relay drive signal at time T ₆ . Since this failure mode is a failure mode related to fire safety, it needs to be turned off earliest, and times T ₆ and T ₅ may be turned off at the same time. In addition, as the failure mode at this time,
The open mode failure of the TRIAC, the open mode failure of the TRIAC gate driver, the disconnection of the relay winding, the open mode failure of the driver, the thermistor open mode failure, etc. are considered.

As described above, according to the present invention, the operation of the relay, triac and thermistor is diagnosed by the combination of ON / OFF of the relay and the triac when the power is turned on or when the printing operation is not performed. Since it is possible to perform the diagnosis, the components can be diagnosed at a low temperature of the electric heating element, so that the safety of the device can be dramatically improved.

<Second Embodiment> Next, a second embodiment of the present invention will be described with reference to FIGS. The same parts as those in the first embodiment are designated by the same reference numerals and the description thereof will be omitted.

The first embodiment has shown the case where the operation diagnosis is carried out from the detection voltage by the thermistor, but the present embodiment shows the case where the current transformer is provided and the failure detection is carried out from this detection voltage.

FIG. 7 is a block diagram showing a schematic configuration of the electric heating element control unit of this embodiment. In FIG. 7, 108 is a current transformer, and the detection voltage is read by the CPU 105.

Therefore, in this embodiment, as shown in the flow chart of FIG. 8, the failure of the relay, triac, thermistor, etc. is detected by detecting the current. The timing chart in the normal state is as shown in FIG.

As described above, in this embodiment, by detecting the current, even if the relay and the thermistor fail, and the triac and the thermistor fail, it is possible to quickly respond.

<Third Embodiment> Next, a third embodiment of the present invention will be described with reference to FIG. In FIG. 10, reference numeral 211 denotes a thermistor which is a temperature detecting means and is in contact with the surface of the fixing roller. As the surface temperature of the fixing roller increases, the resistance value of the thermistor 211 decreases. 212 is
It is a resistor which is a voltage dividing means for converting the resistance value of the thermistor 211 into a voltage value. Reference numeral 213 denotes a CPU having an A / D converter input, which inputs the thermistor voltage divided by the resistor 212, converts it into a temperature detected by the thermistor, and controls the lighting of the halogen heater according to the temperature.

The CPU 213 also has a port 2 output. A transistor 215 is connected to the port 2 output via the transistor 214, and the port 2 output is set to HI.
By setting to GH, the transistor 214 is turned on, and the transistor 215 is also turned on. As a result, the power supply voltage is applied across the transistor 21 across the thermistor 211.
A voltage value obtained by subtracting the saturation collector-emitter voltage of 5, that is, almost the power supply voltage is applied. At this time, if the thermistor 211 is not broken, the thermistor 211 self-heats and the resistance value decreases.

On the other hand, the transistor 214 is turned off and the transistor 215 is also turned off by setting the port 2 output of the CPU 213 to HIGH and then setting the port 2 output to LOW. As a result, the CPU 213
The thermistor voltage divided by the resistor 212 is input to the A / D converter input of.

If the thermistor 211 is not broken,
The thermistor 21 while the port 2 output is HIGH
No. 1 self-heats, the A / D converter input voltage becomes a low value, and conversely, if the thermistor 211 is disconnected, A / D
The D converter input voltage is pulled up by the resistor 212 and becomes the power supply voltage.

Therefore, by monitoring the A / D converter input voltage, it becomes possible to discriminate whether or not the thermistor 211 is disconnected.

<Fourth Embodiment> Next, a fourth embodiment of the present invention will be described with reference to FIG. The same parts as those in the third embodiment are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 11, 221 and 222 are resistors for converting the resistance value of the thermistor 211 into a voltage value. Also in this embodiment, by setting the output of the port 2 of the CPU 213 to HIGH, the transistor 214 is turned on,
Further, the transistor 215 is also turned on. This makes the resistor 221 equivalent to a short-circuited state, and the thermistor 2
The value of the current flowing through 11 increases. At this time, if the thermistor 211 is not broken, the thermistor 211 self-heats and the resistance value decreases.

The difference from the first embodiment is that the presence of the resistor 222 makes it possible to monitor the change in the thermistor voltage in real time. When the A / D converter input voltage does not change or the power supply voltage is shown. Has
It can be judged that the thermistor is broken.

After the detection of disconnection of the thermistor is completed, CP
By making the port 2 output of U213 LOW, the transistor 214 is turned off and the transistor 215 is also turned off. As a result, the thermistor voltage divided by the resistors 221 and 222 is input to the A / D converter input of the CPU 213. In this state, energization of the halogen heater may be started.

<Fifth Embodiment> Next, a fifth embodiment of the present invention will be described with reference to FIG. The same parts as those in the third embodiment are designated by the same reference numerals and the description thereof will be omitted.

Also in this embodiment, the output of the port 2 of the CPU 213 is set to HIGH to turn on the transistor 231 and also turn on the transistor 232. As a result, a current flows through the thermistor 211 via the resistor 233. By making the resistance value of the resistor 233 small, if the thermistor 211 is not broken, the thermistor 21
No. 1 self-heats, and the resistance value decreases.

The CPU 213 monitors the change in the thermistor voltage in real time, and if the A / D converter input voltage does not change or indicates the power supply voltage, it may determine that the thermistor is disconnected.

After the detection of disconnection of the thermistor is completed, CP
By setting the port 2 output of U213 to LOW, the inverter 234 turns on the transistor 235 and also turns on the transistor 236. This makes CP
The thermistor voltage divided by the resistor 237 is input to the A / D converter input of U213. In this state, energization of the halogen heater may be started.

[0057]

As described above, according to the first invention of the present application, the switching means by the combination of ON / OFF of the energization control means and the switching means when the power is turned on or when the printing operation is not performed, Since it becomes possible to diagnose the operation of each of the energization control means and the temperature detection means,
Since the diagnosis can be performed at a low temperature of the electric heating element, it is possible to dramatically improve the safety of the device.

Further, according to the second aspect of the present invention, since the self-heating means is provided to detect the failure of the temperature detecting means, it is possible to prevent smoking and ignition in the apparatus in advance and save a lot of time and power. It is possible to detect disconnection of the temperature detecting means without consumption. Further, the disconnection of the temperature detecting means and the disconnection of the electric heating element can be reliably separated.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an electric heating element control unit of a first embodiment of the present invention.

FIG. 2 is a timing chart at the time of failure detection operation when the switching unit, the energization control unit, and the temperature detection unit are functioning normally in the first embodiment device.

FIG. 3 is a schematic flowchart of failure detection control in the device of the first embodiment.

FIG. 4 is a timing chart when there is a failure in the switching means in the first embodiment device.

FIG. 5 is a timing chart when the energization control unit has a failure in the device of the first embodiment.

FIG. 6 is a timing chart when there is a failure in all or any of the switching means, the energization control means, and the temperature detection means in the first embodiment device.

FIG. 7 is a block diagram showing a schematic configuration of an electric heating element control unit of a second embodiment of the present invention.

FIG. 8 is a schematic flowchart of a failure detection control in the second embodiment device.

FIG. 9 is a timing chart at the time of a failure detection operation when the switching means, the energization control means, and the temperature detection means are functioning normally in the second embodiment device.

FIG. 10 is a connection diagram showing a schematic configuration of a control unit, a temperature detection unit, and a self-heating unit of the third embodiment device of the present invention.

FIG. 11 is a connection diagram showing a schematic configuration of a control unit, a temperature detection unit, and a self-heating unit of a fourth embodiment device of the present invention.

FIG. 12 is a connection diagram showing a schematic configuration of a control unit, a temperature detection unit, and a self-heating unit of a fifth embodiment device of the present invention.

FIG. 13 is a block diagram showing a schematic configuration of a conventional device.

[Explanation of symbols]

 101 commercial power supply 102 triac (energization control means) 103 halogen heater (electric heating element) 104 thermistor (temperature detection means) 105 CPU (control means) 106 relay (switching means) 211 thermistor (temperature detection means) 212, 221, 237 resistance (Voltage dividing unit) 213 CPU (Control unit)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location B41J 29/00 29/46 Z 8804-2C G03G 15/00 103 8804-2C B41J 29/00 U

Claims (2)

[Claims] 1. An electric heating element provided in a fixing device for generating heat by applying a commercial power source, a temperature detecting means for detecting a temperature of the electric heating element, and a connection between the electric heating element and the commercial power source. Switching means for switching the electric heating element to the energized or non-energized state, and energization control means for controlling the energization period in one cycle of the AC waveform of the commercial power source to energize or de-energize the electric heating element. In the image forming apparatus including the switching means and the temperature control means for appropriately driving the energization control means based on the signal from the temperature detection means, the temperature control means does not perform a normal printing operation. At this time, when the switching means and the energization control means are alternately driven for a predetermined period such that only one of them switches the electric heating element to the energized state, and It is set so that the temperature of the electric heating element when driven for a predetermined period so as to be in an electric state is detected by the temperature detecting means to determine a failure of the switching means, the energization controlling means, and the temperature detecting means. An image forming apparatus characterized by the above. 2. An electric heating element provided in a fixing device, a temperature detecting means disposed near the electric heating element, the electric resistance of which changes with temperature, and the electric heating element based on the electric resistance value of the temperature detecting means. An image having control means for determining the temperature of the temperature detection means, and voltage dividing means connected to the temperature detection means for inputting to the control means the electric resistance value of the temperature detection means as a predetermined voltage value. In the forming apparatus, the control means is connected to a self-heating means for bypassing the voltage dividing means and applying a predetermined voltage to the temperature control means to cause the temperature detecting means to self-heat. Is set so that a predetermined voltage is applied to the temperature detecting means by the self-heating means, and a failure of the temperature detecting means is judged by a change in the electric resistance of the temperature detecting means input at that time. An image forming apparatus characterized by the above.