JP5008361B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a technology for protecting a heating element provided in a fixing device of an image forming apparatus.

  In general, an electrophotographic image forming apparatus includes a thermal fixing device for fixing an unfixed image (toner image) on a recording material. Thermal fixing devices often employ halogen heaters and ceramic heaters as heating elements (Patent Documents 1 to 4).

The heat fixing device controls the electric power supplied to the heating element based on the temperature of the heating element detected by the thermistor temperature sensing element, and maintains the temperature of the heating element at the target temperature.
JP-A-63-313182 Japanese Patent Laid-Open No. 2-157878 JP-A-4-44075 JP-A-4-204980

  By the way, it is desirable to mount a protection circuit in the image forming apparatus in order to prevent the heating element and other members from being damaged due to excessive temperature rise. As the protection circuit, for example, a thermal fuse or a thermo switch can be considered. However, since these elements require a certain amount of operation time to respond, if the temperature rising rate is too fast, there is a possibility that the heating elements and the like cannot be sufficiently protected.

A protection circuit that cuts off the power supply when the value of the current supplied to the heating element exceeds a predetermined threshold value is also conceivable. However, such a protection circuit, the value of current instantaneously when the energization is stopped falls below the threshold value, immediately energized from being restarted. This cannot suppress an excessive temperature rise of the heating element. It is also conceivable to introduce a latch circuit in such a protection circuit in order to maintain the cut-off state. However, the latch circuit may malfunction due to an overshoot or the like. Furthermore, the latch circuit becomes a factor of cost increase.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus and a protection method capable of sufficiently protecting a heating element as compared with the related art. Other objects will become apparent throughout the specification.

The present invention
A fixing unit for heating and fixing an unfixed image on the recording material to the recording material;
A thermo switch or a thermal fuse that senses an excessive temperature rise of the fixing unit and operates to cut off a power supply circuit to the fixing unit;
A current detection circuit for detecting a current flowing through the fixing unit via the power supply circuit;
The output of the current detection circuit is input, a protection circuit that compares the output of the current detection circuit with a current threshold and shuts off the power supply circuit when the output of the current detection circuit is greater than the current threshold;
In an image forming apparatus having
The protection circuit can switch the current threshold to a first current threshold and a second current threshold lower than the first current threshold, and when the output of the current detection circuit becomes greater than the first current threshold, the power An operation of switching off the supply circuit and switching the current threshold value from the first current threshold value to the second current threshold value, and when the output of the current detection circuit becomes smaller than the second current threshold value by shutting off the power supply circuit, An operation of connecting a supply circuit and switching the current threshold from the second current threshold to the first current threshold is repeated.
The image forming apparatus is characterized in that the thermo switch or the thermal fuse is activated during the repetitive operation of the protection circuit to shut off the power supply circuit .

  In the present invention, for example, when a fixing runaway state occurs in which protection cannot be made in time at the operating speed of the protection means based on temperature, the supply of power to the heating element is limited for a certain time or more. Thereby, since the interruption | blocking time of electricity supply to a heat generating body can be taken long enough, the temperature increase at an abnormal speed | rate can be prevented. Further, it is possible to suppress damage to the heating element and the constituent members of the fixing device.

  An embodiment of the present invention is shown below. Of course, the individual embodiments described below will be helpful in understanding various concepts, such as the superordinate concept, intermediate concept and subordinate concept of the present invention. Further, the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments.

[Embodiment 1]
FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. Here, a laser printer is taken as an example of the image forming apparatus 100.

  The paper feed cassette 101 stores a large number of recording materials S. The recording material S is sometimes referred to as paper, sheet, transfer paper, transfer material, or recording medium. The paper feed roller 102 is driven by a paper feed solenoid (not shown), separates the recording materials S stacked in the paper feed cassette 101 one by one, and feeds them to the transport roller 103. The conveyance roller 103 conveys the recording material S further downstream.

  The registration roller 105 is a type of conveyance roller that conveys the recording material S. In particular, the registration roller 105 is used to adjust the timing of conveying the leading edge of the recording material S to the image forming unit (the transfer roller 106 and the photosensitive drum 107).

  In the image forming unit, the recording material S to which the image of the developer (eg, toner) is transferred is conveyed to the fixing device 108. The fixing device 108 includes a pressure roller 109 for heating and fixing a toner image onto a recording material, a fixing film 110, and a ceramic heater (hereinafter simply referred to as a heater) 111. The fixing unit 110 and the heater 111 constitute a heating unit. The recording material S is pressure-fixed and then discharged to the outside by the paper discharge roller 112.

  2A and 2B are schematic cross-sectional views of the fixing device 108 according to the embodiment. The heater 111 is supported by the film guide 201. A fixing film 110 made of a heat-resistant material having a cylindrical shape is externally fitted to the film guide 201. The excessive temperature rise prevention unit 202 is, for example, a thermostat. The excessive temperature rise prevention unit 202 is in contact with the surface of the insulating substrate or the protective layer of the heater 111. Although illustration is omitted, the temperature detecting element is also in contact with the surface of the heater 111 in the same manner.

  Here, the heating element of the heater 111 may be on the side opposite to the nip portion as shown in FIG. 2A, or may be on the nip portion side as shown in FIG. 2B.

  FIG. 3 is a diagram showing a drive and control circuit for the ceramic heater according to the present embodiment. This embodiment includes two protection circuits that operate as a safety device. When the temperature of the heating element exceeds the temperature threshold, the first protection circuit limits the supply of electric power to the heating element after the time necessary for operation has elapsed. In addition, the second protection circuit limits the supply of power to the heating element for a certain time or more when protection by the first protection circuit is not in time because the heating rate of the heating element is faster than a specific speed. The restriction includes not only completely stopping the supply of power but also reducing the power to the extent that protection can be realized.

  The AC power source 1 is a commercial AC power source connected to the image forming apparatus 100. The current from the AC power source 1 is supplied to the heating elements 3 and 20 of the heater 111 via the AC filter 2, the current transformer 25, and the relay 41. As described above, the primary side of the current transformer 25 is connected to the path from the power source to the heater 111.

  The power supply control to the heating element 3 is achieved by switching between energization (ON) and shut-off (OFF) in the triac 4. Resistors 5 and 6 are bias resistors for the triac 4.

  The phototriac coupler 7 is an element for ensuring a creepage distance between the primary and secondary. The triac 4 is turned on by energizing the light emitting diode provided in the phototriac coupler 7. The resistor 8 is a resistor for limiting the current of the phototriac coupler 7. The transistor 9 functions to turn on / off the phototriac coupler 7. The transistor 9 operates according to the ON1 signal input from the engine controller 11 via the resistor 10.

  The supply control of power to the heating element 20 is achieved by switching between energization and interruption of the triac 13. The resistors 14 and 15 are bias resistors for the triac 13.

  The phototriac coupler 16 is an element for ensuring a creepage distance between the primary and secondary. The triac 13 is switched on by energizing the light-emitting diodes included in the phototriac coupler 16. The resistor 17 is a resistor for limiting the current of the phototriac coupler 16. The transistor 18 functions to turn on / off the phototriac coupler 16. The transistor 18 operates in accordance with the ON2 signal input from the engine controller 11 via the resistor 19.

  The current from the AC power supply 1 is also input to the zero cross detection circuit 12 via the AC filter 2. The zero cross detection circuit 12 notifies the engine controller 11 as a pulse signal that the commercial power supply voltage is equal to or lower than a predetermined threshold value. Hereinafter, this signal sent to the engine controller 11 is referred to as a ZEROX signal. The engine controller 11 detects the edge of the pulse of the ZEROX signal, and turns on or off the triac 4 or 13 according to the detection result.

  The heater current supplied to the heating elements 3 and 20 is converted into a voltage by the current transformer 25. This voltage is input to a current detection circuit 27 that functions as a current detection means via a bleeder resistor 26 connected to the secondary side of the current transformer 25. The current detection circuit 27 converts the waveform of the heater current converted into a voltage into an effective value or a square value thereof, and sends it to the engine controller 11 as an HCRRT signal. Thus, the HCRRT signal represents the effective value of the heater current or its square value.

  The temperature detection element 21 is, for example, a thermistor temperature sensing element that detects the temperature of the heater 111. An insulator is provided between the temperature detection element 21 and the heating elements 3 and 20. An insulation distance is secured between the temperature detection element 21 and the heating elements 3 and 20 by the insulator.

  The temperature detected by the temperature detection element 21 is detected as a partial pressure between the resistor 22 and the temperature detection element 21. This partial pressure is input to the engine controller 11 as a TH signal. The engine controller 11 monitors the temperature of the heater 111 with a TH signal.

  The engine controller 11 calculates the ratio of power to be supplied to the heating elements 3 and 20 based on the measured temperature and the set temperature. Furthermore, the engine controller 11 converts the power ratio into a corresponding phase angle or wave number. Then, the engine controller 11 sends an ON1 signal to the transistor 9 or an ON2 signal to the transistor 18 according to the phase angle or wave number.

  The engine controller 11 calculates the upper limit of the power ratio based on the HCRRT signal notified from the current detection circuit 27, and controls the power supplied to the heater 111 to be equal to or lower than the upper limit power ratio.

  FIG. 4 is a diagram illustrating an example of a table used for phase control according to the embodiment. The engine controller 11 reads a phase angle corresponding to the calculated power ratio by referring to a table stored in a nonvolatile memory or the like, and executes phase control based on the read phase angle.

  By the way, if the power supply control mechanism breaks down while supplying power to the heating elements 3 and 20, the heating elements 3 and 20 may fall into thermal runaway. In order to suppress this, the excessive temperature rise prevention unit 202 described above is disposed on the heater 111. The excessive temperature rise prevention unit 202 can be realized by a thermal fuse or a thermo switch, for example. When the excessive temperature rise prevention unit 202 reaches a predetermined temperature or higher, the thermoswitch is opened, and the power supply to the heating elements 3 and 20 is cut off. That is, the excessive temperature rise prevention unit 202 functions as a first protection circuit.

  The drive circuit for the relay 41 includes resistors 44 and 45 and transistors 43 and 46. The diode 42 is provided to prevent destruction of the element due to the counter electromotive force.

  When the engine controller 11 changes the RLD signal to Low level, the engine controller 11 turns off the transistor 46, turns on the transistor 43, and turns on the relay 41 accordingly. Thereby, the current to the heating elements 3 and 20 is energized.

  On the other hand, the engine controller 11 also monitors whether or not the temperature of the heater 111 has reached a predetermined abnormally high temperature. If the TH signal indicates an abnormally high temperature, the engine controller 11 changes the RLD signal to a high level. As a result, the relay 41 is turned off, and the current to the heating elements 3 and 20 is interrupted.

  The second protection circuit is realized by the relay 41, the transistor 43, the threshold comparison switching circuit 300, and the like. When the detected current value exceeds the first current threshold, the signal output from the threshold comparison switching circuit 300 becomes Low. As a result, the transistor 43 is turned off, and then the relay 41 is turned off. Therefore, the energization to the ceramic heater 111 is cut off. Note that the first current threshold value is less than a current value that may damage the heating element, and is a value that is greater than or equal to a current value necessary for the operation of the image forming apparatus.

  The condition for outputting the Low signal from the threshold comparison switching circuit 300 is that the current detection circuit 27 detects a current (effective value) larger than the first and second current threshold values set in advance. The second current threshold is a value defined such that the first protection circuit operates before the heating element is damaged. Of course, it goes without saying that the first current threshold is larger than the second current threshold.

  The threshold comparison switching circuit 300 outputs a HIGH signal if the detected current value is equal to or less than the second current threshold. As a result, the relay 41 is turned on, and the supply of current to the heating element is resumed. As described above, the threshold comparison switching circuit 300, the relay 41, and the like function as supply restarting means for restarting the supply of power to the heating element.

  The threshold comparison switching circuit 300 sets a comparator that compares the detected current value with a preset current threshold, and sets the current threshold compared by the comparator as the first current threshold or the second current threshold. A setting circuit; When the detected current value exceeds the first current threshold, the setting circuit sets the comparison target current threshold to the second current threshold. Further, the setting circuit sets the current threshold value to be compared to the first current threshold value when a current equal to or lower than the second current threshold value is detected.

  Thus, since this embodiment does not use a latch circuit that may malfunction, a highly reliable device can be provided. Further, the cost can be reduced as compared with a device using a latch circuit.

  FIG. 5A is a schematic cross-sectional view of the heater 111 according to the embodiment. The heater 111 includes an insulating substrate 501, heating elements 3 and 20, and a protective layer 502. The insulating substrate 501 is ceramic-based (SiC, AlN, Al2O3, etc.). The heating elements 3 and 20 are formed on the surface of the insulating substrate 501 by paste printing or the like. The protective layer 502 is glass or the like for protecting the two heating elements 3 and 20.

  FIG. 5B is a schematic plan view of the heater 111 according to the embodiment. The temperature detection element 21 and the excessive temperature rise prevention unit 202 are provided on the protective layer 502. The temperature detection element 21 and the excessive temperature rise prevention unit 202 are disposed at positions that are symmetrical with respect to the center in the length direction of the heating elements 3a and 20a. Further, as can be seen from FIG. 5B, the sheet is disposed closer to the center than the area (small size paper area) through which the recording material of the minimum width that passes through the fixing device 108 passes.

  The heating element 3 includes a heating element 3a that generates heat when power is supplied, electrodes 20c and 3d, and conductive parts 3b and 3b 'that connect the electrode and the heating part. The heating element 20 includes a heating element 20a, electrodes 20c and 20d, and conductive parts 20b and 20b 'that connect the electrodes and the heating part. The electrode 20c is a common electrode and is connected to both the heating elements 3 and 20.

  The HOT-side terminal of the AC power supply 1 is connected to the common electrode 20 c through the excessive temperature rise prevention unit 202. The electrode 20 d is connected to the Neutral terminal of the AC power supply 1 through the triac 4. The electrode 3 d is connected to the Neutral terminal of the AC power supply 1 through the triac 13.

  FIG. 6 is an exemplary circuit diagram of the threshold comparison switching circuit according to the embodiment. In the following, the value of the current supplied to the heating element is converted into a voltage and comparison is performed. This is because the circuit configuration can be simplified by adopting the voltage value.

  The comparator 73 compares the voltage threshold defined by the voltage dividing resistors 71, 72, and 82 with the voltage value (CURRMS) corresponding to the current value from the current detection circuit 27. The voltage threshold defined by the voltage dividing resistors 71 and 72 is the first voltage threshold corresponding to the first current threshold.

  If the detected voltage value CURRMS is greater than the first voltage threshold, the comparator 73 outputs Low. When the output of the comparator 73 becomes low, the transistor 43 for driving the relay 41 is turned off, and the relay 41 is also turned off. Therefore, the power supply to the heater 111 is cut off.

  The resistors 76, 77, 79, and 80 are provided to limit the value of current flowing through the base. The resistor 83 is connected between the base and the reference voltage Vref in order to drive the transistor 43. The diode 75 is provided to prevent malfunction of the transistor 43.

  When the output of the comparator 73 becomes Low, in addition to the power supply to the heater 111 being cut off, the pnp junction transistor 78 is turned on. Thereby, the transistor 81 is turned on. When the transistor 81 is turned on, the voltage threshold value input to the non-inverting input terminal of the comparator 73 is determined by the voltage dividing ratio between the combined resistance value of the resistor 72 and the resistor 82 connected in parallel and the voltage dividing resistor 71. That is, this voltage threshold value becomes the second voltage threshold value. When the resistor 72 and the resistor 82 are connected in parallel, the combined resistance value becomes smaller than that of the resistor 72. Therefore, the second voltage threshold is smaller than the first voltage threshold. As described above, when the output of the comparator 73 becomes Low, the voltage threshold is changed from the first voltage threshold to the second voltage threshold.

  After that, the energization of the heater 111 is stopped, so that the voltage value from the current detection circuit 27 decreases. And when this voltage value becomes below a 2nd voltage threshold value, the output of the comparator 73 will become High. Thereby, the transistor 81 is turned off, and the voltage threshold set in the comparator 73 is switched to the first voltage threshold. Further, since the output of the comparator 73 becomes High, the transistor 43 is turned on, the relay 41 is also turned on, and energization to the heater 111 is resumed.

  As described above, it is possible to switch between the first voltage threshold and the second voltage threshold and to give the voltage threshold a hysteresis characteristic. Compared with the case where hysteresis is not provided, the present embodiment can maintain the energization interruption period of the heater 111 relatively long. That is, since the time until the first protection circuit is activated is long, the second protection circuit restricts the supply of power to the heating element for a certain time or longer when the heating rate of the heating element cannot be accommodated. Accordingly, for example, when the fixing device runs away, the heating rate of the heater 111 can be reduced. Further, by operating the excessive temperature rise prevention unit 202 before not only the heater 111 but also other components of the fixing device are damaged, it becomes easy to protect them.

  FIG. 7 is an exemplary flowchart showing a heating element protection method according to the embodiment. Here, it is assumed that the fixing device 108 is in an uncontrollable state (runaway state).

  In step S701, the first protection circuit such as a thermo switch determines whether or not the temperature of the heater 111 is higher than a predetermined threshold temperature. If the heater 111 falls into a runaway state and energization is started, the temperature of the heater 111 will rise above the threshold temperature. Therefore, if the temperature of the heater 111 is high, the safety mechanism of the first protection circuit operates and the energization to the heater 111 is stopped. On the other hand, if the temperature rising rate is faster than a specific rate, the response speed of the first protection circuit is not in time, and the ceramic heater 111 may be damaged. Therefore, if the temperature of the heater 111 is lower than the predetermined threshold temperature, the process proceeds to step S702.

  In step S702, the comparator 73 determines whether or not the voltage value output from the current detection circuit 27 exceeds the first voltage threshold value. If not, the process returns to step S701.

  If so, the process proceeds to step S703, and the comparator 73 outputs Low. In step S704, the relay 41 is switched off by the second protection circuit, and energization of the heater 111 is stopped. When the energization of the heater 111 is stopped, the output voltage of the current detection circuit 27 starts to decrease. In step S705, the threshold value setting circuit of the second protection circuit switches the voltage threshold value of the comparator 73 from the first voltage threshold value to the second voltage threshold value.

  In step S705, the comparator 73 determines whether or not the output voltage value of the current detection circuit 27 is equal to or lower than the second threshold value. If the voltage value is not less than or equal to the second threshold value, the process returns to step S704, and the state where the energization is stopped is maintained.

  On the other hand, if the voltage value is equal to or lower than the second threshold value, the process advances to step S707, and the comparator 73 outputs High. In step S708, the relay 41, which is a part of the energization control circuit, is turned on by the second protection circuit, and energization to the heater 111 is resumed. In step S709, the threshold value setting circuit of the second protection circuit switches the voltage threshold value of the comparator 73 from the second voltage threshold value to the first voltage threshold value. Thereafter, the process returns to step S701.

  FIG. 8 is a diagram illustrating the relationship among the output of the current detection circuit, the output of the comparator, the threshold value, and the heater temperature according to the embodiment. At t1, the heater 111 enters a runaway state and the output voltage of the current detection circuit 27 rises. When the output voltage value becomes larger than the first voltage threshold at t2, the energization of the heater 111 is cut off. At t2, the voltage threshold value of the comparator 73 is set to the second voltage threshold value. Since the energization of the heater 111 is stopped from t2 to t4, the output voltage of the current detection circuit 27 gradually decreases.

  Finally, when the output voltage value of the current detection circuit 27 becomes equal to or lower than the second voltage threshold at t4, the voltage threshold of the comparator 73 is set to the first voltage threshold. Thereby, the energization to the heater 111 is also resumed. Further, it has been experimentally known that the temperature curve of the heater 111 is faster in the energized state than in the disconnected state. Therefore, the temperature of the heater 111 gradually increases while the energization and interruption of the heater 111 are repeated. Finally, when the temperature of the heater 111 exceeds the operating temperature of the first protection circuit, the energization of the heater 111 is completely cut off.

  According to the present embodiment, at the operation speed of the first protection circuit based on temperature, when a fixing runaway state occurs in which protection of the heating element or the like is not in time, supply of power to the heating element is limited for a certain time or more. A second protection circuit is provided. Thereby, since the interruption | blocking period of the electricity supply to a heat generating body can be taken long, the temperature rise at an abnormal speed | rate can be prevented. Further, it is possible to suppress damage to the heating element and the constituent members of the fixing device.

  In particular, in the second protection circuit, the current detection circuit 27 detects the value of the current supplied to the heating element. Further, when the detected current value exceeds the first current threshold, the comparator 73 and the relay 41 limit the supply of power to the heating element. When the detected current value is equal to or smaller than the second current threshold value that is smaller than the first current threshold value, the comparator 73 and the relay 41 restart the supply of power to the heating element.

  In this embodiment, since a latch circuit with a possibility of malfunction is not employed, a relatively reliable protection circuit can be provided and cost can be reduced.

  Further, the comparator 73 compares the detected current value with the current threshold value, and switches the current threshold value between the first and second current threshold values. It is possible to realize a second protection circuit that restricts the supply of power for a certain time or more.

[Embodiment 2]
FIG. 9 is an exemplary circuit diagram of another threshold comparison switching circuit according to the embodiment. Another example of realization of the second protection circuit will be described with reference to FIG.

  The comparator 903 compares the first voltage threshold defined by the voltage dividing resistors 901 and 902 with the voltage value (CURRMS) corresponding to the current value from the current detection circuit 27. If the detected voltage value CURRMS is greater than the first voltage threshold, the comparator 903 outputs Low. The pull-up resistor 905 is a resistor for determining the output voltage value of the comparator 903.

  The comparator 908 compares the second voltage threshold defined by the voltage dividing resistors 906 and 907 with the voltage value (CURRMS) corresponding to the current value from the current detection circuit 27. If the voltage value CURRMS is less than or equal to the second voltage threshold, the comparator 908 outputs Low. Each resistance value is set so that the first voltage threshold is larger than the second voltage threshold.

  Resistors 910 and 911 are resistors for determining the output voltage value of the comparator 908. Here, when both the comparator 903 and the comparator 908 are High, the values of the resistors 910 and 911 are set so that the output voltage of the comparator 903 is higher than the output voltage of the comparator 908.

  The comparator 912 is a comparator that compares the output voltages of the comparator 903 and the comparator 908. The output of the comparator 903 when the output of the comparator 912 becomes Low is determined by the voltage dividing resistance values of the resistor 905 and the resistor 913. Each resistance value is set so that the voltage value input from the comparator 903 when the comparator 912 becomes Low is smaller than the voltage value input from the comparator 908.

  FIG. 10 is a diagram illustrating a relationship among the output of the current detection circuit, the output of each comparator, and the temperature of the heater according to the embodiment. The circuit operation will be described with reference to FIG.

  When current starts to flow through the heater 111 and the output voltage of the current detection circuit 27 exceeds the second voltage threshold at t10, the comparator 908 switches from Low to High. At this stage, the output of the comparator 903 is still high. As described above, the high level of the comparator 903 is set higher than the high level of the comparator 908. For this reason, the output of the comparator 912 becomes High, and the energization state of the heater 111 is maintained.

  When the output voltage of the current detection circuit 27 exceeds the first voltage threshold at t11, the output of the comparator 903 is switched to Low. At this time, since the output voltage from the comparator 908 becomes larger than the output voltage from the comparator 903, the output of the comparator 912 becomes Low, and the energization to the heater 111 is cut off.

  The output of the current detection circuit 27 has a certain time constant. Therefore, even when the energization is stopped, the output of the current detection circuit 27 does not decrease immediately. When a certain period of time elapses from t11 and becomes t12, the output of the current detection circuit 27 falls below the first voltage threshold. Therefore, at t12, the output of the comparator 903 becomes High again. However, at this time, since the output of the comparator 912 remains low, the value of the output voltage of the comparator 903 becomes a value determined by the voltage dividing resistance values of the resistors 905 and 913. As described above, the output voltage value at this time is set to be lower than the output voltage value of the comparator 908 determined from the voltage dividing resistance values of the resistors 910 and 911. Therefore, the output of the comparator 912 is maintained in the Low state, and the state where the energization to the heater 111 is cut off is maintained.

  The output of the current detection circuit 27 further decreases and falls below the second voltage threshold at t13. At this time, the output of the comparator 908 becomes Low. As a result, the output of the comparator 912 becomes High, and energization of the heater 111 is started again. Thereafter, at t15, the output of the current detection circuit 27 again exceeds the first voltage threshold value, and the above control cycle is repeated.

  Even if such a configuration is adopted, the heating element can be suitably protected as in the first embodiment.

  FIG. 11 is an exemplary flowchart showing another method for protecting the heating element according to the embodiment. In step S1101, the first protection circuit such as a thermo switch determines whether the temperature of the heater 111 is higher than a predetermined threshold temperature. If it is higher, the safety mechanism of the first protection circuit operates, and energization of the heater 111 is stopped. On the other hand, if it is lower, the process proceeds to step S1102.

  In step S1102, the comparator 903 determines whether the output voltage value of the current detection circuit 27 is greater than the first voltage threshold value. If not, the process returns to step S1101 to continue energization. On the other hand, if larger, the process proceeds to step S1103. In step S1103, the comparator 908 output is greater than the comparator 903 output. As a result, in step S1104, the comparator 912 outputs Low. In step S1105, the transistor 43 is turned off, the relay 41 is also turned off, and energization to the heater 111 is stopped. When the energization to the heater 111 is stopped, the output of the current detection circuit 27 starts to decrease.

  In step S1106, the comparator 908 determines whether or not the output voltage of the current detection circuit 27 is equal to or lower than the second voltage threshold value. If not, the process returns to step S1103 to continue energization. On the other hand, if it is below, the process proceeds to step S1107, and the output of the comparator 903 becomes larger than the output of the comparator 908.

  In step S1108, the comparator 912 outputs High. In step S1109, the transistor 43 is turned on, the relay 41 is also turned on, and energization of the heater 111 is resumed. After that, the process proceeds to step S1101.

  According to the present embodiment, the first comparator 903 outputs Low when the voltage value corresponding to the current value detected by the current detection circuit 27 exceeds the first voltage threshold. On the other hand, the first comparator outputs High when the voltage value is equal to or lower than the first voltage threshold value. The second comparator 908 outputs High when the voltage value corresponding to the current value detected by the current detection circuit 27 exceeds the second voltage threshold. The second comparator outputs Low when the voltage value is equal to or lower than the second voltage threshold.

  Further, the third comparator 912 includes a first terminal to which an output from the first comparator and an output from the third comparator are input, and a second terminal to which an output from the second comparator is input. Further, the third comparator switches the output of the third comparator to Low when the first comparator becomes High. However, even if the first comparator switches to Low again, the third comparator continues to output Low until the second comparator switches to Low. Thereby, the electric current to a heat generating body is interrupted | blocked. Therefore, the second embodiment also has the same effect as the first embodiment.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. It is a schematic sectional drawing of the fixing device which concerns on embodiment. It is a schematic sectional drawing of the fixing device which concerns on embodiment. It is a figure which shows the drive and control circuit of the ceramic heater which concerns on this embodiment. It is a figure which shows an example of the table used for the phase control which concerns on embodiment. It is a schematic sectional drawing of the ceramic heater 111 which concerns on embodiment. It is a schematic plan view of the ceramic heater 111 which concerns on embodiment. It is an exemplary circuit diagram of a threshold value comparison switching circuit according to the embodiment. 5 is an exemplary flowchart illustrating a method for protecting a heating element according to an embodiment. It is the figure which illustrated the relationship between the output of the current detection circuit which concerns on embodiment, the output of a comparator, a threshold value, and the temperature of a heater. It is an exemplary circuit diagram of another threshold value comparison switching circuit according to the embodiment. It is a figure which shows the relationship between the output of the current detection circuit which concerns on embodiment, the output of each comparator, and the temperature of a heater. It is an exemplary flowchart which shows the other protection method of the heat generating body which concerns on embodiment.

Claims (2)

  A fixing unit for heating and fixing an unfixed image on the recording material to the recording material;
  A thermo switch or a thermal fuse that senses an excessive temperature rise of the fixing unit and operates to cut off a power supply circuit to the fixing unit;
  A current detection circuit for detecting a current flowing through the fixing unit via the power supply circuit;
  The output of the current detection circuit is input, a protection circuit that compares the output of the current detection circuit with a current threshold and shuts off the power supply circuit when the output of the current detection circuit is greater than the current threshold;
In an image forming apparatus having
  The protection circuit can switch the current threshold to a first current threshold and a second current threshold lower than the first current threshold, and when the output of the current detection circuit becomes greater than the first current threshold, the power An operation of switching off the supply circuit and switching the current threshold value from the first current threshold value to the second current threshold value, and when the output of the current detection circuit becomes smaller than the second current threshold value by shutting off the power supply circuit An operation of connecting a supply circuit and switching the current threshold from the second current threshold to the first current threshold is repeated.
  The image forming apparatus, wherein the thermo switch or the thermal fuse is activated during the repetitive operation of the protection circuit to shut off the power supply circuit.   The image forming apparatus according to claim 1, wherein the fixing unit includes a cylindrical film and a heater that is in contact with an inner surface of the film and is supplied with power through the power supply circuit.

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