JP2009042595A - Temperature control device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a temperature control device with an inexpensive second safety circuit which detects the abnormal state of a CPU and keeps a fixing heater turned off in addition to a safety circuit by means of hardware without a CPU. <P>SOLUTION: The temperature control device 9 has: a temperature detecting section 23 which detects a fixing temperature; the CPU 41 which outputs a heater control signal HC; and the safety circuit 304 which forcibly stops supply of power to the fixing heater 21. The temperature control device 9 includes a differentiation circuit46 which differentiates the pulse signal PO of a predetermined period TO output from the CPU 41; a timer circuit 49 which outputs a forcible stop signal FS when a predetermined time T1 elapses from the application of a power source voltage Vin; a gate circuit which turns off the fixing heater 21 by the output signal of the timer circuit 49 or the heater control signal HC. The output voltage of the differentiating circuit 46 is supplied to the timer circuit 49 as a power source voltage Vin. One period TO of the pulse signal is set shorter than the predetermined time T1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a temperature control device incorporated in an image forming apparatus having a thermal fixing device such as a copying machine or a printer, and relates to a temperature detection unit that detects a fixing temperature, and the fixing temperature detected by the temperature detection unit is a target temperature. A temperature that includes a CPU that outputs a heater control signal for driving and controlling the fixing heater, and a safety circuit that forcibly stops power supply to the fixing heater when the fixing temperature reaches an abnormal overheat temperature. The present invention relates to a control device.

  An image forming apparatus adopting an electrophotographic system includes a fixing heater that is driven and controlled by a heater control signal output from a control unit having a CPU, and a fixing heater in order to melt and fix the toner image on a sheet. A fixing device including an internal fixing roller and a pressure roller pressed against the fixing roller is incorporated.

  The fixing roller is controlled to a predetermined target temperature by a fixing heater that is energized and controlled by the control unit. However, if the energization state of the fixing heater is maintained due to an abnormality of the control unit, the fixing roller is overheated to an abnormally high temperature. Since there is a possibility of causing damage to the apparatus, conventionally, a technique for avoiding an abnormal overheating state by a hardware circuit not including a control unit has been proposed.

  For example, there is a safety circuit that connects a thermo switch for monitoring the fixing temperature in series with the energization line to the fixing heater, and forcibly cuts off the power supply to the fixing heater when an abnormal overheat temperature is detected by the thermo switch. Yes.

  However, when the above-described safety circuit is employed, since the thermoswitch is closed and the fixing heater is energized again when the fixing temperature becomes equal to or lower than the abnormal overheating temperature, the fixing temperature is abnormally overheated as shown in FIG. There is a possibility that the fixing roller, the pressure roller, and the devices and resin members installed around the fixing device may be damaged by high heat, or may be distorted or deformed.

  Therefore, in Patent Document 1, the temperature of the fixing heater is monitored, the energization control of the fixing heater and the power on / off control of the power system are performed, and when an abnormality in the fixing heater temperature is detected when the fixing heater is energized, the fixing heater is detected. A single MPU that prohibits energization of the power supply and a power system power supply that is turned off, and an excessive temperature rise that detects an excessive increase in the fixing heater temperature and forcibly turns off the power system power according to the temperature overheating condition Even if the temperature monitoring means of the MPU does not operate normally even if the temperature of the fixing heater rises due to abnormality of the fixing heater control means, etc., the power system is detected by the overtemperature detection / control means. An image forming apparatus characterized by forcibly turning off a power supply has been proposed.

  Specifically, the overtemperature detection / control means includes a watchdog timer circuit that operates when a predetermined time elapses when the temperature of the fixing heater detected by the thermistor is abnormally high, a D-type flip-flop that latches the output signal, and a flip-flop And a relay circuit that cuts off the power supply line to the fixing heater in response to the output signal.

  However, in the above-described over-temperature detection / control means, the flip-flop is reset by the reset signal output from the MPU after the abnormality is detected, so that if an abnormality occurs in the MPU and an incorrect reset signal is output, the fixing is performed. There was a possibility that energization to the heater could not be prevented. Further, in order to cope with a malfunction of the flip-flop due to noise or the like, it is necessary to determine whether or not the malfunction occurs and to output a reset signal, so that there is a problem that the control load on the MPU increases.

Japanese Patent Laid-Open No. 10-307514

  In view of the above-described problems, the object of the present invention is to provide an inexpensive second safety circuit that detects an abnormality of the CPU and maintains the fixing heater in an OFF state in addition to a hardware safety circuit that does not involve a CPU. It is in providing a temperature control device.

  In order to achieve the above-mentioned object, a first characteristic configuration of the temperature control device according to the present invention includes a temperature detection unit for detecting a fixing temperature and the temperature detection as described in claim 1 of the document of the claims. A CPU that outputs a heater control signal for driving and controlling the fixing heater so that the fixing temperature detected by the unit reaches a target temperature, and forcibly stops power supply to the fixing heater when the fixing temperature reaches an abnormal overheat temperature A temperature control device configured to include a safety circuit for differentiating a pulse signal having a predetermined cycle output from the CPU, and a heater control signal when a predetermined time elapses after the power supply voltage is applied. Regardless of the above, there is provided a timer circuit for outputting a forced stop signal, and a gate circuit for turning off the fixing heater by an output signal of the timer circuit or the heater control signal. The output voltage of the road is supplied to the timer circuit as the power supply voltage lies in that one period of the pulse signal is shorter than the predetermined time.

  The pulse signal output from the CPU at a predetermined cycle is differentiated by a differentiation circuit, and the output voltage of the differentiation circuit is input to the timer circuit as a power supply voltage. The output voltage rises sharply in the positive direction from the predetermined voltage value at the rising edge of the pulse signal, then changes to a spike shape that decreases to the predetermined voltage value, and sharply decreases in the negative direction from the predetermined voltage value at the falling edge. After that, it changes in a spike shape rising to a predetermined voltage value. Since the power supply to the timer circuit is periodically interrupted at the timing when the power supply voltage changes in a spike shape in the negative direction, the timer circuit is periodically reset.

  Here, the predetermined cycle of the pulse signal output from the CPU is set to be shorter than a predetermined time until the forced stop signal is output from the timer circuit after the power supply voltage is applied. Therefore, when the CPU is operating normally, the timer circuit is reset before outputting the forced stop signal, so that the forced stop signal is not output from the timer circuit.

  On the other hand, if an abnormality such as CPU runaway or output port latch-up occurs, the pulse signal is fixed at high level or low level, and a constant output voltage is applied from the differentiation circuit to the timer circuit as the power supply voltage. When a predetermined time elapses after such a power supply voltage is applied, a forced stop signal is output from the timer circuit, and the energization to the fixing heater is forcibly turned off by the gate circuit.

  Therefore, according to the above-described configuration, inconvenience that the temperature of the fixing roller is maintained before and after the abnormal overheat temperature detected by the safety circuit in the case of an abnormality such as CPU runaway. Such a circuit has a small number of parts and can be constructed very inexpensively.

  In addition to the first feature configuration described above, the second feature configuration includes an integration circuit that operates when the power supply voltage is applied, and the integration circuit, in addition to the first feature configuration described above. This is because it is composed of a logic circuit whose output level is inverted when the predetermined time elapses based on the output signal.

  When the CPU is operating normally, the integration circuit is supplied with a power supply voltage that varies in a spiked manner in the negative direction every predetermined period, so that the output voltage of the integration circuit decreases every logical period, The output level of the circuit does not rise to a voltage value that reverses.

  On the other hand, a power supply voltage of a predetermined voltage value is continuously applied to the integration circuit when an abnormality such as a runaway of the CPU occurs. Therefore, when a predetermined time elapses after the power supply voltage is applied, the output voltage of the integration circuit is The output level rises to a voltage value that inverts, and a signal with the output level inverted is output from the logic circuit.

  The integration circuit can be constructed at a low cost by combining a few components such as a resistor and a capacitor, for example, so that an inexpensive temperature control device can be realized.

  In the third feature configuration, as described in claim 3, in addition to the first feature configuration described above, the output level of the timer circuit is inverted when a predetermined time elapses after the power supply voltage is applied. This is because it is composed of a reset IC.

  The reset IC is configured to invert the level and output a signal when a predetermined time elapses after the power supply voltage is applied.

  When the CPU is operating normally, the reset IC is supplied with a power supply voltage that changes in a spiked manner in the negative direction every predetermined period. Therefore, since the reset IC is reset by cutting off the power supply at every predetermined period, a signal whose level is inverted is not output from the reset IC. On the other hand, a power supply voltage having a predetermined voltage value is continuously applied to the reset IC when the CPU is out of control or the like, so that a signal whose level is inverted is output from the reset IC when a predetermined time elapses after the power supply voltage is applied. The

  According to the above configuration, an inexpensive temperature control device can be realized by using an inexpensive reset IC that is mass-produced.

  As described above, according to the present invention, in addition to a hardware safety circuit that does not involve a CPU, a temperature provided with an inexpensive second safety circuit that detects an abnormality of the CPU and maintains an OFF state of the fixing heater. A control device can be provided.

  Hereinafter, a color printer which is an example of an image forming apparatus including the temperature control device of the present invention will be described.

  As shown in FIG. 2, a color printer 100 that employs an electrophotographic method is input from an operation unit 110 on which operation keys for inputting a liquid crystal screen, printing conditions, and the like are arranged, and a personal computer or the like via a network or the like. The image forming unit 120 forms a toner image based on the received image data and the sheet conveyed from either the standard cassette 141 or the two-stage option cassettes 142 and 143 constituting the sheet storage unit 140. Functional blocks such as a transfer unit 130 that transfers the formed toner image, a fixing unit 200 that melts and fixes the toner image on the paper, and a control unit 400 that controls these functional blocks and executes a predetermined image forming process. And a power supply unit 300 that supplies necessary power to the control unit 400 and the like.

  The image forming unit 120 forms an electrostatic latent image by exposing the surface of the photoconductor by scanning a laser beam based on the input image data, and a charging device that uniformly charges the surface of the photoconductor. A photoconductor unit including an exposure device and a developing device that visualizes the formed electrostatic latent image as a toner image is provided for each of the M, C, Y, and K toner colors.

  The transfer unit 130 includes an intermediate transfer belt 132 that superimposes and supports toner images formed by the photosensitive units of the image forming unit 120, a support roller 131 that rotatably supports the intermediate transfer belt 132, and an intermediate transfer belt 132. The image forming apparatus includes a transfer roller 133 that transfers a carried toner image onto a sheet conveyed from the sheet storage unit 140, a blade 134 that removes residual toner on the intermediate transfer belt 132, and the like.

  As shown in FIG. 3, the fixing unit 200 includes a fixing roller 20 in which a fixing heater 21 is housed, and a pressure roller 22 pressed against the fixing roller 20, and serves as a temperature detection unit that detects a fixing temperature on the fixing roller 20. Thermistors 23 are arranged in contact with each other.

  As shown in FIG. 1, the thermistor 23 is connected in series with the resistor R 24, and the divided voltage value of the DC voltage Vdc by the thermistor 23 and the resistor R 24 is input to the control unit 400. In the present embodiment, an NTC thermistor is used as the thermistor 23, and the partial pressure value increases as the temperature increases.

  The power supply unit 300 includes a noise filter 301 provided on the input side of a commercial power supply line, a relay switch 302 as a power switch, and steps down an input AC voltage, and AC / DC converts the stepped-down AC voltage into a DC voltage Vdc. Thus, a power supply circuit 303 that supplies the control unit 400 is provided. A power supply line for supplying power to the fixing heater 21 of the fixing unit 200 is branched immediately after the relay switch 302.

  As shown in FIG. 4, the power supply circuit 303 has a power transformer T30 that receives an AC voltage of AC100V input from a commercial power supply on the primary side and outputs a stepped-down AC voltage AC26V and an AC voltage AC12V from the secondary side, A first DC voltage output unit 31 that AC / DC converts AC voltage AC26V and outputs DC voltage DC24V, and a second DC voltage output unit that AC / DC converts AC voltage AC12V and outputs DC voltage DC3.3V 35 etc. are arranged on the substrate.

  The first DC voltage output unit 31 includes a rectifier 32 configured by a diode bridge, a smoothing capacitor C33, a DC regulator 34, and the like. The AC voltage AC26V is full-wave rectified by the rectifier 32 and smoothed by the smoothing capacitor C33. The DC regulator 34 performs DC / DC conversion and outputs a DC voltage DC24V.

  The second DC voltage output unit 35 includes a rectifier 36 configured by a diode bridge, a smoothing capacitor C37, a DC regulator 38, and the like. The AC voltage AC12V is full-wave rectified by the rectifier 36 and smoothed by the smoothing capacitor C37. The DC regulator 38 performs DC / DC conversion and outputs a DC voltage DC 3.3V.

  In this embodiment, the DC voltage Vdc supplied to the control unit 400 is set to the DC voltage DC3.3V output from the second DC voltage output unit 35, but is limited to such a value. Instead, it can be set as appropriate according to the operating voltage of the controller 400. For example, the DC voltage Vdc may be set to the DC voltage DC5V.

  As shown in FIG. 1, the control unit 400 includes a microcomputer 40 mounted on a substrate, peripheral circuits, and the like, and is supplied with a DC voltage Vdc from a power supply circuit 303. The microcomputer 40 incorporates a CPU 41, a ROM storing a control program, a RAM used as a work area for the CPU 41, an input / output circuit, and the like. The microcomputer 40 controls each functional block based on a control program executed by the CPU 41 and executes a predetermined image forming process. For example, the microcomputer 40 detects the fixing temperature based on the divided voltage value of the DC voltage Vdc generated by the thermistor 23 and the resistor R24.

  As shown in FIG. 1, the temperature control device 9 according to the present invention for controlling the fixing temperature of the fixing unit 200 includes a thermistor 23 for detecting the fixing temperature, and fixing so that the fixing temperature detected by the thermistor 23 becomes a target temperature. A CPU 41 that outputs a heater control signal for driving and controlling the heater 21 and a thermo switch 304 as a safety circuit that forcibly stops power supply to the fixing heater 21 when the fixing temperature reaches an abnormal overheat temperature are provided.

  When the color printer 100 is turned on and the control program is executed by the CPU 41, the microcomputer 40 performs processing such as initialization of the RAM and setting of the input / output port, and then the fixing temperature becomes the target temperature. Further, the fixing temperature is detected based on the divided value of the DC voltage Vdc by the thermistor 23 and the resistor R24, and as shown in FIG. 1, a heater control signal HC for driving and controlling the fixing heater 21 is generated. Further, as shown in FIG. 5, the microcomputer 40 outputs a pulse signal P0 at a predetermined period T0.

  As shown in FIG. 1, the output port from which the microcomputer 40 outputs the heater control signal HC is connected to the base of the transistor Q42, and the collector of the transistor Q42 and the remote terminal of the triac 305 are connected. Therefore, when the heater control signal HC is input to the base of the transistor Q42, the triac 305 is turned on or off.

  As shown in FIG. 6, when the fixing temperature is lower than the target temperature and the fixing heater 21 is turned on, the microcomputer 40 outputs a low level heater control signal HC to increase the fixing temperature, and the fixing temperature is higher than the target temperature. When the fixing heater 21 is turned off, a high level heater control signal HC is output to lower the fixing temperature.

  If an abnormality such as CPU runaway due to noise or the like occurs and the output port is latched up and the heater control signal HC is latched at a low level, power supply to the fixing heater 21 is continued and the fixing temperature rises. When the temperature reaches the abnormal overheat temperature, the thermo switch 304 is opened, the power supply to the fixing heater 21 is forcibly stopped, and the fixing temperature is prevented from rising. However, when the power supply to the fixing heater 21 is stopped and the fixing temperature falls below the abnormal overheating temperature, the thermo switch 304 is closed, the power supply to the fixing heater 21 is resumed, and the fixing temperature rises. Therefore, the fixing temperature is maintained before and after the abnormal overheating temperature.

  If the fixing temperature is maintained around the abnormal overheating temperature, the fixing roller 20 and the pressure roller 22 that constitute the fixing unit 200, and the devices and resin members installed around the fixing unit 200 are damaged by high heat. Or there is a possibility of causing distortion and deformation.

  Therefore, in order to prevent the fixing temperature from being maintained around the abnormal overheating temperature, as shown in FIG. 1, the temperature control device 9 differentiates the pulse signal P0 having a predetermined period T0 output from the microcomputer 40. The circuit 46, the timer circuit 49 that outputs a forced stop signal FS regardless of the heater control signal HC when a predetermined time elapses after the power supply voltage Vin is applied, and the fixing heater 21 by the output signal of the timer circuit 49 or the heater control signal HC. A gate circuit is provided for turning off.

  As shown in FIG. 1, the differentiating circuit 46 includes a resistor R47 and a capacitor C47. The capacitor C47 is connected to the connection node Nod1 of the transistor Q45 and the resistor R46, and the connection node Nod2 of the resistor R47 and the capacitor C47 is connected to the power supply input terminal of the timer circuit 49.

  As shown in FIG. 1 and FIG. 5, the differentiating circuit 46 differentiates the pulse signal P1 output from the collector of the transistor Q45 that inverts the output level of the pulse signal P0, and uses the output voltage as the power supply voltage Vin. Configured to supply. Since the pulse signal P1 is AC-coupled by the capacitor C47, a voltage obtained by superimposing the differential voltage of the pulse signal P1 and the DC voltage Vdc becomes the output voltage of the differentiation circuit 46.

  The output voltage of the differentiating circuit 46 is clamped by the diode D48 connected to the connection node Nod3 of the differentiating circuit 46 and the timer circuit 49, and is limited to a voltage value Vh that is an allowable voltage value that can be applied to the timer circuit 49. Therefore, the timer circuit 49 is not damaged by the application of the power supply voltage Vin that is the output voltage of the differentiating circuit 46.

  As shown in FIGS. 5 and 7, the timer circuit 49 operates when the power supply voltage Vin is applied, and the output level is inverted when a predetermined time T <b> 1 elapses based on the output signal of the integration circuit 50. The logic circuit 54 is used. One cycle T0 of the pulse signal P0 is set shorter than the predetermined time T1.

  The integrating circuit 50 includes a resistor R51 and a capacitor C52 connected in series. When the power supply voltage Vin is applied, the capacitor C52 is charged based on a time constant determined by the resistance value of the resistor R51 and the capacitance of the capacitor C52. When the power supply voltage Vin changes in a spike shape in the negative direction, the charge charged in the capacitor C52 is discharged through the diode D53.

  The logic circuit 54 is composed of a drive circuit having hysteresis characteristics. The input terminal of the drive circuit is connected to the connection node Nod4 of the resistor R51 and the capacitor C52 constituting the integrating circuit 50, and the output terminal is connected to the base of the transistor Q44.

  The voltage of the capacitor C52 charged by the power supply voltage Vin is input to the logic circuit 54. When the voltage exceeds the threshold voltage Vth, the logic circuit 54 inverts the level of the output signal and forcibly supplies power to the fixing heater 21. A high level signal is output as the forced stop signal FS to be stopped. The output signal of the logic circuit 54 is the output signal of the timer circuit 49.

  The collectors of the transistor Q42 and the transistor Q44 are connected to each other to form a wired OR circuit.

  When a low level signal is input from the timer circuit 49 to the transistor Q44, the triac 305 is turned on or off based on the output signal level of the transistor Q42 driven based on the heater control signal HC.

  When a forced stop signal, which is a high level signal, is input from the timer circuit 49 to the transistor Q44, the triac 305 is forcibly turned off even if the heater control signal HC output from the CPU 41 is at a low level, and the fixing heater Power supply to 21 is stopped. That is, the wired OR circuit composed of the transistor Q42 and the transistor Q44 serves as a gate circuit for turning off the fixing heater 21 by the output signal of the timer circuit 49 or the heater control signal HC.

  As shown in FIG. 5, the pulse signal P0 output from the microcomputer 40 for a predetermined period T0 is inverted in output level by the transistor Q45 and input to the differentiation circuit 46 as the pulse signal P1. The pulse signal P1 is differentiated by the differentiation circuit 46, and the output voltage of the differentiation circuit 46 is clamped by the diode D48 and applied to the timer circuit 49 as the power supply voltage Vin.

  The power supply voltage Vin changes in a negative direction every predetermined cycle T0, and the capacitor C52 of the timer circuit 49 is discharged every predetermined cycle T0. Therefore, a forced stop signal FS is output from the logic circuit 54 of the timer circuit 49. Never happen.

  However, if abnormalities such as runaway occur in the CPU 41 due to abnormal noise or the like, the pulse signal P0 output from the microcomputer 40 is maintained at a high level or a low level as shown in FIG. The pulse signal P1 output from Q45 is also maintained at a high level or a low level. Therefore, the differential voltage of the pulse signal P1 by the differentiating circuit 46 becomes 0V, and the output voltage maintained at the DC voltage Vdc is applied from the differentiating circuit 46 to the timer circuit 46 as the power supply voltage Vin.

  The capacitor C52 is charged without being discharged by the power supply voltage Vin maintained at the DC voltage Vdc. When a predetermined time T1 elapses after the power supply voltage Vin is applied, the voltage of the capacitor C52 exceeds the threshold voltage Vth, and the timer circuit 49 From this, a forced stop signal of the fixing heater 21 is output.

  Therefore, as shown in FIG. 6, in the event of an abnormality due to runaway of the CPU 41 or the like, the power supply to the fixing heater 21 is forcibly stopped by the forced stop signal FS output from the timer circuit 49. It will not rise to or be maintained around abnormal overheating temperatures.

  As described above, inconvenience that the temperature of the fixing roller 21 is maintained before and after the abnormal overheating temperature detected by the thermo switch 304 when the CPU 41 is out of control or the like. Such a circuit has a small number of parts and can be constructed very inexpensively.

  Another embodiment will be described below.

  In the above-described embodiment, the timer circuit 49 includes the integration circuit 50 that operates when the power supply voltage Vin is applied, and the logic circuit 54 that reverses the output level after a predetermined time T1 has elapsed based on the output signal of the integration circuit 50. Although configured, it may be configured by a reset IC that is mass-produced and sold at low cost.

  For example, as shown in FIG. 8, a direct current power supply 60 to which a power supply voltage Vin is applied, a reference voltage generation unit 61 that receives a direct current output from the direct current power supply 60 and generates a reference voltage Vref, and a reference The reference voltage Vref of the voltage generator 61 is input to the non-inverting input terminal, and the divided value of the power supply voltage Vin by the resistors R62, R63, and R64 is input to the inverting input terminal, and the output signal level of the comparator 65 is Based on this, the resistor R64 is short-circuited to change the divided voltage value so that the output signal level of the comparator 65 has hysteresis characteristics, the delay circuit 67 that delays the output signal of the comparator 65, and the output signal of the delay circuit 67 A transistor Q68 for outputting a signal based on the power supply voltage Vin, and a predetermined time elapses after the power supply voltage Vin is applied. And S-809xxC Shirizu reset IC6, a product of Seiko Instruments Inc., which are configured to invert the output level is suitable as a timer circuit 49 in the present invention. However, it goes without saying that the reset IC 6 is not limited to this product. The delay time of the delay circuit 67 can be adjusted by increasing or decreasing the capacity of the external capacitor C69 connected to the reset IC 6.

  In the above-described embodiment, it has been described that the differentiation circuit 46 differentiates the pulse signal P0 output from the CPU 41 and supplies it to the timer circuit 49 as the power supply voltage Vin, but the pulse signal P0 It is not necessary to output the power supply voltage Vin supplied to the CPU 49 from the CPU 41, but a predetermined time T1 from when the power supply voltage Vin is applied until the forced stop signal FS is output from the timer circuit 49. Any signal may be used as long as it is output with a shorter period.

  For example, a clock signal output from the CPU 41 to a motor that rotationally drives the photosensitive member of the image forming unit 120 can be used as the pulse signal. In addition, in order to communicate with any one of the slave CPUs provided in each of the two-stage option cassettes 142 and 143 constituting the paper storage unit 140, a polling select signal output from the CPU 41 with a level switched at a predetermined cycle is output. May be used. In this case, the clock signal is not only a clock signal supplied to the motor, and the polling select signal is not only a signal designating the slave CPU of the option cassette 142 or 143 to be communicated, It is also used as a pulse signal for generating the power supply voltage Vin, so that the output port of the microcomputer 40 that is limited in number can be saved or used for other purposes.

  In the above-described embodiment, the temperature control device 9 is composed of a wired OR circuit in which the gate circuit that turns off the fixing heater 21 by the output signal of the timer circuit 49 or the heater control signal HC is composed of the transistor Q42 and the transistor Q44. However, the present invention is not limited to this.

  For example, as shown in FIG. 9A, a three-state buffer 7 to which a heater control signal is input and a transistor Q42 that inverts the output signal level of the three-state buffer 7 to output a signal constitute a gate circuit. When a high level signal is input from 49, the output of the three-state buffer 7 becomes high impedance regardless of the signal level of the heater control signal HC, and the transistor Q42 is turned on and the triac 305 is shut off. As shown in FIG. 9B, a gate circuit is formed by the NOR circuit 8 to which the heater control signal HC and the output signal from the timer circuit 49 are input. When the signal is input, the low level is output from the NOR circuit 8 regardless of the heater control signal HC. No. is output may be one configured to triac 305 is cut off.

  In the above-described embodiment, the temperature control device 9 has been described as including the thermo switch 304 as a safety circuit. However, the present invention is not limited to this, and the safety circuit can be used when the fixing temperature reaches an abnormal overheat temperature. What is necessary is just to forcibly stop the power supply to 21. For example, a thermistor 21, a fixing temperature detected by the thermistor 21 and an abnormal overheating temperature are input, and a high level or low level signal is output when the fixing temperature becomes equal to or higher than the abnormal overheating temperature. Based on this, a relay switch that forcibly stops power supply to the fixing heater 21 may be used.

  In the above-described embodiment, the NTC thermistor is used as the temperature detection unit. However, a PTC thermistor may be used. In this case, the logic level of the gate circuit, the reference voltage value input to the comparator, etc. may be appropriately adjusted according to the characteristics of the temperature detection unit.

  In the above embodiment, the case where the temperature control device 9 of the present invention is applied to the color printer 100 has been described. However, the temperature control device 9 according to the present invention is an arbitrary image forming apparatus provided with a fixing heater such as a copying machine. It goes without saying that it is possible to apply to.

  Each of the above-described embodiments is merely an example of the present invention, and the scope of the present invention is not limited by the description. The specific configuration of each part is appropriately selected within the scope of the effects of the present invention. It goes without saying that changes can be designed.

Explanatory drawing of temperature control device Illustration of printer Illustration of fixing unit Power supply circuit illustration Explanatory diagram of output signal of timer circuit Explanatory diagram of fixing temperature controlled by temperature controller Illustration of timer circuit Explanatory drawing of the timer circuit in another embodiment (A) is explanatory drawing of the gate circuit in another embodiment, (b) is explanatory drawing of the gate circuit in another embodiment. Explanatory diagram of fixing temperature control in the prior art

Explanation of symbols

9: Temperature controller 21: Fixing heater 23: Temperature detector (NTC thermistor)
40: Microcomputer 41: CPU
46: differentiation circuit 49: timer circuit 200: fixing unit 300: power supply unit 304: safety circuit (thermo switch)
305: Triac D48: Diode (diode that clamps the power supply voltage)
Q42: Gate circuit (transistor constituting a wired OR circuit)
Q44: Gate circuit (transistor constituting a wired OR circuit)

Claims (3)

A temperature detecting unit for detecting a fixing temperature; a CPU for outputting a heater control signal for driving and controlling the fixing heater so that the fixing temperature detected by the temperature detecting unit becomes a target temperature; and the fixing temperature becomes an abnormal overheating temperature. A temperature control device configured to include a safety circuit that forcibly stops power supply to the fixing heater when the temperature reaches,
A differentiation circuit for differentiating a pulse signal of a predetermined period output from the CPU, a timer circuit for outputting a forced stop signal regardless of the heater control signal when a predetermined time elapses after the power supply voltage is applied, and A gate circuit for turning off the fixing heater in response to an output signal or the heater control signal, the output voltage of the differentiation circuit being supplied to the timer circuit as the power supply voltage, and one cycle of the pulse signal from the predetermined time A temperature control device set short.   2. The timer circuit includes an integration circuit that operates when the power supply voltage is applied, and a logic circuit that inverts an output level when the predetermined time has elapsed based on an output signal of the integration circuit. Temperature control device.   The temperature control device according to claim 1, wherein the timer circuit includes a reset IC whose output level is inverted when a predetermined time elapses after the power supply voltage is applied.

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