Power supply circuit and image forming apparatus

Abstract

To enable immediate turning on of a heater while detecting an inflow of a DC.SOLUTION: A low voltage power supply 30 comprises: a triac 106 that turns on / off a heater in response to supply of a current from a commercial power supply 60; a relay 104 that turns on / off supply of a current to the triac 106 from the commercial power supply 60; a heater control unit 41 that controls the heater by controlling the triac 106 and the relay 104; a DC detection circuit 150 that detects a DC from the commercial power supply 60; and a DC protection circuit 160 that, when the DC detection circuit 150 has detected a DC, prevents the triac 106 from being turned on even if the heater control unit 41 tries to turn on the triac 106, and also prevents the relay 104 from being turned on even if the heater control unit 41 tries to turn on the relay 104.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a power supply circuit and an image forming apparatus. [Background technology]

[0002] In order to reduce the risk of equipment and power supply circuit failures caused by the input of an abnormal input voltage, a protection circuit is provided in the equipment.

[0003] For example, the image forming apparatus described in Patent Document 1 has a DC detection unit and an input power supply abnormality determination unit. If the image forming apparatus has shifted to an image forming operation when the DC detection unit detects DC, the input power supply abnormality determination unit does not turn on the heater control signal. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2020-197559 A Summary of the Invention [Problem to be solved by the invention]

[0005] However, in a conventional image forming apparatus, an input power supply abnormality determination unit determines whether or not to turn on a heater control signal, and therefore the image forming apparatus transitions to image forming operation after the input power supply abnormality determination unit determines that the heater may be turned on, which means that it takes time to start the image forming operation.

[0006] Therefore, one or more aspects of the present disclosure aim to make it possible to immediately turn on the heater even when an inflow of direct current is detected. [Means for solving the problem]

[0007] A power supply circuit according to one aspect of the present disclosure is characterized in comprising: a triac that receives a current from a commercial power source and turns a heater on or off; a switch that turns on or off the supply of current from the commercial power source to the triac; a heater control unit that controls the heater by controlling the triac and the switch; a DC detection circuit that detects DC from the commercial power source; and a DC protection circuit that, when the DC detection circuit detects DC, prevents the triac from being turned on even if the heater control unit attempts to turn the triac on, and prevents the switch from being turned on even if the heater control unit attempts to turn the switch on.

[0008] An image forming apparatus according to one aspect of the present disclosure includes a heater and the above-described power supply circuit. Effect of the Invention

[0009] According to one or more aspects of the present disclosure, the heater can be turned on immediately even when an inflow of direct current is detected. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a vertical sectional view illustrating a schematic configuration of an image forming apparatus according to first and second embodiments. [Diagram 2] 4 is a block diagram illustrating a schematic configuration of a low-voltage power supply and a main control unit in the image forming apparatus according to the first and second embodiments. FIG. [Diagram 3] 2 is a circuit diagram of a DC detection circuit according to the first embodiment. [Figure 4] FIG. 2 is a circuit diagram of a DC protection circuit. [Diagram 5] 1A and 1B are block diagrams illustrating an example of a hardware configuration. [Figure 6] 5 is a flowchart showing an operation of the image forming apparatus at the start of printing. [Figure 7] FIG. 11 is a circuit diagram of a DC detection circuit according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Embodiment 1 FIG. 1 is a vertical cross-sectional view illustrating a schematic configuration of an image forming apparatus 100 according to the first embodiment. The image forming apparatus 100 according to the first embodiment is an example of a color image forming apparatus that obtains a color image by overlapping and printing toners, which are developers of four colors, black, yellow, magenta, and cyan. However, the image forming apparatus 100 is not limited to this example, and the image forming apparatus 100 may be a monochrome image forming apparatus that is only black, or a color image forming apparatus that uses other colors.

[0012] In Fig. 1, the capital letter "K" at the end of the reference symbol means black, the capital letter "Y" means yellow, the capital letter "M" means magenta, and the capital letter "C" means cyan. In the following description, when there is no need to particularly distinguish between these colors, these capital letters will be omitted.

[0013] As shown in the figure, image forming apparatus 100 includes photosensitive drums 2K, 2Y, 2M, and 2C as image carriers, chargers 3K, 3Y, 3M, and 3C as charging sections, exposure devices 4K, 4Y, 4M, and 4C as exposure sections, and development devices 5K, 5Y, 5M, and 5C as development sections.

[0014] The image forming apparatus 100 also includes transfer rollers 6K, 6Y, 6M, and 6C as a transfer section, a transfer belt 8, a drive roller 9, an idle roller 10, a fuser 11 as a fixing section, a first pair of conveying rollers 12a and 12b, a second pair of conveying rollers 13a and 13b, a discharge roller pair 14a and 14b, a hopping roller 15, a write sensor 16, a discharge sensor 17, a support plate member 18, and a spring 19.

[0015] The image forming apparatus 100 further includes a low-voltage power supply 30, a main control unit 40, and a display unit 50. In the following description, the main control unit 40 will also be referred to simply as a control unit, and the low-voltage power supply 30 will also be referred to as a power supply unit.

[0016] The photosensitive drum 2 carries an image. For example, the photosensitive drum 2 is uniformly charged by the charger 3 and exposed by the exposure device 4. As a result, an electrostatic latent image is formed on the photosensitive drum 2. Then, a developer is applied by the developing device 5 to the electrostatic latent image formed on the photosensitive drum 2, thereby forming a toner image as a developer image, which is an image to be transferred.

[0017] The charger 3 charges the corresponding photosensitive drum 2 negatively. The exposure device 4 exposes the corresponding photosensitive drum 2 to light, thereby writing an electrostatic latent image onto the photosensitive drum 2 . The developing unit 5 visualizes the electrostatic latent image by attaching toner, which is a developer, to the electrostatic latent image on the corresponding photosensitive drum 2. The toner here is, for example, negatively charged.

[0018] The transfer roller 6 is disposed inside the transfer belt 8, which is formed in the shape of an endless belt, and is biased by an elastic member such as a spring (not shown) so as to press the transfer roller 6 against the corresponding photosensitive drum 2, sandwiching the transfer belt 8 therebetween.

[0019] The transfer belt 8 is supported by the outer circumferential surfaces of a drive roller 9 and an idle roller 10. The transfer belt 8 is pulled between the drive roller 9 and the idle roller 10 so that the contact surface between the photosensitive drum 2 and the transfer belt 8 is flat.

[0020] The drive roller 9 is connected to a driving device (not shown) and rotates around a shaft. When the transfer belt 8 moves in conjunction with the rotation of the drive roller 9, the idle roller 10 rotates in the same direction as the rotation direction of the drive roller 9 in conjunction with the movement of the transfer belt 8.

[0021] The fixing unit 11 has a fixing roller 21 equipped with a heater 20 as an internal heat source, a backup roller 22 pressed against the fixing roller 21 by a biasing means, and a thermistor 23 as a temperature detection unit that detects the temperature of the fixing unit 11. The fixing unit 11 fixes the toner image transferred to the recording medium 24 by heat and pressure. The heater 20 generates heat to heat the recording medium 24 to which the toner image has been transferred.

[0022] The thermistor 23 detects the temperature of the fixing unit 11 and notifies the main control unit 40 of the temperature. The main control unit 40 uses the temperature to control the on / off of the heater 20 so as to keep the temperature of the fixing unit 11 constant.

[0023] 1 indicates a transport path for the recording medium 24. Along the transport path, a first transport roller pair 12a, 12b, a second transport roller pair 13a, 13b, and a write sensor 16 are disposed upstream of the transfer belt 8, and a discharge sensor 17 and a discharge roller pair 14a, 14b are disposed downstream of the fixing unit 11.

[0024] When the recording medium 24 is transported along the transport path, the write sensor 16 and the discharge sensor 17 detect a predetermined position of the recording medium 24, for example, the write sensor 16 detects the leading end position of the recording medium 24, and the discharge sensor 17 detects the trailing end position, and provide a detection signal to the main control unit 40.

[0025] A recording medium 24 is placed on the upper surface of the support plate member 18. A spring 19, which is a biasing member that pushes the support plate member 18 upward, is provided below the support plate member 18. The recording medium 24 placed on the upper surface of the support plate member 18 is pressed against the hopping roller 15 by the biasing force of the spring 19. The hopping roller 15 rotates in a direction to push the recording medium 24 out onto the transport path, thereby pushing the recording medium out onto the transport path.

[0026] The photosensitive drum 2, the hopping roller 15, the first conveying roller pair 12a, 12b, the second conveying roller pair 13a, 13b, the drive roller 9, the fixing unit 11 (fixing roller 21 and back-up roller 11b), and the discharge roller pair 14a, 14b are rotated by a motor or the like (not shown).

[0027] FIG. 2 is a block diagram that illustrates a schematic configuration of the low-voltage power supply 30 and the main control unit 40 in the image forming apparatus 100 according to the first embodiment. FIG. 2 shows a circuit configuration in which the parts related to the features of the first embodiment are extracted from the low-voltage power supply 30 as a power supply circuit and the main control unit 40.

[0028] A commercial power source 60 serving as an AC power source is connected to a power cord 61 . The power cord 61 is connected to an inlet 62 , which is connected to the input portion 31 of the low-voltage power supply 30 .

[0029] The LINE side of the input unit 31 is connected to the FuseA101. The FuseA 101 is connected to the filter 102 , and the NEUTRAL side of the input unit 31 is connected to the filter 102 .

[0030] The filter 102 is connected to the FuseB 103 and the relay 104 . The relay 104 is connected to the LINE side of the heater AC output section 32. The heater AC output section 32 is connected to the heater 20.

[0031] FuseB 103 is connected to an input side of a rectifier bridge 105, and filter 102 is connected to another input side of rectifier bridge 105, a triac 106, a resistor 107, and a resistor 108. The resistor 108 is connected to a bidirectional photocoupler 109 .

[0032] The bidirectional photocoupler 109 is connected to the relay 104. The emitter of the bidirectional photocoupler 109 is connected to the GND of the low-voltage power supply 30. The collector of the bidirectional photocoupler 109 is connected to a resistor 110 and the R-CHK terminal 33 of the low-voltage power supply 30.

[0033] Resistor 110 is connected to the +5V terminal 34 of the low voltage power supply 30 . The positive electrode on the output side of the rectifier bridge 105 is connected to a capacitor 121 and a transformer 122 of a power factor correction circuit 120 .

[0034] A triac 106 serving as a heater circuit triac ON-OFF section is connected to a resistor 111 and the NEUTRAL side of the heater AC output section 32 . The resistor 111 is connected to the gate pin of the triac 106 and to the phototriac 112 . The resistor 107 is connected to a phototriac 112 .

[0035] The cathode of the phototriac 112 is connected to an ACON-N terminal 44 of a heater control unit 41 included in the main control unit 40. The anode of the phototriac 112 is connected to the collector of a transistor 161 (described later) of the DC protection circuit 160 , and the emitter of the transistor 161 is connected to an H-GUARD terminal 45 of the heater control unit 41 .

[0036] The negative terminal of the output side of the rectifier bridge 105 is connected to a capacitor 121 and the input side of a transformer 122 . The output side of the transformer 122 is connected to the anode of the diode 123 . The other output side of the transformer 122 is connected to a resistor 124 , which is connected to a capacitor 125 .

[0037] The OUT pin of the power factor correction circuit control IC 126 is connected to the gate of a FET (Field Effect Transistor) 127 . The drain of the FET 127 is connected to the anode of the diode 123. The source of the FET 127 is connected to the IS pin of the power factor correction circuit control IC 126 and to a resistor 128.

[0038] The FB pin of the power factor correction circuit control IC 126 is connected to a resistor 129a and a resistor 129b. Capacitor 125 , resistor 129 b , resistor 128 , and the GND pin of power factor correction circuit control IC 126 are connected to the negative terminal of the output side of rectifier bridge 105 . The cathode of the diode 123 is connected to the main transformer 113, the sub-transformer 114, the positive electrode of the electrolytic capacitor 115, the resistor 116, and a resistor 129a.

[0039] The OUT pin of the main power supply control IC 117 is connected to the gate of the FET 118 . The main transformer 113 is connected to the drain of the FET 118 .

[0040] The source of the FET 118 is connected to the IS pin of the main power supply control IC 117 and to a resistor 119 . The FB pin of the main power supply control IC 117 is connected to the collector of the photocoupler 130 . The GND pin of the main power supply control IC 117 , the resistor 119 , the negative terminal of the electrolytic capacitor 115 , and the emitter of the photocoupler 130 are connected to the negative terminal of the output side of the rectifier bridge 105 .

[0041] The VCC pin of the main power supply control IC 117 is connected to the VCC pin of the power factor correction circuit control IC 126 and to the emitter of a transistor 181 of the control IC power supply switching unit 180 . The resistor 182 is connected to the collector of the transistor 181. The resistor 182 is connected to the collector of the photocoupler 183.

[0042] The emitter of the photocoupler 183 is connected to the base of the transistor 181. The anode of the photocoupler 183 is connected to the resistor 184. Resistor 184 is connected to the +5V terminal 34 of the low voltage power supply 30 . The cathode of the photocoupler 183 is connected to the collector of a transistor 185, and the base of the transistor 185 is connected to a resistor 186. The resistor 186 is connected to the POWER SAVE-N terminal 35 of the low-voltage power supply 30. The emitter of the transistor 185 is connected to the GND of the low-voltage power supply 30. The collector of the transistor 181 is connected to the VCC pin of the sub-power supply control IC131.

[0043] The OUT pin of the sub-power supply control IC 131 is connected to the gate of the FET 132 . The sub-transformer 114 is connected to the drain of the FET 132 . The source of the FET 132 is connected to the IS pin of the sub-power supply control IC 131 and to a resistor 133 . The VIN pin of the sub-power supply control IC 131 is connected to a resistor 116 . The FB pin of the sub-power supply control IC 131 is connected to the collector of the photocoupler 146 .

[0044] The auxiliary winding of the sub-transformer 114 is connected to the anode of a diode 134 , and the cathode of the diode 134 is connected to the VCC pin of the sub-power supply control IC 131 and the positive electrode of an electrolytic capacitor 135 . The auxiliary winding of the sub-transformer 114 is connected to the negative terminal of the electrolytic capacitor 135 , the GND pin of the sub-power supply control IC 131 , the emitter of the photocoupler 146 , the resistor 133 , and the negative terminal of the output side of the rectifier bridge 105 .

[0045] The output side of the main transformer 113 is connected to the anode of a diode 136 , and the cathode of the diode 136 is connected to the positive pole of an electrolytic capacitor 137 , a resistor 138 , a resistor 139 a , and a +24 V terminal 36 of the low-voltage power supply 30 . The other output side of the main transformer 113 is connected to the anode of the variable shunt regulator 140, the negative electrode of the electrolytic capacitor 137, and the resistor 139b. The anode of the photocoupler 130 is connected to a resistor 138 . The cathode of the photocoupler 130 is connected to the cathode of the variable shunt regulator 140 .

[0046] The reference of the variable shunt regulator 140 is connected to resistor 139a and resistor 139b. The output side of the sub-transformer 114 is connected to the anode of the diode 141, and the cathode of the diode 141 is connected to the positive pole of the electrolytic capacitor 142, a resistor 143, a resistor 144a, a resistor 184, and a +5V terminal 34 of the low-voltage power supply 30.

[0047] The other output side of the sub-transformer 114 is connected to the anode of the variable shunt regulator 145 , the negative electrode of the electrolytic capacitor 142 , the resistor 144 b , and the GND of the low-voltage power supply 30 . The anode of the photocoupler 146 is connected to the resistor 143 . The cathode of the photocoupler 146 is connected to the cathode of the variable shunt regulator 145 .

[0048] The reference of variable shunt regulator 145 is connected to resistor 144a and resistor 144b. The +24V terminal 36 of the low voltage power supply 30 is connected to the input of a voltage conversion section 42 of the main control section 40. The +5V terminal 34 of the low voltage power supply 30 is connected to the input of a power supply control section 43 of the main control section 40.

[0049] The GND of the low-voltage power supply 30 is connected to the GND of the voltage conversion unit 42 of the main control unit 40 and to the power supply control unit 43. The power supply control unit 43 is connected to a main soft switch 25 for turning on and off the main power supply of the image forming apparatus 100. The voltage converted by the voltage conversion unit 42 in the main control unit 40 is supplied to each circuit of the main control unit 40.

[0050] The ACON-N terminal 44 of the low-voltage power supply 30 is connected to a heater control unit 41 in the main control unit 40 . The heater control unit 41 is connected to a thermistor 23 in the fixing unit 11 for detecting the temperature of the fixing unit. The R-CHK terminal 33 of the low voltage power supply 30 is connected to a heater control unit 41 in the main control unit 40 .

[0051] The positive electrode of the output side of the rectifier bridge 105 is connected to a portion P01 of the DC detection circuit 150. The negative electrode of the output side of the rectifier bridge 105 is connected to a portion P02 of the DC detection circuit 150.

[0052] The portion P03 of the DC detection circuit 150 is connected to the portion P13 of the DC protection circuit 160, and the portion P04 of the DC detection circuit 150 is connected to the portion P14 of the DC protection circuit 160.

[0053] The portion P 11 of the DC protection circuit 160 is connected to the anode of the phototriac 112 , and the portion P 12 of the DC protection circuit 160 is connected to the H-GUARD terminal 45 of the low-voltage power supply 30 . The portion P15 of the DC protection circuit 160 is connected to one end of a relay ON / OFF circuit 190, and the other end of the relay ON / OFF circuit 190 is connected to the RLY-ON terminal 37 of the low-voltage power supply 30.

[0054] The portion P16 of the DC protection circuit 160 is connected to the ACZEROX-N terminal 38 of the low-voltage power supply 30, and the portion P17 of the DC protection circuit 160 is connected to GND of the low-voltage power supply 30. The portion P18 of the DC protection circuit 160 is connected to the +5V terminal 34 of the low voltage power supply 30.

[0055] The ACZEROX-N terminal 38 , the RLY-ON terminal 37 , the H-GUARD terminal 45 , the ACON-N terminal 44 , and the R-CHK terminal 33 are connected to a heater control unit 41 in a main control unit 40 .

[0056] FIG. 3 is a circuit diagram of the DC detection circuit 150 according to the first embodiment. The portion P 01 of the DC detection circuit 150 is connected to one end of a capacitor 152 , and the other end of the capacitor 152 is connected to one end of a resistor 153 . The other end of the resistor 153 is connected to the anode of a photocoupler 154 , and the cathode of the photocoupler 154 is connected to a portion P 02 of the DC detection circuit 150 .

[0057] The collector of the photocoupler 154 is connected to the portion P03 of the DC detection circuit 150, and the emitter of the photocoupler 154 is connected to the portion P04 of the DC detection circuit 150. A circuit 151 composed of a capacitor 152 and a resistor 153 serves as a DC detection section.

[0058] In this circuit example, the DC detection circuit 150 not only operates as a circuit for detecting DC, but can also be used as a zero-cross detection circuit.

[0059] FIG. 4 is a circuit diagram showing a DC protection circuit 160 serving as a DC protection unit. The portion P 11 of the DC protection circuit 160 is connected to the collector of the transistor 161 . The portion P 12 of the DC protection circuit 160 is connected to the emitter of the transistor 161 and a resistor 162 . The portion P13 of the DC protection circuit 160 is connected to a resistor 163 and a resistor 164. The portion P14 and the portion P17 of the DC protection circuit 160 are connected to GND. The portion P 15 of the DC protection circuit 160 is connected to the emitter of a transistor 165 and to a resistor 166 . The portion P16 of the DC protection circuit 160 is connected to a resistor 164. Part P18 of the DC protection circuit 160 is connected to the +5V terminal 34.

[0060] Resistor 163 is connected to the base pin of transistor 167 , the collector of which is connected to resistor 168 and resistor 169 . Resistor 169 is connected to the base of transistor 170 . The collector of transistor 170 is connected to resistor 171 , capacitor 172 , resistor 173 , and resistor 174 .

[0061] Resistor 173 is connected to resistor 162 and to the base of transistor 161 . Resistor 174 is connected to the base of transistor 165 and to resistor 166 . Resistor 168 , resistor 171 , and resistor 164 are connected to +5V terminal 34 . The emitter of transistor 167, the emitter of transistor 170, the emitter of transistor 165, and capacitor 172 are connected to GND.

[0062] Resistor 163 , transistor 167 , resistor 169 , transistor 170 and capacitor 172 constitute a control circuit 175 for controlling the switching of transistor 161 and transistor 165 .

[0063] Returning to FIG. 2, a relay ON / OFF circuit 190 serving as a heater circuit relay ON / OFF section includes a coil 191 of the relay 104 and a diode 192. One end of the coil 191 is connected to the anode of the diode 192 , and the other end of the coil 191 is connected to the cathode of the diode 192 .

[0064] As described above, the low-voltage power supply 30 includes the triac 106 that receives a current supply from the commercial power supply 60 and turns the heater 20 on or off, the relay 104 that functions as a switch that turns on or off the supply of current from the commercial power supply 60 to the triac 106, the heater control unit 41 that controls the heater 20 by controlling the triac 106 and the relay 104, a DC detection circuit 150 that detects DC from the commercial power supply 60, and a DC protection circuit 160 that, when the DC detection circuit 150 detects DC, prevents the triac 106 from being turned on even if the heater control unit 41 attempts to turn the relay 104 on, and prevents the relay 104 from being turned on even if the heater control unit 41 attempts to turn the relay 104 on.

[0065] Here, the DC detection circuit 150 is provided with a capacitor 152 in a branch path branched off from the supply path of the current from the commercial power supply 60. This allows the DC detection circuit 150 to detect the DC from the commercial power supply 60 when the current stops flowing to the stage downstream of the capacitor 152. In the example of Fig. 2, a branch path is formed between points d1 and d2 downstream of the rectifier bridge 105 in the supply path through which the current is supplied from the commercial power supply 60 to the main control unit 40.

[0066] In addition, the DC protection circuit 160 includes a transistor 161 as a first switching element that cuts off an ACON-N signal as a triac-on signal that is sent from the heater control unit 41 to the triac 106 to turn on the triac 106 when the DC detection circuit 150 detects DC, and a transistor 165 as a second switching element that cuts off an RLY-ON signal as a switch-on signal that is sent from the heater control unit 41 to the relay 104 to turn on the relay 104 when the DC detection circuit 150 detects DC.

[0067] Specifically, when the DC detection circuit 150 detects DC from the commercial power supply 60, it outputs a low-level signal as a detection signal to the DC protection circuit 160, and when the DC detection circuit 150 detects AC from the commercial power supply 60, it outputs a signal that alternates between low and high levels as a detection signal to the DC protection circuit 160. The DC protection circuit 160 includes a control circuit 175 that outputs a high-level signal as a control signal when the detection signal is a low-level signal, and outputs a low-level signal as a control signal when the detection signal is a signal that alternates between low and high levels. The transistor 161 blocks the ACON-N signal when the control signal is high, and passes the ACON-N signal when the control signal is low. The transistor 165 blocks the RLY-ON signal when the control signal is high, and passes the RLY-ON signal when the control signal is low. Here, the control circuit 175 includes a capacitor 172, and by charging the capacitor 172 during the period when the detection signal is at a high level, when the detection signal is a signal that alternates between a low level and a high level, the control circuit 175 outputs a low level signal as a control signal.

[0068] A part or the whole of the main control unit 40 described above can be configured, for example, as shown in Fig. 5(A), by a memory 80 and a processor 81 such as a CPU (Central Processing Unit) that executes a program stored in the memory 80. Such a program may be provided through a network, or may be provided by being recorded on a recording medium. That is, such a program may be provided, for example, as a program product.

[0069] In addition, a part or the whole of the main control unit 40 may be configured, for example, as shown in FIG. 5(B), by a processing circuit 82 such as a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit) or an FPGA (Field Programmable Gate Array). As described above, the main control unit 40 can be configured with a processing circuit network.

[0070] Next, the operation of the image forming apparatus 100 will be described with reference to FIGS. The ON-OFF control of the heater 20 during printing (image formation) is performed by controlling the relay 104 with the RLY-ON signal, which is an output signal from the heater control unit 41 of the main control unit 40, and by controlling the triac 106 with the ACON-N signal, which is its output signal. The RLY-ON signal is input to the low-voltage power supply 30 via the RLY-ON terminal 37, and the ACON-N signal is input to the low-voltage power supply 30 via the ACON-N terminal 44.

[0071] Generally, a triac can control the ON / OFF state when AC is input, but when DC is input, it can only be turned ON, and cannot be turned OFF. Therefore, if the power supply during printing switches to DC, the triac cannot be turned OFF, and it remains ON, so the heater remains ON.

[0072] The same is true for relays; with a typical relay, in the case of direct current, arc discharge continues to occur between the contacts when the relay is turned off, so the circuit cannot be turned off and remains on.

[0073] When a direct current is supplied from the commercial power source 60 due to an accident on the power supply side, the operation is as follows.

[0074] The supplied DC passes through the power cord 61, the inlet 62, the input section 31 of the low-voltage power supply 30, FuseA 101, the filter 102, FuseB 103, the rectifier bridge 105, the capacitor 121, the transformer 122, the diode 123 and the resistor 116 and is then supplied to the sub-power supply control IC 131. When the DC is supplied, the sub-power supply control IC 131 starts up and the switching operation of the FET 132 begins. At that time, a voltage is generated in the auxiliary winding of the sub-transformer 114. The voltage is rectified by the diode 134, smoothed by the electrolytic capacitor 135, and input to the sub-power supply control IC 131 where it is used as the operating power supply VCC.

[0075] When the FET 132 starts a switching operation, a voltage is generated on the secondary side of the sub-transformer 114. The voltage generated on the secondary side is rectified by the diode 141 and smoothed by the electrolytic capacitor 142 to obtain a stepped-down DC voltage. This voltage is fed back to the FB pin of the sub-power supply control IC 131 by resistors 144a, 144b, the variable shunt regulator 145, resistor 143, and photocoupler 146, and the secondary side of the sub-transformer 114 is controlled to output +5V.

[0076] The IS pin of the sub-power supply control IC 131 receives an input of a voltage that monitors the current status. In the event of an overcurrent, the sub-power supply control IC 131 stops operating and stops outputting to the +5V terminal 34.

[0077] When +5V is output from the low voltage power supply 30, the power supply control unit 43 starts operating. The power supply control unit 43 switches the output of the POWERSAVE-N signal to H level. The POWERSAVE-N signal is input from the POWERSAVE-N terminal 35 to the low-voltage power supply 30.

[0078] When the POWERSAVE-N signal goes to H level, a current flows to the base of the transistor 185 via the resistor 186, and the transistor 185 turns ON. When the transistor 185 is turned on, a current flows from the resistor 184 connected to the +5V terminal 34 to the light-emitting side of the photocoupler 183 .

[0079] When a current flows through the light-emitting side of the photocoupler 183, the phototransistor of the photocoupler 183 turns ON. As a result, a current flows from the voltage output from the auxiliary winding of the sub-transformer 114 through the resistor 182 to the base of the transistor 181, turning the transistor 181 ON.

[0080] When the transistor 181 is turned ON, the voltage output from the auxiliary winding of the sub-transformer 114 is supplied to the VCC pin of the main power supply control IC 117 and the VCC pin of the power factor correction circuit control IC 126 . When a voltage is supplied to the VCC pin of the power factor correction circuit control IC 126, the power factor correction circuit control IC 126 starts operating. When the power factor correction circuit control IC 126 starts operating, a switching signal is output from the OUT pin of the power factor correction circuit control IC 126, and the FET 127 starts switching operation.

[0081] When the FET 127 starts switching operation, the voltage input to the inlet 62 is boosted by the back electromotive force generated by the transformer 122, and the boosted voltage is rectified by the diode 123 and smoothed by the electrolytic capacitor 115. The voltage is divided by resistors 129a and 129b, and the divided voltage is input to the FB pin of the power factor correction circuit control IC 126. The power factor correction circuit control IC 126 performs feedback control using the voltage input to the FB pin, and the output from the power factor correction circuit 120 is kept at a constant voltage.

[0082] When a voltage is supplied to the VCC pin of the main power supply control IC 117, the main power supply control IC 117 starts operating. When the main power supply control IC 117 starts operating, a switching signal is output from the OUT pin of the main power supply control IC 117, and the FET 118 starts switching operation. When the FET 118 starts switching operation, the main transformer 113 starts operating, and a voltage is generated on the secondary side of the main transformer 113.

[0083] The voltage generated on the secondary side is rectified by diode 136 and smoothed by electrolytic capacitor 137 to obtain a stepped-down DC voltage. This voltage is fed back to the FB pin of main power supply control IC 117 by resistors 139a, 139b, variable shunt regulator 140, resistor 138, and photocoupler 130. The main power supply control IC 117 controls the output from +24V terminal 36 to +24V by the voltage input to the FB pin. The IS pin of main power supply control IC 117 receives an input of a voltage that monitors the current state, and if the voltage indicates an overcurrent, the main power supply control IC 117 stops operation and stops the output from +24V terminal 36.

[0084] The above operations are the same whether the power source is DC or AC. In other words, whether the power source is AC or DC, the output voltage is a stepped-down DC voltage, and this voltage is supplied to the main control unit 40.

[0085] The operation of the image forming apparatus 100 when it transitions to a printing state after the voltage is supplied to the main control unit 40 by the above-mentioned operation will be described with reference to FIG.

[0086] FIG. 6 is a flowchart showing the operation of image forming apparatus 100 at the start of printing. First, when the image forming apparatus 100 transitions to a printing state, the heater control unit 41 sets the RLY-ON signal output from the RLY-ON terminal 37 to 0 V (S10). In this state, the contact of the relay 104 is turned OFF.

[0087] Next, the heater control unit 41 starts counting the number of falling edges of the R-CHK signal output from the R-CHK terminal 33 (S11). The counting time is one cycle (50 Hz: 20 ms), and the heater control unit 41 determines whether or not 20 ms has elapsed (S12). If the counted time has elapsed 20 ms (Yes in S12), the process proceeds to step S13.

[0088] In step S13, heater control unit 41 determines whether the count of the number of falling edges of the R-CHK signal exceeds 0. If the count of the number of falling edges of the R-CHK signal exceeds 0, the contacts of relay 104 are welded. Therefore, if the count of the number of falling edges of the R-CHK signal exceeds 0 (Yes in S13), heater control unit 41 advances the process to step S14 and performs error processing. Here, as error processing, printing by image forming apparatus 100 is stopped. If the count of the number of falling edges of the R-CHK signal is 0 (No in S13), there is no problem, so the process advances to step S15.

[0089] In step S15, the heater control unit 41 sets the RLY-ON signal to 5 V, turning the relay 104 ON.

[0090] Next, the heater control unit 41 sets a heater-on timer (not shown) and starts measuring the heater-on time (S16).

[0091] Then, heater control unit 41 sets the H-GUARD signal output from H-GUARD terminal 45 to +5V and the ACON-N signal output from ACON-N terminal 44 to the L level, turning ON triac 106 (S17). When the collector voltage of transistor 170 becomes the L level, transistor 161 of DC protection circuit 160 turns ON, so that setting the H-GUARD signal to 5V allows current to flow. As a result, triac 106 turns ON.

[0092] Next, the heater control unit 41 checks the time of a heater ON timer (not shown) and determines whether the heater ON time exceeds a predetermined threshold value (S18). If the heater ON time exceeds the predetermined threshold value (Yes in S18), there is a high possibility that a malfunction has occurred, so the heater control unit 41 advances the process to step S14 and performs error processing. Here, too, printing by the image forming apparatus 100 is stopped as an error processing. On the other hand, if the heater ON time does not exceed the predetermined threshold value (No in S18), the process advances to step S19.

[0093] In step S19, heater control unit 41 checks the temperature of thermistor 23 and determines whether the temperature has reached the print target temperature. If the temperature has not reached the print target temperature (No in S19), the process returns to step S17, and if the temperature has reached the print target temperature (Yes in S19), the process proceeds to step S20.

[0094] In step S20, the heater control unit 41 sets the ACON-N signal to H level to turn the triac 106 off. After that, the print control continues.

[0095] Here, in step S15, when AC is applied to the input unit 31, the relay 104 is turned ON, but when DC is applied to the input unit 31, the relay 104 is not turned ON. This is due to the DC detection circuit 150 and the DC protection circuit 160. These circuits operate as follows.

[0096] First, the case where a DC voltage is applied to the input section 31 will be described. When a DC voltage is applied to the input unit 31, a positive voltage is applied to resistor 153 of DC detection circuit 150, and a negative voltage is applied to the cathode of photocoupler 154. The DC voltage is cut off by the presence of capacitor 152 in DC detection circuit 150. Therefore, no current flows through resistor 153 and the light-emitting side of photocoupler 154, and the phototransistor of photocoupler 154 does not turn ON.

[0097] When the phototransistor of photocoupler 154 is not turned ON, the collector of the phototransistor of photocoupler 154 becomes H level because it is pulled up to +5 V by resistor 164 of DC protection circuit 160. In this case, a voltage is applied to resistor 163 of DC protection circuit 160, so that a base current flows through transistor 167 of DC protection circuit 160, and transistor 167 turns ON.

[0098] When the transistor 167 is turned on, the collector voltage of the transistor 167 becomes the L level. When the collector voltage of the transistor 167 becomes the L level, the transistor 170 of the DC protection circuit 160 becomes the OFF state.

[0099] When the transistor 170 is turned off, the collector voltage of the transistor 170 becomes H level. When the collector voltage of the transistor 170 becomes H level, the transistor 165 of the DC protection circuit 160 becomes OFF, so that even if the RLY-ON signal is set to 5 V, no current can flow through the coil 191, and the relay 104 does not turn ON.

[0100] Next, a case where an AC voltage is applied to the input unit 31 will be described. A full-wave rectified voltage is applied between resistor 153 of DC detection circuit 150 and the cathode of photocoupler 154. DC detection circuit 150 has capacitor 152, but since the input voltage is not DC, the voltage is not cut and a current flows through resistor 153 and the light-emitting side of photocoupler 154.

[0101] At this time, since the voltage is full-wave rectified, the phototransistor of the photocoupler 154 repeatedly turns ON and OFF. When the phototransistor of the photocoupler 154 is repeatedly turned on and off, the collector of the phototransistor of the photocoupler 154 is pulled up to +5 V by the resistor 164 of the DC protection circuit 160, and therefore repeatedly changes to the HL level.

[0102] Since the voltage of the resistor 163 of the DC protection circuit 160 becomes the HL level, the base current repeatedly flows and does not flow through the transistor 167 of the DC protection circuit 160. As a result, the transistor 167 repeatedly switches between the ON state and the OFF state.

[0103] When the transistor 167 alternates between the ON state and the OFF state, the collector voltage of the transistor 167 alternates between the HL level. When the collector voltage of the transistor 167 alternates between the HL level, the transistor 170 of the DC protection circuit 160 alternates between the ON state and the OFF state.

[0104] When the transistor 170 is ON, the collector voltage of the transistor 170 is at L level. When it is OFF, the collector voltage of 122b remains at L level until charging of the capacitor 172 connected to the transistor 170 is completed. As a result, the collector voltage of the transistor 170 is always at L level at this time.

[0105] When the collector voltage of the transistor 170 becomes the L level, the transistor 165 of the DC protection circuit 160 turns ON. Therefore, when the RLY-ON signal is set to 5 V, a current can flow through the coil 191, and the relay 104 turns ON.

[0106] Also, in step S17, when an AC voltage is applied to the input unit 31, the triac 106 turns ON, but when a DC voltage is applied to the input unit 31, the triac 106 does not turn ON. This is due to the DC detection circuit 150 and the DC protection circuit 160. These circuits operate as follows.

[0107] First, the case where a DC voltage is applied will be described. When a DC voltage is applied to the input unit 31, a positive voltage is applied to resistor 153 of DC detection circuit 150, and a negative voltage is applied to the cathode of photocoupler 154. Since DC detection circuit 150 includes capacitor 152, the DC voltage is cut off and no current flows through resistor 153 and the light-emitting side of photocoupler 154. As a result, the phototransistor of photocoupler 154 does not turn ON.

[0108] If the phototransistor of photocoupler 154 does not turn ON, the collector of the phototransistor of photocoupler 154 will be at H level because it is pulled up to +5 V by resistor 164 in DC protection circuit 160. As a result, a voltage is applied to resistor 163 of DC protection circuit 160, a base current flows through transistor 167 of DC protection circuit 160, and transistor 167 turns ON.

[0109] When the transistor 167 is turned on, the collector voltage of the transistor 167 goes to the L level. When the collector voltage of the transistor 167 goes to the L level, the transistor 170 of the DC protection circuit 160 goes to the OFF state.

[0110] When the transistor 170 is turned off, the collector voltage of the transistor 170 becomes H level. When the collector voltage of the transistor 170 becomes H level, the transistor 161 of the DC protection circuit 160 turns off. Therefore, even if the H-GUARD signal is set to 5V, no current can flow through the coil 191, and the triac 106 does not turn on.

[0111] Next, a case where an AC voltage is applied to the input unit 31 will be described. A full-wave rectified voltage is applied between resistor 153 of DC detection circuit 150 and the cathode of photocoupler 154. DC detection circuit 150 has capacitor 152, but since the input voltage is not DC, the voltage is not cut and a current flows through resistor 153 and the light-emitting side of photocoupler 154.

[0112] At this time, since the voltage is full-wave rectified, the phototransistor of photocoupler 154 repeatedly turns ON and OFF. When the phototransistor of photocoupler 154 repeatedly turns ON and OFF, the collector of the phototransistor of photocoupler 154 repeatedly changes to the HL level because it is pulled up to +5 V by resistor 164 of DC protection circuit 160.

[0113] When the voltage of the resistor 163 of the DC protection circuit 160 becomes the HL level, the base current also alternates between flowing and not flowing through the transistor 167 of the DC protection circuit 160. As a result, the transistor 167 alternates between the ON state and the OFF state.

[0114] When the transistor 167 alternates between the ON state and the OFF state, the collector voltage of the transistor 167 alternates between the HL level. When the collector voltage of the transistor 167 alternates between the HL level, the transistor 170 of the DC protection circuit 160 alternates between the ON state and the OFF state.

[0115] When transistor 170 is ON, the collector voltage of transistor 170 is at L level. When it is OFF, the collector voltage of 122b remains at L level until charging of capacitor 172 connected to transistor 170 is completed. Therefore, the collector voltage of transistor 170 is always at L level at this time.

[0116] When the collector voltage of the transistor 170 becomes the L level, the transistor 161 of the DC protection circuit 160 turns ON. Therefore, when the H-GUARD signal is set to 5 V, a current can flow through the coil 191, and the triac 106 turns ON.

[0117] As described above, according to the first embodiment, DC detection is performed by hardware, which eliminates the need for firmware to monitor DC detection every time printing starts. This eliminates the impact on warm-up time and first printout time, allowing for quick printing start.

[0118] Embodiment 2 As shown in FIG. 1, the configuration of the image forming apparatus 200 according to the second embodiment is similar to that of the image forming apparatus 100 according to the first embodiment, except for the low-voltage power supply 70.

[0119] As shown in FIG. 2, the image forming apparatus 200 according to the second embodiment includes a low-voltage power supply 70 and a main control unit 40. The main control unit 40 of the image forming apparatus 200 according to the second embodiment is similar to the main control unit 40 of the image forming apparatus 100 according to the first embodiment. Moreover, the low-voltage power supply 70 in the second embodiment is configured similarly to the low-voltage power supply 30 in the first embodiment, except for the DC detection circuit 250.

[0120] FIG. 7 is a circuit diagram of a DC detection circuit 250 according to the second embodiment. The DC detection circuit 250 includes a capacitor 152, a resistor 153, a photocoupler 154, a diode 255, a resistor 256, and a switching circuit 257 as a switching unit. The capacitor 152, resistor 153, and photocoupler 154 of the DC detection circuit 250 in the second embodiment are similar to the capacitor 152, resistor 153, and photocoupler 154 of the DC detection circuit 150 in the first embodiment.

[0121] The portion P01 of the DC detection circuit 250 is connected to the anode of the diode 255 and the anode of the diode 257a of the switching circuit 257. The cathode of the diode 255 is connected to one end of the capacitor 152 , and the other end of the capacitor 152 is connected to the drain of the FET 257 b of the switching circuit 257 and one end of the resistor 256 . Here, a single diode 255 is used, but a diode bridge for full wave rectification may be used in place of the diode 255.

[0122] The other end of the resistor 256 is connected to the source of the FET 257 b of the switching circuit 257 and one end of the resistor 153 . The other end of the resistor 153 is connected to the anode of the photocoupler 154 .

[0123] The cathode of the diode 257a of the switching circuit 257 is connected to a capacitor 257c, a cathode of a Zener diode 257d, a resistor 257e, and a resistor 257f. Resistor 257e is connected to the gate of FET 257b and to the collector of transistor 257g. Resistor 257f is connected to the collector of transistor 257h and to resistor 257i, which is connected to the base of transistor 257g.

[0124] The anode of the Zener diode 257d is connected to a resistor 257j, which is connected to a resistor 257k and a resistor 257l. Resistor 257l is connected to the base of transistor 257h. The portion P02 of the DC detection circuit 250 is connected to the capacitor 257c, the resistor 257k, the emitter of the transistor 257h, and the emitter of the transistor 257g.

[0125] In the switching circuit 257, the Zener diode 257d is used to detect the voltage, but other voltage detection circuits or elements may be used as long as they are capable of detecting the voltage. Furthermore, although the FET 257b is used as the switching element, not only semiconductors such as FETs or transistors but also mechanical relays or the like may be used as long as they can be used as the switching element. The DC detection circuit 250 can also be used as a zero-cross detection circuit.

[0126] Next, the operation of the DC detection circuit 250 will be described, particularly the operation that differs from that of the first embodiment. The DC detection circuit 250 always consumes power when power is input. This is because the DC detection circuit 250 always detects DC. In this respect, the second embodiment is similar to the first embodiment.

[0127] The power supply circuit shown in Fig. 2 is a universal power supply that can handle a wide range of input voltages. Such a power supply can handle inputs from AC 100V to AC 240V. In the DC detection circuit 150 according to the first embodiment shown in Fig. 3, the resistance value of the resistor 153 is fixed, so that the power consumption is higher when AC 240V is input than when AC 100V is input.

[0128] In the second embodiment, a switching circuit 257 is provided so that the power consumption remains the same whether AC 100V is input or AC 240V is input.

[0129] When a voltage is applied to the portion P01 and the portion P02 of the DC detection circuit 250, a current flows through the diode 255, the capacitor 152, the resistor 256 or between the drain and source of the FET 257b of the switching circuit 257, the resistor 153, and the photocoupler 154.

[0130] Whether a current flows through the resistor 256 or between the drain and source of the FET 257b depends on the voltage value applied to the portion P01 and the portion P02 of the DC detection circuit 250. When the input voltage is less than 180 V, a current flows between the drain and source of FET 257b, and when the input voltage is 200 V or more, a current flows through resistor 256. This operation is as follows.

[0131] When a voltage is applied to the portion P01 and the portion P02 of the DC detection circuit 250, the voltage is half-wave rectified by the diode 257a and smoothed into a DC voltage by the capacitor 257c.

[0132] When the smoothed DC voltage is 200 V or more, a Zener current flows through the Zener diode 257d. Here, for example, it is assumed that the Zener voltage of the Zener diode 257d is 180 V. In this case, a Zener current flows through the resistors 257j and 257k, and a current also flows through the resistor 257l. This causes a base current to flow through the transistor 257h, and the transistor 257h is turned on.

[0133] As a result, no current flows through the resistor 257i and no base current flows through the transistor 257g, so that the transistor 257g is turned off. When the transistor 257g is in the OFF state, the voltage of the gate of the FET 257b is kept at a high voltage. Since the FET 257b here is a P-channel FET, it is not turned ON.

[0134] On the other hand, when the DC voltage smoothed by the capacitor 257c is less than 200 V, no Zener current flows through the Zener diode 257d. In this case, no Zener current flows through the resistors 257j and 257k, and no current flows through the resistor 257l. As a result, no base current flows through the transistor 257h, and the transistor 257h is turned off.

[0135] As a result, a current flows through the resistors 257f and 257i, and a current flows through the base of the transistor 257g, turning the transistor 257g on. When the transistor 257g is turned on, the voltage at the gate of the FET 257b becomes a low voltage, and the FET 257b is turned on.

[0136] As described above, when the input voltage is 200V or more, the resistors 256 and 153 act as two limiting resistors, and when the input voltage is less than 200V, the FET 257b connected in parallel to the resistor 256 is turned on, so that the resistor 256 is bypassed and only the resistor 153 acts as a limiting resistor. In this example, the Zener voltage of the Zener diode 257d is 180V, but this voltage value is merely an example, and it is also possible to adjust the switching voltage by using another Zener voltage.

[0137] In the second embodiment, the DC detection circuit 250 includes a resistor 153 as a first resistor connected to the branch path, a resistor 256 as a second resistor connected to the branch path, and a switching circuit 257 that, when the voltage from the commercial power supply 60 is equal to or higher than a predetermined threshold, causes a current from the commercial power supply 60 to flow through the resistor 256, and, when the voltage from the commercial power supply 60 is less than the predetermined threshold, causes the current from the commercial power supply 60 to flow bypassing the resistor 256 so that the current from the commercial power supply 60 does not flow through the resistor 256.

[0138] When the limiting resistance of the DC detection circuit 150 is fixed as in the first embodiment, when an input voltage exceeding 200V is applied, the power consumed by the DC detection circuit 150 becomes larger than when 100V is applied. In contrast to this, in the second embodiment, by providing the switching circuit 257, when the input voltage exceeds 200V, the limiting resistance value also increases, so that even if the input voltage exceeds 200V, the power consumption can be the same as when a voltage of 100V is input.

[0139] Also, in the second embodiment, DC detection is performed by hardware, so there is no need for firmware to monitor DC detection every time printing starts. This eliminates the impact on warm-up time and first printout time, enabling printing to start quickly.

[0140] In the above-described first and second embodiments, a color printer has been described as an example, but the first and second embodiments can also be applied to a monochrome printer or a multifunction printer that uses a heater circuit. [Explanation of symbols]

[0141] 100,200 image forming apparatus, 2 photosensitive drum, 3 charger, 4 exposure unit, 5 developing unit, 6 transfer roller, 8 transfer belt, 9 drive roller, 10 idle roller, 11 fixing unit, 12a, 12b first conveying roller pair, 13a, 13b second conveying roller pair, 14a, 14b discharge roller pair, 15 hopping roller, 16 write sensor, 17 discharge sensor, 18 support plate member, 19 spring, 30 low voltage power supply, 40 main control unit, 41 heater control unit, 50 display unit, 60 commercial power supply, 104 relay, 106 triac, 150 DC detection circuit, 152 capacitor, 153 resistor, 154 photocoupler, 255 diode, 256 Resistors, 257 Switching circuits, 160 DC protection circuits, 161, 165, 167, 170 Transistors, 162, 163, 164, 166, 168, 169, 171, 173, 174 Resistors, 172 Capacitors, 175 Control circuits.

Claims

1. A triac that receives current from a commercial power source and turns the heater on or off; a switch that turns on or off the supply of current from the commercial power supply to the triac; a heater control unit that controls the triac and the switch to control the heater; a DC detection circuit for detecting a DC current from the commercial power source; a DC protection circuit that prevents the triac from being turned on even if the heater control unit tries to turn on the triac when the DC detection circuit detects a direct current, and prevents the switch from being turned on even if the heater control unit tries to turn on the switch. A power supply circuit comprising:

2. The DC detection circuit detects DC from the commercial power source when a current stops flowing downstream of the capacitor by providing a capacitor in a branch path branched from a current supply path from the commercial power source.

2. The power supply circuit according to claim 1 .

3. The DC detection circuit includes: a first resistor connected to the branch; a second resistor connected to the branch; a switching circuit that, when a voltage from the commercial power supply is equal to or higher than a predetermined threshold, causes a current from the commercial power supply to flow through the second resistor, and, when a voltage from the commercial power supply is less than a predetermined threshold, causes the current from the commercial power supply to flow bypassing the second resistor so as not to flow through the second resistor.

3. The power supply circuit according to claim 2 .

4. The DC protection circuit includes: a first switching element that cuts off a triac-on signal, which is a signal sent from the heater control unit to the triac in order to turn on the triac when the direct current detection circuit detects a direct current; a second switching element that cuts off a switch-on signal, which is a signal sent from the heater control unit to the switch, in order to turn on the switch when the DC detection circuit detects DC.

4. The power supply circuit according to claim 1, wherein:

5. the DC detection circuit provides, as a detection signal, to the DC protection circuit when it detects DC from the commercial power supply, and provides, as a detection signal, a signal that alternates between a low level and a high level when it detects AC from the commercial power supply; The DC protection circuit includes: a control circuit that outputs, as a control signal, a signal that becomes high level when the detection signal is a low level signal, and a signal that becomes low level when the detection signal is a signal that alternates between a low level and a high level; a first switching element that blocks a triac-on signal, which is a signal sent from the heater control unit to the triac in order to turn on the triac, when the control signal is at a high level, and passes the triac-on signal when the control signal is at a low level; a second switching element that blocks a switch-on signal, which is a signal sent from the heater control unit to the switch in order to turn on the switch, when the control signal is at a high level, and passes the switch-on signal when the control signal is at a low level.

2. The power supply circuit according to claim 1 .

6. The control circuit includes a capacitor, and when the detection signal is a signal that alternates between a low level and a high level, the control circuit outputs a low level signal as the control signal by charging the capacitor during a period when the detection signal is at a high level.

6. The power supply circuit according to claim 5 .

7. The heater; The power supply circuit according to any one of claims 1 to 6. An image forming apparatus comprising:

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