Image forming apparatus, method for controlling image forming apparatus, and control program of image forming apparatus

Abstract

To provide a technology capable of protecting a fixing heater even in the case where the fixing heater is connected to either of two or more kinds of power sources having different voltages.SOLUTION: An image forming apparatus includes a voltage detection part 204 for detecting voltage of a power source 108, a current detection part 212 for detecting a current value in a heat source 220, a switch 222 for switching on / off of supply of power to the heat source 220, and a control part 107 for controlling the switch 222. The control part 107 acquires the voltage from the voltage detection part 204, determines a threshold of current to be supplied to the heat source 220 on the basis of the voltage, acquires the current value from the current detection part 212, determines whether the current value is equal to or more than the threshold, and outputs a signal S204 to block the supply of the power to the heat source 220 to the switch 222 on the basis of the determination that the current value is equal to or more than the threshold.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to an image forming apparatus, and more particularly to a technique for protecting a fixing heater. [Background technology]

[0002] A worldwide fixing heater may be used in an image forming apparatus. The worldwide fixing heater is a fixing heater that is assumed to be used worldwide. The worldwide fixing heater is, for example, a fixing heater of 100V specification. An image forming apparatus equipped with a worldwide fixing heater may be connected to a plurality of power sources of various voltages (110V, 120V, 230V).

[0003] In a conventional fixing heater protection method, a thermostat arranged near the fixing heater monitors the temperature of the fixing heater. If the fixing heater reaches an abnormal temperature, the thermostat cuts off the power supply to the fixing heater. This conventional heater protection method is based on the assumption that a 100V fixing heater is connected to a 100V power supply. However, as described above, an image forming apparatus equipped with a worldwide fixing heater may be connected to a power supply with a voltage higher than 100V.

[0004] The thermostat has a slow temperature rise speed. Therefore, when an image forming apparatus is connected to a high-voltage power supply such as 200V, a time lag may occur between when the temperature of the fixing heater rises and when the thermostat detects an abnormal temperature. As a result, when an image forming apparatus is connected to a high-voltage power supply such as 200V, protection of the fixing heater by cutting off the supply of power may not be completed in time.

[0005] Regarding technology for protecting a fixing heater, for example, Japanese Patent Application Laid-Open No. 2008-003469 (Patent Document 1) discloses a heating device and an image forming apparatus that "can be supplied worldwide as a common product regardless of whether the commercial power source is 100V or 200V, and can appropriately switch operation modes without causing problems such as errors or breakdowns, thereby achieving good fixing of images" ([Abstract]).

[0006] Another example of a technique for protecting a fixing heater is disclosed in JP 2017-90570 A (Patent Document 2). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2008-003469 A [Patent Document 2] JP 2017-90570 A Summary of the Invention [Problem to be solved by the invention]

[0008] According to the technology disclosed in Patent Document 1, the image forming apparatus can only protect the fixing heater when the image forming apparatus is connected to a power source of either 100V or 200V. Moreover, to protect the fixing heater using the method disclosed in Patent Document 1, the fixing heater, which is divided into two, must be switched between series and parallel. Furthermore, according to the technology disclosed in Patent Document 2, the fixing heater can only be protected when the image forming apparatus is connected to a power source of one type of voltage. Therefore, there is a need for a technology that can protect the fixing heater even when the fixing heater is connected to any of two or more power sources of different voltages.

[0009] The present disclosure has been made in consideration of the above-described background, and an object of one aspect is to protect the fixing heater when the fixing heater is connected to any of two or more power sources having different voltages. [Means for solving the problem]

[0010] According to one embodiment, there is provided an image forming apparatus. The image forming apparatus includes a voltage detection unit that detects a voltage of a power supply, a current detection unit that detects a current value in a heat source, a switch that switches on and off the supply of power to the heat source, and a control unit that controls the switch. The control unit obtains the voltage from the voltage detection unit, determines a threshold value of the current supplied to the heat source based on the voltage, obtains the current value from the current detection unit, determines whether the current value is equal to or greater than the threshold value, and outputs a signal to the switch to cut off the supply of power to the heat source based on the determination that the current value is equal to or greater than the threshold value.

[0011] In one aspect, the control unit, based on a change in the voltage of the power supply, selects a threshold value according to the changed voltage.

[0012] In one aspect, the image forming apparatus is configured to be connectable to at least both an external power supply of a first voltage and an external power supply of a second voltage higher than the first voltage.

[0013] In one aspect, the signal is a pulse width modulation (PWM) signal. The control unit reduces the threshold value and increases the duty ratio of the signal based on a drop in the voltage of the power supply.

[0014] In one aspect, the signal is a PWM signal, and the control unit increases the threshold value and decreases the duty ratio of the signal based on an increase in the voltage of the power supply.

[0015] In one aspect, the image forming apparatus further includes a bypass switch for switching whether or not the current to the heat source passes through a path via a thermostat. The control unit switches the bypass switch so that the current to the heat source passes through the path via the thermostat based on the image forming apparatus being connected to an external power supply of a first voltage, and switches the bypass switch so that the current to the heat source does not pass through the path via the thermostat based on the image forming apparatus being connected to an external power supply of a second voltage higher than the first voltage.

[0016] In one aspect, the control unit selects a first threshold value in a predetermined period after the supply of power to the heat source starts, and selects a second threshold value smaller than the first threshold value after the predetermined period has elapsed.

[0017] In one aspect, the control unit changes the threshold value based on a change in the flow rate of current flowing through the heat source due to deterioration of the heat source over time.

[0018] In one aspect, changing the threshold value based on a change in the flow rate of current flowing through the heat source due to deterioration of the heat source over time includes changing the threshold value based on a difference between the flow rate of current passing through the heat source in a first period and the flow rate of current passing through the heat source in a second period that is after the first period.

[0019] In one aspect, the control unit changes the threshold value based on the temperature and / or humidity around the heat source.

[0020] According to another embodiment, there is provided a method for controlling an image forming apparatus, the method including: acquiring a voltage of a power supply, determining a threshold value of a current supplied to a heat source based on the voltage, acquiring a current value in the heat source, determining whether the current value is equal to or greater than the threshold value, and cutting off the supply of power to the heat source based on the determination that the current value is equal to or greater than the threshold value.

[0021] According to yet another embodiment, there is provided a control program for an image forming apparatus, the control program causing an image reading apparatus to acquire a voltage of a power supply, determine a threshold value of a current supplied to a heat source based on the voltage, acquire a current value in the heat source, determine whether the current value is equal to or greater than the threshold value, and, based on the determination that the current value is equal to or greater than the threshold value, cut off the supply of power to the heat source. Effect of the Invention

[0022] According to an embodiment, even when the fixing heater is connected to any one of two or more power sources having different voltages, the image forming apparatus can protect the fixing heater.

[0023] The above and other objects, features, aspects and advantages of the present disclosure will become apparent from the following detailed description of the disclosure taken in conjunction with the accompanying drawings. [Brief description of the drawings]

[0024] [Figure 1] 1 shows an example of a configuration of an image forming apparatus 100 according to the present embodiment. [Diagram 2] 2 is a diagram showing an example of the configuration of a peripheral circuit 200 of a fixing device 105. FIG. [Diagram 3] 13 is a diagram showing an example of the relationship between the voltage of the power supply unit 108 and the threshold value of the current supplied to the heat source 220. FIG. [Figure 4] FIG. 11 is a diagram showing an example of a case where a current supplied to a heat source 220 is equal to or greater than a threshold in an image forming apparatus 100 according to the present embodiment. [Diagram 5] 13 is a diagram showing another example of the configuration of the circuitry around fixing device 105 in image forming apparatus 100 according to the present embodiment. FIG. [Figure 6] FIG. 11 is a diagram illustrating an example of a process for changing a threshold value based on an inrush current. [Figure 7] FIG. 11 is a diagram showing an example of a change in threshold value based on aging deterioration of a heat source. [Figure 8] FIG. 11 is a diagram showing an example of a change in threshold value based on the surrounding environment. [Figure 9] This is a flowchart showing an example of the procedure of internal processing executed by the image forming apparatus 100 according to the present embodiment.

Embodiments for Carrying Out the Invention

[0025] Hereinafter, embodiments of the technical idea according to the present disclosure will be described with reference to the drawings. In the following description, the same parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions thereof will not be repeated. Also, each embodiment, each modification example, each software configuration, each hardware configuration, each function, and each process, etc. may be selectively combined as appropriate.

[0026] <A. Configuration of the Image Forming Apparatus> FIG. 1 is a diagram showing an example of the configuration of the image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 is an arbitrary apparatus that prints an image on a medium using toner. The apparatus is, for example, an MFP (Multifunction Peripheral) or the like. Also, the image forming apparatus 100 can be configured to print an image on an arbitrary medium such as paper or resin.

[0027] The image forming apparatus 100 includes a paper feed tray 102, a registration roller 103, a toner image forming unit 104, a fixing device 105, an operation input unit 106, a control unit 107, a power supply unit 108, a medium 109, various sensors 110, and a conveyance roller 120.

[0028] The paper feed tray 102 loads and accommodates the medium 109 for forming an image. The paper feed tray 102 can be switched between an open state in which the medium 109 can be replenished from the outside of the image forming apparatus 100 and a closed state in which the medium 109 cannot be replenished from the outside of the image forming apparatus 100. There may be a plurality of paper feed trays 102. The medium 109 loaded on the paper feed tray 102 is conveyed by the conveyance roller 120 toward the downstream of the conveyance path.

[0029] The registration roller 103 adjusts the transport speed of the medium 109. The registration roller 103 guides the medium 109 transported from the transport roller 120 one sheet at a time to the printing position along the dotted line which is the transport path. The registration roller 103 stops the medium 109 temporarily until the timing for printing arrives. The registration roller 103 may include a metal roller and a rubber roller.

[0030] Toner image forming unit 104 synthesizes images of four colors, Y (yellow), M (magenta), C (cyan), and K (black), in a tandem manner. In one aspect, toner image forming unit 104 may synthesize images of any color, three or less, or five or more. Toner image forming unit 104 forms a toner image on medium 109 conveyed by registration roller 103. More specifically, toner image forming unit 104 forms a toner image of each color on a photoconductor corresponding to each color.

[0031] Next, the toner image forming unit 104 transfers the toner images on each photoconductor to an intermediate transfer belt. The toner images of each color are superimposed on the intermediate transfer belt to form a full-color toner image. The full-color toner image on the intermediate transfer belt is transferred to the medium 109 being conveyed by a transfer roller.

[0032] The fixing device 105 applies heat and pressure to the medium 109 onto which the toner image has been transferred by the toner image forming unit 104. This fixes the toner image to the medium 109. More specifically, the fixing device 105 is made up of two rollers, one of which includes a heat source inside. The heat source here is a fixing heater such as a halogen heater. The heat source included in the fixing device 105 may be called a fixing heater or a heater.

[0033] The toner on the surface of the medium 109 sandwiched between the two rollers is melted by the heat generated by the heat source. Then, as the medium 109 passes through the fixing device 105, the melted toner solidifies and is fixed to the medium 109.

[0034] The control unit 107 controls the overall operation of the image forming apparatus 100. The control unit 107 may include a processor that executes a control program, a read only memory (ROM) that stores the control program and the like, and a random access memory (RAM) that constitutes a work area for the CPU. Furthermore, the control unit 107 may include a non-volatile storage device.

[0035] In one aspect, the processor may include at least one Central Processing Unit (CPU), at least one Graphics Processing Unit (GPU), at least one Field Programmable Gate Array (FPGA), at least one Application Specific Integrated Circuit (ASIC), at least one Artificial Intelligence (AI) chip, or a combination thereof.

[0036] The ROM may be a non-volatile memory and may be rewritable. The ROM may be realized by a hard disk drive (HDD), a solid state drive (SSD), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM), a flash memory, etc. The RAM may be realized by a dynamic random access memory (DRAM), a static random access memory (SRAM), etc.

[0037] Operation input unit 106 accepts input of print settings and / or operation settings of image forming apparatus 100 for medium 109 input by a user of image forming apparatus 100. Operation input unit 106 outputs various settings input by the user of image forming apparatus 100 to control unit 107.

[0038] As an example, the operation input unit 106 may include a display unit (not shown) and an operation unit (not shown). The display unit may be a liquid crystal monitor, an organic EL (Electro Luminescence) monitor, or the like. The liquid crystal monitor, the organic EL monitor, or the like includes a touch sensor, and is capable of displaying an operation menu and receiving input by touch from the user. The operation unit includes a plurality of buttons, and is capable of receiving input from the user in the same manner as a touch panel. In one aspect, the operation unit may include a touch sensor that is overlaid on the display unit.

[0039] The power supply unit 108 supplies power to each component of the image forming apparatus 100 under the control of the control unit 107. As an example, the power supply unit 108 is a power supply circuit, and is configured to be connectable to a commercial external power supply. The voltage of the commercial external power supply varies depending on the country. For example, a power supply with a commercial external voltage of 100V to 250V is often used.

[0040] The various sensors 110 may include an optical sensor, an ultrasonic sensor, any other sensor, or a combination of these. Under the control of the control unit 107, the various sensors 110 detect the physical properties of the medium 109 using different methods and output information related to the detected physical properties to the control unit 107. The detection positions of the various sensors 110 are upstream of the registration rollers 103 along the transport path. The optical sensor outputs information related to the transmittance of light passing through the medium 109. The ultrasonic sensor outputs information related to the transmittance of ultrasonic waves passing through the medium 109.

[0041] FIG. 2 is a diagram showing an example of the configuration of a circuit 200 around the fixing device 105. The configuration and operation of the circuit 200 will be described in detail with reference to FIG. 2. The circuit 200 is a circuit that is assumed to be connected to commercial power sources having different voltages around the world. In other words, the circuit 200 is a circuit that is assumed to use a worldwide fixing heater in an image forming apparatus. Therefore, the circuit 200 has a function of appropriately protecting the heat source of the fixing device 105 even when it is connected to a high-voltage power source such as 200V or 240V, not just 100V.

[0042] The circuit 200 includes a power supply unit 108, a voltage detection unit 204, a diode 206, a diode 208, a coil 210, a current detection unit 212, the fixing device 105, a switch 222, and a control unit 107. The fixing device 105 also includes a temperature detection unit 216, a heating roller 218, and a heat source 220. The temperature detection unit 216 may be located outside the fixing device 105. The circuit 200 may also include a humidity detection unit (not shown).

[0043] The power supply unit 108 operates as an AC power supply and supplies power to each unit of the image forming apparatus 100. The power supply unit 108 is connected to a commercial external power supply. In one aspect, the power supply unit 108 itself may be a commercial external power supply. In this case, the power supply unit 108 is present outside the circuit 200. The voltage of the commercial external power supply may differ depending on the country. For example, a power supply of 100V to 250V is used as the commercial external voltage. The power supply unit 108 is configured to be connectable to at least any of an external power supply of a first voltage (e.g., a 100V power supply) and an external power supply of a second voltage higher than the first voltage (e.g., a 200V power supply). Furthermore, the power supply unit 108 may be configured to be connectable to any of three or more power supplies of different voltages, such as an external power supply of a third voltage and an external power supply of a fourth voltage.

[0044] Both ends of the voltage detection unit 204 are connected to the power supply unit 108 via a circuit. The voltage detection unit 204 detects the voltage of the power supply unit 108. The voltage detection unit 204 outputs a signal S201 indicating the detected voltage to the control unit 107.

[0045] Both ends of diode 206 are connected to power supply unit 108 via a circuit so as to be in parallel with voltage detection unit 204. Diode 206 is a rectifier diode, and converts AC current input from power supply unit 108 into DC current. The current converted into DC current flows into downstream coil 210.

[0046] The coil 210 is provided downstream of the diode 206 and upstream of the fixing device 105. The coil 210 smoothes the current converted into a direct current by the diode 206. The coil 210 also suppresses an inrush current from flowing into the fixing device 105.

[0047] The current detection unit 212 is provided downstream of the coil 210. The current detection unit 212 detects the current value of the current flowing into the heat source 220. The current detection unit 212 outputs a signal S202 indicating the detected current value of the current flowing into the heat source 220 to the control unit 107.

[0048] Fixing device 105 is provided downstream of current detection unit 212. The DC current smoothed by passing through coil 210 flows into heat source 220. This causes the temperature of heat source 220 to rise. Heat source 220 is located inside heating roller 218. As the temperature of heat source 220, which is a fixing heater, rises, the temperature of heating roller 218 also rises.

[0049] The temperature detection unit 216 detects the temperature of the heat source 220. Furthermore, the temperature detection unit 216 outputs a signal S203 indicating the detected temperature of the heat source 220 to the control unit 107.

[0050] The switch 222 is provided downstream of the fixing device 105. The switch 222 switches on and off the supply of power to the heat source 220 based on a signal S204 from the control unit 107. More specifically, the switch 222 switches on and off the connection of the circuit from the fixing device 105 to the diode 206 based on the signal S204, which is a PWM signal output from the control unit 107. In one aspect, the switch 222 may be a transistor.

[0051] Diode 208 is a freewheeling diode for discharging an electromotive force (surge voltage) generated in coil 210 when switch 222 is turned off. One end of diode 208 is connected to a circuit between diode 206 and coil 210. The other end of diode 208 is connected to a circuit between fixing device 105 and switch 222.

[0052] In the circuit 200, the control unit 107 controls the switch 222 to prevent the temperature of the heat source 220 from rising too much. As described above, the control unit 107 acquires the signal S201 indicating the voltage of the power supply unit 108 from the voltage detection unit 204. The control unit 107 determines the threshold value of the current supplied to the heat source 220 based on the acquired signal S201. That is, the control unit 107 determines the threshold value of the current supplied to the heat source 220 based on the voltage of the power supply unit 108.

[0053] Furthermore, the control unit 107 acquires a signal S202 indicating the current value of the current flowing into the fixing device 105 from the current detection unit 212. The control unit 107 can calculate the current value of the current flowing into the fixing device 105 from the signal S202. Furthermore, the control unit 107 compares the current value of the current flowing into the fixing device 105 with a threshold value.

[0054] Based on the determination that the current value of the current flowing into the fixing device 105 is equal to or greater than the threshold value, the control unit 107 outputs a signal S204 to the switch 222 to cut off the supply of power to the heat source 220. By turning off the switch 222, the control unit 107 can prevent the temperature of the heat source 220 from rising too much.

[0055] Assume that control unit 107 determines that the current value of the current flowing into fixing device 105 is less than the threshold value. In this case, control unit 107 adjusts the current flowing into fixing device 105 so that the value of the current flowing into fixing device 105 is within a predetermined range. More specifically, control unit 107 estimates the temperature of heat source 220 from signal S203 acquired from temperature detection unit 216. Control unit 107 adjusts signal S204 output to switch 222 so that the temperature of heat source 220 is within a predetermined range.

[0056] In normal operation, the control unit 107 determines the duty ratio of the signal S204 based on the signals S201 and S203. More specifically, the control unit 107 determines the duty ratio so that the temperature of the heat source 220 becomes the set temperature in each operation mode, such as printing on normal paper or printing on thick paper. The duty ratio indicates the proportion of the signal that is on in one period of the signal. The control unit 107 transmits the signal S204 having the determined duty ratio to the switch 222. The switch 222 starts to turn on and off based on the signal S204.

[0057] When switch 222 is turned on, the current supplied from power supply unit 108 is rectified by diode 206. The current then flows into coil 210 and heat source 220. During this time, coil 210 stores a portion of the current flowing through it as magnetic energy.

[0058] On the other hand, when switch 222 is turned off, the supply of power from power supply unit 108 to heat source 220 is cut off. Also, the magnetic energy stored in coil 210 is released as a current and starts to flow to heat source 220. This current returns to coil 210 via diode 208. As an example, control unit 107 can cut off the supply of power to heat source 220 by setting the duty ratio of signal S204 to 0.

[0059] By the above operation, the power necessary to maintain the set temperature of each operation mode is supplied to the heat source 220. As a result, the temperatures of the heat source 220 and the heating roller 218 are maintained at the set temperatures in each operation mode. Further, the control unit 107 can cut off the power supply to the heat source 220 based on detecting that an excessive current is flowing into the heat source 220.

[0060] As described above, the image forming apparatus 100 includes a voltage detection unit 204 that detects the voltage of the power supply (power supply unit 108), a current detection unit 212 that detects the current value in the heat source 220, a switch 222 that switches on and off the supply of power to the heat source 220, and a control unit 107 that controls the switch 222. The control unit 107 acquires a voltage (signal S201) from the voltage detection unit 204 and determines a threshold value of the current supplied to the heat source 220 based on the voltage. Further, the control unit 107 acquires a current value (signal S202) from the current detection unit 212 and determines whether the current value is equal to or greater than the threshold value. Furthermore, the control unit 107 outputs a signal (signal S204) for cutting off the power supply to the heat source 220 to the switch 222 based on determining that the current value is equal to or greater than the threshold value.

[0061] In this way, the control unit 107 estimates the temperature rise of the heat source 220 based on the voltage of the power supply unit 108 and the current value flowing into the heat source 220. Thereby, compared with the case of using a thermostat with a slow temperature rising speed, the control unit 107 can detect or predict the temperature rise of the heat source 220 at a higher speed. Therefore, even when the image forming apparatus 100 is connected to a high-voltage power supply such as 200V and the temperature of the heat source 220 rises rapidly, the control unit 107 can protect the heat source 220.

[0062] As described above, the image forming apparatus 100 is configured to be connectable to at least an external power supply of a first voltage (for example, a 100V power supply) and an external power supply of a second voltage higher than the first voltage (for example, a 200V power supply).

[0063] <B. Threshold Value of Current Flowing into Heat Source> 3 is a diagram showing an example of the relationship between the voltage of power supply unit 108 and the threshold of the current supplied to heat source 220. When the voltage changes, the current flowing through heat source 220 also changes. For example, assume that the voltage of power supply unit 108 changes from 100V to 200V. In this case, the current flowing through heat source 220 doubles. That is, the amount of current flowing through heat source 220 changes significantly depending on the voltage of power supply unit 108. For this reason, it is desirable for control unit 107 to change the threshold of the current flowing through heat source 220 for each voltage of power supply unit 108.

[0064] Therefore, control unit 107 changes the threshold of the current flowing through heat source 220 based on the voltage of power supply unit 108. As an example, image forming apparatus 100 uses a first threshold when connected to a power supply of a low first voltage (e.g., 100V). Also, image forming apparatus 100 uses a second threshold that is higher than the first threshold when connected to a power supply of a second voltage (e.g., 200V) that is higher than the first voltage, such as 200V.

[0065] However, changing the threshold value also changes the power flowing to the heat source 220. Therefore, in order to keep the power flowing to the heat source 220 constant, the image forming apparatus 100 changes the duty ratio of the PWM signal in addition to the threshold value based on the change in the voltage of the power supply unit 108.

[0066] 3 indicates a current that flows to heat source 220 based on a PWM signal when power supply unit 108 is connected to an external power supply of a first voltage. Similarly, current pulse 306B indicates a current pulse that flows to heat source 220 based on a PWM signal when power supply unit 108 is connected to an external power supply of a second voltage higher than the first voltage. Using current pulse 306A and current pulse 306B as examples, it will be described how the threshold value and the PWM signal change as the voltage of power supply unit 108 changes.

[0067] The current pulse 306A is a pulse generated by turning on and off the switch 222 when the voltage of the power supply unit 108 is a first voltage. A length 310A of the current pulse 306A matches a length 320A of a section in which the PWM signal 308A that controls the switch 222 is on. A peak value of the current pulse 306A is determined by the voltage of the power supply unit 108. A line 304A is a curve that connects the peak values ​​of the current pulse 306A in the normal range. The first threshold 302A can be set to, for example, a value slightly larger than the line 304A.

[0068] The current pulse 306B is a pulse generated by turning on and off the switch 222 when the voltage of the power supply unit 108 is the second voltage. The length 310B of the current pulse 306B matches the length 320B of the section in which the PWM signal 308B that controls the switch 222 is on. The peak value of the current pulse 306B is determined by the voltage of the power supply unit 108. The line 304B is a curve that connects the peak values ​​of the current pulse 306B in the normal range. The second threshold 302B can be set to, for example, a value slightly larger than the line 304B.

[0069] Comparing current pulses 306A and 306B, it can be seen that the peak of current pulse 306B is significantly higher than the peak of current pulse 306A. This difference in the peaks of the currents is due to the difference in the voltage of power supply unit 108. In a circuit, if the power supply voltage is doubled, other conditions are the same, the current flowing through the circuit will also double. Therefore, first threshold 302A is not suitable for use as the threshold of current pulse 306B.

[0070] Therefore, control unit 107 adjusts the threshold of the current flowing through heat source 220 based on the change in the voltage of power supply unit 108. When the voltage of power supply unit 108 is low, control unit 107 reduces the threshold of the current flowing through heat source 220. When the voltage of power supply unit 108 is high, control unit 107 increases the threshold of the current flowing through heat source 220.

[0071] However, simply increasing the threshold value changes the current flowing into heat source 220. As a result, the power supplied to heat source 220 may also change. Therefore, control unit 107 reduces the duty ratio of the PWM signal in order to keep the power supplied to heat source 220 constant.

[0072] As an example, assume that the voltage of the power supply unit 108 has doubled. In this case, the peak value of the current is doubled. Therefore, the control unit 107 halves the duty ratio of the PWM signal. By doing so, the power (=current*voltage) supplied to the heat source 220 becomes constant regardless of the voltage of the power supply unit 108. That is, the control unit 107 can keep the upper limit of the amount of power supplied to the heat source 220 constant by changing the threshold of the current according to the voltage of the power supply unit 108 and changing the duty ratio of the PWM signal. In the example of FIG. 3, based on the change in the voltage of the power supply unit 108 from the first voltage to the higher second voltage, the control unit 107 increases the threshold and decreases the duty ratio of the PWM signal (signal S204).

[0073] As described above, control unit 107 selects or calculates a threshold value according to the changed voltage based on the change in the voltage of the power supply (power supply unit 108). More specifically, control unit 107 increases the threshold value based on an increase in the voltage of the power supply (power supply unit 108). Conversely, control unit 107 decreases the threshold value based on a decrease in the voltage of the power supply (power supply unit 108). In one aspect, control unit 107 may select the threshold value from a table that associates the power supply voltage with the threshold value. In another aspect, control unit 107 may calculate the threshold value from a mathematical formula that uses the power supply voltage as an input.

[0074] As described above, the signal S204 is a PWM signal. The control unit 107 reduces the threshold value based on a drop in the voltage of the power supply (power supply unit 108). Furthermore, the control unit 107 increases the duty ratio of the signal.

[0075] Conversely, the control unit 107 increases the threshold value based on an increase in the voltage of the power supply (power supply unit 108). Furthermore, the control unit 107 decreases the duty ratio of the signal S204.

[0076] 4 is a diagram showing an example of a case where the current supplied to heat source 220 is equal to or greater than a threshold in image forming apparatus 100 according to the present embodiment. An example of a process in which control unit 107 cuts off the supply of power to heat source 220 will be described with reference to FIG.

[0077] For example, it is assumed that the voltage of the power supply unit 108 is a relatively low first voltage (such as 100V). In this case, the control unit 107 sets a threshold indicating an upper limit of the current flowing to the heat source 220 to a first threshold 302A corresponding to the first voltage. At this time, the control unit 107 determines whether or not the peak value of the current pulse 400A of the current flowing to the heat source 220 is equal to or greater than the first threshold 302A. Based on the determination that the peak value of the current pulse 400A of the current flowing to the heat source 220 is equal to or greater than the first threshold 302A, the control unit 107 sets the duty ratio of the PWM signal (signal S204) to 0. That is, the control unit 107 outputs a signal S204 to the switch 222 to cut off the supply of power to the heat source 220. Based on the acquisition of the signal S204, the switch 222 cuts off the supply of power to the heat source 220.

[0078] For example, it is assumed that the voltage of the power supply unit 108 is a relatively high second voltage (such as 200V). In this case, the control unit 107 sets a threshold indicating an upper limit of the current flowing to the heat source 220 to the second threshold 302B corresponding to the second voltage. At this time, the control unit 107 determines whether or not the peak value of the current pulse 400B of the current flowing to the heat source 220 is equal to or greater than the second threshold 302B. Based on the determination that the peak value of the current pulse 400B of the current flowing to the heat source 220 is equal to or greater than the second threshold 302B, the control unit 107 sets the duty ratio of the PWM signal (signal S204) to 0. That is, the control unit 107 outputs a signal S204 to the switch 222 to cut off the supply of power to the heat source 220. Based on the acquisition of the signal S204, the switch 222 cuts off the supply of power to the heat source 220.

[0079] As described above, the threshold value of the current flowing through the heat source 220 changes according to the voltage of the power supply unit 108. That is, based on the voltage of the power supply unit 108, the upper limit value (threshold value) of the current determined to be abnormal changes.

[0080] <C. Modification Example> FIG. 5 is a diagram showing another example of the configuration of the circuit around the fixing device 105 in the image forming apparatus 100 according to the present embodiment. The circuit 500 includes a bypass switch 502 and a thermostat 504 in addition to the components of the circuit 200. In FIG. 5, the same components as those included in the circuit 200 are denoted by the same reference numerals. Their names and functions are also the same. Therefore, the description thereof will not be repeated.

[0081] The bypass switch 502 is provided downstream of the current detection unit 212 and upstream of the fixing device 105. The bypass switch 502 switches the path of the current flowing into the fixing device 105. The bypass switch 502 can switch between two paths. The first path is a path in which current flows through the heating roller 218 without passing through the thermostat 504, as in the circuit 200. The second circuit is a path in which current flows through the heating roller 218 via the thermostat 504. The control unit 107 switches the path through which current flows by controlling the bypass switch 502.

[0082] The thermostat 504 is provided downstream of the bypass switch 502 and upstream of the heating roller 218. When the image forming apparatus 100 is connected to a power supply having a low first voltage such as 100V, the image forming apparatus 100 can protect the heat source 220 by using the thermostat 504 in the conventional manner. The method of protecting the heat source 220 using the thermostat 504 is a hard protection without the intervention of control. Therefore, the method of protecting the heat source 220 by the thermostat 504 has the advantage that the certainty of protecting the heat source 220 is higher than that of the soft protection with the intervention of control.

[0083] Therefore, when image forming apparatus 100 is connected to a power source with a low first voltage such as 100V, control unit 107 switches intermediate switch 502 so that the current passes through thermostat 504. Specifically, control unit 107 outputs signal S501 for switching the current path to intermediate switch 502. At that time, control unit 107 may disable the function of current detection unit 212.

[0084] Furthermore, when image forming apparatus 100 is connected to a power source with a high second voltage such as 200V, control unit 107 switches bypass switch 502 so that the current flows to heating roller 218 without passing through thermostat 504. Specifically, control unit 107 outputs signal S501 for switching the current path to bypass switch 502. When this switching process is performed, control unit 107 sets current detection unit 212 to an available state.

[0085] As described above, image forming apparatus 100 further includes bypass switch 502 for switching whether or not the current to heat source 220 passes through the path via thermostat 504. Based on the fact that image forming apparatus 100 is connected to an external power supply of a first voltage, control unit 107 switches bypass switch 502 so that the current to heat source 220 passes through the path via thermostat 504. Based on the fact that image forming apparatus 100 is connected to an external power supply of a second voltage that is higher than the first voltage, control unit 107 switches bypass switch 502 so that the current to heat source 220 does not pass through the path via thermostat 504.

[0086] As described above, when image forming apparatus 100 is connected to a low first voltage such as 100 V, image forming apparatus 100 uses thermostat 504 to protect heat source 220. Thermostat 504 does not require control by control unit 107 and can operate independently as hardware. Therefore, by using thermostat 504, image forming apparatus 100 can more reliably protect heat source 220.

[0087] Also, when the image forming apparatus 100 is connected to a high second voltage such as 200V, the image forming apparatus 100 uses the current detection unit 212 to protect the heat source 220. The current detection unit 212 can quickly detect changes in the current flowing into the heat source 220. The image forming apparatus 100 can immediately detect an increase in the temperature of the heat source 220 from the change in the current. That is, by using the current detection unit 212, the image forming apparatus 100 can protect the heat source 220 even when the temperature of the heat source 220 rises rapidly.

[0088] <D. Adjustment of the Threshold Value of the Current Flowing through the Heat Source> FIG. 6 is a diagram showing an example of a threshold change process based on an inrush current. Immediately after the image forming apparatus 100 is connected to an external power source, a large inrush current temporarily flows from the power supply unit 108 into the image forming apparatus 100. Therefore, due to the inrush current, the amount of current flowing through the heat source 220 and the current detection unit 212 temporarily changes greatly. As a result, immediately after the image forming apparatus 100 is connected to an external power source, there is a possibility that a current equal to or greater than the threshold value flows into the heat source 220. However, this is only temporary, and the current flowing into the heat source 220 immediately drops to a normal value.

[0089] As described above, due to the occurrence of the inrush current, a current equal to or greater than the threshold value temporarily flows into the heat source 220. Therefore, each time the inrush current occurs, the control unit 107 may cut off the power supply to the heat source 220. Therefore, it is desirable for the control unit 107 to change the threshold value of the current flowing through the heat source 220 while the inrush current is flowing. Therefore, the control unit 107 may temporarily increase the threshold value of the current flowing through the heat source 220 so that the magnitude of the inrush current does not exceed the threshold value of the current flowing into the heat source 220.

[0090] As an example, the control unit 107 may temporarily increase the threshold value for a predetermined period after the image forming apparatus 100 is connected to an external power source. The predetermined period is the period during which the inrush current occurs. The predetermined period may be obtained through experiments or the like. Also, the predetermined period may be an arbitrary period such as 50 ms.

[0091] The current pulse 608 is a collection of small current pulses in a normal range. Normally, the second threshold 604 may be set to, for example, a value slightly larger than the current pulse 608. On the other hand, while an inrush current is flowing into the image forming apparatus 100, the current pulse 606 is larger than the current pulse 608 under normal conditions.

[0092] Therefore, control unit 107 increases first threshold value 602 for a predetermined period 610 after image forming apparatus 100 is connected to an external power source. This allows control unit 107 to prevent cutting off the power supply to heat source 220 due to detection of an inrush current. In one aspect, predetermined period 610 may be determined by experiment or the like at the time of product design.

[0093] As described above, the control unit 107 selects or calculates the first threshold 602 during a predetermined period 610 after the start of the supply of power to the heat source 220. After the predetermined period 610 has elapsed, the control unit 107 selects or calculates the second threshold 604 that is smaller than the first threshold 602. The first threshold 602 is a threshold corresponding to an inrush current, and the second threshold 604 is a threshold corresponding to a normal current. In one aspect, the control unit 107 may select the threshold from a table that associates the power supply voltage with the threshold. In another aspect, the control unit 107 may calculate the threshold from a mathematical formula that uses the power supply voltage as an input.

[0094] 7 is a diagram showing an example of a change in the threshold value based on the deterioration of the heat source over time. The resistance of the heat source 220 may increase due to deterioration over time. That is, the amount of current flowing through the heat source 220 and the current detection unit 212 decreases due to the deterioration of the heat source 220 over time. That is, the normal value of the current flowing into the heat source 220 decreases due to the deterioration of the heat source 220 over time.

[0095] This causes a large discrepancy between the normal value of the current flowing into the heat source 220 and the threshold value of the current flowing into the heat source 220. Therefore, even if an excessive current flows into the heat source 220 and the heat source 220 reaches an abnormal temperature, the value of the current flowing into the heat source 220 does not exceed the threshold value, and the abnormality is likely not to be detected.

[0096] Therefore, it is desirable for control unit 107 to change the threshold of the current flowing through heat source 220 based on the deterioration over time of heat source 220. Therefore, control unit 107 changes the threshold of the current flowing through heat source 220 in order to appropriately protect heat source 220. More specifically, control unit 107 reduces the threshold of the current flowing through heat source 220 based on the deterioration over time of heat source 220.

[0097] As an example, the control unit 107 may permanently reduce the threshold value based on detection of a decrease in the current flowing through the heat source 220 due to deterioration of the heat source 220. The control unit 107 may detect deterioration of the heat source 220 by comparing the current value of the current flowing through the heat source 220 before deterioration with the current current value of the current flowing through the heat source 220. In one aspect, the control unit 107 may periodically compare the current value of the current flowing through the heat source 220 before deterioration with the current current value of the current flowing through the heat source 220. In one aspect, the control unit 107 may set the changed threshold value to a value obtained by adding an arbitrary current value, such as 10 A, to the current value of the current flowing through the heat source 220 after deterioration. The current value of the current flowing through the heat source 220 after deterioration means a normal value of the current flowing through the heat source 220 after deterioration.

[0098] The first period 710 is a period in which the heat source 220 has not deteriorated over time. In the first period 710, the control unit 107 may use a first threshold value 702 for the heat source 220 that has not deteriorated. The second period 712 is a period in which the heat source 220 has deteriorated over time. In the second period 712, the control unit 107 may use a second threshold value 704 that is smaller than the first threshold value 702. The control unit 107 may detect deterioration of the heat source 220 and a decrease in the current flowing through the heat source 220 by the signal S202 obtained from the current detection unit 212.

[0099] Control unit 107 reduces the threshold value based on detection of a decrease in the current flowing through heat source 220 due to aging deterioration of heat source 220. That is, control unit 107 may use second threshold value 704 that is smaller than first threshold value 702 based on aging deterioration of heat source 220. In this way, even if the current flowing through heat source 220 changes due to aging deterioration of heat source 220, control unit 107 may appropriately cut off the supply of power to heat source 220 to protect heat source 220.

[0100] As described above, the control unit 107 changes the first threshold value 702 based on the change in the current flowing through the heat source 220 due to aging deterioration of the heat source 220. Furthermore, changing the first threshold value 702 based on the change in the current flowing through the heat source 220 due to aging deterioration of the heat source 220 includes changing the first threshold value 702 based on the difference between the current value of the current that passed through the heat source 220 in a first period 710 and the current value of the current that passed through the heat source 220 in a second period 712 after the first period 710.

[0101] 8 is a diagram showing an example of a change in threshold value based on the surrounding environment. The resistance of the heat source 220 may change depending on the surrounding environment. For example, the resistance of the heat source 220 may change depending on the temperature and / or humidity around the heat source 220. The resistance of the heat source 220 may increase, particularly due to an increase in the surrounding temperature. That is, even if the detected voltage of the heat source 220 is the same, the current flowing through the heat source 220 and the current detection unit 212 decreases due to an increase in the temperature around the heat source 220.

[0102] This may cause a large discrepancy between the normal value of the current flowing into heat source 220 and the threshold value of the current flowing into heat source 220. In other words, even if heat source 220 reaches an abnormal temperature, the value of the current flowing into heat source 220 may not exceed the threshold value, and control unit 107 may not be able to properly detect the abnormality.

[0103] Therefore, it is desirable for control unit 107 to change the threshold of the current flowing through heat source 220 based on an increase in the temperature around heat source 220. Therefore, in order to appropriately protect heat source 220, control unit 107 changes the threshold of the current flowing through heat source 220 according to the temperature around heat source 220.

[0104] As an example, the control unit 107 determines whether the temperature around the heat source 220 obtained from the temperature detection unit 216 is equal to or higher than a threshold temperature of 35° C. The threshold temperature can be set to any temperature, such as 35° C.

[0105] Table 800 is a table that stores a threshold value for each temperature (environmental temperature) around heat source 220. As an example, table 800 includes a first threshold value 802 that is used when the environmental temperature is less than a predetermined temperature. Table 800 also includes a second threshold value 804 that is used when the environmental temperature is equal to or greater than a predetermined temperature. The predetermined temperature may be set to any temperature, for example, 35°C. In the example of FIG. 8, table 800 stores threshold values ​​for two temperature ranges, less than 35°C and equal to or greater than 35°C, but this is merely an example. In one aspect, table 800 may store threshold values ​​corresponding to three or more temperature ranges.

[0106] Based on Table 800, when the temperature around the heat source 220 is less than a predetermined temperature, the control unit 107 determines the threshold value of the current flowing into the heat source 220 as the first threshold value 802. When the temperature around the heat source 220 is equal to or higher than the predetermined temperature, the control unit 107 sets the threshold value of the current flowing into the heat source 220 as the second threshold value 804. The second threshold value 804 is smaller than the first threshold value 802. The first threshold value 802 and the second threshold value 804 can be set to arbitrary values. In one aspect, Table 800 may be stored in the ROM or storage device of the control unit 107. In this case, the control unit 107 can refer to Table 800 in the ROM or storage device. In other aspects, Table 800 may include the threshold values themselves or correction values for the threshold values. For example, when the temperature around the heat source 220 is less than the threshold temperature, the control unit 107 adds a first correction value to the threshold value. Conversely, when the temperature around the heat source 220 is equal to or higher than the threshold temperature, the control unit 107 adds a second correction value to the threshold value.

[0107] Also, the control unit 107 may change the threshold value according to the temperature and / or humidity around the heat source 220. By doing so, even if the resistance of the heat source 220 changes due to a change in the environment around the heat source 220, the control unit 107 can appropriately cut off the power supply to the heat source 220 and protect the heat source 220.

[0108] As described above, the control unit 107 changes the threshold value based on the environment around the heat source 220. More specifically, the control unit 107 can change the threshold value based on the temperature around the heat source 220. Also, the control unit 107 can change the threshold value based on the humidity around the heat source 220. Furthermore, the control unit 107 can change the threshold value based on both the temperature and humidity around the heat source 220.

[0109] <E. Procedure of Internal Processing of Image Forming Apparatus> Fig. 9 is a flowchart showing an example of a procedure of a process executed by image forming apparatus 100 according to the present embodiment. In one aspect, control unit 107 may load a program for executing the process of Fig. 9 from a storage device or a ROM into a RAM and execute the program. In another aspect, some or all of the process may be realized as a combination of circuit elements configured to execute the process.

[0110] In step S910, the control unit 107 acquires the voltage of the power supply unit 108 from the voltage detection unit 204. More specifically, the control unit 107 acquires a signal S201 indicating the voltage of the power supply unit 108 from the voltage detection unit 204. The control unit 107 can detect the voltage of the power supply unit 108 based on the signal S201.

[0111] In step S920, control unit 107 determines whether the voltage of power supply unit 108 is greater than a first voltage. The first voltage may be any voltage, such as 100V, 110V, etc. If control unit 107 determines that the voltage of power supply unit 108 is greater than the first voltage (YES in step S920), control unit 107 transfers control to step S940. If not (NO in step S920), control unit 107 transfers control to step S930.

[0112] In step S930, the control unit 107 sets the threshold of the current flowing into the heat source 220 to the first threshold for the first voltage.

[0113] In step S940, the control unit 107 sets the threshold of the current flowing into the heat source 220 to a second threshold for the second voltage. The second voltage is a voltage higher than the first voltage.

[0114] In a certain aspect, the control unit 107 may change the threshold of the current flowing through the heat source 220 according to the voltage of the power supply unit 108. As an example, the control unit 107 may hold a table in which each of a plurality of voltages is associated with a threshold corresponding to each voltage. In this case, the control unit 107 may select the threshold corresponding to the voltage of the power supply unit 108 from the table. As another example, the control unit 107 may determine the threshold according to various power supply voltages by substituting the voltage of the heat source 220 into a pre-integrated trained AI or the like and obtaining the output threshold. Furthermore, as another example, the control unit 107 may execute a program that uses the voltage of the power supply unit 108 as an input and performs calculation of a formula for calculating the threshold. For example, the formula may be obtained from experimental results such as the rate of temperature rise of the heat source 220 when a certain voltage is applied to the heat source 220.

[0115] For example, when the voltage of power supply unit 108 is 100V, control unit 107 may set the threshold to a threshold for 100V (first threshold 302A). Similarly, when the voltage of power supply unit 108 is 130V, control unit 107 may set the threshold to a value for 130V. Furthermore, when the voltage of power supply unit 108 is 220V, control unit 107 may set the threshold to a value for 220V. In this way, control unit 107 may set the threshold to a value according to the individual power supply voltage.

[0116] In step S950, control unit 107 acquires the current value of the current flowing into heat source 220 from current detection unit 212. More specifically, control unit 107 acquires signal S202 indicating the current value of the current flowing into heat source 220 from current detection unit 212. Control unit 107 calculates or detects the current value of the current flowing into heat source 220 based on signal S202.

[0117] In step S960, control unit 107 determines whether or not the current value of the current flowing into heat source 220 is equal to or greater than a threshold value. If control unit 107 determines that the current value of the current flowing into heat source 220 is equal to or greater than the threshold value (YES in step S960), control proceeds to step S970. If not (NO in step S960), control unit 107 executes the process of step S960. In another aspect, control unit 107 may determine whether or not the current value of the current flowing into heat source 220 has exceeded a threshold value.

[0118] In step S970, the control unit 107 cuts off the supply of power (current flow) to the heat source 220. More specifically, the control unit 107 outputs a signal S204 for cutting off the supply of power to the heat source 220 to the switch 222. The signal S204 is a PWM signal with a duty ratio of 0. When the PWM signal with a duty ratio of 0 is input, the switch 222 cuts off the circuit connecting the power supply unit 108 and the heat source 220. As a result, the supply of power to the heat source 220 is cut off.

[0119] In one aspect, image forming apparatus 100 may execute a switching process of bypass switch 502 in steps S920 to S940. As an example, it is assumed that the voltage of power supply unit 108 is a low voltage such as 100V. In this case, control unit 107 controls bypass switch 502 to connect power supply unit 108 to heat source 220 via thermostat 504. As another example, it is assumed that the voltage of power supply unit 108 is a high voltage such as 200V. In this case, control unit 107 controls bypass switch 502 to connect power supply unit 108 to heat source 220 without passing through thermostat 504. Furthermore, control unit 107 may change the threshold value according to the voltage of power supply unit 108 after enabling current detection unit 212.

[0120] As described above, the control unit 107 acquires the voltage of the power supply (step S910), and determines the threshold of the current supplied to the heat source 220 based on the voltage (step S920) (steps S930 and S940). The control unit 107 acquires the current value in the heat source 220 (step S950), and determines whether the current value is equal to or greater than the threshold (step S960). Furthermore, the control unit 107 cuts off the supply of power to the heat source 220 based on the determination that the current value is equal to or greater than the threshold (step S970). In this way, the control unit 107 can quickly detect a temperature rise of the heat source 220 based on the voltage of the power supply unit 108 and the threshold of the current flowing into the heat source 220. This allows the control unit 107 to appropriately protect the heat source 220 regardless of the type of voltage of the commercial power supply.

[0121] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is indicated by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims. In addition, the disclosure contents described in the embodiments and each modified example are intended to be implemented, as far as possible, either alone or in combination. [Explanation of symbols]

[0122] 100 Image forming apparatus, 102 Paper feed tray, 103 Registration roller, 104 Toner image forming section, 105 Fixing device, 106 Operation input section, 107 Control section, 108 Power supply section, 109 Media, 110 Various sensors, 120 Conveying roller, 200, 500 Circuit, 204 Voltage detection section, 212 Current detection section, 206, 208 Diode, 210 Coil, 216 Temperature detection section, 218 Heating roller, 220 Heat source, 222 Switch, 302A, 602, 702, 802 First threshold, 302B, 604, 704, 804 Second threshold, 304A, 304B Line, 306A, 306B, 400A, 400B, 606, 608 Current pulse, 308A, 308B, S201, S202, S203, S204, S501 signal, 310A, 310B, 320A, 320B length, 502 via switch, 504 thermostat, 710 first period, 712 second period, 800 table.

Claims

1. a voltage detection unit that detects a voltage of a power supply; a current detection unit that detects a current value in the heat source; A switch for switching on and off the supply of power to the heat source; A control unit that controls the switch, The control unit is acquiring the voltage from the voltage detection unit; determining a threshold for a current provided to the heat source based on the voltage; acquiring the current value from the current detection unit; determining whether the current value is equal to or greater than the threshold value; The image forming apparatus outputs a signal to the switch to cut off the supply of power to the heat source based on the determination that the current value is equal to or greater than the threshold value.

2. The image forming apparatus according to claim 1 , wherein the control unit is configured to select the threshold value according to a voltage after the change based on the change in the voltage of the power supply.

3. 3. The image forming apparatus according to claim 2, wherein the image forming apparatus is configured to be connectable to at least both an external power source of a first voltage and an external power source of a second voltage higher than the first voltage.

4. The signal is a PWM (Pulse Width Modulation) signal, The image forming apparatus according to claim 3 , wherein the control unit decreases the threshold value and increases a duty ratio of the signal based on the decrease in the voltage of the power supply.

5. the signal is a PWM signal, The image forming apparatus according to claim 3 , wherein the control unit increases the threshold value and decreases a duty ratio of the signal based on an increase in the voltage of the power supply.

6. the image forming apparatus further includes a bypass switch for switching whether or not a current to the heat source passes through a path via a thermostat; The control unit is switching the bypass switch so that a current to the heat source passes through a path that passes through the thermostat based on the image forming apparatus being connected to an external power supply of the first voltage; 4. The image forming apparatus according to claim 3, wherein the bypass switch is switched so that current to the heat source does not pass through a path that passes through the thermostat based on the image forming apparatus being connected to an external power source of the second voltage higher than the first voltage.

7. The control unit is selecting a first threshold value at a predetermined time period after initiation of supplying power to the heat source; 4. The image forming apparatus according to claim 1, further comprising: a second threshold value that is smaller than the first threshold value, selected after the predetermined period has elapsed.

8. 4. The image forming apparatus according to claim 1, wherein the control unit changes the threshold value based on a change in a flow rate of the current flowing through the heat source due to deterioration of the heat source over time.

9. 9. The image forming apparatus of claim 8, wherein changing the threshold value based on a change in the flow rate of current flowing through the heat source due to deterioration of the heat source over time includes changing the threshold value based on a difference between the flow rate of current passing through the heat source in a first period and the flow rate of current passing through the heat source in a second period after the first period.

10. The image forming apparatus according to claim 1 , wherein the control unit changes the threshold value based on an ambient temperature and / or humidity of the heat source.

11. A control method for an image forming apparatus, comprising: Obtaining a voltage of a power source; determining a threshold value for a current provided to a heat source based on the voltage; Obtaining a current value in the heat source; determining whether the current value is greater than or equal to the threshold; and cutting off the supply of power to the heat source based on the determination that the current value is equal to or greater than the threshold value.

12. A control program for an image forming apparatus, Obtaining a voltage of a power source; determining a threshold value for a current provided to a heat source based on the voltage; Obtaining a current value in the heat source; determining whether the current value is greater than or equal to the threshold; and cutting off the supply of power to the heat source based on the determination that the current value is equal to or greater than the threshold value.

Images