JPH04287079A - Image forming device - Google Patents

Abstract

PURPOSE:To heighten safety by surely evading the overheat state of a heater, etc., in a safety circuit to prevent fuming and firing, etc., from occurring especially. CONSTITUTION:A conduction cutoff means is operated by judging the overheat state of a heating means 101 based on the temperature detection result of the heating means 101 in addition to a means 102 which detects the temperature of the heating means such as a fixing heater 101, etc., a means 104 which controls the presence/absence of a conduction instruction to the fixing heater 101 by the temperature detection result, a means 105 which performs the opening/closing control of a conduction circuit by the presence/absence of the conduction instruction, a means 107 which detects the fact that an abnormal state occurs which sets a conducting state in spite of the conduction instruction to the heating means 101, a means 109 which stores and holds the abnormal state, and the conduction cutoff means 106 which stops conduction to the fixing heater 101 by a stored and held result. Also, the preset value of duration is varied corresponding to the overheat temperature of a fixing device in addition to a means 17 which measures the duration of the overheat state, and a means 19 which operates the conduction cutoff means by judging the continuance of the overheat state.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to a safety circuit for preventing smoke and ignition.

0002

2. Description of the Related Art Image forming apparatuses that visualize images stored in memory by fixing toner on printing paper are typified by laser beam printers and copying machines. Since then, it has been widely used. A printing operation in an image forming apparatus is completed by making the toner visible on the printing paper through known electrophotographic techniques, that is, the processes of exposure, development, and transfer, and finally fixing the toner on the printing paper. In the process of fixing the toner to the print paper, a method using heat, a method using pressure, or a method using both heat and pressure is selected. If a thermal fixing method is selected, there is a possibility of a fire accident occurring because the printing paper, which can become an ignitable substance, is heated, so ensuring safety in this case is an extremely important technology. FIG. 10 shows a conventional fixing heater control circuit and a circuit for ensuring the above-mentioned safety.

[0003] Reference numeral 1101 is a current transformer that detects the flowing current, and a current proportional to the current flowing in the AC primary circuit is obtained in the secondary winding. The current transformer 1101 maintains insulation between the AC primary and secondary. Next, the resistor 1102 converts the secondary winding current value into a current/voltage. 1103 is a rectifier/smoothing circuit including a diode, a capacitor, and a resistor, and a comparator 1104 in the next stage.
For input, current transformer 1101 and resistor 110
Convert the AC voltage obtained in step 2 to DC voltage. Although the diagram shows a half-wave rectifier circuit, it may be a full-wave rectifier circuit.

Comparator 1104 has a non-inverting input terminal and an inverting input terminal, and when the inverting input voltage is greater than the non-inverting input voltage, the output becomes LOW. In the figure, since the non-inverting input terminal is grounded, the current transformer 1101
When a current is detected at , the output of comparator 1104 becomes LOW.

[0005] Reference numeral 1105 denotes a relay as current cutoff means, and when the transistor 1106 is turned on, current flows through the relay winding and the relay contacts are closed. 1107 is a flip-flop, and when the set terminal S is LOW and the reset terminal R is LOW, the inverted output bar Q is HIGH.
Therefore, the transistor 1106 is turned on. Since a capacitor and a resistor are connected to the reset terminal R, the power supply O
At N, it becomes LOW, and after a time determined by a time constant has elapsed, it becomes HIGH. When the reset terminal R is HIGH and the set terminal S is HIGH, the inverted output and bar Q are LOW.
The state becomes OW, and at this time, the transistor 1106 is turned off and the relay contact is opened.

Once set, the flip-flop 1107 will not be reset until the power is turned on again. The conditions for the output of the NOR gate 1108 to be HIGH are that the output of the comparator 1104 is LOW and the port output of the CPU 1113 is LOW, and even though the CPU 1113 is not outputting an energization instruction to the fixing unit 1112, the current transformer is This corresponds to the case where current detection by 1101 is performed.

[0007] Reference numeral 1109 is a triac that controls the opening and closing of the energized circuit, and controls the opening and closing of the AC primary circuit. 1
110 is a phototriac coupler, and when the built-in LED lights up, the triac 1109 fires, and A
C primary circuit is closed. Photo triac coupler 111
0 maintains insulation between AC primary and secondary. The transistor 1111 is connected when the port output of the CPU 1113 is HI.
It is turned on when it is GH, and at this time, the LED of the phototriac coupler 1110 lights up.

[0008] 1112 is a fixing device, and electric parts are A
It consists of a heater inserted into the C primary circuit and a thermistor that monitors the temperature of the heater. Fixed resistance 111
The resistance value of the thermistor is converted into a voltage value by the voltage dividing circuit 4 and input to the A/D conversion circuit. In the diagram, A/
Although a CPU with a built-in D conversion circuit is used, the A/D conversion circuit may be externally attached.

[0009] The CPU 1113 knows the temperature of the heater using a thermistor, and if the temperature is high, the triac 11
09 to open the AC primary circuit, and if the temperature is low, start firing of the triac 1109 to close the AC primary circuit. This controls the heater of the fixing device 1112 to maintain a constant temperature.

[0010] If the triac 1109 fails due to a short circuit in the AC primary circuit, the heater is energized even though the CPU 1113 has not outputted an energization instruction to the fixing device 1112. At this time, the current flowing through the heater is detected by the current transformer 1101, and the NO.
When the output of the R gate 1108 becomes HIGH, this abnormal state is stored and held in the flip-flop 1107,
Relay 1105 is cut off. As a result, the fuser 1
Abnormal energization of 112 is prevented, and the occurrence of a fire accident can be prevented.

[0011]

[Problems to be Solved by the Invention] However, in the above conventional example, if the CPU 1113 that controls temperature goes out of control with the port output set to HIGH, or if the secondary winding of the current transformer 1101 is disconnected, the fuser 11
If a problem occurs in which the current flowing to the fixing device 12 cannot be detected, there is a possibility that the fixing heater continues to be heated, which may cause a fire accident.

The present invention has been made in order to solve the problems of the prior art described above, and its purpose is to prevent heating means from being turned off due to a malfunction other than an abnormal state in which the current is turned on even though there is no command to turn on the power. It is an object of the present invention to provide an image forming apparatus in which power supply is forcibly cut off even when heating continues, thereby more reliably avoiding a dangerous overheating state of a fixing device, and increasing safety.

[0013]

[Means for Solving the Problems] The present invention has been made to solve the above-mentioned problems of the prior art, and provides a heating means for thermally fixing a recording medium such as toner to printing paper, and the heating means described above. means for detecting the temperature of the heating means; means for controlling the presence or absence of an instruction to energize the heating means based on the temperature detection result; means for controlling the opening and closing of the energizing circuit depending on the presence or absence of the energization instruction; means for detecting the occurrence of an abnormal state in which the heating means is energized even though there is no instruction to energize the heating means; means for storing and retaining the abnormal state; In the image forming apparatus, the image forming apparatus has a current supply cutoff means for stopping power supply to the heating means according to the above, and a means for determining an overheating state based on a temperature detection result of the heating means, and a means for immediately activating the power supply cutoff means based on the determination result. It is characterized by comprising means.

The means for determining the overheating state may include means for starting measurement of the duration of the overheating state based on the temperature detection result of the heating means, and means for determining the overheating state when the duration exceeds a set value. It is effective to comprise the following: and to change the set value of the duration time according to the superheating temperature of the heating means.

[0015]

[Operation] If the heating means overheats when no abnormality is detected by the abnormal current detection means, the current cutoff means is activated to cut off the current to the heating means.

[0016] Furthermore, in an overheated state, particularly in a dangerous temperature range, the set value of the continuation time is shortened to promptly cut off the current supply.

[0017]

[Embodiments] [Embodiment 1] The present invention will be explained below based on the illustrated embodiments.

In this embodiment, the power is cut off each time an overheating state of the fixing device is detected, and the power cutoff circuit is restored when the overheating state is resolved.

FIG. 1 shows a circuit diagram for explaining this embodiment.

Reference numeral 211 is a current detection circuit using a current transformer. A current proportional to the current flowing through the AC primary circuit is obtained in the secondary winding by the current transformer T, and this current is converted from current to voltage by the resistor R1. Next, by passing through a rectifier/smoothing circuit including a diode D1, a capacitor C1, and a resistor R2, the AC voltage is converted into a DC voltage for input to the next stage comparator CP1. Since the non-inverting input terminal of the comparator CP1 is grounded, when current is detected by the current transformer T, the output of the comparator CP1 becomes LOW.

[0021] Reference numeral 212 denotes a relay as current cutoff means, and when the transistor 213 is turned on, the AC primary circuit is closed.

Reference numeral 214 denotes an abnormal state holding circuit, which is composed of a flip-flop FF and a NOR gate 241. The reset terminal R becomes LOW when the power is turned on, and becomes HIGH after a certain period of time has elapsed. At this time, the inverted output Q
is HIGH, and becomes LOW when the set terminal S becomes HIGH. Once set, the flip-flop FF will not be reset until the power is turned on again. The condition in which the output of the NOR gate 241 becomes HIGH is when current is detected by the current transformer T even though the CPU 220 has not outputted an energization instruction to the fixing device 218.

215 is an overheating state detection comparator, and when the heater in the fixing device 218 overheats and the resistance value of the thermistor decreases, the output of the comparator 215 becomes LOW.
becomes. When the overheating condition is resolved, comparator 2
The output of 15 returns to HIGH. The overheating state detection value can be determined by adjusting the ratio of resistors R3 and R4 connected to the inverting input terminal.

216 is an AND gate, and when the inverted output bar Q of the flip-flop FF is HIGH and the output of the comparator 215 is HIGH, the AND gate 21
The output of 6 becomes HIGH, and transistor 213 turns on.
This closes the AC primary circuit of relay 212.

Reference numeral 217 is an opening/closing control means for controlling opening/closing of the AC primary circuit. CPU220 port output is HIG
At the time of H, the transistor Tr1 is turned on and the built-in LED of the phototriac coupler PC lights up, so that the triac TRC is fired and the AC primary circuit is closed. vice versa,
When the port output of the CPU 220 is set to LOW, the AC primary circuit is opened.

218 is a fixing device, and the electrical parts are AC
It consists of a heater H inserted into the primary circuit and a thermistor Th that monitors the temperature of the heater H. A voltage dividing circuit with a fixed resistor 219 converts the resistance value of the thermistor Th into a voltage value, which is input to the A/D conversion circuit built into the CPU 220 and the comparator 215.

The CPU 220 knows the temperature of the heater H by the thermistor Th, and if the temperature is high, it stops firing the triac TRC and opens the AC primary circuit, and if the temperature is low, it starts the triac TRC and opens the AC primary circuit. Close the circuit. This controls the heater H of the fixing device 218 to maintain a constant temperature.

Similar to the conventional example, the triac TRC is
The AC primary circuit malfunctions due to a short circuit, and the AC
If the PU220 goes out of control with the port output set to HIGH, the current flowing through the heater H is detected by the current transformer T, and the output of the NOR gate 241 becomes HIGH.
As a result, this abnormal state is recorded and held in the flip-flop FF, and the relay 212 is cut off.

According to this embodiment, even when the above-mentioned malfunction occurs, the relay 212 is cut off by the comparator 215 detecting overheating.

As a result, it goes without saying that the relay 212 will be cut off even if, for example, the secondary winding of the current transformer T is disconnected and a malfunction occurs in which the energized current cannot be detected.

[Embodiment 2] In this embodiment, the occurrence of an overheating state in the fixing device is memorized and retained, and thereafter the energization cutoff circuit is activated, and the energization cutoff circuit is not restored until the power of the apparatus is turned on again.

FIG. 2 shows a circuit diagram explaining this embodiment.

211, 212, 213, 217, in FIG.
Components marked 218, 219, and 220 are those of Example 1.
The configuration is similar to that shown in FIG.

A flip-flop 221 maintains an abnormal state. The flip-flop 221 is the OR gate 2
It is set when the 22 output is HIGH, and once set, it will not be reset until the power is turned on again.

The condition for the output of the OR gate 222 to be HIGH is that either the output of the NOR gate 223 is HIGH or the output of the overheating state detection comparator 224 is HIGH.

The condition that the output of the NOR gate 223 becomes HIGH is that the current transformer T
This is a case where current detection is performed by

The condition under which the output of the overheat state detection comparator 224 becomes HIGH is when the heater H in the fixing device 218 overheats and the resistance value of the thermistor Th decreases.

In other words, the conditions for storing and holding the abnormal state in the flip-flop 221 are when the fuser 218 is energized even though the CPU 220 has not outputted an energization instruction to the fuser 218; This happens either when the heater H in 218 overheats.

According to this embodiment, when the overheating is detected by the comparator 224, the relay 212 is cut off, which causes a problem in which, for example, the secondary winding of the current transformer T is disconnected, and the flowing current cannot be detected. Needless to say, even in this case, the relay 212 is cut off.

The difference from the first embodiment is that re-energization is not permitted even if the overheating condition is resolved. The use of Embodiment 1 and Embodiment 2 depends on the heat capacity of the fixing device 218 used. In other words, if the heat capacity of the fuser 218 is large and the overshoot during heating is large, the form of Embodiment 1 is adopted, and the fuser 218 is
When the heat capacity of 18 is small, the form of Example 2 is used in order to improve safety.

[Embodiment 3] In this embodiment, when the temperature of the fuser is overheated and the temperature is above a certain level, it is stored in memory, and from then on, the energization cutoff circuit is activated, and the energization cutoff circuit is restored until the power of the device is turned on again. I won't let you.

[0042] If the overheating state is below a certain temperature, the current is cut off every time it is detected, and the current cutoff circuit is restored when the overheating state is resolved.

FIG. 3 shows a circuit diagram explaining this embodiment.

In FIG. 3, 211, 212, 213, 217
, 218, 219, and 220 are the same as those shown in FIG. 1 showing the first embodiment.

This embodiment differs from the first and second embodiments in that two types of set voltages (detected temperatures) are used and overheating state detection comparators 235 and 236 are provided.

Assuming that the power supply voltage is V0, the inverted input voltage of the comparator 235 is V1, the non-inverted input voltage of the comparator 236 is V2, and the resistors that divide these voltages are R37, R38, and R39, the following equation is obtained.

V1=V0×(R38+R39)/(R37+R3
8+R39)V2 =V0×R39/(R37+R3
8+R39) V1 > V2 The resistance value of the thermistor Th becomes smaller as the temperature becomes higher. In the circuit configuration shown in FIG. 3, the comparator 235
The non-inverting input voltage of the comparator 236 and the inverting input voltage of the comparator 236 both decrease as the temperature increases.

Furthermore, since V1 > V2, the voltage V2 is used as a reference voltage for determining a higher overheating temperature.

First, in the normal temperature range, the output of the comparator 235 is HIGH, the output of the comparator 236 is LOW, the flip-flop 232 is not set, and both inputs of the AND gate 231 are HIGH, so the contact of the relay 212 is is closed.

Even in an overheated state, if the temperature is in a low temperature range, only the output of the comparator 235 changes and becomes LOW, and the flip-flop 232 is not set, but the relay contact is opened. When the temperature returns to the normal temperature range from this state, the relay contact closes again.

If the superheat temperature is high, comparator 2
35 output is LOW, comparator 236 output is HI
GH, the flip-flop 232 is set and the relay contact is opened. Even if the temperature returns to normal from this state, the relay contacts will not close.

The conditions to be stored and held in the flip-flop 232 as an abnormal state are when the CPU 220 is energized even though the fuser 218 has not outputted an energization instruction to the fuser 218, or when the fuser 218 is in an energized state. This is one of the cases where the heater H reaches a high overheating temperature, and the only way to recover from this state is to turn on the power again.

Furthermore, an overheated state in a low temperature range is not stored as an abnormal state, but is restored when the overheated state is resolved.

According to this embodiment, even in an overheated state,
By setting two types of detection temperatures, it is possible to both prevent overshoot during heating and achieve high safety.

[Embodiment 4] In this embodiment, one capacitor, multiple comparators, transistors, and resistors are used, and the comparator is switched depending on the overheating temperature of the fuser, and the charging path of the capacitor by the resistor is increased or decreased. By doing so, the set value of the overheating duration time is equivalently changed. FIG. 4 shows a fixing safety circuit of an image forming apparatus according to Embodiment 1 of the present invention. That is, the fixing device 103, which is composed of a fixing heater 101 that is a heating means for printing paper and a thermistor 102 that detects the temperature of the fixing heater, and the detected temperature of the thermistor 102 are input as a voltage value, and based on this result, the heater energization control circuit Instructs the opening and closing of a certain triac 105, and in the event of some abnormal situation,
A CPU 104 is provided that immediately controls a relay 106 that is a heater current cutoff circuit. Further, an overheating state detection circuit 20 includes a current detection circuit including a current transformer 107 as means for detecting the heater current, and a comparator 108 for detecting an overheating state of the heater 101.
0, an abnormal current detection circuit as a means for detecting the energizing current even though the CPU 104 has not outputted an energization instruction, and an abnormal current detection circuit as a means for recording and retaining the detection of an abnormal state or an overheating state of the heater 101. A flip-flop 109 is provided. The circuits of each part will be explained in detail below. 103 is a fixing device, and electrical parts are:
It consists of a heater 101 inserted into the AC primary circuit and a thermistor 102 that monitors the temperature of the heater. The thermistor 10 is connected by a voltage divider circuit with a fixed resistor 110.
The resistance value of 2 is converted into a voltage value and input to the A/D conversion circuit. The figure shows a CPU 104 with a built-in A/D conversion circuit.
However, the A/D conversion circuit may be externally attached. The CPU 104 is connected to the heater 1 by the thermistor 102.
It functions as a means to know the temperature of 01 and to control whether or not to instruct the heater 101 as a heating means, and if the temperature is high, the ignition of the triac 105 is stopped and the AC primary circuit is opened, and the temperature is low. Triac 105 in case
starts ignition and closes the AC primary circuit. This controls the heater 101 of the fixing device 103 to maintain a constant temperature. The CPU 104 outputs the firing signal of the triac 105 in a pulse form from the port 2 output. The reason why it is pulsed is that the firing signal of the triac 105 is HI.
This is to avoid the possibility that the fixing heater 101 may continue to be heated due to runaway of the CPU 104 when set to GH active or LOW active. Also, C
An open collector output from a transistor 111 is provided from the CPU board on which the PU 104 is mounted. The reason for the open collector output is to prevent disconnection of the harness between the CPU board and the safety circuit. In other words, by installing the pull-up resistor 112 on the safety circuit side,
If the harness is disconnected, it will not malfunction because it is connected to the power line with low impedance. 113 is a DC cut capacitor. As a result, when the CPU 104 runs out of control or the harness is disconnected, the signal that becomes HIGH level can be cut off. The next stage, a resistor 114, a diode 115, and a capacitor 116, is a rectifier/smoothing circuit for obtaining the envelope of the pulse waveform. The transistor 117 is a phototriac coupler 11
8 LED drive and abnormal current timing capacitor 12
Discharge the charge of 8. Phototriac coupler 118
When the built-in LED lights up, the triac 105 fires and the AC primary circuit closes. By using the phototriac coupler 118 with a built-in zero-cross comparator, it is possible to suppress noise generation when the triac 105 is turned on. 107 is a current transformer, and a current value proportional to the current of the AC primary circuit is obtained in the secondary winding. Next, the resistor 119 converts the secondary winding current value into a current/voltage. Resistors 120, 121, 122, 123 and comparators 124, 125 constitute a window comparator. The comparators 124 and 125 have open collector outputs, and when the inverted input voltage exceeds the non-inverted input voltage, current is sucked from the output terminals. The two comparators 124 and 125 are wired-ORed, and a LOW level is created by a resistor 126. The window width is determined by the ratio of the four resistors, and when the input voltages of the comparators 124 and 125 are above and below the window width, the output becomes LOW level. Transistor 127 is a switch that changes charging and discharging of capacitor 128, and this transistor 127 is OFF.
In the case of F, the capacitor 12 is connected via resistors 129 and 130.
It is charged to 8. When the transistor 127 is ON, the capacitor 1 is connected via the resistor 130 and the transistor 127.
28 is discharged. Charging of the capacitor 128 progresses, and the potential across the capacitor 128 increases to the level of the transistor 13.
1 base-emitter voltage and Zener diode 132
When the sum of the Zener voltage of
Do N. Further, the explanation of the overheating state detection circuit 200 is shown in FIG.
will be described later using

Flip-flop 109 is used only for setting, and transistor 131 is ON or transistor 1 is ON.
When 9 is turned ON, the memory is retained. In other words, CPU1
04 does not output an energization instruction to the fixing heater 101, but the fuser heater 101 is energized, or the fuser heater 101 remains overheated for a certain period of time or more. 109 stores the fact that it is in an abnormal state. 106 is a relay in which transistor 139 is ON and transistor 140 is ON.
When is ON, current flows through the relay winding and the relay contacts close. By setting the port 1 output of the CPU 104 to HIGH, the transistor 141 is turned on, and thereby the transistor 140 is turned on. After detecting that some kind of failure has occurred, the CPU 104 sets the port 1 output to LO.
By setting it to W, the fixing heater 101 can be immediately shut off. The reason why the port 1 output is enabled HIGH is that when the port 1 output harness is disconnected,
This is because the resistor inserted in parallel between the base and emitter of the transistor 141 allows the relay contact to settle in the direction of opening, that is, the direction of high safety. When flip-flop 109 is set, transistor 139 is turned off and the relay contact is opened. FIG. 5 shows a heater overheating state detection circuit for explaining this embodiment.

Reference numerals 11a, 11b, and 11c are overheat state detection comparators as means for starting measurement of the duration of the overheat state, and all non-inverting input terminals are connected to one end of a thermistor. When the fixing heater overheats, the resistance value of the thermistor decreases, and the non-inverting input voltage falls below the inverting input voltage, the comparator output becomes LOW. Once the overheating condition is removed, the comparator output returns to HIGH. The overheating state detection value can be determined by adjusting the ratio of the resistors connected to the inverting input terminal.

[0058] The power supply voltage is set to V0, and the comparator 11a
Let the inverted input voltage of the comparator 11b be V1, the inverted input voltage of the comparator 11b be V2, and the inverted input voltage of the comparator 11c be V3, and the resistors that divide these voltages be R1.
2, R13, R14, and R15, the following formula is obtained.

[0059] V1 = V0 × (R13 + R14 + R15)
/(R12+R13+R14+R15) V2
=V0×(R14+R15)/(R12+R13+R
14+R15) V3 = V0 × (R15)/
(R12+R13+R14+R15)V1>V2>
Since V3 V1 > V2 > V3, the voltage V3
is the reference voltage for determining a higher overheating temperature.

First, in the normal temperature range, all outputs of comparators 11a, 11b, and 11c are HIGH, and transistors 16a, 16b, and 16c are turned on. At this time, resistors 18a, 18b connected to the power supply,
The current flowing through transistor 18c is transmitted through transistors 16a, 1
6b and 16c, the capacitor 17, which serves as a means for measuring the duration of the overheating state, is not charged. Therefore,
The potential of the capacitor 17 does not rise, and the transistor 19, which serves as a means for determining the continuation of the overheating state and activating the current cutoff means, is not turned on, so that the flip-flop 109 is not set to be in an abnormal state.

In the overheated state, if the temperature is in a low temperature range, the output of the comparator 11a becomes LOW, and only the transistor 16a is turned off. At this time, the current flowing through the resistor 18a connected to the power supply is transferred to the capacitor 17.
to charge. Therefore, the potential of the capacitor 17 increases,
When the sum of the base-emitter voltage of the transistor 19 and the Zener voltage of the Zener diode 20 is exceeded, the transistor 19 is turned on. As a result, the flip-flop 109 is set to be in an abnormal state, the relay 106 is cut off, and the power supply to the fixing heater 101 is stopped.

In this state, the time constant for the capacitor 17 is the product of the resistance value of the resistor 18a and the capacitance value of the capacitor 17, and has a large value.

When the overheating temperature is moderate, the outputs of the comparators 11a and 11b become LOW, and the transistors 16a and 16b are turned off. At this time, the current flowing through the resistors 18a and 18b connected to the power source charges the capacitor 17, and finally the current supply to the fixing heater 101 is stopped.

In this state, the time constant for the capacitor 17 is the product of the parallel resistance value of the resistor 18a and the resistor 18b and the capacitance value of the capacitor 17.
When the resistance values are the same, the resistance value becomes 1/2 of the above-mentioned case where the superheat temperature is low.

If the superheat temperature is high, comparator 1
All outputs of 1a, 11b, and 11c become LOW, and transistors 16a, 16b, and 16c are turned off. At this time, resistors 18a, 18b, 18c connected to the power supply
The current flowing through the capacitor 17 charges the capacitor 17, and eventually the fixing heater 101 stops being energized.

In this state, the time constant for the capacitor 17 is the product of the parallel resistance value of the resistor 18a, resistor 18b, and resistor 18c and the capacitance value of the capacitor 17.
, 18b, and 18c are equal, the resistance value becomes 1/3 of the above-described low overheating temperature.

[Embodiment 5] In this embodiment, one capacitor, a plurality of comparators, a transistor, and a constant current element are used. The comparator is switched depending on the overheating temperature of the fuser, and the capacitor is controlled by the constant current element. By increasing or decreasing the number of charging paths, the set value of the overheating duration time is equivalently changed.

FIG. 6 shows a heater overheat state detection circuit for explaining this embodiment.

The other circuits are the same as the circuit shown in FIG. 4, so their explanation will be omitted.

Further, in the drawings, the same reference numerals as in FIG. 5 are given to the parts where the same parts as in FIG. 5 are used.

First, in the normal temperature range, all outputs of comparators 11a, 11b, and 11c are HIGH, and transistors 16a, 16b, and 16c are turned on. At this time, constant current elements 21a and 2 connected to the power source
The current flowing through transistors 1b and 21c is
The capacitor 17 is not charged via a, 16b, and 16c. Therefore, the potential of the capacitor 17 does not rise and the transistor 19 does not turn on, so the flip-flop 1
09 is not set if there is an abnormal state.

In the overheated state, if the temperature is in a low temperature range, the output of the comparator 11a becomes LOW, and only the transistor 16a is turned off. At this time, the current flowing through the constant current element 21a connected to the power source charges the capacitor 17. Therefore, the potential of the capacitor 17 increases, and when it exceeds the sum of the base-emitter voltage of the transistor 19 and the Zener voltage of the Zener diode 20, the transistor 19 is turned on. As a result, the flip-flop 109 is set to be in an abnormal state, the relay 106 is cut off, and the power supply to the fixing heater 101 is stopped.

In this state, the time until the relay 106 is cut off is inversely proportional to the constant current value of the constant current element 21a and proportional to the capacitance value of the capacitor 17, and therefore takes a large value.

When the overheating temperature is moderate, the outputs of the comparators 11a and 11b become LOW, and the transistors 16a and 16b are turned off. At this time, the current flowing through the constant current elements 21a and 21b connected to the power source charges the capacitor 17, and finally the fixing heater 101
energization is stopped.

In this state, the time until the relay 106 is cut off is inversely proportional to the sum of the constant current value of the constant current element 21a and the constant current value of the constant current element 21b, and is proportional to the capacitance value of the capacitor 17. , when the constant current value of the constant current element 21a and the constant current value of the constant current element 21b are equal, the temperature becomes 1/2 of the above-mentioned low overheating temperature.

If the superheat temperature is high, comparator 1
All outputs of 1a, 11b, and 11c become LOW, and transistors 16a, 16b, and 16c are turned off. At this time, constant current elements 21a, 21b,
The current flowing through 21c charges the capacitor 17,
Eventually, the power supply to the fixing heater 101 is stopped.

The time until the relay 106 is cut off in this state is inversely proportional to the sum of the constant current values of the constant current elements 21a, 21b, and 21c, and proportional to the capacitance value of the capacitor 17. , 21b, and 21c are equal, 1 in the case of the low overheating temperature described above.
/3.

In this embodiment, a constant current diode is used as the constant current element, but the constant current circuit may be constructed by a combination of a constant voltage circuit, a resistor, and a current mirror circuit.

Compared to Example 4, replacing the resistor with a constant current element increases the cost, but the advantage is that the capacitor can be charged with a constant current, making it easier to calculate the set value of the overheating duration time. There is.

[Embodiment 6] In this embodiment, an A/D converter and a digital counter are used to change the output value of the A/D converter according to the overheating temperature of the fixing device, and this value is changed to the output value of the digital counter. By setting the initial value, equivalently,
Change the set value of overheating duration.

FIG. 7 shows a heater overheat state detection circuit for explaining this embodiment.

The other circuits are the same as those in the fourth embodiment shown in FIG. 4, so their explanation will be omitted.

Reference numeral 31 denotes an operational amplifier, and by arranging resistors around it, an amplifier having a transfer function shown in the following formula is realized. Note that the input voltage of the amplifier is ei, and the output voltage is eo.

eo=5-ei The reason for preparing the amplifier having the above transfer function is that when the fixing heater 101 overheats and the resistance value of the thermistor 102 decreases, the input voltage ei of the amplifier decreases. By setting the input voltage to a large value, the initial value of the digital counter is increased and the counter overflows quickly.

32 is a 4-bit A/D converter with built-in sample/hold, which holds the input voltage at the rising edge of the sampling clock and converts the A/D converter in a span of 0 to +5V.
/D conversion. The most significant bit is the MSB and the least significant bit is the LSB.

33 is a sequencer that controls the circuit shown in FIG.

34 is a clock supply circuit.

A reset circuit 35 includes a capacitor and a resistor, and resets the sequencer 33 when the power is turned on.

FIG. 8 shows the internal circuit of the sequencer,
A timing chart is shown in FIG.

[0090] Four JK flip-flops 41, 42,
By constructing a Johnson counter using 43 and 44, eight states are realized.

First, set the negative logic reset input *RST to LO.
By setting W, all flip-flops 41, 42,
43 and 44 are cleared, and the output Q becomes LOW. (State 1) At this time, all outputs of the sequencer 33 are initialized. That is, the SAMPLE output is LOW, the ENT output is LOW, the *CLR output is LOW, and the *LOAD output is HIGH.

When *RST becomes HIGH, the output Q1 of the flip-flop 41 becomes HIGH. (State 2) Next, wait for the COMP terminal to become HIGH. The condition for the COMP terminal to become HIGH is that the overheat state detection comparator 36 in FIG. 7 detects overheating of the fixing heater 101 and the output of the comparator 36 becomes HIGH. When the COMP terminal becomes HIGH, the output Q2 of the flip-flop 42 becomes HIGH. (State 3) In state 3, the SAMPLE signal is set to HIGH to provide a sampling clock to the A/D converter 32 in FIG. The A/D converter 32 activates the fixing heater 10 at the rising edge of the SAMPLE signal.
The input voltage VIN, which has an overheating temperature of 1, is sampled/held, A/D converted, and then output as 4 bits.

[0093] At the next clock, a transition is made to the next state. (State 4) In state 4, the output Q3 of the flip-flop 43 is set to HIGH, and the SAMPLE signal is set to LOW.

[0094] At the next clock, a transition is made to the next state. (State 5) In state 5, the output Q4 of the flip-flop 44 is set to HIGH, and the clear signal *CLR is set to HIGH.
IGH. As a result, the counter 37 in FIG.
Clear the . Note that the ENT input of the counter is L.
Since it remains OW, the count operation does not start.

[0095] At the next clock, a transition is made to the next state. (State 6) In state 6, the output Q1 of the flip-flop 41 is set to LOW, and the load signal *LOAD is set to LOW.
OW. Thereby, the counter 37 internally loads the A/D conversion value of the A/D converter 32 as an initial value.

[0096] At the next clock, a transition is made to the next state. (State 7) In state 7, the output Q2 of the flip-flop 42 is set to LOW, and the load signal *LOAD is set to HIGH.
IGH.

[0097] At the next clock, a transition is made to the next state. (State 8) In state 8, the output Q3 of the flip-flop 43 is set to LOW, and the count enable signal E is set to LOW.
Set NT to HIGH. In this state, in FIG.
If the output of the overheating state detection comparator 36 is HIGH, the counter 37 starts counting from the set initial value.

[0098] When the COMP terminal becomes LOW, a transition is made to the next state. (State 1) That is, when the overheating state is eliminated, the output of the overheating state detection comparator 36 in FIG. 7 becomes LOW, and the state returns to state 1. At this point, the counter 37 stops counting and the internal count value is cleared.

In state 8, when the number of clocks equal to the counter size minus the counter initial value is input, the counter overflows and the ripple carry output RCO, which is a carry signal, becomes HIGH. Conversely, if the period of state 8 is short, the ripple carry output RCO is set to H.
The counter is cleared without setting it to IGH.

[0100] This means that the set value of the overheating continuation time is equivalently changed in accordance with the value of the overheating temperature.

36 is an overheating state detection comparator;
The inverting input terminal is connected to one end of the thermistor 102. When the fixing heater 101 overheats and the resistance value of the thermistor 102 decreases, the inverted input voltage becomes lower than the non-inverted input voltage, so the output of the comparator 36 becomes HIGH. Once the overheating condition is resolved, the comparator 36 output returns to LOW. The overheating state detection value can be determined by adjusting the ratio of the resistors connected to the non-inverting input terminal.

37 is a counter, and the execution conditions for the counting operation are that a clock serving as a counting base is supplied, and that the ENT which is the count enable terminal is supplied.
, ENP are both HIGH, and the negative logic clear terminal *CLR is HIGH.

[0103] Before starting the counting operation, the counter 37
By setting the LOAD terminal to LOW, 4-bit initial values are loaded from the A, B, C, and D terminals. Note that the A terminal is the LSB, and the D terminal is the MSB.

When the counting operation conditions are met, the counter 37 starts counting from the set initial value, and when the counter size is exceeded, the ripple carry output RCO, which is a carry signal, becomes HIGH.

The measurement time for the continuation of the overheating state is determined by the frequency of the clock supply circuit or the initial value of the counter.

38 is an inverter, 39 is a diode OR gate, and the output of the inverter 38 is LO
When W or the output of the flip-flop 109 is LOW, the relay drive transistor 139 is turned OFF,
By opening the relay contact, power supply to the fixing heater 101 is cut off.

Although the circuit scale is increased compared to Examples 4 and 5, since no capacitor is used, time measurement can be performed with high accuracy, and there are advantages such as no change in measurement time over time.

[0108]

[Effects of the Invention] As explained above, according to the present invention,
If the means for managing temperature and controlling the presence or absence of energization instructions goes out of control while outputting energization instructions to the fuser, or if a malfunction occurs in the circuit that detects the energization to the fuser that makes it undetectable. Also, since it is possible to operate a circuit that cuts off power to the fuser, it is effective in more reliably preventing the occurrence of fire accidents.

[0109] Furthermore, if a means is provided to change the set value of the duration according to the overheating temperature of the fixing unit, the current can be cut off immediately in the particularly dangerous temperature range of the overheating state, resulting in safer operation. An image forming apparatus can be provided.

For example, even if a heating means such as a fixing heater becomes overheated and a cut-off means such as a relay is cut off, and the power is immediately turned on again to recover, the printing paper will not reach its ignition point. , it is possible to avoid equipment ignition and smoke generation.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram for realizing a first embodiment of the present invention.

FIG. 2 is a circuit diagram for realizing a second embodiment of the present invention.

FIG. 3 is a circuit diagram for realizing Embodiment 3 of the present invention.

FIG. 4 is an overall circuit diagram to which an overheating state detection circuit according to a fourth embodiment of the present invention is applied.

FIG. 5 is a circuit diagram for realizing a fourth embodiment of the present invention.

FIG. 6 is a circuit diagram for realizing a fifth embodiment of the present invention.

FIG. 7 is a circuit diagram for realizing a sixth embodiment of the present invention.

8 is a circuit diagram of a sequencer 33 that controls the circuit shown in FIG. 7. FIG.

9 is a timing chart of the sequencer 33 shown in FIG. 7. FIG.

FIG. 10 is a circuit diagram for explaining a conventional technique.

[Explanation of symbols]

211 Energizing current detection circuit 212 Relay 213 Transistor 214 Abnormal state holding circuit 215 Overheating state detection comparator 216 AND gate 217 Opening/closing control means 218 Fixing device 220 CPU 101 Fixing heater (heating means) 102 Thermistor (temperature detecting means) 104
CPU 105 Triac (heater energization control circuit) 106
Relay (energization cutoff means) 107 Current transformer (energization current detection means) 10
9 Flip-flop (memory holding means) 200 Heating state detection circuit 11a, 11b, 11c Comparator (means to start measuring the duration time of overheating state) 17 Capacitor (means to measure the duration time of overheating state) 19 Transistor (in the energized state (Means for determining whether to continue and activating the energization cutoff means)

Claims (3)

[Claims] Claims: 1. A heating means for thermally fixing a recording medium such as toner to a printing paper, a means for detecting the temperature of the heating means, and controlling whether or not to instruct the heating means to be energized based on the temperature detection result. means for controlling the opening and closing of the energizing circuit depending on the presence or absence of the energization instruction; and means for detecting the energizing current to the heating means; the heating means is energized even though there is no energization instruction; An image forming apparatus comprising means for detecting occurrence of an abnormal state, means for storing and retaining the abnormal state, and energization cutoff means for stopping energization to the heating means according to the result of storing the memory, the heating means An image forming apparatus comprising: means for determining an overheating state based on a temperature detection result; and means for immediately operating the energization cutoff means based on the determination result. 2. The means for determining the overheating state includes means for starting measuring the duration of the overheating state based on the temperature detection result of the heating means, and means for determining the overheating state when the duration exceeds a set value. The image forming apparatus according to claim 1, further comprising the following. 3. The image forming apparatus according to claim 1, further comprising means for changing the set value of the duration according to the superheating temperature of the heating means.