JPH04318587A - Image forming device - Google Patents

Abstract

PURPOSE:To provide an image forming device in which malfunction caused by a noise is prevented and abnormal overheat state is surely detected. CONSTITUTION:The output of a comparator 11 becomes a LOW level when a temperature detected by a thermistor 102 is equal to or above a set value, and it becomes a HIGH level when the temperature is under the set value. By turning off a transistor 12 when the output is at the LOW level, a capacitor 13 is charged through resistances 14 and 15. In the case that charging is continued in an abnormal state and potential at both ends of the capacitor 13 exceeds the sum of voltage between the base and the emitter of the transistor 16 and Zener voltage of a Zener diode 17, the transistor 16 is turned on and a flip-flop 109 is set. When the flip-flop 109 is set, a transistor 134 is turned off and the contact of a relay 106 is opened and the energizing of a fixing heater 101 is stopped.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a safety circuit for preventing smoke and fire from occurring in an image forming apparatus.

0002

[Prior Art] Image forming apparatuses, which visualize images stored in memory by fixing toner (developer) on print paper, are typified by laser beam printers and copying machines. Widely used.

[0003] The printing operation in the image forming apparatus described above 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. . As a fixing method, there are a method using heat, a method using pressure, and a method using a combination of heat and pressure, and any one of them is selected and used.

[0004] If a method using heat is selected,
Ensuring safety at this time is an extremely important technology, as there is a possibility of a fire occurring due to the printing paper being heated, which can become an ignitable substance.

FIG. 4 shows a control circuit for the fixing device heater and a circuit for ensuring the above-mentioned safety (hereinafter referred to as a safety circuit).

[0006] In FIG. 4, a fixing device 103 includes a fixing heater 101, which is a heating means for printing paper, and a thermistor 102, which detects the temperature of the fixing heater. The temperature detected by the thermistor 102 is input as a voltage value to the CPU 104, which is a control means, and based on this result, the CPU
104 is a triac 105 which is a heater energization control circuit.
In addition, in the event of any abnormality, it immediately controls the relay 106, which is a heater energization cutoff circuit. Detection of the abnormal situation is carried out by the current transformer 107.
a current detection circuit that detects the heater current;
This is performed by an overheating state detection circuit that includes a comparator 108 and detects an overheating state of the heater, and a flip-flop 109. The flip-flop 109 is connected to the CP
The fact that the energizing current was detected even though the U did not output the energizing instruction, or that the overheating state of the heater was detected is stored and retained.

[0007] The circuits of each part will be explained in detail below.

The electrical components of the fixing device 103 are comprised of a heater 101 inserted into an AC primary circuit and a thermistor 102 that monitors the temperature of the heater 101. The thermistor 1 is connected by a voltage divider circuit with a fixed resistor 110
The resistance value of 02 is converted into a voltage value and input to the A/D conversion circuit. In the figure, a CPU with a built-in A/D conversion circuit is used, but the A/D conversion circuit may be externally attached.

The CPU 104 knows the temperature of the heater 101 by the thermistor 102, and if the temperature is high, it stops firing the triac 105 and opens the AC primary circuit, and if the temperature is low, it starts firing the triac 105. to close the AC primary circuit. This controls the heater 101 of the fixing device 103 to maintain a constant temperature.

[0010] The CPU 104 outputs a triac firing signal in the form of a pulse from the port 2 output. The reason why it is pulsed is that the firing signal of the triac 105 is set to HIG.
This is to avoid the possibility of the fixing heater continuing to be heated due to runaway of the CPU when set to H active or LOW active.

[0011] Furthermore, the CPU board on which the CPU 104 is mounted has an open collector output from a transistor 111. The reason for using open collector output is
This is a measure against disconnection of the harness between the CPU board and the safety circuit. That is, by installing the pull-up resistor 112 on the safety circuit side, when the harness is disconnected, it is connected to the power supply line with low impedance, so that malfunction does not occur.

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.

[0013] The next stage, a resistor 114, a diode 115,
The capacitor 116 is a rectifier/smoothing circuit, and is used to obtain the envelope of the pulse waveform.

The transistor 117 discharges the charge of the LED drive of the phototriac coupler 118 and the capacitor 128 for measuring abnormal current flow in the current detection circuit, which will be described later. When the built-in LED of the phototriac coupler 118 is lit by the transistor 117, the triac 105 is fired and the AC primary circuit is closed. By using a phototriac coupler 118 with a built-in zero-cross comparator, it is possible to suppress noise generation when the triac is fired.

Next, although the CPU 104 does not output an energization instruction to the heater 101, the heater 101
A current detection circuit that detects an abnormality when the current is turned on will be described.

Reference numeral 107 denotes a current transformer, in which a current value proportional to the current of the AC primary circuit is obtained in the secondary winding, and a resistor 119 connected to the secondary winding changes the current value of the secondary winding current to / A voltage conversion is performed. By inputting this voltage value to the comparators 124 and 125, the heater 10
The energization state of 1 is determined.

The comparators 124 and 125 are resistors 12
0, 121, 122, and 123 to form a window comparator, and the window width is determined by the ratio of the four resistors 120, 121, 122, and 123. When the input voltages to the comparators 124 and 125 are above and below the window width, this indicates that the heater 101 is energized, and the outputs of the two wired-ORed comparators become LOW level. In other words, when the input voltage is above or below the window width, the inverting input voltage of one of the comparators exceeds the non-inverting input voltage, and the current flowing through the resistor 126 connected to the open collector output terminal is sucked from the output terminal. The above-mentioned LOW level is created. In this manner, when the output of the comparator is at the LOW level, that is, when the heater is energized, the transistor 127 connected to the output is turned off.

The transistor 127 is a switch for charging and discharging the capacitor 128 for measuring abnormal energization, and when it is ON, the capacitor 128 is discharged via the resistor 130 and the transistor 127, and when it is OFF, the capacitor 128 is discharged. is connected to capacitor 128 via resistors 129 and 130.
is charged to. However, the capacitor 128
It is connected to the collector of the above-mentioned triac ignition transistor 117 via a diode 133, and when the transistor 117 is on, the capacitor 128 is not charged. In other words, if the heater 101 is energized by receiving the energization command normally,
The capacitor 128 is not charged, and is only charged in an abnormal state in which the heater 101 is energized even though there is no energization command.

As charging of the capacitor 128 progresses in this abnormal state, the potential across the capacitor 128 exceeds the sum of the base-emitter voltage of the transistor 131 and the Zener voltage of the Zener diode 132, turning the transistor 131 ON. As a result, the set-dedicated flip-flop 109 is set, and the abnormal state is stored and held. As described above, the current detection circuit consists of capacitor 1
A time constant circuit constituted by resistors 129 and 130 sets the flip-flop 109 when abnormal energization continues for a certain period of time or more.

Next, an explanation will be given of an overheating state detection circuit that detects when the heater is abnormally overheated.

As described above, the signal from the thermistor 102 is input to the A/D conversion circuit of the CPU 104, but is also input to the comparator 108 constituting the overheat state detection circuit. Therefore, when the voltage input to the comparator 108 is less than a predetermined value, the transistor 133 is turned on and the flip-flop 109 is set. As a result, the abnormal state is stored in the flip-flop 109. In this way, abnormal energization is detected by the current detection circuit or overheating state detection circuit. Next, the processing after abnormal energization is detected will be explained.

[0022] In a normal state, the transistor 135 and the transistor 134 are turned on, so that the relay contact of the relay 106 constituting the AC primary circuit is closed. The transistor 135 is turned on by setting the port 1 output of the CPU 104 to HIGH, and the transistor 134 is turned on when the flip-flop 109 is not set. Therefore, when abnormal energization is detected by the current detection circuit or the overheat state detection circuit and the flip-flop 109 is set, the transistor 134 is turned off and the relay contact is opened. As a result, the power supply to the heater 101 is forcibly cut off, making it possible to prevent a fire accident due to abnormal power supply. Also, the port 1 output above is HIGH.
Since it is enabled, even if the port 1 output harness is disconnected, the relay contact is opened by the resistor inserted in parallel between the base and emitter of the transistor 136, so safety is maintained.

[0023]

[Problems to be Solved by the Invention] However, according to the above conventional example, due to the structure of the apparatus, the fixing device, the CPU board, and the safety circuit may be installed in separate locations, and the overheating state detection comparator As the input harness becomes longer, noise resistance tends to decrease.
There has been a problem in that even though there is no overheating condition, the flip-flop retains the memory that it is in an abnormal condition, making the fixing operation impossible.

Furthermore, even in the case where the overheating state is not memorized, the relay contacts repeat opening and closing each time the comparator malfunctions, resulting in problems such as shortening the life of the relay contacts and generating noise when the contacts open and close. Ta.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and provide an image forming apparatus that can reliably detect an abnormal overheating state without causing malfunctions due to noise.

[0026]

[Means for Solving the Problems] According to the present invention, the above object is achieved by providing a heating means for heating an unfixed developer image on a recording material, a temperature detecting means for detecting the temperature of the heating means, and a temperature detecting means for detecting the temperature of the heating means. Comparing means for comparing the temperature detected by the detecting means with a preset temperature and outputting a predetermined output when it is determined that the detected temperature is higher than the set temperature;
In the image forming apparatus, the image forming apparatus includes a cutoff means for cutting off driving of the heating means when the comparison means produces a predetermined output, the cutoff means measuring the time during which the comparison means outputs the predetermined output. This is achieved by having a time measuring means and setting the heating means to stop driving when the measured time exceeds a predetermined value.

[0027]

[Operation] According to the present invention, the temperature of the heating means is detected by the temperature detecting means and is inputted to the comparing means to compare it with the set temperature. As a result of the comparison, if the temperature is higher than the set temperature, the comparison means performs a predetermined output. The cutoff means measures the outputted time using the time measurement means, and cuts off the driving of the heating means when the measured time exceeds a predetermined value. That is, when the overheating state continues for a certain period of time or more, the occurrence of a fire accident is reliably prevented, and in the case of false detection by the comparison means due to the influence of noise, a normal fixing operation is performed.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First to third embodiments of the present invention will be explained based on the drawings. <First Embodiment> First, a first embodiment of the present invention will be described based on FIG. 1. Note that parts common to the conventional device shown in FIG. 4 are denoted by the same reference numerals, and description thereof will be omitted.

In this embodiment, a time constant circuit including a resistor and a capacitor is used to detect that the overheating state of the fixing device continues for a certain period of time or more.

The part indicated by A in FIG. 1 is the heater overheat state detection circuit of this embodiment.

Reference numeral 11 denotes an overheating state detection comparator serving as comparison means, the non-inverting input terminal of which is connected to one end of the thermistor. When the fixing heater 101 serving as a heating unit heats up and the resistance value of the thermistor 102 serving as a temperature detecting unit decreases, the non-inverting input voltage becomes lower than the inverting input voltage, so the output of the comparator 11 becomes LOW. When the overheating condition is eliminated, the output of the comparator 11 returns to HIGH. The overheating state detection value can be determined by adjusting the ratio of the resistors connected to the inverting input terminal.

The transistor 12 is a switch for charging and discharging the capacitor 13.
is OFF, capacitor 1 is connected via resistors 14 and 15.
It is charged to 3. When transistor 12 is ON, capacitor 13 is discharged via resistor 15 and transistor 12.

Charging of the capacitor 13 progresses, and the potential across the capacitor 13 increases to the base of the transistor 16.
When the sum of the emitter voltage and the Zener voltage of the Zener diode 17 is exceeded, the transistor 16 is turned on. The collector of the transistor 16 is connected to the set terminal of the flip-flop 109 via the resistor 18, and sets the flip-flop 109. In this way, since the flip-flop 109 is not set unless the capacitor 13 is charged, it is possible to measure that the overheating state of the fixing device has continued for a certain period of time or more.

Note that the time constant for measuring the duration of the overheating state is determined by the capacitance value of the capacitor 13 and the resistors 14 and 15.
It can be determined by the resistance value of .

According to this embodiment, when the harness serving as the input to the overheat state detection comparator 11 becomes long, noise resistance can be improved by increasing the above-mentioned time constant. <Second Embodiment> Next, a second embodiment of the present invention will be described based on FIG. 2. Note that FIG. 2 shows only the parts corresponding to the parts indicated by A in FIG. 1, and the explanation of parts common to other apparatuses in FIG. 1 is omitted.

This embodiment differs from the first embodiment in that it uses a time constant circuit including a constant current element and a capacitor to detect when the overheating state of the fixing device has continued for a certain period of time or more.

In FIG. 2, 21 is an overheating state detection comparator, the non-inverting input terminal of which is connected to one end of a thermistor. When the fixing heater overheats and the resistance value of the thermistor decreases, the non-inverting input voltage becomes lower than the inverting input voltage, so 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.

The transistor 22 is a switch for switching between charging and discharging the capacitor 23. When the transistor 22 is OFF, the capacitor is charged with a constant current via the constant current diode 24. When the transistor is ON,
The capacitor is discharged via the current limiting resistor 25 and the transistor.

When the charging of the capacitor progresses and the potential across the capacitor exceeds the sum of the base-emitter voltage of the transistor 26 and the Zener voltage of the Zener diode 27, the transistor 26 is turned on. Since the collector of the transistor 26 is connected to the set terminal of the flip-flop via the resistor 28, the transistor 26
6 is turned ON, that is, after the overheating state of the fixing device continues for a certain period of time or more, the abnormal state is stored and retained in the flip-flop.

The time constant for measuring the duration of the overheating state is determined by the capacitance value of the capacitor 23 and the constant current diode 24.
Determined by the current standard value of

In this embodiment as well, when the harness serving as the input to the overheating state detection comparator becomes long, the noise resistance can be improved by increasing the above-mentioned time constant.

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

According to this embodiment, compared to the first embodiment, replacing the resistor with a constant current element increases the cost, but since the capacitor is charged with a constant current, the time constant calculation is easy. There is. <Third Embodiment> Next, a third embodiment of the present invention will be described based on FIG. 3. Note that explanations of parts common to the first embodiment will be omitted.

This embodiment differs from the first embodiment in that a digital counter is used instead of the time constant circuit to detect when the overheating state of the fixing device has continued for a certain period of time or more.

In FIG. 3, reference numeral 31 denotes an overheating state detection comparator, the inverting input terminal of which is connected to one end of a thermistor. When the fixing heater overheats and the resistance value of the thermistor decreases, the inverted input voltage becomes lower than the non-inverted input voltage, so the comparator output becomes HIGH. Once the overheating condition is resolved, the comparator 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.

32 and 33 are counters, and the conditions for executing the counting operation are that a clock serving as a counting base is supplied, that the count enable terminals ENT and ENP are both HIGH, and that the negative logic clear terminal

[0047]

[Outside 1] is HIGH.

34 is a clock supply circuit. 35 is a reset circuit including a capacitor and a resistor, which clears the counter value when the power is turned on.

The counter 32 starts counting when the counting operation conditions are met, and when the counter size is exceeded, the ripple carry RCO, which is a carry signal, is set to HIGH.
shall be. At this time, the next stage counter 33 starts counting operation. When the counter 33 also exceeds the counter size, the RCO is set to HIGH.

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

[0051] 36 is an inverter. 37 is an OR gate using a diode, and when the output of the inverter 36 is LOW or the output of the flip-flop 109 is LOW.
In the case of W, the relay drive transistor 134 is turned off, and power supply to the fixing heater is cut off.

According to this embodiment, although the circuit scale is increased compared to the first and second embodiments, since no capacitor is used, time measurement can be carried out with high precision, and there is no change in the measurement time over time. There are advantages such as

[0053]

[Effects of the Invention] As explained above, according to the present invention,
Since the cutoff means includes a time measurement means for measuring the output time of the comparison means, and the drive of the heating means is cut off when the output time exceeds a predetermined value, the fixing operation becomes impossible due to malfunction due to noise or the like. can avoid the situation,
In addition, when abnormal energization occurs, the driving of the heating means can be reliably cut off to prevent a fire from occurring.

Furthermore, since malfunctions due to noise and the like do not occur, unnecessary opening and closing of the relay contacts can be prevented, thereby reducing the lifespan of the relay contacts and preventing noise from occurring when the contacts open and close.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of a device according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a device according to a third embodiment of the present invention.

FIG. 4 is a circuit diagram of a conventional device.

[Explanation of symbols]

11 Comparison means (comparator) 101 Heating means (fixing heater)

Claims (1)

[Claims] 1. A heating means for heating an unfixed developer image on a recording material, a temperature detection means for detecting the temperature of the heating means, and a temperature detected by the temperature detection means is higher than a preset temperature. In the image forming apparatus, the image forming apparatus includes a comparison means that outputs a predetermined output when it is determined that the comparison means outputs a predetermined output, and a cutoff means that cuts off the driving of the heating means when the predetermined output of the comparison means is received. The comparing means has a time measuring means for measuring the time when a predetermined output is performed,
An image forming apparatus characterized in that the heating means is set to shut off when the measured time exceeds a predetermined value.