JPH0415471B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a temperature control device for controlling the temperature of a fixing device or the like used in an image forming apparatus or the like.

[Prior art]

Various heat generating members are used in electronic devices. For example, a fixing device in an image forming device (electronic copying device) is a device that fixes an unfixed toner image transferred to a transfer paper onto the transfer paper as a stable image using high temperature. Generally, such a member is required to have a constant temperature, and a constant temperature necessary for fixing is maintained by controlling on/off of a heating element, which will be described later, using a temperature sensing element such as a thermistor. However, if a failure (breakage) occurs in the above-mentioned heating element, not only a fixing failure occurs but also damage to the apparatus itself.

To prevent this, various control circuits have been devised.

FIG. 4 shows a circuit diagram of a conventional temperature control device equipped with a heating element disconnection detection circuit.

The circuit in the same figure consists of a thermistor TH and resistors R ₁ , R ₂ ,
Bridge circuit consisting of _R3 , comparator _Q1 , trigger circuit 1, transistor _Q2 , and resistor
Temperature control circuit consisting of R ₄ to _{R 6} , triax 2, heater H, AC power supply 3, current detection circuit 4
The heater disconnection detection circuit consists of the following:

This circuit uses a comparator _Q1 to compare the voltage VN, which is the normal power supply voltage _VCC divided by the resistor _R3 and thermistor _TH , with _the reference voltage _VI , which is divided by the resistors R1 _{and R2} .
This is a circuit that controls on/off the triac ₂ via the transistor Q 2 based on the comparison output, and conducts or cuts off alternating current to the heater H. This operating current detection circuit 4 includes a comparator
The current detection circuit 4 monitors the output of the comparator _{Q 1 and} outputs a disconnection signal a if no current is supplied from the AC power supply 3 when the output of the comparator Q 1 is a high signal, indicating that the heater H is disconnected. This is to inform you that the

[Problems with conventional technology]

However, in the conventional temperature control device, although it is possible to detect the disconnection of the heater H, the current detection circuit 4
Since it is connected to the AC power supply 3, which is a commercial power supply, the current detection circuit 4 needs to be constructed of a high-voltage circuit, which is extremely disadvantageous in terms of cost. In addition, it is necessary to match the power supply specifications of each exporting country, making it impossible to standardize parts.

[Purpose of the invention]

In view of the above-mentioned conventional drawbacks, the present invention detects disconnection of a heater connected to a commercial power supply using only a low-voltage control circuit, standardizes the current detection circuit without being affected by the power supply specifications of each country, and reduces the cost of the device. The purpose of the present invention is to provide a temperature control device that makes it possible to lower the temperature.

[Key points of the invention]

In order to achieve the above object, the present invention includes a heated member heated by a heat source, a detection means for detecting the temperature of the heated member, and an output of the detection means and a reference potential for setting the temperature to a predetermined temperature. and a control means for controlling the energization of the heat source by comparing the temperature, the potential changing means for changing the reference potential in order to switch the predetermined temperature to a lower temperature side at a predetermined period after reaching the predetermined temperature. , inputting the output of the potential changing means and the output of the control means,
The apparatus is characterized by further comprising a determination means for determining whether the heat source is disconnected on the condition that the control means outputs a signal for heating the heat source when the reference potential is switched to a low temperature side.

[Embodiments of the invention]

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is an external view of a fixing device in which the temperature control device of the present invention is used. In the same figure,
Reference numeral 5 denotes a heating roll which has a heat source 6 such as a heater inside, and whose peripheral surface is coated with a resin 5a such as Teflon (registered trademark) having heat resistance and toner release properties, and rotates in the direction shown. A pressure roll 7 whose circumferential surface is covered with heat-resistant and elastic silicone rubber or the like is pressed to the lower part of the heating roll 5 with appropriate pressure to form a nip portion 8, and the nip portion 8 is formed in a state in which the pressure roll 7 is pressed against the heating roll 5. It works together with the same role 5. Then, by passing the paper on which the toner image has been formed into the nip portion 8 from the direction of arrow A,
The paper is held and pressed, and at this time, the toner image is thermally fused onto the paper by the heating action of the heat source 6 and the pressing action of the pressure roll 7, thereby performing a fixing operation.
The tip of a peeling claw 9 is brought into contact with the paper discharge side of the circumferential surface of the heating roll 5 to peel the paper discharged from the nip section 8 from the heating roll 5, and then the paper is further removed by the paper discharge roll 10. Eject outside the machine.

Further, a thermistor 11 for temperature detection is attached to the circumferential surface of the heating roll 5 in contact with the circumferential surface of the heating roll 5, and this thermistor 11 is attached to the arm 12. This arm 12 is rotatably provided around a shaft 12a.

A temperature control circuit shown in FIG. 1 is provided as a temperature control device in the fixing device having the external configuration as described above.

In the figure, a thermistor 11 which is a temperature sensing element
The series circuit of resistors R 7 and R ₇ is connected in parallel with the series circuit of resistors R ₈ , R ₉ , and R ₁₀ to form a bridge circuit.
Further, the connection point between the thermistor 11 and the resistor _R7 is connected to the non-inverting input of the comparator _Q3 , and the connection point between the resistor _R8 and the resistor _R9 is connected to the inverting input of the comparator _Q3 . Furthermore, the output of comparator _Q3 is connected to the connection point between resistor _R11 and resistor _R12 .
The output of the comparator Q ₃ is also connected to the input of the AND gate 19 and the ready circuit 14 . resistance
A resistor _R13 is further connected to _R12 , and the above resistor
R ₁₁ , R ₁₂ , and R ₁₃ form a series circuit. Furthermore, the base terminal of the transistor _Q4 is connected to the connection point between the resistors _R12 and _R13 . The emitter terminal of this transistor _Q4 is grounded, and the trigger circuit 15 is connected to the collector terminal.

Furthermore, the trigger output of this trigger circuit 15 is
The gate terminal of triax 16 is connected, and this triax 16 and heater H' (heat source 6 in Fig. 2) are connected.
and AC power supply 17 constitute a closed circuit.

On the other hand, the output of the above-mentioned ready circuit 14 is output from the oscillator 1.
The output of the oscillator 18 is connected via the inverter 20 to the input of the AND gate 19 mentioned above. Then, the AND gate 19 outputs a disconnection signal b of the heater H' when the conditions described later are met. Furthermore, the output of the oscillator 18 mentioned above is a resistor.
It is also connected to the base terminal of transistor _Q5 via _R14 . The collector terminal of this transistor _Q5 is the resistor that makes up the bridge circuit mentioned above.
It is connected to the connection point of R ₉ and R ₁₀ , and the emitter terminal is grounded.

The circuit operation of the temperature control device of this embodiment having the circuit configured as described above will be explained below.

First, turn on the switch (not shown), and
Power is supplied between 1 and 22 to put the circuit into operation. When the power is turned on, the surface temperature of the heating roll 5 described above is the same as room temperature. Therefore, the resistance of thermistor 11 is high, and the voltage V _N ' at the connection point between resistor R ₇ and thermistor 11 is equal to the voltage at the connection point between resistors R ₈ and R ₉ .
higher than V _I ′. Therefore, the output of comparator _Q3 becomes a high signal, turning on transistor _Q4 , turning on trigger circuit 15, and turning on triac 16.
Supply a high signal to the gate of Through this series of operations, the triac 16 becomes conductive, and the AC power source 1
An alternating current flows through a closed circuit consisting of 7, heater H', and triax 16. Therefore, the heater
The heating roll 5 is heated by H' (heat source 6),
The temperature of the circumferential surface of the heating roll 5 gradually increases.

The heating roll 5 is gradually heated by this heating operation, and when the thermistor 11 detects the fixable temperature of the toner (approximately 180°C), the temperature is determined in advance based on the ratio of the resistance value of the resistor R ₈ and the resistance value of the resistor R ₉ . Since the voltage value (V _I ′) corresponding to 180°C is applied to the inverting input terminal of comparator Q ₃ (however, at this time, as will be explained later, transistor Q ₅ is turned on and one end of resistor R ₉ is connected to ground potential). ), the magnitudes of the input voltages V _N ′ and V _I ′ of comparator Q ₃ are the opposite of the above,
The output of comparator _Q3 becomes a low signal. Therefore, the transistor _Q4 is turned off, the trigger circuit 15 is also turned off, and the AC current flowing through the closed circuit of the AC power supply 17, heater H', and triax 16 is cut off.

This circuit operation causes heater H' to stop heating operation. When the heating operation of the heater H' stops and the temperature detected by the thermistor 11 decreases, the output of the comparator _Q3 is inverted again and the above operation is repeated.
The temperature of the heating roll 5 is controlled to a predetermined temperature (180° C.). On the other hand, in the ready circuit 14, the comparator
Once the output of _Q3 changes from a high signal to a low signal, it outputs a high signal to the oscillator 18 instead of the previous low signal, as shown in the time chart and temperature characteristic diagram of FIG. In response to the input of this high signal, the oscillator 18 outputs a pulse signal of 0.2 seconds to the AND gate 19 every 10 seconds, for example. Therefore, transistor _Q5 is turned off when the output of oscillator 18 is _a low signal as shown in FIG.
A series circuit of R ₁₀ is formed. Therefore, the reference voltage V _I '' is applied to the inverting input terminal of the comparator Q _3. This reference voltage V _I '' is a resistance value corresponding to when the temperature detected by the thermistor 11 is 150°C _. The resistance value is set to invert the output. Therefore, when the output of the oscillator 18 is a low signal _, the temperature of the heating roll 5 is 150
Outputs a high signal only when the temperature is below ℃. The temperature of the heating roll 5 becomes 150° C. or less only when the heater H' is disconnected unless the power is turned off.
For example, even if the transfer paper passes through the fixing device, the third
As shown in period T in the figure, the temperature of the heating roll 5 never drops below 150°C. Also, this 150
The temperature in degrees Celsius is the minimum temperature required for the fixing device to thermally fix the toner, and normally around 180 degrees Celsius is desirable for fixing the toner, but if it is over 150 degrees Celsius, it may cause fusing problems, etc. Never.

On the other hand, the pulse signal (0.2 seconds/10 seconds) from the oscillator 18 is inputted to the AND gate 19 via the inverter 20 as well as the high or low signal from the comparator _Q3 . When a high signal is input from the inverter 20, check the output of the comparator _Q3 . At this time, if the output of comparator Q ₃ is a low signal,
In other words, if the state is at time T ₁ in the time chart/temperature characteristic diagram in Figure 3, the AND gate 19
The disconnection signal b is never output from the circuit. However, if _the temperature _{characteristic} curve B in _{FIG .}
As time passes, the temperature of the heating roll 5 decreases, and when a high signal output from the comparator Q ₃ is detected at time T ₄ , the AND gate 19 outputs a disconnection signal b. This disconnection signal b is output to a control device such as a microcomputer (not shown), and processes such as prohibiting copying and displaying an abnormality are performed.

As described above, the temperature control device of this embodiment sets two reference voltage values corresponding to the fixable temperature and the normal fixing temperature in the comparator _Q3 , and controls the comparator Q at the potential of the fixable temperature at a predetermined period. ₃ compares the temperature detected by the thermistor 11, and if the temperature of the heating roll 5 is below a predetermined temperature (if the comparator Q ₃ outputs a high signal), the AND gate 1
9 outputs a disconnection signal b to detect that the heater H' is disconnected.

Although the above-described embodiments have been described with respect to a fixing device, the present invention is not limited to a fixing device, but can also be applied to electrical equipment having a temperature control device that feedback-controls the temperature of a heater used in a heating device, etc. Of course, it can be used for.

Further, in this embodiment, the temperature of the heating roll 5 (the temperature detected by the thermistor 11) corresponding to the reference voltage compared by the comparator _Q3 is set to 180°C and 150°C, but the temperature is not limited to these temperatures.

〔Effect of the invention〕

As explained in detail above, according to the present invention, the heater disconnection detection circuit can be detected using only a simple low-voltage circuit such as an AND gate, so there is no need for a high-voltage circuit as a circuit, and it is possible to standardize parts. This makes it possible to reduce the cost of the device. In addition, two-point temperature comparison (two-point voltage comparison) is possible with a single comparator.
becomes possible.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram of the temperature control device of this embodiment, Fig. 2 is a configuration diagram of a fixing device in which the temperature control device of this embodiment is used, and Fig. 3 is a time chart and a time chart according to the time chart. FIG. 4, which is a temperature characteristic diagram of the ivy heating roll, is a circuit diagram of a conventional temperature control device. 5... Heating roll, 6... Heat source, 11... Thermistor, 14... Ready circuit, 17... AC power supply, 18... Oscillator, 19... AND gate, H,
H′...Heater, _Q3 ...Comparator, _Q5 ...Transistor.

Claims (1)

[Claims] 1. A heated member heated by a heat source, a detection means for detecting the temperature of the heated member, and a comparison between the output of the detection means and a reference potential for setting a predetermined temperature to control energization of the heat source. a temperature control device comprising a control means, a potential changing means for changing the reference potential to switch the predetermined temperature to a lower temperature side at a predetermined period after reaching the predetermined temperature; and an output of the potential changing means and the control. and determining means for determining disconnection of the heat source on the condition that the control means outputs a signal for heating the heat source when the reference potential is switched to a low temperature side. Features temperature control device.