JPS6044689B2 - Temperature control device temperature measuring element disconnection detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to detecting disconnection of a temperature measuring element in a heating element temperature control device in a copying machine or the like.

It is not easy to detect disconnection in a temperature measuring element such as a thermistor, which has a high resistance at low temperatures.
In order to facilitate such disconnection detection, the present invention raises the temperature of the temperature sensing element to a certain extent by energizing the heating element without detecting disconnection for a scheduled time after the power is turned on, and then lowers its resistance value. In other words, the present invention relates to a device that starts wire breakage detection after the temperature measuring element is in a state where it is easy to detect wire breakage. Thermistors, which are widely used as temperature measuring and temperature control elements, have extremely high resistance at low temperatures, so conventionally the current flowing through them has been amplified and then used to detect disconnection. This method also has the disadvantage that it is susceptible to noise and the like because it generates a very small current at low temperatures. In other words, after the power is turned on, the heating element is in a cold state, and the temperature of the thermistor that detects the temperature is also low, so its resistance value is very high, and the difference in resistance value when the wire is disconnected is It was unclear, it was difficult to detect a disconnection, and there was a high possibility of misjudgment. The present invention
This is particularly effective when a thermistor, etc. shows high resistance when the power is turned on.The heating element is heated without detecting a disconnection for a scheduled time after the power is turned on, and the temperature of the thermistor, etc. rises to a certain extent after the scheduled time. By starting the disconnection detection after the above-mentioned period, the disconnection detection is made easier.

Hereinafter, the present invention will be explained in detail with reference to FIG.

In Fig. 1, 1 is a temperature measuring element (hereinafter represented by a thermistor) having a negative temperature coefficient of resistance, and 2 is a heated body (
A comparator for temperature control (not shown) outputs "H" when the resistance of the thermistor 1 is large (that is, when the temperature of the thermistor and the heated object is low).

3 is a control circuit for the heater 5, for example SSR (
solid state relays) etc. can be used, and the input is
When the voltage is H, the heater 5 is energized. 4 is a timer circuit that operates the relay 6 for a scheduled time after the power is turned on, and its contact 6
Close a.

7 is a comparator, which is “H” during the scheduled time after the power is turned on.
'Produces an output and makes transistor Q3 conductive.

A charging/discharging capacitor 8 is charged with a time constant determined by the capacitance C and the resistor R7 after the power is turned on, and discharged with the time constant of the resistor R4 and the resistor R7 when the power is turned off.

By setting the resistor R6 to a lower resistance than R7, charging can be performed slowly and discharging can be performed quickly. 9 is a buffer inverter which detects the output of the transistor Q1 and outputs a power low when a disconnection is detected.

The operation of the circuit described above is as follows.

When the power switches S1 and S2 are closed, since the capacitor 8 is uncharged, the output of the comparator 7 becomes "゜H", turning on the transistor Q3.The relay 6 operates and the contact 6a closes. The current from the power supply V+ is R1→
The current flows from R2 to the base of Q1, turning on transistor Q1. Here, the value of R2 is set so that the output of comparator 2 is ゜゜H
By setting the control circuit 3 to 5, the control circuit 3 is driven and the heater 5 is energized, thereby increasing the temperature of the heated object. As time passes, charging of the capacitor 8 progresses, and its potential increases. When the potential becomes equal to or higher than the voltage divided by R8 and R9, the output voltage of the comparator 7 becomes low, turning off the transistor Q3. This opens the relay 6 and opens its contacts 6a. Here, if thermistor 1 is disconnected, transistor Q1
No current flows through the base of Q2, and its collector becomes high level, turning on transistor Q2. As a result, the output of the buffer inverter 9 becomes "L", and the terminal 10 outputs a disconnection detection signal.On the other hand, if the thermistor 1 is not disconnected, even if the contact 6a is open, the circuit is connected to the transistor Q1 through the thermistor 1. Since the current is supplied and the transistor Q1 remains on, the output of the buffer inverter 9 remains at "°H" and does not change, and no disconnection detection signal is output. As described above, according to the present invention, disconnection can be detected easily and reliably with an extremely simple configuration, and problems such as misjudgment due to noise are eliminated.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of one embodiment of the present invention.

Claims (1)

[Claims] 1. A temperature control device comprising a heater, a negative resistance temperature coefficient temperature measuring element whose resistance value changes according to heat generation of the heater, and means for controlling energization to the heater in response to a change in resistance of the temperature measuring element, a fixed resistor connected in parallel with the temperature sensing element via a relay contact; a timer circuit that generates an output for a scheduled time after power is turned on to the heater; and a timer circuit that is excited by the output of the timer circuit and connects the relay contact a relay means that closes only for a predetermined time after power is turned on to the heater; a switching means that is controlled on and off according to the value of the current flowing through the temperature sensing element and the fixed resistor connected in parallel; and the switching means. A temperature measuring element disconnection detection device for a temperature control device, comprising means for outputting a disconnection detection signal of the temperature measuring element according to an on/off state of the means.