Heat Control Circuit Using a Thermistor and a TRIAC

Feb 152017

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Heat Control Circuit Using a Thermistor and a TRIAC

This heat control circuit, which uses a thermistor and a TRIAC, is designed to control the temperature of a room. It can do so by using a heat source, such as an electric oven heating element, or a cooling device, such as a fan.

The trigger circuit uses a unijunction transistor (UJT) to introduce a conduction angle to the TRIAC. The size of this angle depends on the room temperature, which is measured using a thermal resistance or thermistor (RT). The thermistor has a value of 2 kilohms at 25 °C.

A bridge diode rectifier and a Zener diode power the circuit and maintain stable voltage for the trigger circuit.

The resistor R2 is adjusted so that the PNP bipolar transistor Q1 does not enter the cutoff state at a given temperature. When Q1 is in the cutoff state, no current charges capacitor C1. Therefore, the UJT and the TRIACs are also in the cutoff state.

If this transistor is saturated (on), it will charge capacitor C1 and trigger the UJT when the voltage reaches Vp. The time it takes for the UJT to reach the Vp voltage depends on the RT thermistor.

An increase in temperature decreases the value of RT, which consequently decreases the collector current of the transistor by increasing the charge time of the capacitor and decreasing the conduction angle. Conversely, decreasing the temperature increases the driving angle.

To reverse the operation of this circuit and how it responds to temperature, exchange resistors RT and R2.

You may be interested in: Thermistor-Controlled AC Fan Circuit.