Temperature controller using SCR
The temperature control circuit is used to regulate the temperature. Figure shows the temperature control circuit using thermostat as temperature detector and SCR as a switching device. The mercury in glass thermostat is extremely sensitive temperature measuring instrument which is capable of sensing changes in temperature of the order of 0.1°C.
Mode I: When the temperature is less than the desired value, the mercury in the glass thermostat is not able to short the electrodes A & B. Therefore the SCR receives the gate signal in both the half cycles & it will be triggered. Hence the heater will be connected to AC source in respective half-cycles. Thus heating is continued and temperature increases to desired level.
Mode II: As the temperature increases, the mercury level increases and when it reaches the desired value, the electrode A and B are short circuited through mercury. This will short circuit the gate to cathode, hence no trigger pulse can be received by SCR. Hence it is not turned-on in the next half-cycle & maintained off. Thus the heater gets disconnected and will remain disconnected from the circuit. The heating then stops and temperature is not allowed to rise beyond desired level. After some time when temperature falls to certain level and mercury switch gets opened, i.e the gate-cathode short-circuit is opened, the pulse is provided to SCR in each half-cycle and heater gets connected to source to produce heat and raise the temperature.