UJT as Over Voltage Detector Circuit Diagram:

UJT as Over Voltage Detector Circuit Diagram – A simple dc overvoltage detector circuit is given in Fig. 26.96. It operates on the fact that the device remains switched off as long as the input voltage remain less than the peak point voltage V_P of the UJT and as soon as input volt­age exceeds V_P, the device is switched on.

Let the input voltage V_in to be kept constant. For it V_BB is to be chosen in such a way that this is slightly lesser than V_in. As soon as V_in exceeds V_P, then UJT is switched on, and consequently the capacitor commences discharging through low resistance path E and B₁.

The current flowing in pilot lamp L lights it, which is an indication of an overvoltage across the circuit.