dThis simple circuit can do a lot and immediately break the power from the appliance if there is any increase or decrease in the Mains voltage. It similar to the working of A Stabilizer to sense high and low voltage fluctuations. The trick is simple and the circuit costs only Rs.100.
The idea behind the circuit is that any voltage increase or decrease in the 230 volt mains lines, will change the voltage in the Primary of the transformer. A corresponding voltage change also appears in the secondary also. This is used in the circuit to operate the relay.
When the voltage level is normal, say 230 volts, relay will be energized since T2 conducting. This is set by the preset VR2. Load will get power through the Common and NO contacts of the relay. When the voltage drops, T2 turns off because of the setting of VR2 and relay de-energize to cut off power to the appliance. When the voltage increases again, relay will be activated again.
VR1 is used to set the high voltage limit. When the voltage is normal or low , Zener diode ZD will not conduct ( the limit is set by VR1). When the voltage rises above this set limit, ZD conducts and T1 switches on. This drops the voltage to VR2 and T2 switches off. Relay again deactivates.