We are aware that when the line voltage become more than 250 volts and less than 160 volts, it can damage out TV sets, refrigerators etc. Hence, we use voltage stabilizers which are costly. The circuit described here is low-cost. The assembling cost comes around $1 only. If the voltage exceeds the mentioned range it automatically cuts-off the supply so that the life of the TV set and other devices can be extended.
The common point of the relay is connected to the phase line and form the normal point it is connected to the device. The neutral will have a direct line from the mains supply.
To adjust VR1 and VR2, you have to take the help of a manual voltage stabilizer. First adjust the manual voltage stabilizer so that its output gives 160 volts. Then adjust VR1 so that the relay goes to the excited position. Now adjust the manual voltage stabilizer to give an output of 250 volts. Then adjust VR2 so that the relay just goes to the excited position.
This circuit requires nearly 75mA current at 12 volts. You may not get a transformer of this range in the market. Therefore, take a 6-volt, 300 mA centre-tapped transformer which is easily available in the market. Now obtain 12 volts from the transformer using full wave rectifier from the two end points of the secondary winding.
Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)
R1, R5 = 47 KΩ
R2, R3, R6, R7 = 150 Ω
R4 = 100 Ω, 0.5W
VR1, VR2 = 68 KΩ
T1, T3 = BC158
T2, T4 = BC148
D1 – D4 = 1N4148
D4 = 1N4007
ZD1 = 9V, 1W
RL1= 6V, 300 Ω Relay