Auto Changeover from Generator to mains supply

Because of energy crisis Load shedding is a common problem in many countries. Sudden power fluctuations, surge and high voltage may spoil sophisticated household appliances like TV, VCR, VCP and music system. This circuit provides protection against these problems and automatically changes the power supply from Generator output to mains supply and also switches off the generator.

Circuit descriptions

This circuit is self explanatory. When lines supply voltage crosses the preset level, Zener diode ZD4 break down and thus transistor T2 conducts, T3 does not conduct, causing the relay to de-energise. Voltage surge at the time of power resumption is protected by the delay circuit around transistor T1.

All the portable petrol and kerosene generators have a connection from CDI. To stop the engine of the generator-set, this connection is to be grounded momentarily (through the switch). This lead is to be connected with the ground through the N/C point of the relay.

auto changeoverfrom genset to mains supply

Instead of the one transformer, two transformers are used in series to ensure better life of the unit. Value of capacitor C2 can be increased or decreased for the variation of delay time. A negative feedback resistance, R6 (15 K-ohm to 100 k-ohm) can be incorporated between transistor T3 and T4 to overcome the damage that could be caused by voltage fluctuations.

Resistors (all ¼-watt, ± 5% Carbon)

R1 = 10 Ω

R2 = 1 KΩ

R3 = 820 Ω

R4 = 57 KΩ

R5 = 47 Ω

R6 = (15 KΩ to 100 KΩ)


C1 = 100 µF/25V

C2 = 1000 µF/12V

C3 = 100 µF/18V


T1 = BC147

T2, T3 = SL100

D1, D2 = 1N4001

D3 = 1N4007

ZD1 = 5.6V/400mW

ZD2 = 7.8V/400mW


X1, X2 = 230V AC primary to 12 V, 500mA secondary transformers

RL1 = 12V/450 Ω relay

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