Power Guard


Power Guard Under and Over Voltage Protector

Under and Over Voltage Protector

A few circuits on under/over voltage protection have appeared recently in EFY. But this circuit is simple, very easy to construct and inexpensive. It can be added to existing stabilisers, in which case DC power supply can be taken from the original stabiliser circuit and only a few additional components, including one relay, are required to make the circuit.

Power Guard Under and Over Voltage Protector circuit

PARTS LIST

R1
1KΩ

R2
820Ω

R3
10KΩ 1/2W

C1
1000µF 25V

C2
250µF 25V

VR1
4.7KΩ

VR2
4.7KΩ

T1
HL 100

T2
HL 100

RL
12V 450Ω

Z1
Bz 148

Z2
Bz 148

D1
BY 126 × 4

T
12V 500mA

When supply voltage is within limit. T2 conducting and T1 is in a non-conducting state. At low voltage, zener Z2 will not breakdown and T2 will not conduct, which results in de-energising the relay. This adjustment is done by adjusting VR2.

At high voltage, zener Z1 will breakdown and T1 will go in conducting state and the relay will de-energise. This adjustment is done by adjusting VR1. Normally-open contact of the relay is used in control circuit. Red LED gives high low voltage cut-off  indication.


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