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

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.

Please send your ideas, those are very important for our success…

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6 comments on “Power Guard
  1. Arup Basak says:

    Use a good macro mode camera for taking pcb images please. otherwise, good post

  2. pasanlaksiri says:

    You have some nice posts. All the best,

  3. ananda says:

    Dear Sir,
    May i have the value of the BZ148 as it is not possible to get it from the net.



  4. hilmi says:

    how the capacity in watt to lose operated

  5. ya.vasch1950 says:

    Яку напругу видає трансформатор?.

  6. Christian says:

    Nice circuit but there is a little fault that will make it not to work the last transistor cannot switch on the relay because Insufficient current at the base you need to add two more transistor in this way

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