Over Voltage Protection Circuit, Auto Cut Circuit

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 overvoltage autocut circuit diagram

This Autocut circuit is very easy, simple and very useful.

Only One transistor and some other components are used in this Overvoltage protection Auto circuit.

Load is connected with NC (normally connected ) Pin of the relay.

This circuit is a very useful auto cut circuit to the protection of any electrical or electronics appliance from high voltage. If any device needs 12v to work properly then this can be damaged by increasing the voltage. This circuit automatically cuts the current supply to any appliance from the input circuit. And when the voltage decreased at a normal level then automatic on the supply. So it helps to protect the circuit damage by the high voltage. If you use this circuit for Autocut on over 6v then use 6 v relay and circuit need 6v .

Before using this circuit set the preset and check the relay or output to at which input voltage level relay gets activated. Increase and Check the input voltage by multimeter and set the preset at the required position. This circuit can be used at both AC and DC appliance because it uses relay, but this circuit works only on DC supply power.


At first, when the voltage under or below the maximum limit ( at which it adjusted) the maximum voltage dropped through the Resistor R2 to the ground and the Base of Transistor does not get enough power to switch on the transistor. At this stage, the transistor in OFF state and Relay not get supply and it is not activated.

when the Input voltage of this circuit is increasing and cross a level than the input voltage through resistor R1 and preset also increased and even after dropping the voltage through R3, enough amount of voltage is at the Base of Transistor Turned On it and Supply passed Between Emitter and collector. Now Relay is activated and supply is Disconnected to the appliance . And that is protected from high voltage supply.

Before using it should be adjusted by preset and tested the input voltage and set required the auto cut level according to suitable.


Transformer 12v 1A -1

BC547 Transistor – 1

1000uf 35v capacitor -1

1N4007 Diode – 1


2.2kΩ-2,    33kΩ – 1   

Preset – 5K- 1

Also Read Overvoltage Autocutoff circuit using 741 opamp

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