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UnderVoltage and Overvoltage Protection circuit

Over-voltage and Under-voltage Protection Circuit for  High and Low Voltage Cutoff.

This voltage sensor circuit can be used to protects any appliance to voltage fluctuation. When the input voltage goes high then the relay will be activated and also when the input voltage is below then required level then relay activated.

NoteLoad is connected with NC and COM pin of Relay

Dual comparator ic LM358 is used to make this circuit diagram. Here both two comparators of 358 ic are used. Actually AC supply is not detected by the circuit, Circuit is detected AC supply through DC converter. So we need DC power for circuit operation. When the AC supply is fluctuating high and low then the circuit is also getting a variable DC power and then the circuit is able to sense the voltage condition. 

When the input voltage is greater than the reference voltage then the output of the comparator is high and if the input voltage is lower than the reference voltage then the output is LOW. This mechanism of the comparator is used in this circuit to sense and compare the input supply voltage.

Reference voltage means the input at the inverting pin (pin 2 & 6 ). Here voltage of the inverting input pin is fixed 5.1v by Zener diode. You can choose other values by different Zener diodes below the value of input voltage.

At the normal voltage level. The output of the comparator is LOW because the input voltage is lower than the reference voltage, And the relay is not activated, the Appliance is ON. IF AC voltage increases then the input voltage at pin 3 is also increased. When the input voltage goes high then the reference voltage at pin 2, The Relay activates and pin of the relay is changed from NC to NO and the connected load appliance is disconnected.

In this circuit reference voltage is 4v. so as soon input voltage go more than 4v then the relay is activated.


In the 2nd Comparator (U2B) the Zener Diode is connected with non-inverting input.

Assume that the AC supply is decreased, when ac supply goes LOW, then the Reference voltage of comparator 2 is also get decreased as compared to the input voltage. Input voltage is fixed by a 4v Zener diode. so as the reference voltage is decreased below the input voltage then the output of the comparator goes HIGH. Relay is activated again and the connected load is disconnected and turned off.

So Output Of comparator is LOW And Load Gets Supply voltage – If only the input voltage Of comparator 1 is below the 5.1v. And the reference voltage of comparator 2 is more than 4v. The required cutoff level is set by adjusting the preset.

If a comparator output is HIGH and another Comparator output is LOW then Only High signal is passed and the relay gets the only high signal, No LOW/Negative signal because the base of the transistor is connected through a Diode.

so this is a protection circuit with high and low voltage. You can use 6.8v and 6.0v Zener diode. And can useLM324 opamp Comparator ic for this circuit.

Over-voltage and Under-voltage Protection Circuit diagram

overvoltage and undervoltage cutoff protection circuit




Vin> Vref= High Output (+vcc output)

Vin<Vref = LOW Output (-ve vcc output)     Vin = input voltage , Vref= reference voltage

Adjust the cutoff level by the preset adjustment.


IC– LM358-1

Zener Diode

5.1v -1











1000uf 35v-1

Diode– 5

Relay 9v-1

7809 regulator ic-1

transformer -12v 250ma-1

Transistor BC547-1


Also Read 10 A high current adjustable voltage regulator 

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