Note- Connect the load with NC pin of relay
When the the input voltage at Non- inverting pin of op-amp 741 is cross over the set reference voltage . The output of this ic at pin 6 is goes to High and relay is activated and then changed the relay pin from NC (normally connected) to NO (normally open).Appliance get disconnected And a Reference voltage is given through the zener diode to fix a level.
VCC terminal pin 7 of comparator 741 is connected through the regulator ic 7812 to provide the fixed supply . But the input signal at pin 3 and pin 4 is given before regulated ic to detecting the fluctuation of supply. Pin 4 is connected with negative supply. Give 12v DC to this circuit.
At first if the input supply voltage in the circuit is below and under the required level then the output of ic 741 is LOW , and connected transistor remains turn off and relay is not activated. Appliances are On. Connect the load with NC.
As the input supply is increase then the voltage at pin 3 also increased, and when the value of voltage at pin 3 is greater than pin 2 , then the output voltage of 741 goes High , and now the relay is activated and supply is disconnected.
Autocut off level is set by the adjusting the prest by moving it . And after adjusting the cutoff level this circuit is used to sense the adjusted maximum input voltage level. This circit is useful to protection of any appliance by the High voltage supply.In this circuit one 741 ic is used as comparator to detect and compare the input signal with respect to reference voltage. Reference voltage (Vref) is voltage, with level of input signal we want to compare . In this circuit Reference voltage is given at inverting terminal by the zener diode to fix constant level to make sensitive. This circuit can be used AC or DC Load But this circuit needs only DC.
Input voltage (At pin 4) < Reference Voltage (At Pin 2) - Then Output at pin 6 is High and vice-versa.
Pin of 7812 (Front view)
Pin 1= input (Line voltage), Pin2= Ground (Common), Pin 3= VREG(Out)
Zener Diode 6.8v (1W)-1
0.1uf - 1