This circuit protects refrigerators as well as other appliances from over and under-voltage. Operational amplifier IC LM358 is used here as a comparator. IC LM358 consists of 2 parts operational amplifier.
- NI Multisim
- ver 13
The ac mains is reduce by isolation transformers.
The unregulated power supply is connected to the series combination of resistors R1 and R2 and potmeter R3. The same supply is also connected to a 6.8V zener diode (ZD1) through resistor R4.Preset R3 is adjusted such that for the normal supply of 180V to 240V. The voltage at the inverting terminal (pin 2) of operational amplifier N1 is less than 6.8V. Hence the output of the operational amplifier is highand transistor T1 remains on. The relay, which is connected to the collector of transistor Q1, also remains energised. As the AC supply to the electrical appliances is given through the normally closed (N/C) terminal of the relay, the supply is connected during normal operation.
When the AC voltage increases beyond 240V, the voltage at the inverting terminal (pin 2) of operational amplifier N1 increases. The voltage at the non-inverting terminal is still 6.8V because of the zener diode. Thus now if the voltage at pin 2 of the operational amplifier is higher than 6.8V, the output of the operational amplifier goes down low to drive transistor Q1 off and hence de-energise relay RL. This circuits will be alarm when occur. Thus the appliances are protected against over-voltage. Thus the appliances are protected against over-voltage.