This is a easy built and very low-cost circuit purposed to protect your electronic appliances / devices, such as Personal Computer, TV, recording devices, refrigerator, and other devices during sudden tripping and resumption of home mains supply. Appliances like refrigerators and air-conditioners are more inclined to damage due to such conditions.
This electronic appliance protector circuit switches off the mains supply to the load as soon as the power trips. The electrical supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilised.
The circuit involves a step-down transformer followed by a full-wave rectifier and smoothing capacitor C1 which acts as a supply source for relay RL1. Initially, when the circuit is switched on, the power supply path to the stepdown transformer X1 as well as the load is incomplete, as the relay is in de-energised state. To energise the relay, press switch S1 for a short duration. This completes the path for the supply to transformer X1 as also the load via closed contacts of switch S1. Meanwhile, the supply to relay becomes available and it gets energised to provide a parallel path for the supply to the transformer as well as the load.
If there is any interruption in the power supply, the supply to the transformer is not available and the relay de-energises. Thus, once the supply is interrupted even for a brief period, the relay is de-energised and you need to press switch S1 almost instantly (when the supply continues) to make it accessible to the load.
Very-short-duration (say, 1 to 5 milliseconds) interferences or changes won’t influence the circuit because of presence of large value capacitor which has to discharge via the relay coil. Therefore the circuit gives suitable security against flighty power supply conditions.