Electronic and electrical appliances used in most homes in the country are designed to operate at a nominal 230
V AC, which ought to be acceptable within certain tolerable upper and lower limits. Excessive fluctuations
beyond these limits may cause the appliance to malfunction or get irreparably damaged. Most protective devices
are imported as well as expensive. The circuit presented is a relatively cheap automatic mains voltage switch
(AVS) protector that senses undesirable and harmful fluctuations in mains voltages and disconnects the
appliance whenever the mains voltage supply goes above or below pre-settable and safe tolerable limits or
‘window’. The appliance is automatically reconnected when mains power returns to normal within that
‘window’. The design features anti-surge protection, a transformerless power supply for the circuitry, a window
comparator for the detection of low and excessively high mains voltages to the load and a switching relay
capable of switching high currents, ideal for compressor-based appliances. A delay section for control of
spurious low voltage drops as well as mains stability is incorporated. The design provides a safe operating
voltage range of 180V to 250V and offers practically the same functions as those of the imported brands.
Basically, the design consists of various circuit building blocks and circuit techniques that would be of interest
to the electronic hobbyist
