17-10-2014, 04:28 PM
Abstracts: This over/under voltage cut-out will save your costly electrical and electronic appliances from the adverse effects of very high and very low mains voltages. Preset VR1 is adjusted for under voltage (say 140 volts) cut-out by observing that LED just lights up when mains voltage is slightly greater than 140V AC. At this setting the output at pin 3 of IC is low and transistor T1 is in cut-off state. As a result RESET pin 4 of IC is held high since it is connected to Vcc via 100 kilo-ohm resistor R4. Preset VR2 is adjusted for over voltage cut-out by observing that LED2 just extinguishes. When the mains voltage is slightly less than 240V AC with RESET pin 4 of IC2 high, the output pin 3 is also high. As a result transistor T2 conducts and energizes relay RL1, connecting load to power supply via its N/O contacts. This is the situation as long as mains voltage is greater than 135V AC but less than 240V AC. The unregulated power supply is connected to the series combination of resistors R1 and R2 and potentiometer VR1. The same supply is also connected to a 6.8V zener diode (ZD1) through resistor R3. Preset VR1 is adjusted such that for the normal supply of 135V to 240V. Power failure simple circuit given here switches off the mains supply to the load as soon as the power trips. The supply can be resumed only by manual intervention. Thus, the supply may be switched on only after it has stabilized.