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L77, Over Voltage Protection

This circuit protects your television as well as other electrical appliances from over-voltage. It uses operational amplifier µA741 (IC1) as a comparator. The unregulated power supply is connected to resistor R3 and preset VR1 through resistor R2. Zener diode ZD1 provides reference voltage of 5.1V to the inverting input (pin 2) of IC1.


  • IC1 = UA741

  • IC2 = 7812 (Voltage regulator)

  • D1-D5 = 1N4007, D6 = 1N4148

  • R1 = 47R, R2 = 100R, R3 = 330R

  • R4 = 100K, R5 = 1.8K, R6, R7 = 1K

  • VR1 = 10K, VR2 = 22K

  • T1 = BC547, T2 = SL100

  • ZD1 = 5.1V Zener

  • C1, C4, C5 = 0.01uf, C2 = 1000uf 25v

  • C3 = 100uf 25v

  • 12v Relay

  • Transformer (9-0-9) if 7.5-0-7.5 is not available

The non-inverting input (pin 3) of IC1 senses voltage fluctuation in the mains. Preset VR1 is adjusted such that for mains supply below 240V AC, the voltage at the non-inverting terminal of IC1 is less than 5.1V.

Hence the output of IC1 is zero and transistor T1 is in non-conducting state. At the same time transistor T2 conducts to energized relay RL1 to connect the mains to the load.

When AC mains is beyond 240V, the voltage at pin 3 of IC1 goes above 5.1V. The high output of IC1 drives transistor T1 and transistor T2 stops conducting to de energize the relay. Hence the appliance turns off.

Preset VR2 is used for proper biasing of transistor T1. The AC mains supply is stepped down by transformer X1 to deliver a secondary output of 7.5V-0-7.5V AC, 1A. The output of the transformer is rectified by a full-wave rectifier comprising diodes D1 through D4. Capacitors C1 and C2 act as filters to eliminate ripples. Regulator IC 7812 is used to provide regulated 12V supply.

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