Power-Back Surge Protector is a simple yet effective solution for protecting your valuable and sensitive electric/electronic systems. Power-Back Surge typically occurs when power returns after a power-cut (black out) and connected equipments receives a surge of electricity at an over-voltage level, which can be very damaging. Usually, power-back surges are created by the utility, when it restores supply at an above normal voltage level inorder to compensate for the demand as connected equipment restarts at the same time. The author took this little circuit personally to protect numerous lab equipments including his fave DSO! The circuit is designed to be constructed in a small module structure with an onboard electromagnetic relay as the primary switching device. The design is centered around (again) the famed 555 timer, wired as a bi-polar latch switch with its two comparator inputs tied together and biased at 1/2 Vcc through a resistive-potential divider. Since the threshold comparator will trip at 2/3 Vcc and the trigger comparator will trip at 1/3 Vcc, the bias provided by the resistors are centered within the comparators trip limits. The module does need a source of external power for operation, and so a “clean” 5v dc supply is preferred here as the power supply for the entire circuit. However, note that the onboard miniature relay cannot drive grid-supply powered “goliath” loads without the help of an external heavy-duty switching relay. It is better to use a T90 type (5v dc) heavy-duty relay as the external relay, because the type can handle too much power efficiently. Working of the circuit is very simple. Initially when the circuit is powered up or when power supply is resumed, the relay remains in the de-energized state. This prevents the power supply from reaching to the connected load. When the push-to-on switch (a good quality momentary push button switch required) is depressed, the circuit turns to active mode, relay is energized, and hence power supply is extended to the connected load. The red LED in the circuit is the “power in” status indicator, and the green LED is the “power out” status indicator. http://www.electroschematics.com/12335/power-back-surge-protection-circuit/