Power-Back Surge Protection Circuit
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.
Author used an “O/E/N” pcb relay (part # 46-05-2CE) in the prototype, and a “Tyco” T90 pcb relay (part# T90N5D12-5) as the optional external relay. Photograph of the initial prototype with some components soldered at the bottom side of the perfboard (however, without the red indicator, push button switch, and headers) is shown below. Sorry have to be the potato picture as the finished system is in use now!
Schematic of the Power Surge Protector Circuit
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.
(system overview)
Author used an “O/E/N” pcb relay (part # 46-05-2CE) in the prototype, and a “Tyco” T90 pcb relay (part# T90N5D12-5) as the optional external relay. Photograph of the initial prototype with some components soldered at the bottom side of the perfboard (however, without the red indicator, push button switch, and headers) is shown below. Sorry have to be the potato picture as the finished system is in use now!
