Simple Mains AC Over Voltage and Under Voltage Protection Circuit Explained

Our Mains AC domestic power may be full of ups and downs or fluctuations, which may be quite fatal to our sophisticated appliances like TV sets, DVD players, compotes etc. A very simple circuit presented when installed in the house electrical may help in restricting the issue to a great extent.

Here we will learn two designs of over and under voltage circuits, the first based on transistors and the other one using an op amp.

Using Transistors:

You will be surprised to know that a nice little circuit for the said protections can be built using just a couple of transistors and a few other passive components.

 Looking at the figure we can see a very simple arrangement where T1 and T2 are fixed as an inverter configuration, meaning T2 responds oppositely to T1.

Please refer the circuit diagram.

In simple words when T1 conducts, T2 switches OFF and vice versa.

The sensing voltage which is derived from the DC supply voltage itself is fed to the base of T1 via preset P1. 

The preset is used so that the tripping thresholds can be determined precisely and the circuit understands when to execute the control actions. P1 is set for detecting high voltage limits.

Initially when the voltage is within the safe window, T1 remains switched OFF and this allows the required biasing voltage to pass through P2 and reach T2, keeping it switched ON.

Therefore the relay is also kept activated and the connected load receives the required AC voltage.

However in case suppose, the mains voltage exceeds the safe limit, the sensing sample voltage at the base of T1 also rises above the set threshold, T1 immediately conducts and grounds the base of T2. 

This results in switching OFF of T2 and also the relay and the corresponding load. The system thus restricts the dangerous voltage from reaching the load and safeguards it as expected from it.

Now suppose the mains voltage goes too low, T1 is already switched OFF and at this situation T2 also stops conducting due to the settings of P2, which is set so that T2 stops conducting when the Mains input goes below a certain unsafe level.

Thus the relay is once again tripped OFF, cutting of power to the load and prompting the required safety measures.

Though the circuit is reasonably accurate, the window threshold is too wide, meaning the circuit triggers only for voltage levels above 260V and below 200V, or above 130V and below 100 V for 120 V normal supply inputs.

 Therefore, the circuit may not be very useful for folks who might be looking for absolutely accurate tripping points and controls which can be optimized as per ones personal preference. 

To make this possible a couple of op amps may be required to be included instead of transistors.

Parts List for the above AC mains over voltage, under voltage protection circuit.

R1, R2 = 1K,

P1, P2 = 10K,

T1, T2 = BC547B,

C1 = 220uF/25V

RELAY = 12V, 400 OHMS, SPDT,

D1 = 1N4007

TR1 = 0-12V, 500mA

Using an Op Amp:

 The above discussed operations can also be done using an op amp, with precise results.

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