# How to Make an Automatic Voltage Stabilizer? Circuit, Construction Explained

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## Introduction

There are huge varieties of voltage stabilizers available in the market, and surely it’s not a big deal to procure one, according to the needs. But off course it can be very amusing to build one at home all by you and see it actually working. The circuit of an Automatic Voltage Stabilizer (AVS) described in this article is in fact very simple in design, reasonably precise and will give a good protection to the electronic gadget that is connected to it. It will particularly guard them against the dangerous high voltages and also from the possible brownouts (low voltages).The output will stay in the range of 200VAC – 255VAC with an input voltage of 175VAC – 280VAC.

## How does a Voltage Stabilizer Work?

In one of my previous articles you must have learnt regarding the functioning of an auto transformer. There we have studied how an auto transformer may be used to produce voltages higher and lower than the input supply mains AC voltage. An auto transformer in fact plays the most important part in a voltage stabilizer circuit.

The circuit of a voltage stabilizer basically consists of a voltage sensor. It is configured to detect the rise or fall of the AC mains voltage to dangerous levels. As soon it senses a risky input voltage, it immediately energizes the relays connected to it. These relays in turn swaps and switches the appropriate winding terminals of the auto transformer to correct and stabilize the output voltage. Thus the appliance which is connected to the output of the voltage stabilizer circuit always receives a safe, tolerable voltage and is able to function reliably, irrespective of the fluctuating input voltages.

Let’s proceed to know what are the parts required to build it and also its construction details.

## Parts Required

You will require the following parts for the circuit:

• Resistor ¼ watt, CFR R1 = 2 K 7,

• Preset P1 = 10 K Linear,

• Transistor T1 = BC 547,

• Zener diode Z1 = 3 V / 400 mW,

• Diode D1, D2 = 1N4007,

• Capacitor = 220uF / 25 V

• Relay RL1 = 12 V / DPDT mini (double pole, double throw),

• Transformer T1 = 12 – 0 – 12 V / 5 amps. T2 = 0 – 12 volts / 500 mA (input as per country specifications)

• General purpose board = 3” by 3”

## Construction Clues

With the help of the given circuit schematic (on the next page) the construction of this simple AVS circuit may be completed through the following easy steps:

• In the given piece of general purpose board, insert the transistor, solder and cut its leads.

• Proceed by fixing and soldering the rest of the associated parts along with the relay around the transistor.

• Interlink all of them as per the circuit schematic.

• Finally connect the primary and the secondary wires of the transformer to the relay contacts as shown in the diagram.

The next page covers the circuit and the construction details of this automatic voltage stabilizer circuit.

## Circuit Description

The functioning of this simple voltage stabilizer circuit may be understood from the following points:

Referring to the figure (Click to Enlarge) we see that Transistor T1 forms the main active part of the entire circuit.

The voltage from smaller transformer is rectified by D1 and filtered by C1 to produce the required operating power for the control circuit comprising of transistor T1, preset P1, zener diode Z1 and the DPDT relay.

The above voltage is also used as the basic reference or the sensing voltage. Because this voltage will vary proportionately with the applied input voltage variations.

For example, if normally the operating DC voltage is around 12 volts, an increase or decrease of the input AC mains voltage by say 25 volts would proportionately increase or decrease the DC volts to 14 or 10 volts respectively.

Preset P1 is set such that the transistor conducts and operates the relay whenever the input AC mains tends to deviate above the exact normal voltage (110 or 225 volts) and vice versa.

If the input voltage crosses the above limit, T1 conducts and activates the relay. The relay contacts connect the relevant connections of the power stabilizer transformer to deduct 25 volts from the input i.e. bring the output to about 205 volts. From here on if the mains voltage keeps on increasing, the ouput to the appliances will be 25 volts below it. It means, even if the voltage reaches as high as 260 V, the output will reach only up to260 - 25 = 235 volts.

Exactly opposite will happen in case the input AC drops below the normal level, i.e. in this case 25 volts will be added to the output, and even if the input keeps on dropping and reaches 180 volts, the output will reach only up to 180 + 25 = 205 volts.

The present design is very simple and basic, therefore the stabilization cannot be very precise. But certainly it will keep the output within 200 and 250 volts against input extreme voltages of 180 to 275 volts (or within 100 and 125 against 90 and 130 volts).

## How to Test it?

The completed circuit board of simple voltage stabilizer may be tested in the following method:

• For the testing procedure you will require a 0 – 12 volts universal DC variable power supply.

• You can assume the maximum 12 volts of the power supply to be equivalent to an input of approximately 230 VAC. We will take this voltage as the trip or the change-over voltage of the stabilizer.

• Connect the power supply to the supply terminals of the completed circuit board.

• Keep the voltage of the power supply to its maximum position of 12 volts.

• Carefully adjust the preset so that the relay just gets activated.

• Now on decreasing the power supply voltage by 1 volt i.e. to 11 volts should restore the relay back to its deactivated position.

• This concludes the setting of the unit. It should keep the output voltage in between 200 and 255 volts with extreme input voltages of 175 to 280 volts.

Your voltage stabilizer is now ready and shall protect any domestic electronic appliance connected at its output.