In this article we learn how to make a 2 relay or two stage voltage stabilizer circuit for controlling and regulating 220V or 120V mains voltages through a simple circuit.


In this power stabilizer circuit, one relay is wired to select the high or the low tap from the stabilizer transformer at some particular voltage level; whereas the second relay keeps the normal mains voltage switched in, but the moment there’s a voltage fluctuation it toggles and selects the appropriate HOT tap via the first relay contacts.

A simple power stabilizer circuit discussed here is very easy to build and yet is able to provide a 2-stage correction of the input mains.

A simple method of converting a normal transformer into a stabilizer transformer has also been discussed using circuit schematics.

Simulation and Working

As shown in the adjoining figure, the whole circuit operation can be understood with the following points:

220V Mains Power Stabilizer Circuit

Basically the idea here is to make relay #1 switch at two different mains voltage extremes (high and low), which are considered not suitable for the appliances.

This switching enables this relay to select an appropriately conditioned voltage from another relay through its N/C contacts.

How to Wire the Relay Contacts

The contacts of this second relay #2 makes it sure that it selects a appropriate voltages from the stabilizer transformer and keeps it ready for the relay #1 whenever it toggles during dangerous voltage levels. At normal voltages, relay #1 remains activated and selects the normal voltage through its N/O contacts.

Transistor T1 and T2 are used as voltage sensors. Relay #1 is connected to this configuration at the collector of T2.

As long as the voltage is normal, T1 stays switched off. Consequently T2 at this moment remains switched on. Relay #1 is activated, and its N/O contacts connect the NORMAL AC to the appliance.

If the voltage tends to rise, T1 slowly conducts, and at a certain level (decided by the setting of P1), T1 fully conducts and shuts off T2 and relay #1.

The relay immediately connects the corrected (lowered) voltage supplied by relay #2 through its N/C contacts to the output.

Now, in case of a low voltage T1 and T2 both will stop conducting, producing the same result as above, but this time the supplied voltage from relay #2 to relay #1 will be high, so that the output receives the required corrected level of voltage.

Relay #2 is energized by T3 at a particular voltage level (as per the setting of P3) in between the two voltage extremes. Its contacts are wired to the stabilizer transformer tapping so that it selects the desired voltage appropriately.

How to Assemble the Circuit

The construction of this circuit is very simple. It may be done by with the following steps:

Cut a small piece of a general purpose board (about 10 by 5 mm).

Begin the construction by inserting the transistors first, keeping ample space between them so that the other can be accommodated around each of them. Solder and cut off their leads.

Next, insert the rest of the components and interlink them with each other and the transistors by soldering. Take the help of the circuit schematic for their proper orientations and placements.

Finally, fix the relays to complete the board assembly.

The next page deals with the construction of the power stabilizer transformer and the testing procedure. After these procedures are completed, you may integrate the tested circuit assembly to the appropriate transformers.

The whole set up then may be housed inside a tough metal enclosure and installed for the desired operations.
Parts List

R1, R2, R3 = 1K, 1/4W,


C1 = 1000uF/25V

Z1, Z2, Z3 = 3V, 400mW ZENER DIODE,

T1, T2,T3 = BC 547B,


D1--D4 =1N4007,

TR1 =0-12V, 500mA,


How to Convert an Ordinary Transformer into a Stabilizer Transformer

Convert an Ordinary Transformer into a Stabilizer Transformer

Stabilizer transformers are normally made to order and are not available ready made in the market. Since multiple mains AC voltage taps (high and low) outputs are required from them and also since these are specific for a particular application, it becomes much difficult to procure them ready made.

The present circuit also needs a power regulator transformer, but for the ease of construction a simple method may be incorporated to convert an ordinary power supply transformer into a voltage stabilizer transformer.

As shown in the figure, here we require a normal transformer rated at 25-0-25/ 5 Amp. The centre tap should be split, so that the secondary may consist of two separate windings. Now it’s just a matter of connecting the primary wires to the two secondary windings as shown in the diagram.

Thus, by following the above procedure, you should be able to successfully convert an ordinary transformer into a stabilizer transformer, very handy for the present application.

How to Set Up the Unit

You will require a variable 0-24V/500mA power supply for the set up procedure. It may be completed with the following steps:

Since we know that the AC mains voltages fluctuations will always create a proportionate magnitude of the DC voltage fluctuations from a transformer, we may assume that for input voltages of 210, 230 and 250, the correspondingly obtained equivalent DC voltages should be 11.5, 12.5 and 13.5 respectively.

Now the setting of the relevant presets becomes very simple as per the above voltage levels.

  • Initially keep the both the transformers TR1 and TR2 disconnected from the circuit.

  • Keep the slider of P1, P2, and P3 at somewhere around midway position.

  • Connect the external variable power supply to the circuit. Adjust the voltage to about 12.5.

  • Now slowly start adjusting P3 until RL2 just activates.

  • Decrease the supply voltage to about 11.5 volts (RL2 should deactivate in the course), adjust P1 so that RL1 just deactivates.

  • Gradually increase the supply to about 13.5 – this should make RL1 and RL2 to energize one after the other, indicating the correctness of the above settings.

  • Now slowly adjust P2 so that RL1 again deactivates at this voltage (13.5).

  • Confirm the above settings by varying the input voltage from 11.5 to 13.5 back and forth. You should get the following results:

  • RL1 should deactivate at 11.5 and 13.5 voltage levels, but should remain activated in between these voltages. RL2 should switch ON above 12.5 and switch off below 12 volts.

The setting procedure is now complete.

The final construction of this power regulator unit may be concluded by connecting the tested circuit with the relevant transformers and concealing the whole section inside a well ventilated metallic enclosure as suggested in the previous page.

Need Help? Please send your queries through Comments for quick replies!