automatic voltage stabilizer circuit diagram

A voltage stabilizer is a device which stabilizes the AC voltage and keeps it between 200V to 255V AC. Sometimes a variation of voltage or spikes appear on an AC line, if we use a voltage stabilizer then the extra high voltages or low voltages can cause no problem to the appliances. It protects any electronic device connected to it from getting damaged. Automatic voltage stabilizer is very good example of power electronics projects.

automatic voltage stabilizer

Different varieties of voltage stabilizers are present in the market. But we can also make them at home according to our needs and requirements.

voltage stabilizer important points

Before building this device, the following points and specifications should be kept in mind so that the device we build can work properly and give us the desired results:

  • The range of input voltage should be 150 to 260V.
  • The range of output voltage should be 200V to 240V.
  • There should be no change in the waveform or the frequency of input/output voltages.
  • The material used in it should not be too much expensive otherwise there would be no use of making it at home by going through all the trouble, can just buy a cheap one from market instead. Therefore it should not be expensive.
  • No varistors or variable resistors should be present in the final form of the product.
  • A total of 4 relays are used in the circuit.
  • The auto transformer used has 4 additional tappings set at 165V, 190V, 215V and 240V, all with a difference of about 25V.
  • The microcontroller used is PIC 16F873A.

Automatic voltage stabilizer working

The microcontroller generates the control signals and four relays are used with the autotransformer for the control and conversion of voltage. The input voltage is sensed by the microcontroller and it tries to keeps the output voltage between its specified ranges, by switching the relays. Out of the four relays, two of them switch the connection between the tappings 165V, 190V and 240V, one switches the output connection between the tappings 215 and 240 while the last one is a master on/off relay which disconnects the output in case of low and high cut modes. Relay interfacing with microcontroller is very easy.

voltage stabilizer SENSING OF INPUT VOLATGES

First of all, a bridge rectifier is used to convert the input AC voltage into DC voltage, followed by a large capacitor which smoothes out the DC voltage. And by using a voltage divider circuit we step down the DC voltage so that the microcontroller can accept it.After long speculation and experimentation, the ratio for the resistors of voltage divider circuit was chosen to be (47kΩ*6):3.3kΩ. the circuit in this ratio performs better and the power dissipation is also reduced.

A clamp circuit was connected at the output of voltage divider circuit, this is formed by two diodes. The voltage will be clamped by one of the diodes when it starts working in forward biased condition after receiving high voltage. It would be approximately 5.7V.If low voltage appears at the output of voltage divider, then the other diode starts working in the forward biased condition and clamps the voltage by -0.7. These voltages can then safely go to the ADC of the microcontroller. Schottky diodes can be used to improve the clamping of voltages.

The input impedance for the ADC and the input capacitors are the two things that can affect the proper operation of the circuit:

  • If the input capacitor is very large then its discharge will be slower and we would not be able to get fast or quick response. After using different capacitors, we found out that the capacitor of value 22uF was best suited as its response was efficient in case of DC voltages and also ripples.
  • For the proper measurement of DC level by the ADC of PIC, we connect a capacitor at the voltage divider output. This would provide a parallel capacitance to the internal capacitor of ADC. Sampling time of ADC was also adjusted so that we can obtain accurate results.

CALLIBRATION of automatic voltage stabilizer

For the calibration purposes, we have placed a switch in the circuit. When this switch is activated and we reset the microcontroller, then the controller goes to the calibration mode. This would be the only variable resistor that we have used in the circuit and it is needed because there can be a lot of discrepancies in various components and their outputs in the circuit.  The outputs can be affected by the tolerance in the resistors and variations in the forward drop voltages of the diodes, and also by many other factors. We will connect the variable resistor in our voltage divider circuit and by changing the resistance values we can get the required output.

The variable resistor is not reliable in this circuit and under the conditions of varying high and low voltages, we need consistency in the operation of this circuit for longer periods of time therefore we decided to use no variable resistor in the final product.

automatic voltage stabilizer using microcontroller

When the microcontroller enters the calibration mode, the modified input voltage is shown by the controller. We can measure the real voltage by using a voltmeter. We change the variable resistance and a different voltage is displayed by the microcontroller. The coding of the ADC of microcontroller is done such that the result of ADC is converted to an AC voltage level. A constant is also introduced which is multiplied with the whole expression and when we change the value of variable resistor then the constant value also changes which can be seen by the seven segment display. The microcontroller saves this value in its EEPROM.

When the controller starts, it checks for the calibration. The constant value has been stored in the EEPROM, the controller retrieves the data and now this value will be used in all the further voltage calculations. Upon starting of microcontroller for the very first time, it waits for the calibration, if the switch is pressed and calibration is done then the switch gets opened and constant is stored in EEPROM and further operations are carried out.

After the successful calibration, we can now remove the switch and the variable resistor from the circuit. The switch and variable resistor can only be needed now if we want to recalibrate the circuit, otherwise they are not required in the circuit anymore.

Voltage stabilizer relays and transformer tapping 

Transformer tappings with relays

The above configuration shows the different tappings of transformer with relays. The switching of input is done between 165V, 190V and 240V while for output the tappings are 240V and 215V.We have used a simple autotransformer in this circuit. An auxiliary winding is used to power the circuitry and the turns ratio have also been shown:

Autotransformer with tappings

automatic voltage stabilizer circuit diagram

Both parts of circuit diagram of automatic voltage stabilizer is shown below. You can use these circuits.

automatic voltage stabilizer circuit diagram

automatic voltage stabilizer circuit diagram 2

automatic voltage stabilizer circuit diagram

automatic voltage stabilizer circuit diagram

voltage stabilizer  CIRCUIT operation

For the microcontroller circuit, we use an external crystal of 4 MHz. This is required because there is no internal crystal present in PIC 16F873A. A 5V DC input is used to power the microcontroller. The auxiliary winding of autotransformer is of 12.5V. This voltage will not vary much because the circuit and the relays will work to regulate this voltage also. This AC is converted to DC with the help of a rectifier and then a capacitor filters it. A voltage regulator 7805 is also used which will take in the filtered DC. A decoupling capacitor is also used and is placed close to the microcontroller.

The DC voltage that goes into the 7805 is used in powering the relays also. But not directly, as the voltage is still a bit higher than the rated voltage of relays. So we pass this voltage through four diodes in series which will reduce the voltage by 2.8V. The microcontroller controls the switching of relays, but it cannot provide the current required to operate the relays therefore we use transistors to amplify the value of current.

Coming to the seven segment display, the three seven segments displays used in the circuit are switched one after the other which minimizes the pins required to drive them. But this happens so fast that we cannot comprehend it just by looking at them. The refresh rate is 167Hz, that is, the display refreshes 167 times in a second. To achieve the required brightness, we have connected seven transistors with the seven segment displays.

We have used three LEDs in the circuit also which shows the delay, low cut or high cut or simply normal mode of the controller.This was the whole process of making an automatic voltage stabilizer at home. By following the steps correctly, we hope that you can make one at home too and you can also modify it according to your reqirements.


  1. innocent February 16, 2016
  2. innocent February 16, 2016
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        • ASLAM JAVED March 17, 2016
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  3. innocent February 22, 2016
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