Voltage Multiplier Circuits - Half Wave Voltage Doubler Circuit

Voltage Multiplier Circuits - Voltage Doubler Circuit

With the help of diodes and capacitors we can build a circuit, which will provide the DC output that is multiple of the peak input AC voltage.
Such a circuit is known as the voltage multiplier circuit.

Voltage_Multiplier_CircuitFor Example,
A circuit which provide the DC output that is the twice the peak input of AC voltage is known as Voltage Doubler.
A circuit which provide the DC output that is three times the peak input AC voltage is known as Voltage Tripler.

Remember that,

  • Even though the voltage multiplier circuits provide DC output that is much greater than the peak input AC voltage, there is no power amplification happens.
  • So the law of conservation is maintained.
  • ie, when  a voltage multiplier circuit increases the peak AC input voltage by a factor X, the peak input current will be decreased by the same factor X.
  • So output power and input power will be approximately same.
  • Considering the losses in diodes & capacitors, the output power will be less than input power.

Basic Operating Principle:

The basic concept of the voltage multiplier circuit is to charge each capacitor to the peak AC input value and to arrange the capacitors in such a way that their stored voltages will add.

Half Wave Voltage Doubler:

As shown in the above figure the Half wave voltage doubler circuit consists of two capacitors and two diodes. Here the peak ac input voltage is VS(PK), the output dc voltage is 2VS(PK).

Mode-I : Negative Half cycle:

Voltage_doubler_Negative_half_cycleThe circuit diagram for during the negative half cycle  is shown above.
During negative half cycle, the diode D1 is forward biased( ie short) . Diode D2 is reverse biased(ie, open).

The equivalent circuit is shown below.
Voltage_doubler_negative_half_cycle_EquivalentHere the capacitor C1 will charge until voltage across it becomes equal to AC input voltage peak (VS(PK)]. The capacitor C2 will discharge through the load resistor RL.

Mode-II: Positive half cycle

The circuit diagram for during the positive half cycle  is shown below.
Voltage_doubler_Positive_half_cycleDuring positive half cycle, the diode D1 is reverse biased (ie, open).  Diode D2 is forward biased ( ie short).

The operation can be understood with the help of below equivalent circuit diagram.Voltage_doubler_Positive_half_cycle_Equivalent

  • The capacitor C1 is already charged to VS(PK) and the source voltage VS now act as series aiding voltage sources.
  • Thus the capacitor C2 will be charged to the sum of the series peak voltages ie 2VS(PK).
  • Consequently as the VS returns in negative half-cycle, diode D2 is turned off again.
  • At this moment, the only discharge path for C2 is through the load resistance RL.
  • The time constant (=RLC2) is adjusted, in such as way that C2 has little time to lose any of its charge before the input polarity reverses again.
  • During positive half cycle, D2 is turned on and C2 recharges until voltage across it is again equal to 2VS(PK).

Ie, DC Output Voltage = Vdc= 2VS(PK)

As C2 value is huge, they barely discharge between input cycles. So the output voltage similar to filtered half-ware rectifier output waveform.

Disadvantages of Voltage Multipliers

  1. They have poor voltage regulation.
  2. As the load current increases, the dc output voltage drops drastically.
  3. To maintain the output voltage large filter capacitors are required.

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