Voltage Doubler

Voltage doubler, as the name indicates it can deliver the output voltage which is double as that of the input voltage. It is a voltage multiplier with the voltage multiplication factor equal to 2. The circuit is formed by an oscillating AC input voltage, two capacitors and two diodes. The input is AC voltage and the output will be DC voltage with twice the peak value of the input AC. Heavy and expensive step-up transformers can be replaced in some applications by this voltage doubler.

Types of Voltage Doubler

Next, we are going to discuss about the two types of voltage doubler- Half-wave voltage Doubler and Full-wave voltage Doubler.

Half Wave Voltage Doubler

The figure below, shows, a simple DC voltage doubler circuit. Here, it is clear that both the capacitors and the diodes operate together to create the double voltage output.
half wave voltage doubler

Now, we can go through the working of the half wave DC voltage doubler. All over the positive half cycle of the AC sine wave, the first diode (D1) is conducting. That is forward biased state and it will charge the connected capacitor (C1) equal to the peak value of AC secondary voltage of transformer (VSMAX). This capacitor is unable to get discharged due to the unavailability of a path. So, it will remain in the fully charged condition. Next, all over the negative half cycle, the second diode (D2) is conducting or forward biased state, and the first diode (D1) is non-conducting or in reversed biased state. The reversed biased diode (D1) will block the discharging of the connected capacitor (C1) and the forward biased diode (D2) will charge the connected capacitor (C2).

Here we can apply the Kirchhoff’s voltage law to the outer loop which starts from the bottom of secondary of the transformer (lower end is negative and top end is in positive polarity) in the clockwise direction.

That is voltage across the capacitor; C2 will be equal to the two times the peak value of input transformer secondary voltage (2VSMAX).

Throughout the next positive half cycle of AC input, the second diode (D2) will be open due to the reversed biased condition. So, the second capacitor (C2) will get discharged through the load and the output voltage (Vout) < 2VS MAX. Otherwise, the two capacitors will be in the charged condition as said above. If there is a load, then in the next cycle, the C2 will get recharged again.

Full Wave Voltage Doubler

In the full-wave voltage doubler, the components are same as that of half wave voltage doubler. But different is in the circuit as shown below.
full wave voltage doubler
In this doubler, right through the positive cycle of input AC voltage, the first diode (D1) is in the conducting state. That is forward biased state and it will charge the connected capacitor (C1) equal to the peak value of AC secondary voltage of transformer (VSMAX). At this time, D2 will be in reverse biased condition or non conducting state. Throughout the negative cycle of input AC voltage, the second diode (D2) will be in forward biased state and the second capacitor (C2) gets charged. In the no load condition, the entire voltages of two capacitors are delivered as the output voltage. If there is some load connected across the output terminals, then output voltage (Vout) < 2VS MAX. The output wave form is shown below.
output wave voltage doubler
We can observe that, both the voltage doubler will provide 2VS MAX as the output. There is no need of a centre-tapped transformer. 2VS MAX will be the peak inverse voltage ratings of the circuit diodes.

Advantages of Voltage Doubler

  • Can replace the expensive and heavy transformers.
  • Negative voltage can also be created by reversing the polarity of the connected diodes and capacitors.
  • Can increase the voltage multiplication factor by cascading the similar voltage multipliers.

Application of Voltage Doubler

  • Ion pumps
  • Television CRT
  • X-Ray systems
  • Copy machine
  • Radar equipments
  • Travelling wave tubes etc
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