Voltage Doubler: What is it? (Circuit Diagram, Full Wave & Half Wave Doublers)

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Key learnings:

Voltage Doubler Definition: A voltage doubler is an electronic circuit that generates an output voltage twice as high as its input voltage.
Circuit Design: Voltage doublers utilize two capacitors and two diodes in a setup that converts AC input to a higher DC output.
Half-Wave Operation: In half-wave doublers, each capacitor charges during alternate half-cycles of the AC input, blocked from discharging by diodes, to provide double the input voltage.
Full-Wave Benefits: Full-wave voltage doublers charge capacitors during both cycles of AC, offering a more stable output voltage under no-load conditions.
Practical Applications: Voltage doublers are essential in devices requiring high voltages, such as ion pumps and X-ray systems, due to their efficiency in stepping up voltage.

What is a Voltage Doubler?

A voltage doubler is an electronic circuit that produces an output voltage double the input voltage. It is a voltage multiplier with a multiplication factor of 2. The circuit combines an oscillating AC input with two capacitors and two diodes. The input is AC, and the output is DC at twice the peak value of the input AC voltage.

Voltage doublers can substitute for bulky and costly step-up transformers in various applications.

There are two main types of voltage doublers: half wave voltage doublers and full wave voltage doublers.

Types of 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.

Consider the working of the half-wave DC voltage doubler. Throughout the positive half cycle of the AC sine wave, the first diode (D₁) conducts.

In this forward-biased state it charges its capacitor (C₁) to the peak value of the AC secondary voltage of the transformer (V_SMAX).

This capacitor has no discharge path, so it stays fully charged.

Throughout the negative half cycle, the second diode (D₂) conducts in the forward-biased state while the first diode (D₁) stops conducting in the reverse-biased state.

The reverse-biased D₁ blocks C₁ from discharging, while the forward-biased D₂ charges its capacitor (C₂).

Kirchhoff’s voltage law is applied to the outer loop of the transformer, starting at the negative lower end and moving clockwise to the positive top end.

The voltage across capacitor C₂ equals twice the peak value of the transformer’s secondary input voltage, 2V_SMAX.

On the next positive half cycle of the AC input, the second diode (D₂) is cut off in reverse bias. The second capacitor (C₂) then discharges through the load, and the output voltage (V_out) < 2V_{S MAX}.

Without a load, both capacitors stay charged as described above. With a load, C₂ recharges on the next cycle.

Full Wave Voltage Doubler

In the full-wave voltage doubler, the components are the same as in the half-wave version; the difference is how the circuit connects them, as shown below.

In this doubler, the first diode (D₁) conducts throughout the positive half cycle of the input AC voltage.

In this forward-biased state it charges its capacitor (C₁) to the peak value of the AC secondary voltage of the transformer (V_SMAX).

At this time, D₂ sits in the reverse-biased, non-conducting state. Throughout the negative half cycle of the input AC voltage, the second diode (D₂) turns forward biased, and the second capacitor (C₂) charges.

With no load, the sum of the two capacitor voltages appears as the output voltage.

When a load is connected across the output terminals, the output voltage (V_out) may be less than 2V_{S MAX}.

Both voltage doubler types deliver 2V_{S MAX} at the output, and neither needs a centre-tapped transformer. The circuit diodes must carry a peak inverse voltage rating of 2V_{S MAX}.

Advantages of Voltage Doubler

Voltage doublers offer several advantages:

They can replace expensive, heavy transformers.
Negative voltage can also be created by reversing the polarity of the connected diodes and capacitors.
Cascading similar voltage multipliers raises the voltage multiplication factor.