Making Voltage Doublers and Multipliers

Find out how to construct an AC voltage doubler circuit. Extend it to a tripler circuit and more.

A voltage doubler or voltage multiplier is a circuit easily made from a few components which enables the voltage from an AC power source to be increased. This is particularly useful in situations where the load current is low, but a higher voltage than is provided by the AC source is required.

A voltage tripler for example can be used to increase the voltage from a Sturmey Archer Dynohub (1) such that it can be used to charge a 12V Battery, or to do the same for a low voltage stepper motor (2) wind turbine.

In this article we will put together and test a couple of basic voltage multipler circuits. Links to online resources explaining exactly how voltage multiplier circuits work are provided at the end of the article.

Voltage Doubler Circuit

To make a voltage doubler circuit only two capacitors (3) and two diodes (4) are required costing pennies each. The circuit diagram is displayed below:

Testing the Voltage Doubler

A bicycle bottle dynamo was used as the AC power source (see photo below). Since this is used to power two incandescent light bulbs, it should certainly have enough power to provide 10-20mA of current to the LED we planned to use as the test load.

First the AC voltage of the dynamo was measured directly with a multimeter (5). Spinning the front wheel of the bicycle as quickly as possible by hand, the maximum voltage reading recorded was 5.04 Volts AC.

Putting this AC output voltage through a bridge rectifier (6), the maximum DC voltage was measured at 5.12 Volts DC. It is this DC voltage we are interested in seeing increase using our voltage doubler circuit (which will also act as a rectifier converting the AC electricity from the dynamo into DC).

The voltage doubler was built using prototyping breadboard (7) as per the circuit diagram provided earlier in this article. The wheel was spun ten times and an average taken of the maximum voltages recorded. This average was 10.12 Volts DC - almost double that recorded using the bridge rectifier - proving that the voltage doubler circuit works.

Voltage Tripler

A voltage tripler or other larger voltage multiplier can be built by extending the voltage doubler circuit. One extra capacitor and a diode are required for each increase in multiple - therefore a tripler has a total of three capacitors and three diodes as shown below.

The circuit above was built again on prototyping breadboard, the wheel spun ten times, and the average maximum voltage recorded to be 13.64 Volts DC. Not quite triple, but that could be due to the increased amount of voltage dropped across the diodes in the circuit. Another alternative is that the LED was using all of the available current and emptying the third capacitor C3 before it could reach the predicted 15 VDC. (Remember that every increase in voltage is matched by a fall in current since the overall maximum power output of the dynamo is fixed. See Ohm's Law (8))

Voltage tripler circuit on prototyping breadboard

As a final test, the 510 Ohm resistor (in series with the LED) was replaced with a 1,000 Ohm resistor to halve the current reaching the LED to see why the voltage tripler circuit was falling short. The experiment was repeated and the new average maximum voltage recorded was 14.84 Volts.

Therefore the voltage tripler circuit was working perfectly - the problem was insufficient power from the small bottle dynamo reaching the LED. Had a larger dynamo been used a voltage of around 15 Volts DC would have been recorded first time.

Voltage Multipliers

Getting more multiples of voltage is simply a matter of adding more capacitors and diodes to the existing tripler circuit until the required voltage is reached. As further multiples are added the voltage rating of the capacitors used must be considered - for example in a voltage quadrupler circuit, the fourth capacitor would receive 20V, so 16V-rated capactitors could not be used. In addition the power of the AC source must be considered to calculate if enough current at the increased voltage will be available for the load.

Find out More About Voltage Multipliers

In this article we have not covered the sometimes complicated theory involved in voltage multipliers. However, if you are interested in finding out more there are numerous online resources of which a few are listed below:

Voltage Multipliers (9) from play-hookey.com.
Basic Multiplier Circuits (10) - introduces the Villard cascade (as seen in the voltage tripler above) and discusses the practical limitations of voltage multipliers .
Voltage doubler, tripler, and quadrupler circuits (11) - from coolcircuit.com.

Web Link References

(1) http://www.reuk.co.uk/Sturmey-Archer-Dynohub.htm
(2) http://www.reuk.co.uk/Stepper-Motor-Basics.htm
(3) http://www.reuk.co.uk/buy-1000UF-CAPACITOR-16V.htm
(4) http://www.reuk.co.uk/buy-1N4001.htm
(5) http://www.reuk.co.uk/buy-MULTIMETER.htm
(6) http://www.reuk.co.uk/buy-W005.htm
(7) http://www.reuk.co.uk/buy-BREADBOARD.htm
(8) http://www.reuk.co.uk/Ohms-Law.htm
(9) http://www.play-hookey.com/ac_theory/ps_v_multipliers.html
(10) http://www.kronjaeger.com/hv/hv/src/mul/
(11) http://www.coolcircuit.com/circuit/voltage/index.html

Article from REUK.co.uk:
http://www.reuk.co.uk/Making-Voltage-Doublers-and-Multipliers.htm
Published: 26th April 2007
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