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Making Voltage Doublers and Multipliers

Making Voltage Doublers And Multipliers

Find out how to construct an AC voltage doubler circuit. Extend it to a
tripler circuit and more.
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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. In addition these circuits rectify the AC volage to DC
votlage which can be used to power DC loads and charge batteries.

A
voltage tripler for example can be used to increase the voltage from a 6V
rated Sturmey Archer
Dynohub (or other bicycle dynamo
hubs) so that it can be used to charge a 12V Battery, or to do the same for
a low voltage stepper motor
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 for anyone who wants to find out more.

Voltage Doubler Circuit
To make a voltage doubler circuit only
two capacitors
and two diodes 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.
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, 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 since the doubler will also act as a rectifier converting the AC
voltage output by the dynamo into DC voltage.

The voltage doubler
was built using prototyping
breadboard as per the circuit diagram provided earlier in this article. The
wheel was spun rapidly by hand ten times and an average taken of the maximum
voltages recorded from each spin. This average was 10.12 Volts DC -
almost double that recorded using the bridge rectifier - showing that the
voltage doubler (5.12V to 10.12V) circuit was working.

Voltage Tripler
A voltage tripler or other larger voltage
multiplier can be built simply by extending the voltage doubler circuit with
the addition of extra capacitors and diodes. One extra capacitor and one extra
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)

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.

Increasing Efficiency of Voltage Multipliers - Diode Choice
The
forward voltage drop in a standard silicon diode such as the 1N4001 we
used in our experiments is 0.6-0.7 Volts. This is not insignificant, and the
accumulated drops in voltage make a real difference to the final output voltage
from the multiplier as power is wasted (as dissipated heat) from the diodes. To
increase efficiency, it is better to use Germanium
diodes, or easier to find Schottky
Diodes as these have a voltage drop of just 0.25-0.4V, halving the power
lost in the diodes compared to silicon diodes and therefore increasing the
output voltage.

Find out More About Voltage Multipliers
In this article we have not
covered the sometimes complicated theory involved in voltage multipliers.
If you are interested in finding out more then there are numerous online
resources of which a few are listed below:

Voltage Multipliers from play-hookey.com.
Basic Multiplier Circuits - 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 - from
coolcircuit.com.

And here is a video showing a voltage
quadrupler in action together with the circuit diagram, and a time lapse
showing its construction.

Article Last Modified: 22:21, 24th Sep
2014

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