The Voltage Doubler

The Voltage Doubler
It is possible to increase the output voltage of a transformer although this does reduce its ability to supply current at that voltage. The way that this is done is to feed the positive cycles into one storage capacitor and the negative cycles into a second reservoir capacitor. This may sound a little complicated, but in reality, it isn’t. A circuit for doing this is shown here:

With this circuit, the transformer output is some voltage, say “V” volts of AC current. This output waveform is fed to capacitor “C1” through diode “D1” which lops off the negative part of the cycle. This produces a series of positive half-cycles which charge up capacitor “C1” with a positive voltage of “V”.

The other half of the output is fed to capacitor “C2” through diode “D2” which cuts off the positive part of the cycle, causing capacitor “C2” to develop a voltage of -V across it. As the two capacitors are ‘in series’ and not placed across each other, their voltages add up and produce twice the transformer output voltage.

A word of warning here. The transformer is producing an AC waveform and these are marked with the average voltage of the waveform, which is usually a sine wave. The peak voltage of a sinewave is 41% greater than this, so if your transformer has an AC output of 10 volts, then the peaks fed to the capacitors will be about 14.1 volts. If there is no current draw from the capacitors (that is, with the load switched off), then each capacitor will charge to this 14.1 volts and the overall output voltage will be 28.2 volts and not the 20 volts which you might expect. You need to understand that as this is only a half-wave supply, there will be considerable ripple on the output voltage if the current draw is high.

Using one additional smoothing capacitor and paying attention to the voltage ratings of the capacitors, the 28 volts supply circuit might be like this:

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2012

11/11

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Voltage Doubler

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