|Jochen's High Voltage Page|
|Greinacher voltage doubler circuit. (a) simple version for grounded input voltage, (b) double version for symmetric input voltage. Diodes must be dimensioned for 2x input peak voltage Up, caps only for 1x Up.|
The Greinacher doubler circuit (a) transforms a grounded AC voltage (peak voltage Up) into symmetrical DC voltages of 1x Up each, thus producing 2x Up between outputs. If the input voltage is already symmetrical (e.g. as in an Obit), the Greinacher circuit may be doubled according to (b). This does not change the output voltages but increases the possible output current by a factor of two.
|Villard voltage doubler circuit for positive polarity towards ground. For negative polarity reverse all diodes. Diodes and caps must be dimensioned for 2x Up. For sinusoidal input voltage 1x Up is sufficient for C1.|
In contrast to the Greinacher circuit, the Villard doubler circuit produces 2x Up towards ground at a single output. Besides this, the advantage of the Villard circuit is that it may be cascaded to form a voltage multiplier with (in principle) arbitrary output voltage. The way this works is that after C1 and C2 are fully charged to 1x and 2x Up respectively, there is a pulsating DC voltage oscillating between 0 and 2x Up across D2, which is converted to AC by the subsequent cap and thus acts as input AC of the next stage, and so forth.
|Cascaded Villard doubler circuits. All diodes and caps must be dimensioned for 2x Up. For sinusoidal input voltage, C1 of the first stage may be reduced to 1x Up.|
An n-stage cascade produces 2n x Up output voltage. By choosing an appropriate number of stages, any voltage can be reached. However, this is only valid for negligible current draw. As soon as there is output current, there is also an AC current through the caps, resulting in a voltage drop and a lower input voltage for subsequent stages. In fact, numbers much higher than, say, 10 or 20, are not sensible in practice. More specifically, the formula for the voltage drop is
This means, the voltage drop is the higher, the higher the output current I, the lower the frequency f and the lower the capacity C. The voltage drop also increases with the number of stages cubed, which means for 10 stages it is already 1000x as large as for a single stage!
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