A full-wave voltage doubler power supply.
Diodes and capacitors can be interconnected to deliver a dc output that is approximately twice the positive or negative peak ac input voltage. This is called a voltage-doubler power supply. This circuit works well only when the load draws low current. Otherwise, the voltage regulation is poor; the voltage drops a lot when the current demand is significant.
The best way to build a high-voltage power supply is to use a step-up transformer, not a voltage doubling scheme. Nevertheless, a voltage-doubler power supply can be, and sometimes is, used when the cost of the circuit must be minimized and the demands placed on it are expected to be modest.
Above figure is a simplified diagram of a voltage-doubler power supply. It works on the entire ac cycle, so it is called a full-wave voltage doubler. This circuit subjects the diodes to voltage peaks in the reverse direction that are 2.8 times the applied rms ac voltage. Therefore, the diodes should be rated for PIV of at least 4.2 times the rms ac voltage that appears across the transformer secondary. When the current drawn is low, the dc output voltage of this type of power supply is approximately 2.8 times the rms ac input voltage.
Proper operation of a voltage-doubler power supply depends on the ability of the capacitors to hold a charge under maximum load. The capacitors must have large values, as well as be capable of handling high voltages. The capacitors serve two purposes: to boost the voltage and to filter the out put. The resistors, which have low ohmic values and are connected in series with the diodes, protect the diodes against surge currents that occur when the power supply is first switched on.