Your Mentor: Voltage Multiplier

A voltage multiplier is an electrical circuit that converts AC electrical power from a lower voltage to a higher DC voltage, typically using a network of capacitors and diodes.

Voltage multipliers can be used to generate a few volts for electronic appliances, to millions of volts for purposes such as high-energy physics experiments and lightning safety testing. The most common type of voltage multiplier is the half-wave series multiplier, also called the Villard cascade.

Villard Cascade Voltage Multiplier

Operation:

Assuming that the peak voltage of the AC source is +Us, and that the C values are sufficiently high to allow, when charged, that a current flows with no significant change in voltage, then the (simplified) working of the cascade is as follows:

Steps:

Ø negative peak (−Us): The C1 capacitor is charged through diode D1 to Us V (potential difference between left and right plate of the capacitor is Us)

Ø positive peak (+Us): the potential of C1 adds with that of the source, thus charging C2 to 2Us through D2

Ø negative peak: potential of C1 drops to 0 V thus allowing C3 to be charged through D3 to 2Us.

Ø positive peak: potential of C1 rises to 2Us (analogously to step 2), also charging C4 to 2Us. The output voltage (the sum of voltages under C2 and C4) raises till 4Us.

In reality more cycles are required for C4 to reach the full voltage. Each additional stage of two diodes and two capacitors increases the output voltage by twice the peak AC supply voltage.

Voltage Doubler & Tripler:

A voltage doubler uses two stages to approximately double the DC voltage that would have been obtained from a single-stage rectifier. An example of a voltage doubler is found in the input stage of switch mode power supplies containing a SPDT switch to select either 120 volt or 240 volt supply. In the 120 volt position the input is typically configured as a full-wave voltage doubler by opening one AC connection point of a bridge rectfier, and connecting the input to the junction of two series-connected filter capacitors. For 240 volt operation, the switch configures the system as a full-wave bridge, re-connecting the capacitor center-tap wire to the open AC terminal of a bridge rectfier system. This allows 120 or 240 volt operation with the addition of a simple SPDT switch.

A voltage tripler is a three-stage voltage multiplier. A tripler is a popular type of voltage multiplier. The output voltage of a tripler is in practice below three times the peak input voltage due to their high impedance, caused in part by the fact that as each capacitor in the chain supplies power to the next, it partially discharges, losing voltage doing so.

Triplers were commonly used in color television receivers to provide the high voltage for the cathode ray tube (picture tube). Many 1970s TV sets used open triplers, and the individual diode sticks could be replaced if they failed