4-47
Voltage multipliers may also be used as primary power supplies where a 177 volt-ac input is
rectified to pulsating dc. This dc output voltage may be increased (through use of a voltage multiplier) to
as much as 1000 volts dc. This voltage is generally used as the plate or screen grid voltage for electron
tubes.
If you have studied transformers, you may have learned that when voltage is stepped up, the output
current decreases. This is also true of voltage multipliers. Although the measured output voltage of a
voltage multiplier may be several times greater than the input voltage, once a load is connected the value
of the output voltage decreases. Also any small fluctuation of load impedance causes a large fluctuation in
the output voltage of the multiplier. For this reason, voltage multipliers are used only in special
applications where the load is constant and has a high impedance or where input voltage stability is not
critical.
Voltage multipliers may be classified as voltage doublers, triplers, or quadruplers. The classification
depends on the ratio of the output voltage to the input voltage. For example, a voltage multiplier that
increases the peak input voltage twice is called a voltage doubler. Voltage multipliers increase voltages
through the use of series-aiding voltage sources. This can be compared to the connection of dry cells
(batteries) in series.
The figures used in the explanation of voltage multipliers show a transformer input, even though for
some applications a transformer is not necessary. The input could be directly from the power source or
line voltage. This, of course, does not isolate the equipment from the line and creates a potentially
hazardous condition. Most military equipments use transformers to minimize this hazard.
Figure 4-44 shows the schematic for a half-wave voltage doubler. Notice the similarities between
this schematic and those of half-wave voltage rectifiers. In fact, the doubler shown is made up of two
half-wave voltage rectifiers. C1 and CR1 make up one half-wave rectifier, and C2 and CR2 make up the
other. The schematic of the first half-wave rectifier is indicated by the dark lines in view A of figure 4-45.
The dotted lines and associated components represent the other half-wave rectifier and load resistor.
Figure 4-44.—Half-wave voltage doubler.
