When higher DC voltages are required, a step-up transformer is used. However, higher DC voltages can be produced without a step-up transformer. Circuits that are capable of producing higher DC voltages without the benefit of a transformer are called voltage multipliers. Two voltage multipliers are the voltage doubler and the voltage tripler.

# Half Wave Voltage Doubler Circuit

Figure 1 shows a half-wave voltage doubler. It produces a DC output voltage that is twice the peak value of the input signal. Figure 2 shows the circuit during the negative alternation of the input signal.

Diode D_{1} conducts, and the current flows along the path shown. Capacitor C_{1} charges to the peak value of the input signal. Because there is no discharge path, capacitor C_{1} remains charged.

Figure 3 shows the positive alternation of the input signal. At this point, capacitor C_{1} is charged to the negative peak value. This keeps diode D_{1} reverse biased and forward biases diode D_{2}.

This allows diode D_{2} to conduct, charging capacitor C_{2}. Because capacitor C_{1} is charged to the maximum negative value, capacitor C_{2} charges to twice the peak value of the input signal. As the sine wave changes from the positive half cycle to the negative half cycle, diode D_{2} is cut off. This is because capacitor C_{2} holds diode D_{2 }reverse biased.

Capacitor C_{2} discharges through the load, holding the voltage across the load constant. Therefore, it also acts as a filter capacitor. Capacitor C_{2} recharges only during the positive cycle of the input signal, resulting in a ripple frequency of 60 hertz (hence the name half-wave voltage doubler).

One disadvantage of the half-wave voltage doubler is its hard to filter because of the 60-hertz ripple frequency. Another disadvantage is that capacitor C_{2} must have a voltage rating of at least twice the peak value of the AC input signal.

## Full Wave Voltage Doubler Circuit

A full-wave voltage doubler overcomes some of the disadvantages of the half-wave voltage doubler.

Figure 4 is a schematic of a circuit that operates as a full-wave voltage doubler. Figure 5 shows that, on the positive alternation of the input signal, capacitor C_{1} charges through diode D_{1} to the peak value of the AC input signal.

Figure 6 shows that, on the negative alternation, capacitor C_{2} charges through diode D_{2} to the peak value of the input signal.

When the AC input signal is changing between the peaks of the alternations, capacitors C_{1} and C_{2} discharge in series through the load. Because each capacitor is charged to the peak value of the input signal, the total voltage across the load is two times the peak value of the input signal.

Capacitors C_{1} and C_{2 }are charged during the peaks of the input signal. The ripple frequency is 120 hertz because both capacitors C_{1} and C_{2} are charged during each cycle. Capacitors C_{1} and C_{2} split the output voltage to the load, so each capacitor is subject to the peak value of the input signal.

## Voltage Tripler Circuit

Figure 7 shows the circuit of a voltage tripler. In Figure 8, the positive alternation of the input signal biases diode D_{1} so that it conducts. This charges capacitor C_{1 }to the peak value of the input signal. Capacitor C_{1 }places a positive potential across diode D_{2}.

Figure 9 shows the negative alternation of the input signal. Because diode D_{2} is now forward biased, current flows through it to capacitor C_{1} via capacitor C_{2}. This charges capacitor C_{2} to twice the peak value because of the voltage stored in capacitor C_{1}.

Figure 10 shows the occurrence of the next positive alternation. It places a difference of potential across capacitor C_{2} that is three times the peak value. The top plate of capacitor C_{2} has a positive peak value of two times the peak value.

The anode of diode D_{3} has a positive value of three times the peak value with respect to ground. This charges capacitor C_{3} to three times the peak value. This is the voltage that is applied to the load.