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 D1 conducts, and the current flows along the path shown. Capacitor C1 charges to the peak value of the input signal. Because there is no discharge path, capacitor C1 remains charged.
Figure 3 shows the positive alternation of the input signal. At this point, capacitor C1 is charged to the negative peak value. This keeps diode D1 reverse biased and forward biases diode D2.
This allows diode D2 to conduct, charging capacitor C2. Because capacitor C1 is charged to the maximum negative value, capacitor C2 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 D2 is cut off. This is because capacitor C2 holds diode D2 reverse biased.
Capacitor C2 discharges through the load, holding the voltage across the load constant. Therefore, it also acts as a filter capacitor. Capacitor C2 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 C2 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 C1 charges through diode D1 to the peak value of the AC input signal.
Figure 6 shows that, on the negative alternation, capacitor C2 charges through diode D2 to the peak value of the input signal.
When the AC input signal is changing between the peaks of the alternations, capacitors C1 and C2 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 C1 and C2 are charged during the peaks of the input signal. The ripple frequency is 120 hertz because both capacitors C1 and C2 are charged during each cycle. Capacitors C1 and C2 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 D1 so that it conducts. This charges capacitor C1 to the peak value of the input signal. Capacitor C1 places a positive potential across diode D2.
Figure 9 shows the negative alternation of the input signal. Because diode D2 is now forward biased, current flows through it to capacitor C1 via capacitor C2. This charges capacitor C2 to twice the peak value because of the voltage stored in capacitor C1.
Figure 10 shows the occurrence of the next positive alternation. It places a difference of potential across capacitor C2 that is three times the peak value. The top plate of capacitor C2 has a positive peak value of two times the peak value.
The anode of diode D3 has a positive value of three times the peak value with respect to ground. This charges capacitor C3 to three times the peak value. This is the voltage that is applied to the load.