Voltage Doubler Circuits:

Both full wave and half wave rectifier circuits produce a d.c. voltage less than the a.c. maximum voltage. When higher d.c voltages are needed, a Voltage Doubler Circuits or cascaded rectifier doubler circuits are used. The schematic diagram of voltage doublers are given in Figs 6.3a and b.

In Voltage Doubler Circuits shown in Fig. 6.3a, the capacitor C₁ is charged through rectifier R₁ to a voltage of +V_max with polarity as shown in the figure during the negative half cycle. As the voltage of the transformer rises to positive V_max during the next half cycle, the potential of the other terminal of C₁ rises to a voltage of +2V_max.

Thus, the capacitor C₂ in turn is charged through R₂ to 2V_max. Normally the d.c. output voltage on load will be less than 2V_max , depending on the time constant C₂R_L and the forward charging time constants. The ripple voltage of these circuits will be about 2% for R_L/r ≤ 10 and X/r ≤ 0.25, where X and r are the reactance and resistance of the input transformer. The rectifiers are rated to a peak inverse voltage of 2V_max, and the capacitors C₁ and C₂ must also have the same rating.

Cascaded voltage doublers are used when larger output voltages are needed without changing the input transformer voltage level. A typical voltage doubler is shown in Fig. 6.3b and its input and output waveforms are shown in Fig. 6.3c. The rectifiers R₁ and R₂ with transformer T₁ and capacitors C₁ and C₂ produce an output voltage of 2 V in the same way as described above. This circuit is duplicated and connected in series or cascade to obtain a further voltage doubling to 4 V.

T is an isolating transformer to give an insulation for 2V_max since the transformer T₂ is at a potential of 2V_max above the ground. The voltage distribution along the rectifier string R₁,R₂,R₃ and R₄ is made uniform by having capacitors C₁,C₂,C₃ and C₄ of equal values.

The arrangement may be extended to give 6 V, 8 V, and so on by repeating further stages with suitable isolating transformers. In all the voltage doubler circuits, if valves are used, the filament transformers have to be suitably designed and insulated, as all the cathodes will not be at the same potential from ground. The arrangement becomes cumbersome if more than 4 V is needed with cascaded steps.