The voltage doubler

Chapter The voltage doubler

Teach Yourself Electricity and Electronics Third Edition Book
Pages 748
Views 14,442
Downloads : 46 times
PDF Size : 4.4 MiB

Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • a power supply must deliver a high current. Then, the extra diodes—two for each halfof the cycle, rather than one—dissipate more overall heat energy. When current is usedup as heat, it can’t go to the load. Therefore, center-tap circuits are preferable inhigh-current applications.The voltage doublerBy using diodes and capacitors connected in certain ways, a power supply can be madeto deliver a multiple of the peak ac input voltage. Theoretically, large whole-numbermultiples are possible. But you won’t often see power supplies that make use of multi-plication factors larger than 2.In practice, voltage multipliers are practical only when the load draws low cur-rent. Otherwise, the regulation is poor; the output voltage changes considerably withchanges in the load resistance. This bugaboo gets worse and worse as the multiplicationfactor increases. This is why engineers don’t attempt to make, say, a factor-of-16 volt-age multiplier. For a good high-voltage power supply, the best approach is to use astep-up transformer, not a voltage multiplier.A voltage-doubler circuit is shown in Fig. 21-7. This circuit works on the whole acinput wave cycle, and is therefore called a full-wave voltage doubler. Its dc outputvoltage, when the current drawn is low, is about twice the peak ac input voltage, orabout 2.8 times the rms ac input voltage.The voltage doubler38921-7A full-wave voltage doubler.Notice the capacitors in this circuit. The operation of any voltage multiplier is de-pendent on the ability of these capacitors to hold a charge, even when a load is con-nected to the output of the supply. Thus, the capacitors must have large values. If theintent is to get a high dc voltage from the supply, massive capacitors will be necessary.Also, notice the resistors in series with the diodes. These have low values, similar tothose needed when diodes are connected in parallel. When the supply is switched on,the capacitors draw a huge initial charging current. Without the resistors, it would benecessary to use diodes with astronomical Io ratings. Otherwise the surge currentwould burn them out.