Extracting Extra
Power Supply  Voltages
  I know that many readers will balk at the need for an extra power supply voltage. Part of their distaste comes from the need for an extra transformer (or at the very least an extra transformer winding that this extra voltage seems to entail.) The other part is that many find the extra power supply voltage intellectually dishonest or at least clumsy. "One power supply voltage is best," they tell me. "Really, why?" is my usual response, but it is never satisfactorily answered. Let's look into this a little more deeply.
  Well, the first half of their dislike is plain wrong: an extra transformer or even additional secondary winding is not needed, as the extra power supply voltage can be derived by voltage doubling the existing power supply (as long as there is a center tap). The second part...what can I say about the second part, other than to say that the reason this Webzine exists is to lessen the sway of things like the second part. 

Voltage doublers fall into two types: half-wave and full-wave. Both come close to doubling the normally rectified voltage from a transformer winding.   The half-wave version, however, will not sustain the higher voltage under load as well as the full-wave version. It only receives half of the potential charging that the full-wave does, as it only charges its final capacitor during one half of the 60 cycle sine wave from the transformer winding.

Half-wave voltage doubler circuit 

Full-wave voltage doubler circuit 

  But, beyond a sagging output voltage under load, half-wave voltage doubler suffers from two other defects. The first is that has more  noise. The second is that this noise's fundamental frequency is 60 Hz, which is much harder to filter than the full-wave voltage doubler's 120 Hz frequency (the lower the frequency, the harder to filter). These are definitely negative attributes, but one failing often attributed to it, that this circuit tends to magnetize the power transformer's core, is false. The phrase "half-wave" confuses many.
  Now, what normally happens in a full-wave power supply is that the current flows through the secondary winding in one direction on one half of the AC cycle and then it flows in the other direction the other half of the cycle: up, down, up, down.

   A full-wave bridge center-tapped power supply splits and rectifies the secondary AC voltage. For example, a 140 VAC CT transformer will yield +/- 100 volt rails. By adding a full-wave voltage doubler circuit to the power supply, we create the higher power supply voltage. By adding a second voltage doubler, if needed, we could also create a doubled negative rail  power supply voltage.   Voltage doubler circuits have received some bad press the audio magazines. And some of it  is deserved.

Full-wave bridge    rectifier

pg. 13


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