[sdiy] Powersupply question

[Roy J. Tellason]

On Tuesday 26 July 2005 03:43 am, Mikael Mørup wrote:
> Hi list.
>
> I'm planning on building a bench power supply for SDIY work, i'm tired of
> running a long cable out of the back of the enclosure of my synth for
> testing modules under construction.
>
> I would like the new supply to have both +/- 12V, +/- 15V and +5V to cover
> all bases.
>
> Can i do this with one transformer and rectifier and just parallel the
> regulators after the rectifier ?

You _could_,  but it wouldn't be very efficient.  Typically in a setup like 
this the +5 will have a higher current draw (TTL logic,  LEDs,  etc.) and 
feeding a regulator with the higher input voltage needed for the other 
outputs is gonna waste a lot of power as heat.  Stuff that I've seen uses two 
transformer windings and separate rectifiers.

> Also most designs looks something like transformer -> rectifier bridge ->
> large bypass caps -> small bypass caps -> regulators -> more small bypass
> caps.

Sounds about right to me.

> Can i use the same bypass caps for 3 sets of regulators, so it's just the
> regulators (and the caps after them) that are paralleld or must i copy the
> whole chain (after the rectifier) for each voltage ?

I'm not sure what you're asking here.  Bypass caps at the input to the 
regulators?  You could probably get by with one set if wiring was short.  But 
why scrimp?  Caps are cheap...

While I'm on the subject of power supplies,  I have a few VCR power supplies 
here,  one is a switcher and I believe the others are linear (I'd have to 
pull them out of the box to look).  These might be adaptable to this sort of 
use,  as there's typically an output up in the 30-40V range (for tuning 
voltage and other stuff) and some number of lower voltage outputs.  Anybody 
want to take some of these off my hands?  Contact me offlist and we'll kick 
it around some.