[sdiy] Using PC power supplies

[John Luciani]

On 5/4/06, Iain Duncan <iainduncan at telus.net> wrote:

> For my EWI interface project, I was thinking I would use a PC power
> supply as I can get them cheap and they provide all three of of the
> needed voltages already, +12, -12, +5v.
>
> Does anyone here know how well regulated the output voltage is? Should I
>  plan on adding further regulation of those if this will be generating a
> control volt signal that I would like to keep quite sensitive? And if
> so, can anyone point me the easiest way to do so?
>

Couldn't resist power at 10cents/watt either ;-)

The specifications for PC power supplies are at http://www.formfactors.org
The regulation is typically 5%. In most new PC's there is an on board
regulator that
powers the CPU to a tighter tolerance. Some of the manufacturers
produce supplies with tighter tolerances.

One issue is the minimum loads that these supplies require to maintain
regulation.
If you do not draw sufficient current the supply will go out of
regulation. It is not clear
(at least to me) what happens if the supply goes out of regulation. My
concern was
the output going high and tripping an output voltage OVP (which may require some
sort of reset).

When you design your supply don't forget to pull the PSON# signal low
to turn on the
supply. PSON# is usually controlled by circuitry on the motherboard.
Use the +5VSB
to power any circuitry you need to have on all the time.

If you do not need a lot of power I think you are better off with a
wall-wart and some
switching regulators.

(* jcl *)

--
http://www.luciani.org