[sdiy] Using PC power supplies

[John Luciani]

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

> Do you know what the minimum current one would need to draw is? And
> could I just achieve that by diverting some through some leds or something?

I am not sure which supply your going pick. Check out the formfactors.org site
for specification on the many varieties. For an ATX12V supply you need to
draw around 25Watts. Thats quite a lot of light!

Some of the supplies for the compact systems will be lower minimum power
but may be more expensive.

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

> I don't think I need a lot of power, it's just for control signals. But
> I do need +12v -12v and +5v all at the same time. I though maybe that
> would be difficult with a wall wart. Or at least more difficult than
> finding the right leads on an old pc power supply ... am I off base here?

The PC power supply is probably not worth the effort unless you need a lot
of power. You need to do min loads, enable signals, etc and you waste a lot
of power.

Check out Linear Tech or National Semi for the switching regulators.
Since you don't care about size you may want to use the National
switching regulators. They are lower frequency (100-300KHz) and you
should be able to get thru-hole parts.

Depending on the power and regulation requirements you could get a 15V wall
wart, use a linear regulator to get the +12V, use switchers for the
-12V and +5V.
If your power requirement on the +5V is low use a linear regulator
there as well.
Don't forget that most of the wall-warts are unregulated at low loads.
Make sure that
unloaded output voltage of the wall-warts doesn't exceed the maximum input
voltage of the regulator that you choose.

(* jcl *)

P.S.  If the 5% regulation of the PC power supply was not acceptable
you would have
        needed additional regulators anyway.

--
http://www.luciani.org