PC Power Supply for DIY

[Magnus Danielson]

>>>>> "FB" == Fred Becker <mach25 at indy.net> writes:

 FB> I can get an old IBM PC for $10.  Is the power supply useful for powering a
 FB> synth DIY project?  I would think so.  Would I have to keep the noisy fan?

Recall that these powersupplies are unbalanced in the current
sense... they deliver considerable more on the positive rails compared
to their negative rails. Both the +12V and the +5V have pretty much
ouhmp in them... where as the -5V and -12V is really weak since they
are typhically used only for IO stuff where as +5V is used for all
logic and +12V is used for the fan and the drive motors (some floppy
and disk drives does only use +5V nowdays).

You migth find that these powersupplies will not give you much voltage
and current when just fireing them up on the labbench, and this is
explained by the fact that they require a certain minimum load for
stability. I sometimes use old diskdrives as dummy loads but better
than that (and less noisy... :) is to use a dummy load resistor of 5
ohm at the +5V power... not neat but this is how IBM did it back at
the IBM/AT times...

One way to use such supplies to get proper power at say +/- 12V would
be to use 2 PSUs and hooking the +12V of one of the to the ground of
the other. Then would the ground of the other be -12V... but this is
potentially very dangerous cause you migth have a low-ohmic contact
between ground and case which would then cause a short of the low PSU.
I would not recommend such a interconnection unless it has been
verified that the output sections is really floating to the case (and
other potential shortage points). Recall also that the +5V leads of
the lower PSU will get the -7V potential and how the current of that
cursuit will flow is a diffrent story...

Maybe traditional PSUs isn't to bad after all... :)

Cheers,
Magnus