On Jan 13, 21:27, Louis Schulman wrote:
> Old computer power supplies generally have big old electrolytic
capacitors. When these go bad, they can
> cause real problems, and damage other components.
>
> The literature indicates that many of these only have a working life of
2000 hrs., or a shelf life of ten years.
> Obviously, this will be exceeded in old computers.
I think you may have lost a digit off the working life, Louis, at least if
you're referring to the sort of electrolytics found in PSUs :-)
> So, should these be replaced if they exceed a certain age?
If it ain't broke, don't fix it. As always, there's an exception to prove
the rule. If one of a pair goes, it may be worth replacing both to get the
values to match. Or in some PSUs, if one or more fails, they may all have
got too hot or dried out, and at least for small ones (relatively
inexpensive), I tend to replace the lot. In any case, check the others,
they may be on their way to join it.
The working life is an estimate based on mean time between failures (MTBF).
All this says is that for a large sample, it is statistically likely that
a certain proprtion will fail in a certain time. Variation can be
enormous.
> For the screw terminal type, is it necessary to
> use "computer grade" capacitors, which can be fairly expensive?
Depends. The more expensive ones may differ in a few ways:
-- lower ESR and/or higher ripple current tolerance. This means they won't
waste so much energy (and heat up) if used in a PSU delivering a high
current.
-- higher temperature rating. PSUs often get rather warm, and the higher
temperature rating makes them less prone to drying out.
-- longer working life (related to temperature and other factors).
If one can't find the right value in the right
> size package, how much extra capacitance is acceptable?
Electrolytics often have a wide manufacturing tolerance. -20%+50% is still
not unusual. If it's just a filter capacitor I'd say up to 2x is
acceptable. The real issue is likely to be cost. Also bear in mind that a
capacitor of the same size but a higher capacitance may have other
differences, like ripple current rating. I recently replaced a pair of
7700mfd caps in a DEC PSU; I could only get 10000mfd in the right size. In
capacitor terms, that's close enough to be considered a match.
> And any higher working voltage rating is OK?
Yes.
--
Pete Peter Turnbull
Network Manager
University of York
Received on Mon Jan 14 2002 - 03:22:54 GMT