Re: Replace capacitors

The message <bw+14UA8V2rHFw$+@xxxxxxxxxxxxx>
from Mike Tomlinson <mike@xxxxxxxxxxxxx> contains these words:

In article <313030303737303647AF299047@xxxxxxxxxxxxxxxx>, Johnny B Good
<jcs.computersbutt@xxxxxxxxxxxxxxxx> writes

The second thing
to keep in mind is that they are used in circuits where 'more is
better', i.e a larger value will do no harm, and will generally improve
performance.

Up to a point, Lord Copper.

I've repaired quite a few MoBos over the years and have never had to
actually buy any capacitors since I've acquired quite a large collection
of scrapped MoBos from which to recover perfectly good quality
capacitors from.

I can be pretty confident that this source of supply is no more likely
to fail than those bought in 'brand new' from a component supplier. In
fact, I can be more confident that they'll be just as good, if not
better than, 'brand new' capacitors!

I'm sorry Johnny, I have to disagree here. Electrolytic capacitors
start to deteriorate from the moment they are installed in a circuit.
They have a finite life, usually measured in a few thousands of hours,

The actual service lifetime is very dependent on how close to their
ripple current and temperature limits they are operated at, but I'd have
expected a service life in the region of 25 to 30 thousand hours when
run close to, but not exceeding these limits. The increased service
lifetime can easily exceed an order of magnitude improvement with a
quite modest reduction of temperature and ripple current limits.

and particularly in CPU power circuits, where they: 1) are expected to
absorb very large changes in current as the CPU load changes, and 2) are
subjected to localised heat from the MOSFET transistors to whose legs
they are soldered (a direct heat transfer path), they lead a pretty hard
life. To re-use capacitors in such applications is IMHO a false
economy, especially as they are available cheaply new, branded and with
elevated temperature spec (105degC).

I appreciate your concerns, but the ones taken from scrapped boards are
rated at 105 deg C and have cleared the infant mortality part of the
'bathtub' curve thus, effectively, passing an extended 'burn in' test.

Most likely, restricted to just 1800uF and
2200uF values since this covers most situations (the 1800uF would, for
instance, nicely substitute for 1000, 1200, 1500 uF values).

Too much of a bodge. I'd always try and get as close as poss to the
original installed value. I might sub 1800uF for 1500uF but not vice
versa.

That, too, is my preferred choice (using a slightly higher value than
the original), but, knowing the very wide tolerance range for
electrolytic capacitor values and the tendency to 'over-spec' (typically
by half an order of magnitude or more in this type of application), it's
extremely unlikely that a slightly lower value substitute would cause
any problems whatsoever.

If I'm replacing 1800uF caps, and I can't find enough of that value
which would physically fit, I'd choose 1500uF as a substitute since (all
other considerations aside[1]),

I wouldn't.

a slightly lower value is infinitely
better than the existing high resistance versions I'm trying to replace.

Swapping one problem for another IMHO.

I don't see that. Would you care to expand?

--
Regards, John.

Please remove the "ohggcyht" before replying.
The address has been munged to reject Spam-bots.

.

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