Fireworks in a power supply... (indirectly off topic)

From: John Lawson <jpl15_at_netcom.com>
Date: Mon Jul 5 17:57:54 1999

  This is why gear which has been sitting around a long time should
be 'eased' back into operation... especially minicomputers with
large multiple power supplies:

  Electrolytic capacitors use aluminum foil for the electrodes, and
aluminum oxide for the insulation between them. This insulation is
formed on the surface of the foil by a chemical which is contained
within the capacitor case, soaked up by porous paper wound up in the
spool of aluminum.

  When the capacitor dries out, from a pinhole leak or heat or age,
or if there is internal shorting, heat is generated. If the current
is strong enough, the heat boils the chemical and that usually
breaches the case somehow.

  Most electrolytics have a designed in 'safety-valve' of some kind
or another, either a thin rubber plug in the can, or in the case of
small PC-mount units, the case itself is scored with lines in the
material, so it will rupture there first. Then the guts come
spewing out with a bang, and paper bits and aluminum foil go all over.

  I have had this happen while bending over a board trouble shooting
it; damn good thing I wear glasses because the cap-guts coated both
lenses and would have gone right in my eyes. I have been around
many of the darn things when they pop, and have seen some spectacular
(and very dangerous) situations where the larger sizes have gone off
and destroyed equipment in the process.

  My second PDP-11/04 ran fine for about two weeks, and then one
afternoon the +15V regulator filters woofed their cookies
simultaneously, one blew up and one hissed like a snake... it took
two weeks to get the smell out of the house....

 Then there was the 7805 TO-220 regulator on a Cromemco memory board
I was fixing out on the bench... *it* blew up and embedded a sliver
of epoxy in my cheek, as well as scaring the *%^&$ out of me... I
didn't know I was wounded until blood dripped off my chin onto the
board... I had to go to the emergency room and have it dug out..
what fun. The doctor was so fascinated that I was working on a
computer at home (1978) that he kept stopping to ask questions about
the machine, and what it could do, and....

  But I'm getting off topic, no?

 Cheers

John
Received on Mon Jul 05 1999 - 17:57:54 BST