16 Mar
2005
16 Mar
'05
3:16 p.m.
I replaced a electrolytic cap on a dead dog simple diode bridge, cap, and
resistor PC board power supply once. There was an inductor on the board
too, as I recall. Anyhow, I somehow managed to look at the polarity
markings on the new cap twice and STILL solder it in backwards. It was
marked on the negative side, where the old one had plus signs on the
positive side.
Turned it on and BANG! the aluminum cap hit the ceiling above the bench and
squashed itself flat. There was confetti all over the place. If I had been
leaning over it, you'd be callin' me one eye'd Jack.
At 04:44 PM 3/16/2005 -0500, you wrote:
[Mind] It stands to the everlasting credit of science that by acting
on the human mind it has overcome man's insecurity before himself and
before nature. --Albert Einstein
--... ...-- -.. . -. ----. --.- --.- -...
tpeters at nospam.mixcom.com (remove "nospam") N9QQB (amateur radio)
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43? 7' 17.2" N by 88? 6' 28.9" W, Elevation 815', Grid Square
EN53wc
WAN/LAN/Telcom Analyst, Tech Writer, MCP, Cisco Certified CCNA
Apart from
the shock (and dangerous sudden-hi-energy discharge) risks -
one thing to be aware of when 'bringing up' large analog power supplies is
the condition where the filter capacitor(s), or other big electrolytic
device, develops and internal short, or loses it's chemical polarization,
and the esuing thermal rise causes the case to burst. Most big caps have
built-in rupture vents, or a strategic 'crease' in the aluminum to keep
this from becoming an explosion - but I have been the unfortunate
recipient of more than one suddenly-exploding capacitor.
Not counting the sheer terror of having something go BANG! right
next to
you, the larger units can store considerable
energy, both pressure and
electrical, and I've seen them do a lot of damage, especially the ones
mounted upside-down, which have a tendency to launch off the chassis like
a mortar shell and poke holes in walls.
Then there's the hideous mess that all the 'guts' of the cap makes when
they explode and puke up all over everything.
So the moral of the story is to bring a long-dormant power supply up
slowly, using a variac and loads, and to be vigilant for any signs of cap
leakage, swelling, getting suddenly warm, or hissing sounds coming from
one.
John,
Thanks for the vivid visual images! I suppose I've been lucky so far.
Actually, most all of the stuff I've acquired has recently been powered on
by someone else before it came into my hands, so if it was going to go
BANG!, it probably would have done so before it came into my
possession.
I'd be interested in hearing any horror stories relating to exploding
capacitors and more info on the proper caution that needs to be
exercised when messing with old power supplies.
Ashley