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Re: old capacitors



yes, the std. method is to cut one of the leads and insert a high value
resistor (100 k Ohm) and charge it over night. Observing the gradual
increase in EMF is interesting. Another method (power supplies sand
"heavy" equipment) is to monitor the "HV" while increasing the line with
a Variac; this saves blowing a fuse. MS's method is easier, but less
safe and more time consuming. Note: the oscillator doesn't work if the
cap. is too leaky. In one case I replaced it when I couldn't hear it
oscillate rather than attempting to reform it.

To determine the problem: find how long the cap. "holds a chaarge". does
it fire only immediately after unpluging or will it fire a number of min.
after unplugging? The trigger cap. may also leak in which case this
test won't be definitive.

bc

"Edmiston, Mike" wrote:

The effective insulator (dielectric) in an electrolytic capacitor is =
a
layer of metal oxide that forms on the positive electrode of the
capacitor. When a capacitor has not been used for a long time, this
oxide layer diminishes and we eventually have bare metal plates in th=
e
electrolytic paste without a real good insulator between them. When =
the
oxide-deficient capacitor is used the next time, if you are lucky, th=
e
oxide layer is recreated and the capacitor is reformed (rejuvenated).
The reformation of the oxide layer is not instantaneous and can take
hours. While it is reforming, the capacitor is "leaky." The leakage
current creates heating. If you are unlucky the heating can be
sufficient to boil the electrolytic paste and the capacitor explodes =
or
at least discharges its guts through the blow-out plug.

If you have already plugged the unit into AC power and it survived, y=
ou
were lucky. If a device like a photoflash has been idle for several
years, it might be a good idea to plug it in briefly (a second or two=
)
then let it sit for a while, and repeat this several times over an ho=
ur
or so before leaving it plugged in. That allows some reforming of th=
e
oxide layer without allowing too much prolonged high-power leaking.

Until the capacitor is reformed, the drain on the battery-operated po=
wer
supply might be too much to allow it to work properly. If the unit i=
s
left plugged into the AC source for a couple days, the capacitor shou=
ld
completely reform and then the battery circuit might work. If not, i=
t
could be that the capacitor does need replaced, but it also could be
that the battery-operated power supply is defective.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu