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From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: net.analog
Subject: Re: Effect of overvoltage on electrolytic capacitor
Message-ID: <277@neoucom.UUCP>
Date: Wed, 17-Sep-86 09:35:53 EDT
Article-I.D.: neoucom.277
Posted: Wed Sep 17 09:35:53 1986
Date-Received: Fri, 19-Sep-86 23:29:10 EDT
References: <14924@onfcanim.UUCP>
Distribution: net
Organization: Northeastern Ohio Universities College of Medicine
Lines: 45
Summary: They often go BANG!

Hi,

	I tried to dig up my reference from Mallory, but it seems
to have wandered off.  A call to Hamilton-Avnet or a similar
distributor of components should be able to yield a capacitor spec.
catalog.

	Most electrolytic capacitors are designed to withstand
overvoltages of 20 to 30 per cent for brief periods.  Sorry, I
can't cite numbers on the duty factor for overvoltage.

	A general rule of thumb that I was taught was to design in
electrolytics that had woking voltages about 1.33 times the maximum
expected input, irrespective of the cap.'s so-called surge rating.

	The general symptom of an over stressed electrolytic cap.
is getting hot and then, perhaps, exploding.  The voltage rating of
the cap. is proportional to the thickness of the dielectric paste.
Too much voltage will decompose the paste and cause resistive
heating.  If the cap. is not provided with an emergency vent plug,
it will explode.

	I've seen serveral nasty capacitor explosions.  The
dielectric that gets sprayed inside the affected equipment can also
be corrosive and quite smelly!  The worst offenders are small
electrolytics that have been completely dipped in plastic and thus
have no vent.

	If a capacitor sits around in a parts bin for a long time,
the paste will tend to depolarize somewhat and become partially
conductive.  Its usually a good idea to reform the dielectric in a
capacitor that has been sitting unused for a long time.  This can
be done by connecting the cap in series with a 10K ohm or so
resistor to a power supply.  Charging the cap up slowly gives a
chance for the polarization to reform.  You can get an idea of how
well the capacitor is doing by monitoring the voltage drop across
the series resistor.

  --Bill

Bill Mayhew
Division of Basic Medical Sciences
N. E. Ohio Universities' College of Medicine
Rootstown, OH  44272   USA    (216) 325-2511
(wtm@neoucom.UUCP)