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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: Re: (easy) capacitor question
Message-ID: <6209@aw.sei.cmu.edu>
Date: 13 Jul 88 14:45:55 GMT
References: <24300028@silver>
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Reply-To: rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito)
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In article <24300028@silver> Robert Schofield writes:

>    I have a question that should be easy for those in this group.
>
>    Is the voltage rating of an electrolytic capacitor the maximum it can
>    handle or is it the voltage required to drive the capacitor? (If that
>    is even a correct thing to say) What I really want to know is if I can
>    substitute a 35v,47 microfarad capacitor for a 16v, 47 microfarad
>    capacitor.

Easy, but with several twists:

Yes, but electrolytic capacitors should be used at around 75 to 80% of the
working voltage to maintain maximum quality factor (leakage/life
characteristics).

Electrolytics are formed, that is, have the oxide layer built up to the
right thickness to withstand the impressed voltage. A unit run at a
substantially lower voltage will eventually deform and become a lower
voltage unit (with a thinner oxide layer). In other words, after many years,
you'll have a 16-volt unit anyway. As a bypass component, this will not
matter. The end result is that the (originally) higher voltage unit may take
up more space and have slightly higher leakage in operation. 

The voltage rating (if only one is given) is the recommended maximum
impressed circuit voltage (DC plus repetitive peaks) that can be applied to
obtain the rated electrical characteristics and life. Higher voltages may
result in higher leakage, lower service life, dielectric punch-through
(shorting, although some capacitors are self-healing), or explosion due to
internal heating. Oil-filled AC capacitors are especially impressive in that
regard!

By the way, there are such things as AC capacitors (even electrolytics)
designed to handle the heating due repetitive charge/discharge cycles.


Rich