Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucsd!brian
From: brian@ucsd.Edu (Brian Kantor)
Newsgroups: sci.electronics
Subject: Re: Capacitor questions
Keywords: electrolytic capacitors
Message-ID: <10097@ucsd.Edu>
Date: 3 Nov 89 07:28:23 GMT
References: <109@cupcake.sal.wisc.edu>
Reply-To: brian@ucsd.edu (Brian Kantor)
Organization: The Avant-Garde of the Now, Ltd.
Lines: 29

It's hard to know what sort of fudge-factor you can apply to a part
value without knowing what the part is being used for.  For example,
a capacitor used as a power supply filter or power line decoupling is
usually not critical, and the next highest available capacitance will
almost always be perfectly acceptable.  However, if the capacitor is
being used in a circuit as part of a RC time constant, such as a
high-pass, low-pass, or timer circuit, the value is much more critical.

Voltage ratings on electrolytic capacitors are much less of a problem;
usually you can go to double the voltage rating without a problem.
There are some guidelines; it is wise to keep in mind that if you use a
cap with a voltage rating more than (say) 10 times the working voltage
of the circuit, the electrolytic film can deteriorate and the
capacitor's value will change.  Thus it's probably ok to put a 25 or 50
volt capacitor in place of one originally spec'd at 16 volts, but it
wouldn't be smart to use a 100 volt unit there.  If you don't know
what voltage the original was (perhaps you can't read what's left of the
label after the old one exploded), you can feel pretty safe in most
cases with the next voltage rating above the supply voltage; for example
a 16v or 25v capacitor is a good bet for something like a car radio
that runs off 13.8 volts.

Most of the time you want to stick with the original type of capacitor;
substituting a tantalum cap for an aluminum one can often be done but
usually NOT the other way since the aluminum ones are looser tolerance
and have different impedance, loss, and stability characteristics.
Since tantalum's cost more, there probably was a good reason why they
used the more expensive part in the first place.
	- Brian