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From: cyamamot@girtab.usc.edu (Cliff Yamamoto)
Newsgroups: sci.electronics
Subject: -How do you pick the right capacitor?-
Keywords: filter, capacitor, parallel
Message-ID: <7152@chaph.usc.edu>
Date: 22 Dec 89 08:00:49 GMT
Sender: news@chaph.usc.edu
Reply-To: cyamamot@girtab.usc.edu (Cliff Yamamoto)
Organization: University of Southern California, Los Angeles, CA
Lines: 26

Observation   :	I've read and seen circuits where a large value electrolytic &
		a small value mylar capacitor are wired in parallel for
		filtering.  My understanding is that the small value mylar is
		for high frequency filtering and the large electrolytic for
		low frequency filtering.

Problem       :	I want to filter the AUDIBLE whining noise from the alternator
		in my car  (Yes I know about other methods, but I want to try
		stopping the problem at the source).

Question      :	How are these capacitor values calculated?

I'm asking because I was going to pluck down $30 for a 61,000 uFD 50 Vdc
electrolytic capacitor to use as a filter across the alternator output.  But
then I remembered that a large valued capacitor does not filter ripple
for ALL frequencies (it would probably just filter lower audible frequencies,
not the irritating higher pitched whining sound).

Again, I'd like to ask those analog and power supply gurus out there how
one selects the proper values capacitor(s) for filtering ripples that have
frequencies in the audio as well as RF ranges.  How are these parallel
combinations of capacitors actually chosen?  Is there any kind of formula
to use?  Are there any pros and cons?

Thanks for any info!
Cliff