piclist 2000\06\06\110207a >
Thread: Filter Capacitors
www.piclist.com/techref/logic/dsps.htm?key=filter
picon face BY : Spehro Pefhany email (remove spam text)



At 07:58 AM 6/6/00 -0500, you wrote:
>In many products, I see a large electrolytic capacitor in parallel with a
>small disc or dipped capacitor.  Why is that done?  Obviously, the pair
>does not act as a single large capacitor, or aonly a single one would be
>used.  Just how does that arrangement work?
>
>Mark P

It does act as a single large capacitor. Have you ever (deliberately or
inadvertently) ;-)  taken an electrolytic capacitor apart? The plates are
coiled together. This construction has a significant amount of
inductance, which is like a series inductor (and there is a series
resistance as well) in series with an ideal capacitor.

So, at low frequencies the added impedance of the series inductance is
negligible, the e-cap acts pretty much like an ideal capacitor (
just some series resistance). At very high frequencies
the impedance of the e-cap may start to rise significantly because of the
inductance, and the ceramic capacitor shunting it may begin to dominate the
parallel impedance. Some e-caps are better than others, this is not
always necessary.




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