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From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: Choosing Cap Size in Power Supply
Message-ID: <642@neoucom.UUCP>
Date: Sun, 2-Aug-87 22:40:28 EDT
Article-I.D.: neoucom.642
Posted: Sun Aug  2 22:40:28 1987
Date-Received: Tue, 4-Aug-87 01:37:27 EDT
References: <5705@ut-ngp.UUCP> <1884@kitty.UUCP> <935@sdcc7.ucsd.EDU> <1666@midas.TEK.COM>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 32
Summary: Capacitor dynamic response


To respond to Jeff at Tektonix (see, I know how to spell it):

I think the the original comment about capacitors having poor
dynamic response stems from the fact that aluminum electrolytic
capacitors have a large surface area and use a very long coiled up
set of plates.  The coiled up plates give them a a relatively high
inductance, which in turn prevents them from unleashing the current
that they've stored up terribly quickly.

To get around the inductance of electrolytics, most power supplys
place small (usually less than 1 uF) capacitors in parallel.
Typically, ceramic caps are employed since their small flat disc
shaped plates have characteristically low inductance.  In spite of
what audiophiles would tell you, ceramics are better for this
application than fancy expensive capacitors.  Mica capacitors would
be good for this application, but they are expensive if you don't
happen to have a load in the junk box.

Note that long leads tend to counteract the effect of capacitors,
thus so-called bypass capacitors should be used at the ends of long
leads to counterbalance the leads' inductance.

Tantalum capacitors are polarized capacitors that have a large
capacitance versus size ratio, giving them low inductance.  The
disadvantage of tantalum capacitors is that they are expensive and
tend to be available in only low voltage ratings.  The most popular
use of tantalum capacitors is in aerospace and avionics equipment
where physical volume and weight (mass) must be reduced.

Bill Mayhew, NEOUUCOM
(wtm@neoucom.UUCP)