From: brian@omaha.eel.ufl.edu (Brian Gentry)
Newsgroups: rec.audio.car
Subject: Re: Suitable Capacitor?
Date: 5 Nov 92 21:04:55 GMT
Article-I.D.: eng.1992Nov5.210455.13631
Organization: UF EE Department (student)

In article <gilchr.720958980@ee.ualberta.ca>, gilchr@ee.ualberta.ca (Andrew Gilchrist) writes:
|> brian@hopi.eel.ufl.edu (Brian Gentry) writes:
|> >In article <gilchr.720422686@ee.ualberta.ca>, (Andrew Gilchrist) writes:
|> >[...]
|> >|> 47000 uF should work ok.  You should wire this up at the load end (by the
|> >|> amplifier).  Since this cap is likely a huge polarized electrolytic, make
|> >|> sure you put in another smaller capacitor in parallel (like a 10 uF
|> >|> tantalum cap, or something).
|> 
|> >I am aware that in CROSSOVERS some people use a "high quality" capacitor 
|> >in parallel with an electrolytic because of the non-linear qualities of
|> >electrolytics, but I have never heard of this in a power supply filtering
|> >application.  Why would you suggest doing this? 
|> 
|> I suggest this because it is a very good idea.  Also, it has nothing to do
|> with "non-linear" qualities of electrolytics.  It is because of the way
|> electrolytics (particularly LARGE ones) are constructed.  It gives them
|> a noticable series inductance.  High frequencies are rejected by this.
|> Thus the need for higher frequency bypass capacitors (small caps).
|> Why have you never heard of such a thing in a power supply application?
|> Do you know ANYTHING about power supplies?  Gadzooks, even the
|> Radio-SHack introduction to electronics book (the one with all the
|> happy faces on the electrons) tells you to put a small bypass capacitor
|> across the power pins of all ICs.
|> 
|> Every schematic for any linear regulator I've ever seen instructs the use
|> of a secondary small capacitor in parallel with the big one.

Thank you for your kind reply ;).  Please consider the application we are
discussing.  We are talking about adding extra energy storage capacity to
an existing power system.  This power system is the alternator/battery
in a car.  We are adding capacitors so that more power is available at
a faster rate.  Capacitors can discharge faster than a battery can.  That's
the reason for using a cap in parallel with your battery.  A secondary 
objective could be filtering, but the frequency of the alternator should
not be high enough to be influenced by the ESL of the capacitor.  Besides
the battery usually does a very good job of filtering the output of the
alternator.

This Equivilent Series Inductance as Gordon Walters pointed out, is not
really an inductance at all!  That is why I refered to it as "non-linearity".
I guess to be totaly correct I would have to tell you exactly how the 
disipation factor of the capacitor changes as frequency increases.  I'll
have to go look it up and get back to you.

Yes I do know something about power supplies.  I do not proffess to be
an expert on the subject.  In extremely high frequency a power supply
applications, I can see the need for adding the smaller parallel cap.
I also understand the need for these bypass caps on ICs.  In the case
of the ICs, the internal structure of the IC creates these high
frequencies which makes it necessary to use the bypass caps.  Amplifiers
that use ICs should already have these caps in place.  I know ours at
U.S.AMPS do.

|> Stop reading audio magazines and thinking they know what they're
|> talking about.

For the record, I have never read an article in an audio magazine that
recommended the use of "stiffening caps".  I first heard about the
idea at a local audio shop.  I was merely trying to apply some of the 
electric circuit theory that I've learned to the problem.

We should probably move this discussion to sci.electronics or E-mail
if you wish to continue it.
Newsgroups: rec.audio.car