From: gilchr@ee.ualberta.ca (Andrew Gilchrist)
Subject: Capacitors, etc... CALMLY
Organization: University Of Alberta, Edmonton Canada
Date: Tue, 10 Nov 1992 09:13:25 GMT

yystg@lims01.lerc.nasa.gov (STEVE GRAHAM) writes:

>In article <gilchr@ee>, gilchr@ee.ualberta.ca (Andrew Gilchrist) writes...
>>AAAAAAAAAARRRRGGGGGGGHHHHH!
[ranting, etc. deleted]
>>You people.

>At least I'm not the only one that doesn't understand you.

>We are talking about DC current.  I for one still do not understand the 
>introduction of freqencies that you made.

>We know how to hook up a capacitor, both for crossover and power supply 
>apps. 

>What we (I say we, now that someone else is also having trouble with what 
>you are saying) don't understand is the need to filter out different 
>frequencies, as you suggest doing.

>S. Graham

Okay. I'm perfectly calm. Sometimes I swear people are just trying to upset
me. I can understand your confusion, but I'm not sure how to explain this
in terms you WILL understand. I will try.

When you have a voltage source connected to a load, the conductor that
connects them contributes a resistance, and an inductance.  The resistance
affects the voltage seen at the load. The inductance affects the RATE
at which current can change.  If the inductance is large, the current
can only change slowly.  Thus, if you suddenly switch in a small resistance,
the full current will not be delivered immediately.  A sharp change
in current means high-frequency components.  The sharper the change,
the higher the frequencies.  Whenever I say "high frequency" you can
just imagine to yourself a sharp change in current.  Inductive elements
resist sharp current changes.

Now, a capacitor is similar.  A capacitor resists sharp voltage changes.
If suddenly drop a voltage source across a capacitor, the potential
cannot change instantaneously.  The mechanism by which this happens:
current flows into the capacitor, and the electrons are stored.  As
electrons accumulate, the potential across the capacitor increases.
When the potential across the capacitor equals the potential of the
source, no current flows.

Now the reason we want to put a capacitor on our amps:
When the amp wants to suddenly draw more current, the inductance and
resistance of the charging circuit will try to resist the current change.
The result would normally be a voltage drop at the load.  But, since we
have a capacitor there, the capacitor will resist this voltage drop by
allowing the stored electrons to flow into the load.
That is, as the voltage starts to drop, the capacitor will deliver
current to the load, and thereby stabilize the voltage.

Now, if the series inductance of the capacitor is large, as it is with
large electrolytics, due to the nature of their construction (rather like
two big flat coils), the capacitor will also tend to resist sharp current
changes through is.  Thus, if the current demand changes too much, the
capcitor will not be able to immediately supply it, and the load voltage
will drop.  So, we stick in a smaller capacitor with a much smaller
series inductance to supply current for overly sharp current demand
changes (high frequency demands).  Obviously, this requirement depends
largely on the inductance of the large capacitor, and the nature of the
load.  i.e. your mileage may vary.

My references in previous posts to "filtering high frequeny components",
etc. are completely valid in this context.  It's just different
terminology, and a slightly different perspective (more technical, I
suppose).  To avoid future confusion, I will try to limit myself to
less technical terminology.

Thank you.

Andrew
From: kirk@hpcc01.corp.hp.com (Kirk Lindstrom)
Date: Tue, 10 Nov 1992 19:21:52 GMT
Subject: Re: Suitable Capacitor?
Organization: Shredding the water of SF Bay, HP-OCD