From: John Woodgate
Subject: Re: hi-capacity voltage controlled capacitor?
Date: Sun, 13 Oct 2002 14:17:46 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sun, 13 Oct 2002 15:44:05 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that carltons
wrote (in ) about 'hi-capacity voltage controlled capacitor?', on Sun, 13 Oct
>gather that there is a problem with ESR in some caps. If this has not
>been pointed out before now, there is a reason why the parameter is called
Yes, it's because it attempts to model ALL the losses as if they were
due to a single series resistor. A better model has a series resistor
and a shunt resistor, and you can carry on making better and better
models by adding more resistors.
>Current, as in circuit current, does not flow through a cap. Is
>this magic or a violation of Kirchoff's laws? No. Charge is exchanged
>from one plate to the other of the cap, this current is called
>displacement current. Since this is a real world there is a limit.
Displacement current behaves in every way as far as the outside world is
concerned as if it DID flow through the capacitor (put an ammeter in
series to see that!). It seems unnecessary to stress that that isn't
actually true. There isn't any feature of displacement current itself
that gives rise to ESR.
> It is
>called ESR and is an apparent, mesureable and different parameter from the
>circuit resistance outside the component.
Part of it is a REAL series resistance. Part of it is due to a REAL
shunt resistance, but you could say (if it helps, which I doubt) that
the part of ESR which is actually due to shunt losses is not 'real'.
> Rds, as in FET Rds, is real and
>is part of the normal thinking of circuits and circuit current.
So is ESR, although it, like many other simply-modelled loss
resistances, varies with frequency.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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