Re: parallel capacitors

[John Denker <jsd@MONMOUTH.COM>]

At 09:57 PM 12/1/99 -0500, Ludwik Kowalski didn't like my analysis and asked:

> What kind of frequencies are important in a power supply? Not
> very high, certainly below 1200 Hz in common cases. Therefore
> the explanation is not satisfying. Why should 0.1uF work better
> than 10 uF at f=1200 Hz? Are voltage ratings identical?

I don't know what *you* are trying to filter, but the thing I'm usually
most worried about is
        dV(L) / dI(L)
where V(L) is the voltage at the load and I(L) is the current drawn by the
load.  The risetime on I(L) is a nanosecond or less.  On that timescale,
whatever is going on "in" the power supply module is irrelevant.  When the
original question spoke of "power supplies" I took that to mean the
power-supply busses, not the power-supply module or anything else that
would have a 1200Hz frequency "in" it.