Re: Question about capicitors



PeteS wrote:

Eeyore wrote:
Dana wrote:

Will a capcitor allow DC to pass before it is charged up?
This question is the result of a thread in a computer group, where some
people are claiming a cap acts like a short before it is fully charged.
While some caps depending on construction may exhibit that kind of
operation, I thought caps were constructed to act like an open to DC.


They act 'like a short circuit' to brief impulses. No net DC flows though.

Graham

Indeed so. One of the reasons we use gated power for switchable power
domains is to 'soft-start' the inrush current. For the OP - check out
hotswap controllers; you can find them easily on all the major mfrs -
Linear tech, AD, National, TI, Maxim to name just a few. If you read
the datasheets carefully you'll see that the switch is slowly turned on
to present some series resistance during that 'spike', as Graham puts
it.

A test on one ceramic cap (4.7uF) showed that applying raw power at
9.5VDC gave an initial inrush current > 25A (don't try that with an
improperly rated tantalum ;)

I have in fact seen supply rail decoupling caps literally 'catch fire' due to
switch-on transients.

Graham

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