On Friday, Mar 12, 2004, at 14:54 US/Pacific, Omega Software wrote:

> I saw a 5.5V 1F capacitor. I wondered if, once charged, something
> nasty could happen if it's accidentally shorted? I'd prefer to play on
> the safe side, this device seems scary to me (one whole Farad!).

Well, was it one of those tiny ones based on activate carbon or carbon
aerogel, or did it take up significant numbers of cubic feet?  I've
played with some of the former, and read the data sheets, and haven't
seen any dire warnings about shorting them...

1F @ 5V is only about 12.5 Joules, right?  That's not very much energy
(a common disposable camera photoflash is about 8J at high voltages,
and those are regularly discharged through pretty much a dead short.)

> The short-circuit current can be hundreds of amperes, not? Or do they
> have very high ESR?

The ones aimed at memory backup tend to have a "high" ESR.  There are
new varieties with "very low ESR (ie the Cooper Bussmann Aerogel caps
sold by digikey), but  you still have to take into account the
relatively low voltages involved.  They have an ESR of about 0.2ohms,
which is pretty low, but at the 2.5 to 5 V we're talking about, that's
a short circuit current of "only" 10 amps or so (and... briefly!)

The high voltage caps are a lot scarier (energy goes up as v^2, don't
forget.)

BillW

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