10 Jun
2009
10 Jun
'09
10:44 p.m.
Eric Smith wrote:
Warren Wolfe wrote:
Erm... I doubt this. I saw a 2 farad cap at a
surplus place in Lima,
Ohio. It was rated at 12 volts, and looked like a sawed-off oil drum.
That's
much older technology. As others here have observed, you can now
get a 100F capacitor that is quite compact, though they have voltage
ratings between 2V and 2.7V.
> I think there's some sort of square-cube thing going on here that
> makes ever-increasing values of capacitance ever huger. Not really. For a given technology, the volume is
roughly linearly
related to the product of the capacitance and voltage.
That doesn't make sense. energy = 1/2 * C * V^2. You are saying if you
double the size of the cap you can handle double the voltage, but that
means four times the energy. That isn't right. It is the stored energy
that is linear with the size of the cap (roughly).
I agree that you can double the capacitance if you double the volume,
which means doubling the stored energy. Great. This is demonstrated by
wiring two "x" farad caps in parallel.
But if you wire them in series so they can handle twice the voltage, the
capacitance is also cut in half. E = 1/2 * (1/2 C) * (2V)^2, which
works out to double the 1/2*C*V^2.