|Re: OT (as in Out of Touch) -- 1.5 Farad (!) Capacitor???|
Message #7 Posted by Eric Smith on 17 Aug 2007, 8:01 p.m.,
in response to message #6 by allen
In fact, you can now get "ultracaps" that are rated in the hundreds and even thousands of farads, though generally at not more than 2.7V. You can put several in series to raise the voltage, though that also lowers the capacitance. However, if you want to increase the voltage rating by n, you need more than n capacitors in series, because they won't share the voltage equally.
I'm not convinced that the ones being sold for car stereo use are anything other than a clever new way to part a fool from his money.
More than one vendor brags about using hybrid capacitors combining the characteristics of electrolytic capacitors and carbon capacitors. They claim specs like 5 farads and 0.002 ohm ESR. However, IMNSHO that's basically fraudulent, because it's just a low-capacitance low-ESR capacitor in parallel with a series chain of 6 or more high-capacitance high-ESR (e.g., 0.1 ohm) capacitors. When your amplifier tries to draw 80A from it, you're not going to actually get that 0.002 ohm ESR, because the carbon capacitor with the low ESR isn't actually storing much energy. Almost all of it will actually have to come from the electrolytic stack, with its 0.6 ohm ESR.
While 0.6 ohms might not sound like much, at 80A that will drop 4.8V out of your nominal 13V. That sort of voltage drop is exactly what installing the capacitor was supposed to fix.