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From: karn@petrus.UUCP (Phil R. Karn)
Newsgroups: net.analog
Subject: Re: One farad Caps
Message-ID: <427@petrus.UUCP>
Date: Mon, 29-Jul-85 11:42:44 EDT
Article-I.D.: petrus.427
Posted: Mon Jul 29 11:42:44 1985
Date-Received: Wed, 31-Jul-85 04:30:35 EDT
References: <102500002@hp-pcd.UUCP> <168@kitty.UUCP>
Organization: Bell Communications Research, Inc
Lines: 20

I've never understood the fascination some people seem to have with
"large" capacitors like 1 Farad. What really matters is the ability of
a capacitor to store energy. For a capacitor of a given size, this
is generally much greater at high voltages (and correspondingly lower
capacitances.)  This is because as you increase the thickness of the
dialectric, the capacitance decreases linearly, but the working voltage
increases linearly. Since the energy stored in a capacitor is
0.5 * C * V^2, the net result is a linear increase in energy storage
capacity.

One of the reasons switching power supplies are so small for the amount of
power they carry is that the AC line is directly rectified and filtered at
relatively high voltage, before conversion to the output voltage.  For
example, I have here a Lambda 10A @ 5V switcher. The first cap is 330 uF @
200V. When rectifying the AC line, this cap will store 4.36 J, but it is
only half the size of a 35mm film can. On the other hand, storing this much
energy in a 1F cap would require a working voltage greater than 3V, and it
would certainly be much larger than this.

Phil