Re: general cap questions and BIG Farad caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>

> A Farad is the unit of capacitance. It is one coulomb per volt. A coulomb
> being a measure of charge (current time time). A coulomb is 1 amp second.
>
> So, if we take a 1 farad cap and charge it to say 5 volts, it will store 5
> coulombs of charge. The same cap will store only 1 coulomb of charge if
> charged to 1 volt. In either case the energy storage of a capacitor is 1/2
> voltage squared times capacitance. The assumed units are farads and energy
> is in Joules. You can see that if you double the voltage in a capacitor
you
> do two things- you well, double the voltage it is storing, and you also
> double the charge it can hold. Doubling the voltage across a cap will
> increase energy storage by a factor of 4. 2x voltage times 2x charge = 4
> times the energy.
>
> You will rarely come across a 1 Farad capacitor. more usual values are in
uF
> nF or pF.
>


Actually, 0.5-1 Farad caps are much more common than one would think.  They
are used all the time in a variety of
applications although almost 99.99% of the time, they are low voltage <20
Volts
5V, 1 Farad caps are about half the size of a 35mm film case.

Anyways, engineers rarely call these things caps once they reach these large
capacitance values.

We basically call them batteries, although they behave a bit differently
than their chemical counterparts.

Some common applications of high capacitance capacitors are:

1.  High power car audio systems.  In professional systems, amplifiers can
draw up to 10,000 watts continuous and even much more than that
     at peak transients.  Large capacitor banks (1-10 Farad) are used to
supply that extra juice when the batteries themselves cannot chemically
     deliver it fast enough.

2.  Crank radio systems.  Those crank-up radiosets that you see hikers and
mountaineers using actually charge up a very large capacitor which acts
     like a battery.  Because these radios need to operate under extreme
harsh conditions (especially very cold temperatures), batteries are very
poor
     at low temperatures, so they use large capacitors instead.  The radio i
currently have has two 5V, 0.47Farad caps in it.

3.  Computer memory back-up and storage.  A lot of new computers are doing
away with lithium batteries and using large capacitors as well.  Typically,
you
     can get a 0.47 Farad, 5V capacitor for about $5.00 new.

Dan