On 28 Mar 2005 19:29:43 -0800, "cnctut"
put finger to keyboard and composed:

I've never seen wattage ratings on a capacitor--voltage yes. Is it
simply .5CV^2 and that's it?

That's the energy stored by a capacitor. An ideal capacitor dissipates
no power, so the concept of wattage makes no sense.

I can design a circuit for a particular
capacitance--but how is the physical size of the Cap determined--I've
seen big capacitors with same MF's as little caps? What am I missing?

The size of a capacitor is determined at least partly by its voltage
rating. AFAIK, larger voltages require a greater distance between the
plates to avert dielectric breakdown. All other things being equal, to
maintain the same capacitance at this greater distance also requires a
larger plate area.

I believe the other important factor determining the size of a cap is
the type of dielectric used.

Thanks

Another important consideration for caps is their equivalent series
resistance (ESR) and their ripple current rating. The ripple current
causes ohmic heating of the ESR, which then causes the temperature of
the electrolyte to rise. AFAIK, every 10 degC rise in operating
temperature reduces the life of the cap by half.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.