Re: Cap values?

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

Original poster: "Sean Taylor by way of Terry Fritz <twftesla-at-qwest-dot-net>" <taylorss-at-rose-hulman.edu>

Capacitors are devices that can store and release a charge at
(theoretically) any rate.  The current into/out of a capacitor is related to
how fast the voltage across it's terminals is changing, I = C* dV/dT.
Current equals capacitance times the rate of change of voltage (Volts per
second).

If the two caps you mention both have a changing voltage across the
terminals of 1 V / S, then the 500 uF cap will have a lot more current
flowing through it than the 0.015 uF cap, i.e. 500 uA vs. 15 nA.

A capacitor hooked up to line voltage will have a continuously varying
voltage and current, returning to the same point every 1/60th of a second.
The residual charge on the capacitor after it is removed depends on when
during that 1/60th of a second cycle it is disconnected, at most you'll have
117*sqrt(2), or about 165 volts, or you might end up with 0 volts.
117*sqrt(2) is the peak of the sine wave from the AC, 117 is the RMS value,
Root Mean Square (the root of the average of the sqare of the sine wave),
the equivalent DC voltage that would result in the same power into a
resistive load.  Any other Q's??

Hope that helps, it was probably a little more than you wanted . . .
Sean Taylor



> Hi guys! For me it's been a good read (pup list) for over a year
now.Thanks
> and keep it coming!
> Question The ratings on caps have i  have to assume deal with how fast
they
> cycle.Is there simple way to grasp this,say with one saying 500uf and
another
> being.015?I find with low voltage caps(700acv) ,when connected to 120ac,
the
> cap will charge and dis-charge while remaining connected to 120ac.Remove
the
> 120ac and the cap might or might not have a charge, why?Sorry if this is
> simple stuff,it's not for me. Thanks