# [EFM] Capaciter question

harrybissell at prodigy.net harrybissell at prodigy.net
Mon Mar 20 23:16:51 CET 2000

I'll have to be short and sweet...

Caps can pass a charge difference... ie. AC can get through a cap, but DC cannot.  If you suddenly apply DC to a cap... current flows while the cap is charging, but stops when the DC level is reached. If you think in terms of the frequencies contained in a rectangular (square wave) there is a DC level, and then progressively smaller amounts of higher and higher harmonics. The Harmonics are "AC" and can pass through the cap, as though it were a certain size "resistor" this is called Capacitive Reactance Xc, or AC Impedance

the formula is...

1/ (2pi * frequency in Hertz * capacitance in Farads)=Xc

and the answer is in OHMS.

So the perfect capacitor has a DC resistance approaching
infinity, and an AC impedance that gets smaller and smaller with increasing frequency. The sharp edge of the rectangle wave goes right through (with little impedance  :^) but the DC part cant get through because the impedance at (or near) DC is very very large.

Real world caps have a tiny amount of leakage current that flows at DC... like if there was a resistor in parallel with the cap. This is a flaw and undesirable, but it does happen. Better caps have less of this leakage.

You can solve problems using ohms law and the above formula... at any fixed frequency the cap looks just like a resistor....

Helps ???

H^)  harry

---- On Mar 20 mitchell <mitch at sirius.com> wrote:
>     So I had some time and thought I would try and get back to learning
> some more of this electronics stuff.
>     I had thought I understood how capactiers worked then I read
> something that has made me rethink everything.
>
>     My original understanding was that the cap would build up a charge
> on one side and then when it had reached its maximum charge it would
> release the charge through its otherside. What I was reading has said
> that the charge can not pass through!
>
>     If anyone cares to give me a little bit of enlightenment it would be
> most appreciated.
>
> Thanks
>
> --M
>