Re: Can a capacitor let DC current through?


"vorange" <orangepic@xxxxxxxxx> wrote in message
news:1187578456.973349.156600@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
1st question : Till now, I believed that capacitors only let AC
signals through while blocking DC. But then, I saw a schematic whee
they put a capacitor on the output line of an opamp. The signal into
the opamp was a square wave signal (which I imagine is DC and not
AC). How then does the output of the opamp (presumably DC as well)
pass through the capacitor? This has confused the hell out of me.
Is there something I'm missing here?


There are a few ways to think about this. One way is to realize that a
square wave(or any wave) is composed of a series(which means a sum) of
simple sine waves. A capacitor passes these sine waves. It does attenuate
each frequency but as long as certain conditions are met the ouput will look
like the input.

A second way is to realize that the capacitor doesn't "block" except in
steady state conditions(and this is where your confusion is comming from).
The capacitor will charge and discharge on each cycle of the square wave and
as long as this charging is fast enough the output will still look like a
square wave.


2nd question : Is it fair to say that if a signal goes from say +5 to
-5 volts and then back to +5...etc that is is an AC signal because its
reversing its direction. But if it goes from +10 to 0 volts and then
back to +10 that it is a DC signal because its not reversing
direction?


I would say this would technically be true. AC means alternating current. If
you have a square wave that goes from -5V to +5V then it must in some way
alternate the current when it changes from - to + or + to -. (even if its
just a fA)

If you take the square wave from 0 to +10 then the current never alternates
but just changes in magnitude(but not sign). -5 to 5 never changes
magnitude(for an ideal signal) but only sign.

Jon



.

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