John Fields wrote:
On Sun, 12 Jun 2005 17:15:06 -0700, Don Lancaster
wrote:
Sum a 1 volt peak sinewave with a 0.6 volt dc term and you have a
waveform whose polarity continuously changes but whose average value is
continuous.
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No, you have a waveform with a polarity which changes _periodically_,
making it an AC signal. Do the electrons traversing the circuit
change direction? Yes. Do the electrons in a DC circuit ever change
direction? No.
So you are saying that DC varying from 5 to 15 as the
op referenced is AC? If you put a DC source across
a capacitor and vary the source up and down, sometimes
electrons are flowing into the capacitor, and sometimes
they are flowing out of it. Same with an inductor.
For the record, I don't want to take one side or another
in the debate about AC vs DC in this thread. The waters
are muddy enough already. I view the op's scenario as
DC with an AC signal imposed on it.
This whole discussion of whether it is AC or DC is
a trap and diversion from the original. It does not
matter whether it is AC or DC that the components
see. For example, a capacitor operates the same on DC
as it does on AC. If there is a path for it to charge,
and a source sufficient to charge it, it charges. If
there is a path for it to discharge, and no source applied
sufficient to keep it charged, it discharges. Same thing
for an inductor below saturation.
The op asked about a sinusoidal varying DC, but gave
no info about frequency. He then asks about impedance
of the (unknown) RLC circuit. The answer has to be
arrived at by a consideration of how each component
reacts. To say (not that you said it) the cap won't pass DC
is crap. Connect a 15 V, 500 ohm relay coil to ground,
and the other side to a 470 uF cap. Connect the other
side of the cap to +12. The relay energizes briefly, proving
that the cap did pass DC. Try the same thing with a
supply that starts at 5 volts and increases to 15 volts
at a rate of 1 cycle per hour, and it does not energize.
But the relay coil DOES charge. For the op to understand
the load impedance, he has to understand what each component
does in his circuit. I see no other way to answer his
question, in the absence of specifics.
Ed
Ergo, because of the periodic polarity reversals what you're looking
at is AC.
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Looking at the Fourier terms makes this waveform perfectly clear.
Calling it "AC" or "DC" does not.
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Why go there? Your description was adequate to indicate that polarity
reversals occur, therefore making the signal voltage alternate between
two different polarities, therefore making the current alternate
between polarities as well. That's why it's called "Alternating
Current".
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"AC" or "DC" are gross and meaningless oversimplifications.
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Uh-huh...
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