On Sat, 11 Jun 2005 06:12:09 GMT, Mac wrote:

snip

Yes. DC by definition is zero frequency.

Um, no. DC is Direct Current, i.e., current that flows in one
direction. For example, the output from a rectifier is DC but it
certainly isn't "zero frequency."

No, it is NOT DC. Sometimes when speaking casually people call it DC, but
more often it will be called rectified AC.

Rectified AC _is_ DC. Now you might say, But it has a lot of AC stuff
riding on it and that makes it "rectified AC."

Okay, so let's hang a large capacitor across the rectifier's output.
Now, assuming there is some sort of load connected to the rectifier,
there will still be ripple on the load--so there is still some AC
present. Is this still rectified AC? Using your definition, when does
the signal change from rectified AC to DC?

I agree with you that DC stands for Direct Current. But what is the
logical meaning of that? Who knows. The bottom line is that when a
waveform varies with time, it is NOT DC in popular useage.

As long as there is a finite load on the rectifier that I've
described, anything less than infinite capacitance will permit some
ripple to be present. So, since you say "... When a waveform varies
with time, it is NOT DC in popular useage," then the signal will never
become DC.

The simple truth is that a current flowing in only one direction is,
by definition, direct current. It might have AC riding on it, but if
it's direction doesn't change, it's DC.