Bob Penoyer wrote:
On Sat, 11 Jun 2005 10:36:54 -0400, "Tam/WB2TT"
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."
Actually, DC from a rectifier *is* "zero frequency", to the
degree that it is DC. Of course until the AC is filtered out,
it has both AC and DC components.
The output of a rectifier contains both AC and DC. You put a filter on it to
get close to pure DC.
That is *precisely* correct. (It just doesn't tell enough of
the story to explain the confusion of this "flows in one
direction" definition of DC.)
A rectified AC waveform contains DC and AC components but if the
current isn't changing direction, it isn't alternating current. And,
if it isn't AC, it's DC.
The output of a rectifier until filtered *does* have both AC and
DC, which actually is another way of saying that yes it *does*
change directions.
What? you say!
The problem is that "direction" only has meaning when measured
in comparison some specific point of reference. If you have
three different reference points, one at the DC level, one at
the peak positive swing and one at the peak negative swing, you
have three very different views of "direction" for current flow:
Reference Direction
Point of flow
========= =====================================
Peak Pos All Negative
DC level Equal cycles of Positive and Negative
Peak Neg All Positive
Obviously the only point of reference you are thinking about is
the Peak Negative swing, and just as clearly that is *not*
appropriate. The correct reference point is necessarily the DC
level, and measured from that point of reference the current is
going to go alternately in each direction.
Bingo, there is your AC.
This is very similar to confusing the negative side of a
rectifer circuit with ground. Rectifiers are most often
grounded on the negative side, but that is absolutely arbitrary.
They can be grounded on the postive side, and with some effort
can have ground at other levels either between those two, or
even beyond either of them.
But rather than look at DC as current going all going in one
direction, and AC as anything else, it is *far* easier to view
it as AC is any current that is changing, and DC is anything
else (i.e., the current is steady).
Technically those definitions are exactly the same, but one
leads to a lot of confusion.
--
Floyd L. Davidson http://web.newsguy.com/floyd_davidson
Ukpeagvik (Barrow, Alaska)