Re: I'm Busted for shoplifting

The other part you glossed over is.. fact.

The AC sinewave includes zero crossing points, which is fact, not my or
anyone elses' imaginary test points.

DC doesn't pass through zero. You can imply that you understand an
ocilloscope, but you don't know what zero crossing is.. that's more than you
being just a little confused.
I guess you never saw the "0" beside the center line on a scope screen.
Maybe the scope manufacturers are just supersticious about putting a number
between numbered scales that increase as they are placed farther away from
that point.

You should get together with Dr. Welch to write long, overly simplistic
explantions describing what folks don't understand about transformers and
circuit breakers.
When describing the most simple characteristics of a transformer, be sure to
use a center-tapped utility transformer for an example, and use phrases like
*Then, try to understand..*, and don't forget to include pulse generators
and DC offsets.

My previous post was to explain that transformers can be used without AC
applied to them, since Hinz was/is mistakenly certain that transformers can
only output AC.

When you apply a DC pulse to a transformer, the output is not the same as if
you had applied an AC voltage, because the DC doesn't alternate. It doesn't
cross zero or have any change of polarity potentials (because it's DC, not
magic).

The "it doesn't alternate" means that the observed waveform won't be a
sinewave, and it doesn't cross the flat horizontal trace line. The flat
horizontal trace line represents zero volts, as in no vertical deflection. A
transformer's output resulting from a DC pulse will be on one side of the
line only, whether the potential is increasing or decreasing. It doesn't
cross zero as the potential drops.
The transformer output pulse will go from zero, to a point, and then back to
zero.

An AC signal reverses polarity potentials, and the observed waveform will
change to be both above and below the flat horizontal trace line.
The transformer output will go from zero, to positive, to zero, then
negative, then back to zero, then positive again (as a sane example).

Peek at this explanation.. an AC sinewave has both positive going and
negative going peaks, and between positive and negative is zero. The AC
signal crosses zero to change (alternate).
A DC pulse (as an input and/or output of a transformer) will be of a single
polarity, never crossing zero.

WB
metalworking projects
http://www.kwagmire.com/metal_proj.html
............

"Jerry Foster" <jmfoster711NOSPAM@xxxxxxxxxxxxx> wrote in message
news:O%3Kh.15079$bb1.6659@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

"Wild Bill" <wb_wildbill@xxxxxxxxxxxxxx> wrote in message
news:45f85c0c$0$16390$88260bb3@xxxxxxxxxxxxxxxxxxxx
OH Yes it is, yes it is.. for an AC input (notice that A means
alternating).

A DC pulse doesn't have positive-negative alternating swings/cycles
(positive-zero-negative-zero, the crossing zero characteristics of
alternating current).
A DC pulse will be zero-to-something potential, and not crossing zero.

Go ahead and enlighten us further

WB
metalworking projects
http://www.kwagmire.com/metal_proj.html
...........

<snip>

What you are missing is that there is nothing magic about "zero." It is
simply where (in overly simplistic terms) you stick the black wire of your
meter.

Imagine a transformer with a centertapped secondary (like the one the
power
company puts on the pole outside your house). The centertap is connected
to
an earth ground and we simply refer to it as "ground." Now, if you put
the
black probe of your meter on ground and the red probe on either end of the
transformer secondary, you will read 110 volts. But, if you put the red
probe on one end and the black probe on the other, you will measure 220
volts.

Now, I know someone is going to point out that it doesn't matter which
probe
you put where, you will still get the same reading. Which, in AC, is
true.
But look at it this way. If you are measuring DC, you put the black probe
on some arbitrary point and put the red probe on the same point, you will,
of course, read zero. And it doesn't matter if the point you pick is
connected to some "ground" or not. Now, when you move the red probe to
some
other point, you will measure the difference between that point and the
one
the black probe is on. Maybe it is +15 volts. Maybe, someplace else
is -5
volts. You are measuring the "potential difference." Now, if you leave
the
red probe on the -5 point and move the black probe to the +15 point, your
meter will read, -20 volts. This should be pretty obvious. But now, move
the black probe to, say, the chassis ground. Maybe then you will read
the -5 volt point as +30, the point on which you originally put the black
probe as +35 and the +15 point at +50. All well and good. The point is
that the meter reads the voltage on the red probe with respect to the
voltage on the black probe.

Now, keep the same idea in mind when looking at AC. The meter reads the
AC
voltage on the red probe with respect to the voltage on the black probe.

OK, now to confuse the issue a little farther, what you read with the
typical AC voltmeter is "rms" voltage. Rms (root of the mean square) is a
mathematical term that means the "effective" or DC-equivalent voltage.
The
peak voltage is the rms voltage times the square root of two (appx.
1.414).
So, when we say "110 volts AC," what we mean is a voltage that, starting
from zero, rises to +155.5 volts, falls back through zero to -155.5 volts,
rises back to zero, etc., etc.

Now, bear in mind that zero, in this example, is the centertap of the
transformer. It is where we stick the black probe. Now, if the
centertap,
instead of being connected to "ground" were connected to, say, a 250 volt
DC
source (referenced to ground), absolutely nothing in this example would
change. But, if we moved the black probe to ground, we would see the
voltage swinging between +94.5 and + 405.5 volts. (Actually, a DC offset
on
an AC voltage would drive the meter nuts and what it would read would be
dependent on the internal workings of the meter, so, let us assume we have
switched from a meter to an oscilloscope that can follow these voltage
excursions...).

An important thing to realize here is that the reference voltage of the
secondary of the transformer doesn't have to be the same as the reference
of
the primary. In the example of the power transformer out on the pole, the
system could still work if, instead of connecting the centertap to ground,
they connected one end of the secondary to ground. You would get 110
volts
with respect to ground off the centertap and 220 volts with respect to
ground off the other end. (This would, of course, be a rather inefficient
way to use the transformer and wires, but it would work... And you would
get 110 volts between the centertap and the other end of the secondary,
but,
if you used this to power something, both sides of the power cord would be
hot...)

Now, for anyone who hasn't given up on this long-winded bit of basic
electronics, we (finally!!!) get to the original question.

Suppose you apply pulsating DC to the transformer. Let's say that one
side
of the primary is hooked to ground (zero) and the other side is pulsed on
(let's say to 100 volts) for 10 milliseconds and then off (zero volts) for
10 milliseconds. You could correctly say this is 100 volts DC being
pulsed
at a 50 Hz rate. But, suppose that instead of putting the black
(reference)
probe on ground, you put it on a point that was 50 volts DC above ground.
Your indicator (meter, oscilloscope...) would show the voltage swinging 50
volts above and below the reference, or 50 volts AC!. And, what you would
measure on the secondary is exactly what you would get if you had put a
simple 50 volts AC on the primary.

And, yes, for all you purists, I've glossed over the implications of wave
shape, etc., etc., etc. I said upfront this was a somewhat simplistic
discussion, OK?

But, if you want to understand this stuff, remember, first of all, that
voltage is something measured at one point WITH RESPECT to a second point,
and that second point isn't necessarily "ground." Then, try to understand
the terms, peak voltage, peak to peak voltage and rms voltage. (And peak
is
1.414 times rms only for a sine wave... If you have something other than
a
sine wave, say the pulsating DC, the multiplier will be different.)

But, since electricity is such a ubiquitous part of our lives, I think it
well worth the effort to understand at least the basics about it. And,
while I'm in the soapbox mode, there is a big difference between
understanding and simply rote learning. The latter tends to
misapplication
of what you know, sometimes with amusing but often with disasterous
consequences.

Jerry







--
Posted via a free Usenet account from http://www.teranews.com

.

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