Center-tap Transformer Voltages

[ronaldrc]

What is apparent about scope traces? What you see is what is there!

Rattus

I don't understand if I apply a negative to the hot or pos. probe of an Oscilloscope and a positive
to the common probe would that not make the trace go down? That is what is shown in my illustration
please explain what I am not seeing. I no I'm not the sharpest Knife in the draw but help me on
that maybe I can understand but maybe not. If line #1 is positive at that point in time wouldn't line#2 be negative or not?

 

[rattus]

Rattus

I don't understand if I apply a negative to the hot or pos. probe of an Oscilloscope and a positive
to the common probe would that not make the trace go down? That is what is shown in my illustration
please explain what I am not seeing. I no I'm not the sharpest Knife in the draw but help me on
that maybe I can understand but maybe not. If line #1 is positive at that point in time wouldn't line#2 be negative or not?

Ronald, the ground clip on most scopes is tied to the chassis which is tied to the EGC.

If you clip that to a hot wire, you will likely burn off the minigator clip and throw a breaker. That is one reason for using a neutral/ground reference.

Some people do this by snipping off the EGC prong, but then the chassis is hot. A better way is to use the switch on the scope which inverts the trace.

If you wanted to look at the 240V wave, you would attach the probes to L1 and L2, ground clips to neutral, and set the scope to trace out A-B.

It has been some time since I have used a scope, but I think they haven't changed that much.

 

[ronaldrc]

Rattus did you look at my illustration where not talking about
hooking the common to the hot side.

The common is still on the neutral we are putting the pos. probe on line #2

I will admit its been years since I even seen an oscilloscope.
and I would think that the newer digital scopes would have a
power supply and chassis that is insulated from the grounding conductor.


Ronald

 

[ronaldrc]

A better way is to use the switch on the scope which inverts the trace.

QUOTE]

Well heck in that case when you test from the neutral to line #2 switch that to invert.

And as you have been saying what you see is what you got.

And with that switch in the other direction it is apparent 180 degrees out. :thumbsup:

Ronald

 

[rattus]

A better way is to use the switch on the scope which inverts the trace.

QUOTE]

Well heck in that case when you test from the neutral to line #2 switch that to invert.

And as you have been saying what you see is what you got.

And with that switch in the other direction it is apparent 180 degrees out. :thumbsup:

Ronald

 

[rattus]

A better way is to use the switch on the scope which inverts the trace.

Well heck in that case when you test from the neutral to line #2 switch that to invert.

And as you have been saying what you see is what you got.

And with that switch in the other direction it is apparent 180 degrees out. :thumbsup:

Ronald

Well, not quite Ronald. When you invert trace B, you get the negative of what is really on L2. Which is Vn -V2n and that is the way you like to look at it.

But to get the VOLTAGE on L2 RELATIVE TO NEUTRAL, you do not invert, then what you see is what is there.

Just forget for a moment that this is a transformer. Imagine three terminals in a row, two are red and marked A and B. One is black and is labelled G. You don't know what the source is. You would place the probes on the red terminals, ground clips on the black terminal, and sync on A. Then you would see Vag and Vbg. You will see the waveforms on A and B. Nothing apparent about it.

 

[ronaldrc]

I don't understand what you just said and you don't understand what I am saying

So as the old saying go lets agree to disagree.

Ronald

 

[rattus]

I don't understand what you just said and you don't understand what I am saying

So as the old saying go lets agree to disagree.

Ronald

I am just saying that if you invert a trace, you
see the inverse of what is really there. Really.

If you don't invert, you see what is REALLY there.

To paraphrase Besoeker, no ifs, ands, buts, apparents about it!

 

[ronaldrc]

Okay

Have a great night:Ronald

 

[Besoeker]

To paraphrase Besoeker, no ifs, ands, buts, apparents about it!

Thank you for the reference.
Appreciated.

 

[gar]

111122-1851 EST

ronaldrc:

The word apparent is a problem. The word apparent as it seems to have been used here is implying that you see something, but it is not real.

About scopes. In their original form these were based on a CRT (cathode ray tube) with two sets of deflection plates. One set deflected the spot horizontally and the other vertically. Initially only AC amplifiers were provided. In the 40s DC amplifiers became available, Tektronix.

If you apply a sine wave to each axis then you can display Lissajous curves. If the frequencies and amplitudes are the same, then the results are a straight line, ellipse, or circle. Harmonically related frequencies produce other predictable patterns.

If the horizontal axis is driven by a sawtooth waveform, then x-axis movement is a repetitive movement, usually left to right, and approximately linear with time. Some lost time in the return trace.

To have a steady display the horizontal sweep must be synchronized with something. Thus, if you want to make time measurements you must define the source and how synchronization is accomplished. Suppose the source is a sine wave, then you might use a positive slope zero crossing as the sync point and you might set the sweep rate to display one full cycle of the sine wave.

You can not leave out how the scope is synchronized if you want phase relationships to be meaningfull.

.

 

[pfalcon]

What is apparent about scope traces? What you see is what is there!

Nonsense. Get a better scope. Tie the scope reference to the X1 or X4 and you'll get a different waveform. Which is exactly why a lot of those old farts clipped the ground on the newer models. Cause some idjit decided to tie the scope reference through it.

As to "What you see is what is there!": of course it is. No one ever said otherwise. And when you stand on your head and call that up, objects really do fall up, they're really there also.

I am just saying that if you invert a trace, you
see the inverse of what is really there. Really.

If you don't invert, you see what is REALLY there.

To paraphrase Besoeker, no ifs, ands, buts, apparents about it!

Even with it inverted you see what's REALLY there. It doesn't become fake just because you don't like the reference frame. The inverted image is just as real. And since your time sync is arbitrary to begin with it's no less REAL. How in the world can you claim you managed to sync with absolute zero time to establish absolute zero phase? Dial that scope over half a cycle and you'll see the same thing you'd get by inverting it. You just chose what's positive cause it matches what you call positive on the plug your scope is powered by.

The word apparent is a problem. The word apparent as it seems to have been used here is implying that you see something, but it is not real. ... You can not leave out how the scope is synchronized if you want phase relationships to be meaningfull.

Apparent is the correct term. Apparent does not imply real or not real. It means what it means. Apparent is what you see. As you stated in your post, how you synchronize plays a major part in what you see. Change the sync, change the wave form, change the appearance, change the apparent phase.

And why is this an issue? Rattus is steadfastly analyzing the circuit without consideration of the primary-secondary coupling. As such, his established frame of reference is as valid as any. He WILL see on the scope what he expects to see. I have never argued otherwise at any time.

 

[pfalcon]

Guys,
You're absolutely free to ignore the primary-secondary coupling. You're absolutely free to use the neutral as your frame of reference. You're measurements and equations will all work. They will all be valid. There is no practical reason for someone in the general electrical trade to do otherwise. Since electrical equipment is all referenced back to that frame it's inherently safer to work in that frame. Less confusion.

All the quizzes about "What would you measure under these conditions?" and "What would you see on your scope under these conditions?" are pointless. You will measure and you will see exactly what you expect under those conditions. A magnitude of 120V at each end that are 180 degrees out of phase.

I have only maintained that when the primary-secondary coupling is considered, the frame of reference has been changed. With the coupling present both secondary legs MUST BE IN PHASE. And you're not going to prove or disprove that with any voltmeter or traditionally set-up oscilloscope for that matter. All your standard connections with a voltmeter or oscilloscope inherently ignore the coupling effect. They can't measure it. They don't see it.

 

[gar]

111123-0916 EST

From dictionary.com

apparent

3. according to appearances, initial evidence, incomplete results, etc.; ostensible rather than actual: He was the apparent winner of the election.

Why was the word apparent even used?

What does the primary secondary coupling have to do with the subject? It is really the coupling between the two halves of the secondary that is important. Eliminate the primary and apply a voltage across the total secondary and you get the same result if primary to secondary leakage flux is ignored.

.

 

[rattus]

pfalcon,

Explain to us please why the physical construction of the xformer matters. I think most of us understand basic transformer theory. That is, we know how the voltages got there, but for this exercise, we do not care. For all I know these voltages may have come straight from the generator, or maybe two generators. Maybe from inverters driven by a battery.

If I want to see the waveforms on L1 and L2, I place the probes on those nodes. Apparently, I see what is really there. I do not wish to see the inverse, therefore I do not place a probe on the neutral. Neither do I flip the invert switch on one of the probes. I want to see the waveforms on L1 and L2 which I expect to be inverses of each other, and that is what I see--the real thing. Smarter people than I agree with me.

How would one write loop or node equations without specifying the voltages at the node in question? How would one perform a SPICE simulation?

 

[ronaldrc]

How would one write loop or node equations without specifying the voltages at the node in question? How would one perform a SPICE simulation?

Rattus
I need you to do me a favor forget about the node voltages. And from your understanding of
basic electrical circuitry tell me how that circuit would display on a scope using electron flow theory.

 

[__dan]

pfalcon,

I think most of us understand basic transformer theory. That is, we know how the voltages got there, but for this exercise, we do not care.

Originally Posted by iwire
Well both are 120 volts to ground so the chances of them getting a 240 volt shock without them actually opening a j box or enclosure first are pretty remote.

Originally Posted by stickboy1375
With both legs being 180 degrees apart, is it even possible to be hit with a 240v shock?



Here is the OP's inquiry. This is your audience. Where does that ubiquitous 240 volt come from, the how or why. If he gets shocked by 240, that's a line to line source and the neutral is not in the circuit's voltage or current loop. The neutral is a system reference point but otherwise invisible to the line to line 240 volt load.

Guy gets shocked by 240 and wants to know why. Is the voltage source a series connection of two indentical 120 volt windings, in phase, or would you like to continue to baffle the masses with invocations of phase relationships relative to the unapplied, inconsequential, neutral. Somewhere in the explanation, the underlying fact that the secondary winding turns have doubled by adding a second identical coil in series, you are trying to lose this fact. The phase relationship explanation fails to convey this essential information, the underlying physical principle of how the 240 volt is manufactured.

It was not so long ago, the human race spent thousands of years consulting a priest for the times to plant and harvest grains. Predicting eclipses was in the realm of magic. Look closer, do not ignore the circuit foundation principles, and you will see the second, identical, in phase, winding in series.
This is science, not magic or dogma.


​

 

[rattus]

Rattus
I need you to do me a favor forget about the node voltages. And from your understanding of
basic electrical circuitry tell me how that circuit would display on a scope using electron flow theory.

There would be no difference at all. There is only a difference in the way a circuit is analyzed.

 

[ronaldrc]

Like Bob, Don and a few others realized a long time ago
it is silly to argue this point.



Please don't take offense but guys


I'm joining that crowd.

 

[gar]

111123-1215 EST

Post #1 (OP) said nothing about 240 V and a shock.

.