TK,

I agree with many of your points, however, I don't buy your spiel about small amplitude AC signals superimposed on a DC level as not being "AC" anymore simply because the current "doesn't alternate." This is a non sequitur at its best. According circuit theory, the superimposition of AC and DC does not change their characteristics. It is however, that certain devices such as LEDs and magnetic circuits are sensitive to the DC level, so they stop working as anticipated when saturated with a large DC bias. Thus the very small AC component has no effect as its actions cannot cause a state change due to the large bias present. This effect does not undermine the superimposition theory in the least. Your argument simply doesn't hold water, because you based it upon the definition of the common phrase used to denote a cyclic function, "alternating current." If it doesn't alternate it can't be AC. Bogus. All you need are changes in time for an AC signal to be present. The exact change in the time domain can be decomposed into a Fourier series that describes its harmonic series in the frequency domain. Thus, any minute change in level for either current or electric potential represents "AC." This is the real pedantic view of the situation.

Here's an _old_ video I made concerning the issue of AC vs DC voltages. This was before I found a better video transcoder so I apologise for the poor video quality.
Using AC-coupling on the scope would take the offset signal, and move it down to the channel baseline, allowing you to display an LED that was ON even when the scope trace says it should be solidly off. Or many other interesting effects. So if you see AC coupling being used in a situation where the _vertical measurements_ are important, I strongly advise you to make the presenter tell you exactly why AC coupling is used and what effect it has on the measurements. Ask him to switch momentarily to DC-coupled... if you see a difference in the trace, you are getting snowed.

http://www.youtube.com/watch?v=jg2_yE5dEQg

Quote from: G4RR3ττ on August 07, 2014, 02:03:34 AM
TK,

I agree with many of your points, however, I don't buy your spiel about small amplitude AC signals superimposed on a DC level as not being "AC" anymore simply because the current "doesn't alternate." This is a non sequitur at its best. According circuit theory, the superimposition of AC and DC does not change their characteristics. It is however, that certain devices such as LEDs and magnetic circuits are sensitive to the DC level, so they stop working as anticipated when saturated with a large DC bias. Thus the very small AC component has no effect as its actions cannot cause a state change due to the large bias present. This effect does not undermine the superimposition theory in the least. Your argument simply doesn't hold water, because you based it upon the definition of the common phrase used to denote a cyclic function, "alternating current." If it doesn't alternate it can't be AC. Bogus. All you need are changes in time for an AC signal to be present. The exact change in the time domain can be decomposed into a Fourier series that describes its harmonic series in the frequency domain. Thus, any minute change in level for either current or electric potential represents "AC." This is the real pedantic view of the situation.

So it doesn't bother you at all to see a scope displaying a signal that goes below baseline, but the LED is on 100 percent of the time anyway. OK, fine, you are clearly a pro, I respect that, and I'm not, so I'll not argue the point with words... just with demonstrations. See the above video. Using an LED simply because it unequivocally indicates Current Direction. It could just as easily be done with an ammeter. 

I have to admit that it chokes me to try to say "Alternating Current" when the current does not, in fact, alternate but only fluctuate.

All of this is distracting from the issue of Alek's misuse of AC coupling when making quantitative measurements involving the vertical values, probably including the phase angle measurements which the scope probably does on the zerocrossings not the peaks since the zeros are generally more precise.

TK,

Looking at the manual, the TRMS measurement has to be "engaged." Likely most people don't bother with it, since they generally want to know just the DC or AC level. I think it's safe to say he was only measuring the AC level on the secondary. As for the Tenma DMM across the primary I'm not too sure, but likely the same situation with the TRMS being a separate button that must be pressed.

I do agree with you about the DC-level as a possible issue, however, I will continue to reject the non sequiturs found in your explanations as to how AC stops being AC anymore. Bias, linearity and saturation effects of circuit elements are very important, that much I will unquestionably agree with, but lets not blow things out of proportion. DC on the primary would serve to make the transformer act like a diode, so it's likely not present.

As for your point about the LED... If the current fluctuates there's AC, and current absolutely does not need to change direction for this to happen. The lowering and raising of the DC current amplitude is all that is required. This is because the vector sum of the two signals is what you observe: if the DC level is greater than the P-P excursions of the AC signal, current will never change direction. So you still get your alternating current cake and can eat it too... Your real complaint is that the DC bias has shifted the sensitivity of the transducers output (light emitted vs current input) to a nonlinear operating point, thus changes in light appear non existent. But are in fact present, just at minute levels undetectable to the human eye.

You may want to re-evaluate your personal definition of AC with that used by electrical engineering.

Grr. Of course I know what you mean. This is my own personal bugbear and I can come up with demonstration after demonstration in support of my position, but of course I agree with your statement of the standard position. I do not agree that the standard position uses the words properly, whatever it might actually mean, and I am quite certain that there is a lot of confusion caused in the unwashed masses because of it. For example we will encounter people here who believe that AC coupling is for measuring AC signals, according to your standard definition of AC, and DC coupling is for DC signals. Whatever they may be, because according to your def, any oscillating signal that can be displayed on the scope is AC, I think. So people will use AC coupling all the time. Even though it is almost never appropriate.

For example I invite you to measure my massively OU circuit below. You must keep your scope set to AC coupled, though. Read the current and the "AC" voltage, AC coupled, and then tell me why my brake light bulb is so darn bright.