Power Measurement Basics

[gotoluc]

Hi poynt,

just noticed this topic. Glad you're making these much needed video demo's

I've noticed MarkE scope shot has AC coupling selected on channel 2. Is this not an issue?

Thanks for all your work to make this happen. I'm sure over time you'll get much appreciation for doing this and in the long run you'll save time on explaining to different individuals like me

Looking forward to the show

Luc

[poynt99]

OK, the video is now assembled in the right order. It should make sense now. Here are the links to both parts.

Part 1a: http://www.youtube.com/watch?v=wIbQUUp9S9o
Part 1b: http://www.youtube.com/watch?v=OH9QYimSO7E

Sorry for the inconvenience.

[MarkE]

I have no problem with this method of measuring the load power, because it works. However, it is NOT in the purest theoretical sense the ideal way to perform it. We do it this way because it is practical for most of us.

What remains is an explanation as to why or how you can conclude that the source dissipation is negative. Do your calculations work out there?

This is getting extremely tiresome.  I have diagrammed, I have provided equations, I have explained, and explained and explained and there seems to be zero progress in our communications.  I suggest that you do any of the following:

1. Pick up a text on power engineering.
2. Search the topic of power sign convention in Google.
3. Look at any of the many, many, many references for electrical power convention

You will find that we determine:

In any circuit branch where the positive convention current flow is in the direction of more positive towards less positive voltage drop that circuit branch is absorbing power, and the sign convention for the measured power is positive.  In any circuit branch where the positive convention current flow is against the direction of more positive towards less positive voltage drop that the circuit branch is emitting power, and the convention for the measured power is negative.  IE:  Absorbed / dissipated power is by universally accepted convention positive power, and supplied power is by universally accepted convention negative power just as in the figure below that you keep questioning.

Surely you acknowledge that P = V*I including signs.
Surely from executing any of the above checks you will find that the industry and academic convention for power sign is positive for circuit branches that absorb power, IE are loads and is negative for circuit branches that supply power, IE are sources.
Surely you can observe from your videos that what you did was measure and record negative voltages across each of your loads as you moved your measurements around in the direction of current flow.
Surely you can observe from your videos that multiplying the voltages you measured across each of your loads against the currents yields negative power values.
Surely you can see that by doing that you are going against the power sign convention used by industry and academia.
Surely you can can see that when you finally worked your way around to where you had a probe on the circuit common, the probe that you had there was the red lead, when common practice is to place the black lead there.
Surely you can see that placing the black lead where your audience and 99.99% of the rest of the world places their black leads is going to cause confusion for your audience.
Surely you can see that this is not going to make it easier for people to understand the usefulness of your technique of moving the measurement common from the power source common to the junction of a current sense resistor and the low side of the circuit under test.
Surely, when you tell your audience that you have to invert the current sense scope channel when using your perfectly valid technique you can understand that they may very well continue to protest against that absolutely correct measure due to confusion you are aggravating.
Surely since your goal is to get your audience to understand how to accurately account for the direction of power flow in their circuits, particularly when that direction varies with time, you agree that it is important that you reduce rather than increase the confusion any of your audience may suffer.

I am totally at a loss as to why we are not communicating.  I know that you are a capable engineer.

[MarkE]

Hi poynt,

just noticed this topic. Glad you're making these much needed video demo's

I've noticed MarkE scope shot has AC coupling selected on channel 2. Is this not an issue?

Thanks for all your work to make this happen. I'm sure over time you'll get much appreciation for doing this and in the long run you'll save time on explaining to different individuals like me

Looking forward to the show

Luc

Gotoluc, no it is not an issue at all.  AC coupling does not invert phase.  AC coupling removes information from below the cut off frequency.  In this case the probe has a cut off frequency of about 350Hz and the oscilloscope has a cut off frequency of about 5Hz.  The source signal is a clean 10kHz sine wave and has virtually no energy below 10kHz.  If the signal had a DC offset in it, the AC coupling would have removed that offset, shifting the signal up or down towards an average voltage of zero.

[poynt99]

Luc,

The AC coupling shown there is no problem. Mark is using a real current probe. The P6021A is only good down to 120Hz, so it would not be suitable for your AC measurements.