Power Measurement Basics

[MarkE]

Mark we are communicating fine. It appears however that you have missed an important part of the video.

I would suggest now that my video parts are compiled in the correct order, you go and watch them again.

You will see that in the beginning of the Part1b video, not only do I clearly state and show that the battery power is negative, but I show that I am multiplying the negative load voltages by a negative loop current of -2.7mA.

For example, I clearly state in the video: " -2.7mA times our -5.43V, gives us a positive 14.7mW".

For your quick reference, here is a screen shot of that part of the video, clearly showing the negative wattage on the battery, and the positive wattages on the 3 loads.

I will go back and view them now that you have updated them.  The good news is that the power convention is positive for things absorbing energy.  I think you are still swimming up hill calling either the voltages or current in that direction negative.  Anyone who hooks up a meter to the negative side of the battery and uses the 100 Ohm resistor as the current sense is going to read a positive voltage and wonder what you are thinking.

[poynt99]

I think you are still swimming up hill calling either the voltages or current in that direction negative.  Anyone who hooks up a meter to the negative side of the battery and uses the 100 Ohm resistor as the current sense is going to read a positive voltage and wonder what you are thinking.

That's one way of looking at it Mark.

Here is another:

As you move your + probe from TP to TP starting at bat+ and going in a CW direction (gnd lead is on bat-), is the voltage climbing or falling in magnitude? I would suggest that it is falling. So CW the voltage is dropping. If we were to move the meter probes across the 2k, maintaining the probe orientation (- +), we would see a negative voltage, indicating to us that the right side of the resistor is at a lower potential than the left side. In other words, the right side is dropping wrt the left side.

You may be correct, it might be confusing to some, even though there is absolutely nothing wrong with the math or the method . Let's see what Luc has to say.

The alternative of course is that we simply place our battery voltage probes backwards + to -, and - to + so we can obtain positive voltage drops across the loads. I have also proposed and used this convention in the past. But unless your circuit is really isolated, you can get into problems. That's why the other convention is preferred.

[poynt99]

The good news is that the power convention is positive for things absorbing energy.

It always has been.

Thank you for the confirmation though. This is one other concept that has met with fierce opposition around these forums.

[gotoluc]

Hi poynt,

both videos are clear, simple to understand and it makes sense the way you present it.

It's a good idea to split them up like you did.

Looking forward to the Oscilloscope videos... I guess you'll start with continuous DC circuits, then pulsed DC circuits and work your way up to AC circuits with phase shifts.

Thanks for the great job

Luc

[MarkE]

Poynt99, the videos are much improved.  Unfortunately, the second video skipped a couple of critical steps from 5:00 in and on.   We agree that power measurements better come out positive for loads, and they had better come out negative for sources.  What you did was without saying as much arrange to measure your power source.  And you also failed to mention the resulting measurement would be negative as convention dictates.  This is really going to mess with your audience. 

First, I think that you absolutely must to add to the video to show load power measurement, and you need to show that coming out positive.  That would require inverting both scope channels to be consistent with the negative voltage drops, and negative current that you demonstrated earlier.

Second, you need to explicitly point out that by placing the reference lead of Chl 1 on the battery cathode and the probe on the anode that you are reproducing the meter measurement of the battery voltage drop following the CW convention you chose, and therefore the power that is measured by inverting only Chl 2 is the power "drawn" by the battery which is negative, confirming that it is a source of power rather than something that is absorbing / dissipating power.

I strongly recommend that you also repeat the entire procedure going CCW.  This would serve several purposes.  First, it would demonstrate that from a power transfer standpoint the polarity of voltage and current do not matter as long as they are measured consistently.  It is the relative direction of current versus voltage that determines whether a circuit branch is a source or a load.  Second, it would show that without inversion, and using connections as people are used to seeing them that the loop current is positive and the voltage dropped across the load is positive yielding positive power dissipation in the load in accordance with convention.  Lastly, it would show that to get the source power, it is the voltage channel: Chl 1 that should be inverted, when following otherwise common measurement convention.

If you do that then I think you will be well positioned to deal with explaining the trade-offs between using the circuit common as the instrument common for power measurements, versus moving the instrument common and then inverting the current sense channel to adjust for the resulting inversion in sensed current polarity.