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Author Topic: Power Measurement Basics  (Read 37115 times)

gotoluc

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Re: Power Measurement Basics
« Reply #30 on: January 21, 2014, 03:43:44 AM »
Quote from: MarkE on January 21, 2014, 03:22:57 AM
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.

Thanks MarkE,

still much to learn ;)

I was doing all my AC power calculation with AC Coupling and was told I must use DC coupling to get accurate math data. I was also told to leave probes to DC coupling as I would most never need to use AC coupling.
I hope this is not another debate issue ???

Hope to get all this straight soon

Thanks

Luc
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poynt99

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Re: Power Measurement Basics
« Reply #31 on: January 21, 2014, 03:49:02 AM »
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.
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gotoluc

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Re: Power Measurement Basics
« Reply #32 on: January 21, 2014, 03:52:03 AM »
Quote from: poynt99 on January 21, 2014, 03:31:04 AM
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.

Okay, I see.

Would be nice to see a demo of making a current probe out of a Hall sensor and how to correctly use it.

Maybe once you have your new videos you could start a new topic to keep it clean and use this topic for discussions.

Thanks

Luc
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poynt99

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Re: Power Measurement Basics
« Reply #33 on: January 21, 2014, 03:56:37 AM »
Luc,

Most Hall sensor probes are not capable of high frequency measurements, which is why there are AC only current probes. When you find a very expensive probe that goes from DC to say 100MHz, it most likely is accomplishing this through a combination of AC probe and DC hall sensor.

The two videos are up now. Please review them and let me know your thoughts. See reply#26 above for the new links.
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MarkE

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Re: Power Measurement Basics
« Reply #34 on: January 21, 2014, 04:19:15 AM »
Quote from: gotoluc on January 21, 2014, 03:43:44 AM
Thanks MarkE,

still much to learn ;)

I was doing all my AC power calculation with AC Coupling and was told I must use DC coupling to get accurate math data. I was also told to leave probes to DC coupling as I would most never need to use AC coupling.
I hope this is not another debate issue ???

Hope to get all this straight soon

Thanks

Luc
Gotoluc you are doing the right thing by doing what you can to learn the subject.  Many people make the bad assumption that because they use an pricey piece of equipment that the measurements must be accurate when there are many ways to misapply even the best equipment and get terribly wrong answers.  If you pay attention to people like Poynt99 and tinselkoala you will do well.
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MarkE

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Re: Power Measurement Basics
« Reply #35 on: January 21, 2014, 04:22:56 AM »
Quote from: poynt99 on January 21, 2014, 03:49:02 AM
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.
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poynt99

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Re: Power Measurement Basics
« Reply #36 on: January 21, 2014, 04:46:56 AM »
Quote from: MarkE on January 21, 2014, 04:22:56 AM
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.
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poynt99

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Re: Power Measurement Basics
« Reply #37 on: January 21, 2014, 04:54:21 AM »
Quote from: MarkE on January 21, 2014, 04:22:56 AM
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.
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gotoluc

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Re: Power Measurement Basics
« Reply #38 on: January 21, 2014, 06:30:34 AM »
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
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MarkE

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Re: Power Measurement Basics
« Reply #39 on: January 21, 2014, 06:39:24 AM »
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.

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poynt99

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Re: Power Measurement Basics
« Reply #40 on: January 21, 2014, 03:46:13 PM »
Mark,

For the record, the corrected videos have the identical content as before. When I originally assembled the various parts, they ended up in reverse order or something, and I did not notice. So the correction was done to the order, but the content has not changed, i.e. I did not make any new videos. Once again I apologize for any confusion this may have caused.

In terms of your suggestions and comments, although they are appreciated and stand on their own to augment what I presented, they are quite beyond my original intention. I feel the main points were clearly made in what I presented and from Luc's comment above, I expect clearly understood as well. I do this on my own time, even though I have a couple other important design projects on the go.

In summary, the main points of the Part1 video are:

1) If ever in doubt how to place your measurement probes in a circuit, keep KVL in mind and place your probes accordingly. i.e. move from component to component without flipping the leads. This is more pertinent to DMM measurements because scopes most often limit us as to how we can apply our probe leads.

2) If you must flip one pair of leads when using a scope (as we do when measuring source power), just invert the current channel within the scope itself. If you do not, your phase information will be inverted.

3) Power sources compute to a negative power, while loads compute to a positive power, if measured correctly.

4) 1)-3) are only important if phase or polarity is pertinent to one's argument or claims.
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poynt99

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Re: Power Measurement Basics
« Reply #41 on: January 21, 2014, 03:54:24 PM »
Quote from: gotoluc on January 21, 2014, 06:30:34 AM
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

Thanks Luc.

I trust that you truly understand and agree with everything in the video? Don't hold back if you don't, because all future concepts will be based on this first video.
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MarkE

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Re: Power Measurement Basics
« Reply #42 on: January 21, 2014, 04:06:03 PM »
Quote from: poynt99 on January 21, 2014, 03:46:13 PM
Mark,

For the record, the corrected videos have the identical content as before. When I originally assembled the various parts, they ended up in reverse order or something, and I did not notice. So the correction was done to the order, but the content has not changed, i.e. I did not make any new videos. Once again I apologize for any confusion this may have caused.

In terms of your suggestions and comments, although they are appreciated and stand on their own to augment what I presented, they are quite beyond my original intention. I feel the main points were clearly made in what I presented and from Luc's comment above, I expect clearly understood as well. I do this on my own time, even though I have a couple other important design projects on the go.

In summary, the main points of the Part1 video are:

1) If ever in doubt how to place your measurement probes in a circuit, keep KVL in mind and place your probes accordingly. i.e. move from component to component without flipping the leads. This is more pertinent to DMM measurements because scopes most often limit us as to how we can apply our probe leads.
I agree, but strongly suggest that the initial probe orientation should always be consistent with:  expected current flow through the load towards the circuit common.  This avoids the potential confusion of multiple negations.
Quote

2) If you must flip one pair of leads when using a scope (as we do when measuring source power), just invert the current channel within the scope itself. If you do not, your phase information will be inverted.
This is a place where I think the message and the demonstration are at odds.  Instrument inversions should be applied to make the measured instrument polarity match the assigned voltage and current conventions.  In your example both voltage and current through the load followed negative conventions.  Both channels should be inverted in order to register load power correctly.  Had you assigned voltage and current going the other way, then neither channel should be inverted to register load power correctly.  If one follows these rules then circuit analysis according to KVL exactly matches instrument readings, and power readings always follow established sign convention of positive values corresponding to loads.
Quote

3) Power sources compute to a negative power, while loads compute to a positive power, if measured correctly.
I absolutely agree.
Quote

4) 1)-3) are only important if phase or polarity is pertinent to one's argument or claims.
Obtaining measurement data consistently with established conventions is essential to clear communication.  I personally do not consider the points discussed above as optional.
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gotoluc

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Re: Power Measurement Basics
« Reply #43 on: January 21, 2014, 05:01:05 PM »
Quote from: poynt99 on January 21, 2014, 03:54:24 PM
Thanks Luc.

I trust that you truly understand and agree with everything in the video? Don't hold back if you don't, because all future concepts will be based on this first video.

Since I have no previous training or background in EE I can understand the logic in the measurement practice you've presented. So I think we can move forward from here.

However, I'm noting your logic is affecting those with EE background and wondering how it will all transform?

Thanks for being very open

Luc
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MarkE

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Re: Power Measurement Basics
« Reply #44 on: January 21, 2014, 05:21:50 PM »
Quote from: gotoluc on January 21, 2014, 05:01:05 PM
Since I have no previous training or background in EE I can understand the logic in the measurement practice you've presented. So I think we can move forward from here.

However, I'm noting your logic is affecting those with EE background and wondering how it will all transform?

Thanks for being very open

Luc
GoToLuc Poynt99's engineering is first rate.  He absolutely knows his subject matter.  My entire concerns revolve around how to effectively get his message across.
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