Power Measurement Basics

[poynt99]

In your first diagram, how did you come to the conclusion that the battery power is -10W and the resistor is +10W?

[MarkE]

If "dissipated" power is the only terminology we are permitted to use, then power sources have a "negative" dissipation. This in no way contradicts what I have illustrated in the videos.

Yes, you did a good job of showing that the sign of power from a source must be the opposite sign of power dissipated by a load. 

I disagree that it is confusing to illustrate "voltage drops" and "voltage gains" in a circuit. This is basic electrical theory that everyone should be familiar with, even OU enthusiasts.

I agree that it is an essential concept.  I think the video would have been more consistent had you chosen to follow a CCW convention rather than CW.  This would have shown positive voltage drop values across loads corresponding to positive power dissipation measurements.

The convention that I chose makes sense from the perspective that we "align" ourselves with the supply voltage, because most people are accustomed to placing the positive DMM lead on the positive terminal of the battery. It also makes sense because it is convention to talk of "voltage drops" across resistors and diodes etc., and a negative voltage measurement across them imo coincides perfectly with this phraseology.

This is a point where we differ.  When you measure the voltage across a circuit, do you place your instrument black lead on the circuit common or on the supply rail?  I am sure that you place the reference black lead on the circuit common like we all do.  So now when thinking about Kirchhoff there is a decision to make:  Is that voltage that you read on the voltage rail the voltage that is being supplied by source, or is it the total voltage that is being dropped by the loads?  Convention says that it is the voltage dropped by the loads.  Think about what you would say to a colleague:  "I have 10V across my circuit."  IE: "I have 10V drop across my loads."  The black reference lead is the lead that stays in place.  As we go around the loads in a circuit we observe 10V, 8V, 2V etc all with reference to the circuit common.  This is what I contend your audience has been doing, conforms with convention, and is what they understand.  The mental stumbling block is that the same positive rail voltage is for purposes of KVL a negative voltage drop from the reference to the supply rail.

One can establish any one of two conventions, as long as they stick with that convention throughout the measurement process (i.e. no flipping of the measurement leads is permitted, unless it is re-inverted inside the scope). I chose the convention which is established by how we would normally measure the power source; red lead on positive, black lead on negative.

We absolutely agree on this.  Where we disagree is which of the two conventions is the more common and familiar.  I've made my argument on that point.

Agreed, and is my preferred method for measuring LOAD power, but not for measuring power supply power. For best accuracy, one can compute/subtract the power in the CSR. For power supply power, your method b) is my choice. The video only addressed measuring the power supply (battery) power with the scope, not the LOAD power.

I don't think that point of view came out in the video.  I think it is important that you make that clear.

If one terminal of the power source IS the circuit common (and it often is), then we are not moving the instrument common from the circuit common. As illustrated in the diagrams, the probes are commoned at the negative terminal of the battery.

Right, so think about this whether you are using a meter or a scope: the CW convention that you followed demonstrating voltage drops according to KVL eventually placed the instrument red lead on the circuit common while looking at the loads.  But, what you have said is that you wanted to look at this from the standpoint of the power source.  I submit that for that purpose you should have used a stack of batteries and measured those individual drops, and not gone around the loads.  There is ultimately no mathematical difference in magnitude whether you choose positive voltage drop for dissipation or source.  There is a difference in sign.  If everyone rigorously follows one convention the opportunity for confusion as to whether a particular circuit branch is dissipating or sourcing power is minimized.

Again, the videos have not addressed oscilloscope power measurement of the LOAD yet, only the power source (battery in this case).

I appreciate the time and effort that goes into making a video.  It's obvious to me that you know your engineering.  My comments are directed at helping you to convey your intent in a way that your intended audience is likely to understand.

[poynt99]

I think the video would have been more consistent had you chosen to follow a CCW convention rather than CW. This would have shown positive voltage drop values across loads corresponding to positive power dissipation measurements.

Yes, perhaps you and I would start with the 2k resistor to see what the voltage drop is, and knowing that it was connected to the positive terminal of the battery, we would place the red lead first, then the black lead second, which would result in a + voltage drop across it.

But we are not strictly trying to measure voltage drops here, we are trying to perform power measurements, and a convention to do so must therefore be established. For the battery power measurement, the scope probe is placed across the battery source with the probe tip on the + battery terminal, not the opposite as you seem to be suggesting. Since we have established the orientation of the probes by how the voltage probe is placed across the battery, we must follow this same convention while placing the current probe across the CSR, otherwise the phase of the power computation will be off by 180º. Since we can not actually place the CSR probe correctly due to the common gnd issue, we must physically invert the CSR probe so that the gnd leads of both scope channels are tied together. Now to correct for this physical CSR probe inversion, we simply invert the CSR channel electrically in the scope. Relative phase between the voltage and the current is now restored.

This is a point where we differ.  When you measure the voltage across a circuit, do you place your instrument black lead on the circuit common or on the supply rail?  I am sure that you place the reference black lead on the circuit common like we all do.  So now when thinking about Kirchhoff there is a decision to make:  Is that voltage that you read on the voltage rail the voltage that is being supplied by source, or is it the total voltage that is being dropped by the loads?  Convention says that it is the voltage dropped by the loads.  Think about what you would say to a colleague:  "I have 10V across my circuit."  IE: "I have 10V drop across my loads."  The black reference lead is the lead that stays in place.  As we go around the loads in a circuit we observe 10V, 8V, 2V etc all with reference to the circuit common.  This is what I contend your audience has been doing, conforms with convention, and is what they understand.  The mental stumbling block is that the same positive rail voltage is for purposes of KVL a negative voltage drop from the reference to the supply rail.We absolutely agree on this.

I think you might be missing the point and confusing the issue Mark.

We are not trying to measure nodes in the circuit relative to any other node. We are trying to measure the voltage across each individual component.

Right, so think about this whether you are using a meter or a scope: the CW convention that you followed demonstrating voltage drops according to KVL eventually placed the instrument red lead on the circuit common while looking at the loads.  But, what you have said is that you wanted to look at this from the standpoint of the power source.  I submit that for that purpose you should have used a stack of batteries and measured those individual drops, and not gone around the loads.

Not sure what point you are trying to make here.

There is ultimately no mathematical difference in magnitude whether you choose positive voltage drop for dissipation or source.  There is a difference in sign.  If everyone rigorously follows one convention the opportunity for confusion as to whether a particular circuit branch is dissipating or sourcing power is minimized.

Indeed, one convention should be followed. I presented the convention that most of us here already use, i.e. oscilloscope voltage probe tip on + battery terminal, and gnd lead at circuit common. CSR probe tip on far side of CSR, and CSR probe gnd lead at circuit common. The only caveat with this, and one that I am trying to emphasize, is that the CSR signal is now 180º out of phase and needs to be corrected by inverting the channel in the scope. Of course all this is only important IF phase (i.e. polarity) is pertinent to someone's argument or claim.

I appreciate the time and effort that goes into making a video.  It's obvious to me that you know your engineering.  My comments are directed at helping you to convey your intent in a way that your intended audience is likely to understand.

Your comments are appreciated Mark. Hopefully this discussion will help get the points across better.

[MarkE]

Poynt99 we are in violent agreement that following a consistent convention is essential.  The convention that I have always seen with respect to power is with reference to the loads.  I have never seen a load referred to as a negative power source.

To establish what is needed to insure we don't mess up the phasing, take for example a sinusoidal source with an offset voltage capability.  Our load will be a simple 1 ohm resistor, and we will place in series a 1 mOhm current sense.  The current sense is low-side and we assign node 0, and the connection points for all of our instrument references to the low side of the power source.  Channel 1 measures voltage from reference to the top of the circuit where the source connects to the load.  Channel 2 measures voltage from the reference to the junction of the CSR and the load.

Set the source to 2V p-p with zero offset.
Channel 1 shows 2V p-p in phase with the power source.
Channel 2 shows ~2mVp-p in phase with the power source.
Measured power magnitude at each peak is:  2mV * 1000A/V * 2V = 4W, and -2mV * 1000A/V * -2V = 4W.

Offset the source positive by one Volt:
Channel 1 swings from -1V to +3V in phase with the power source.
Channel 2 swings from -1mV to +3mV in phase with the power source.
Measured power magnitude at each peak is:  3mV * 1000A/V * 3V = 9W, and -1mV * 1000A/V * -1V = 1W.

Offset the source negative by one Volt:
Channel 1 swings from -3V to +1V in phase with the power source.
Channel 2 swings from -3mV to +1mV in phase with the power source.
Measured power magnitude at each peak is:  1mV * 1000A/V * 1V = 1W, and -3mV * 1000A/V * -3V = 9W.

Where you have to watch yourself is when you use the trick of moving the reference node so that it is at the junction of the CSR and the load, rather than the CSR and the power source.

[TinselKoala]

Yes, absolutely.

I hope there will be some time for questions and answers.

For example, a proper Current Probe (Hall effect - transformer type) matched to the oscilloscope clips around a circuit wire at the measurement point and usually needs no ground reference connection to the circuit. The probe body is generally marked with the correct orientation wrt conventional current flow. How does the signal from a probe like this compare in polarity/phase with a reading of voltage drop from an inline CSR at the same location?

Another "poynt" or demonstration that might be nice would be an explanation of the use of differential voltage probes in situations like this one, and also how two passive probes can be used in place of one differential probe to measure signals between arbitrary points in a circuit.