Inspired by many folks here that wish to better understand some of the lesser-known points about power measurement in any circuit.
Part 1 deals with a simple DC circuit but the concepts apply to all power measurements.
This goes out to all those opposing the fact that to retain proper phase information when making power measurements with an oscilloscope, one channel of the oscilloscope must be electrically inverted (because we physically invert one channel). This video also illustrates the little known fact that power sources compute to a negative power, while devices that dissipate power compute to a positive power. These issues are of particular importance when overunity claims are being made that involve the measured polarity of the power source.
Part 1a: http://www.youtube.com/watch?v=wIbQUUp9S9o
Part 1b:
EDIT: Re-assembling the videos, as I messed up the order of several parts. Sorry. OK, half done.
Thumbs up! These are definitely "must watch" items.
Thank you for the effort and time you put into making these clear presentations.
I'm looking forward to the next chapters!
8)
Thanks TK.
My comments:
I think that the videos do a good job of handling Kirchhoff's voltage law. I also think that they do a good job of showing the essential importance of following consistent conventions through a circuit, for both the direction of current and the direction of EMFs.
My major dissents:
1) Power measurement convention is dissipated power. The videos follow a convention of measuring voltage drops across power dissipating devices as negative values and the voltage drop across the battery power source as a positive voltage. I think this potentially adds to the confusion that the audience may have that you are trying to address.
2) Voltage measurement convention across loads is from the circuit common to the powered load terminal. This is the opposite of what was shown in the video.
3) There are two options for measuring voltage and current with non-isolated, single-ended oscilloscope channels and a current sense resistor:
a) Using the circuit common as the instrument common. In this case the voltage drop across the CSR is included in the DUT voltage measurement introducing a small error. This is by far the more common measurement practice used in industry as it places the instrument commons at the circuit common. The general practice is to reduce the error term to the point that it is insignificant, and/or to compensate for it.
b) Using the junction of the DUT and the CSR as the instrument common. This eliminates the voltage drop error of a) but inverts the sense of the measured CSR voltage with respect to positive convention current that flows through the DUT. That inversion as noted in the video can be corrected by setting the oscilloscope to invert. This is a: clever, and accurate way to measure, but moves the instrument commons from the circuit common, making it an unusual, albeit useful practice.
I keep threatening to write a power measurements primer for Revolution-Green.
Quote from: MarkE on January 20, 2014, 10:53:55 AM
My major dissents:
1) Power measurement convention is dissipated power. The videos follow a convention of measuring voltage drops across power dissipating devices as negative values and the voltage drop across the battery power source as a positive voltage. I think this potentially adds to the confusion that the audience may have that you are trying to address.
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.
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.
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.
Quote
2) Voltage measurement convention across loads is from the circuit common to the powered load terminal. This is the opposite of what was shown in the video.
One can establish either 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. If one does not care about polarity (or require it) in their measurements, then of course this is all moot.
Quote
3) There are two options for measuring voltage and current with non-isolated, single-ended oscilloscope channels and a current sense resistor:
a) Using the circuit common as the instrument common. In this case the voltage drop across the CSR is included in the DUT voltage measurement introducing a small error. This is by far the more common measurement practice used in industry as it places the instrument commons at the circuit common. The general practice is to reduce the error term to the point that it is insignificant, and/or to compensate for it.
Agreed, and is my preferred method for measuring LOAD power, but not for measuring power supply power. For best accuracy measuring LOAD power, one can easily 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.
Quote
b) Using the junction of the DUT and the CSR as the instrument common. This eliminates the voltage drop error of a) but inverts the sense of the measured CSR voltage with respect to positive convention current that flows through the DUT. That inversion as noted in the video can be corrected by setting the oscilloscope to invert. This is a: clever, and accurate way to measure, but moves the instrument commons from the circuit common, making it an unusual, albeit useful practice.
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 diagram, the probes are commoned at the negative terminal of the battery. Again, the videos have not addressed oscilloscope power measurement of the LOAD yet, only the power source (battery in this case).
In your first diagram, how did you come to the conclusion that the battery power is -10W and the resistor is +10W?
Quote from: poynt99 on January 20, 2014, 11:31:45 AM
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.
Quote
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.
Quote
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.
Quote
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.
Quote
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.
Quote
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.
QuoteAgain, 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.
Quote from: MarkE on January 20, 2014, 11:57:43 AM
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.
Quote
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.
Quote
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.
Quote
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.
Quote
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.
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.
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.
Quote from: MarkE on January 20, 2014, 01:24:39 PM
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.
I think what you are trying to say is that in your work, you are never interested in measuring the input or source power; you are only interested in measuring the power dissipated in a circuit's loads, correct? I have never suggested that loads compute to a negative power, they don't. However, power sources DO, if measured correctly.
Quote
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.
I'm not sure I understand your point. Have I shown the reference between the CSR and load?
In your scenario above, you are measuring voltage directly across the voltage source, and yes due to the probe configuration your voltage and current traces will be in-phase. However, your probes are placed in a series-opposing configuration, which means that one channel signal is inverted wrt the other.
So since you are measuring directly across the voltage source, you are actually measuring the source power, not the load power (even though they are close to the same in magnitude). When you invert one channel in the scope to correct for the series-opposing probe configuration, your power will compute to be negative, as it should be for where you are measuring, i.e. source power.
In order to properly measure the 1 Ohm
load power, you must move your voltage probes either directly across the load resistor (differential probe), or from the source to the reference, which is the connection we had previously. Now your probes are either in series-adding (differential probe scenario), or they are in a "fixed reference configuration" whereby the CSR drop must be compensated for accuracy. In either of these last two cases, there is no need to invert one channel because the probes are physically "in phase".
How would you go about measuring the power in each of several loads that were in series? What if you were only interested in the total power being used, how would you measure that?
Keep in mind that in these forums, it is important that folks understand how to measure battery/source power and load power, because often the load they are interested in may not be the only component dissipating power in their circuit. Comparison between "input" and "output" power then becomes very relevant.
And there's this:
http://www.overunity.com/14220/power-measurement-basics/msg383962/#msg383962 (http://www.overunity.com/14220/power-measurement-basics/msg383962/#msg383962)
Quote from: TinselKoala on January 20, 2014, 01:25:44 PM
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.
I've attached an oscilloscope capture using a 100 Ohm resistor driven by a function generator and a Tektronix P6021 transformer current probe. I have diagrammed the set-up including with the marking orientation of the P6021. The P6021 output is in phase with the function generator voltage. This is consistent with current flow in the diagrams that both Poynt99 and I have posted. The issue is what convention to follow for the voltage. My experience is that the applied voltage across a load is always used. IE it is the voltage that with an in-phase current, results in power dissipation.
Quote from: poynt99 on January 20, 2014, 11:52:05 AM
In your first diagram, how did you come to the conclusion that the battery power is -10W and the resistor is +10W?
The battery
disssipates -10W. That is the same as saying that it
supplies +10W.
Quote from: MarkE on January 20, 2014, 02:53:47 PM
The battery disssipates -10W. That is the same as saying that it supplies +10W.
You've re-stated basically what the diagram is depicting, but how did you come to the conclusion that the battery dissipates -10W, or the resistor +10W?
Why did you not conclude that the battery was dissipating +10W and the resisitor -10W?
Quote from: poynt99 on January 20, 2014, 02:38:05 PM
I think what you are trying to say is that in your work, you are never interested in measuring the input or source power; you are only interested in measuring the power dissipated in a circuit's loads, correct? I have never suggested that loads compute to a negative power, they don't. However, power sources DO, if measured correctly.
I'm not sure I understand your point. Have I shown the reference between the CSR and load?
In your scenario above, you are measuring voltage directly across the voltage source, and yes due to the probe configuration your voltage and current traces will be in-phase. However, your probes are placed in a series-opposing configuration, which means that one channel signal is inverted wrt the other.
They do not oppose. Both probes are in phase. In order to oppose one must be CCW and the other CW from the reference node.
Quote
So since you are measuring directly across the voltage source, you are actually measuring the source power, not the load power (even though they are close to the same in magnitude). When you invert one channel in the scope to correct for the series-opposing probe configuration, your power will compute to be negative, as it should be for where you are measuring, i.e. source power.
You have declared that I am measuring across the source and then introduced loss that is not shown in the diagram. As the diagram is shown there is no distinction between the voltage across the source and the load. If we introduce a few milliOhms of wiring resistance, then my probes would be on the load side of that wiring resistance. The magnitudes would then change. The signs would remain the same.
The issue entirely revolves around whether we declare positive power that which the source supplies, in which case dissipated power is negative, or do we declare positive power that which the loads dissipate, in which case sources "dissipate" negative power. Industry convention is the latter. Power meters indicate positive power as the power into the load. No one refers to the power that a light bulb or any other kind of load dissipates as negative. If we were to split the 1 Ohm load into two 0.5 Ohm resistors and probe the junction, we would note basically half the source as the
positive voltage drop across the bottom 0.5 Ohm resistor. And we would still report the same current.
Quote
In order to properly measure the 1 Ohm load power, you must move your voltage probes either directly across the load resistor (differential probe), or from the source to the reference, which is the connection we had previously.
You are mixing up multiple issues. The first is what is the convention for positive versus negative power. The second issue is one of instrumentation options of which there are many for anyone with enough budget, and fewer for those who don't. It is critical that the convention is settled before worrying about small instrumentation errors. The convention issue does not change whether we instrument with some piece of junk +/-20% accurate instrument or something good to six digits that is fully isolated and has an infinite CMRR.
QuoteNow your probes are either in series-adding (differential probe scenario), or they are in a "fixed reference configuration" whereby the CSR drop must be compensated for accuracy.
I described where the probes are. They both use the same node 0 reference. Yes, that introduces a miniscule voltage magnitude error if left uncorrected, but no it has nothing, absolutely nothing to do with the selected power convention.
QuoteIn either of these last two cases, there is no need to invert one channel because the probes are physically "in phase".
How would you go about measuring the power in each of several loads that were in series? What if you were only interested in the total power being used, how would you measure that?
Dividing a branch into series pieces does not change the convention or methods.
Quote
Keep in mind that in these forums, it is important that folks understand how to measure battery/source power and load power, because often the load they are interested in may not be the only component dissipating power in their circuit. Comparison between "input" and "output" power then becomes very relevant.
Which is the reason that I object to your choice of positive power as that supplied by a source when the common convention for positive power is the quantity dissipated by loads. If one is intent on educating folks, which is a good thing, teaching them to go against accepted conventions is a recipe for confusion and dissent.
Quote
And there's this:
http://www.overunity.com/14220/power-measurement-basics/msg383962/#msg383962 (http://www.overunity.com/14220/power-measurement-basics/msg383962/#msg383962)
Quote from: poynt99 on January 20, 2014, 02:59:44 PM
You've re-stated basically what the diagram is depicting, but how did you come to the conclusion that the battery dissipates -10W, or the resistor +10W?
Why did you not conclude that the battery was dissipating +10W and the resisitor -10W?
There are two possible conventions. Either works mathematically provided one applies it consistently. I chose the convention used throughout industry and academia.
Quote from: MarkE on January 20, 2014, 03:51:10 PM
There are two possible conventions. Either works mathematically provided one applies it consistently. I chose the convention used throughout industry and academia.
Would you mind showing your calculations?
Quote from: MarkE on January 20, 2014, 03:42:19 PM
They do not oppose. Both probes are in phase. In order to oppose one must be CCW and the other CW from the reference node.
Please show diagrammatically what you mean.
Quote from: MarkE on January 20, 2014, 03:42:19 PM
Which is the reason that I object to your choice of positive power as that supplied by a source when the common convention for positive power is the quantity dissipated by loads.
:o
Mark, please quote me where I stated that a measured source power computes to a positive value.
Quote
If one is intent on educating folks, which is a good thing, teaching them to go against accepted conventions is a recipe for confusion and dissent.
Agreed, which is why I'm continuing with this discussion.
Quote from: poynt99 on January 20, 2014, 04:40:00 PM
Would you mind showing your calculations?
Take the test case I offered driving a 1 Ohm load with a series 1 mOhm CSR. Use the diagram of that I posted. We are assuming lossless wiring and no hidden circuit elements:
Label the nodes: Source negative terminal / CSR bottom Node 0
CSR / load Node 1
Source positive terminal / load top Node 2
Vload = V2 - V1
Vcsr = V1
Iloop = V2/(Rload + Rcsr)
Pload = (V2^2 - V2*V1) / (Rload + Rcsr)
Since Rcsr << Rload, then Rload ~= Rload + Rcsr and V2 ~= V2 - V1, then Pload ~= V2*V1/Rcsr
Quote from: poynt99 on January 20, 2014, 04:51:47 PM
:o
Mark, please quote me where I stated that a measured source power computes to a positive value.
Agreed, which is why I'm continuing with this discussion.
That falls out of a consequence of your procedure that amounts to measuring the voltage rail and the current through the load as your source power. As I have read your posts you seem quite adamant that you are measuring the source power with that method. For the positive polarity power supply, both the voltage measured and the positive convention current are positive. Therefore their product: the measured power is positive.
Quote from: poynt99 on January 20, 2014, 04:42:52 PM
Please show diagrammatically what you mean.
I already have. See once again the diagram on the left, and the diagram on the right. On the left, moving from node zero to either measurement point is CCW. On the right, moving from node zero to the voltage measurement point is CCW and the proxy for the current measurement, the CSR voltage the direction is CW. A common loop current circulating in either direction develops voltage measurements of opposing signs on the two channels.
The diagram on the left is the accepted and vastly predominant method of measuring voltage and currents in a circuit. If one models a non-contact current sensor, a transformer primary replaces the CSR. The transformer imposes a small series voltage burden to AC just like the CSR does at all frequencies.
I just realized that the order of the individual parts of my video are all messed up. I will have to re-assemble and upload them again.
Quote from: MarkE on January 20, 2014, 05:00:02 PM
Take the test case I offered driving a 1 Ohm load with a series 1 mOhm CSR. Use the diagram of that I posted. We are assuming lossless wiring and no hidden circuit elements:
Label the nodes: Source negative terminal / CSR bottom Node 0
CSR / load Node 1
Source positive terminal / load top Node 2
Vload = V2 - V1
Vcsr = V1
Iloop = V2/(Rload + Rcsr)
Pload = (V2^2 - V2*V1) / (Rload + Rcsr)
Since Rcsr << Rload, then Rload ~= Rload + Rcsr and V2 ~= V2 - V1, then Pload ~= V2*V1/Rcsr
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?
Quote from: MarkE on January 20, 2014, 05:26:48 PM
That falls out of a consequence of your procedure that amounts to measuring the voltage rail and the current through the load as your source power. As I have read your posts you seem quite adamant that you are measuring the source power with that method. For the positive polarity power supply, both the voltage measured and the positive convention current are positive. Therefore their product: the measured power is positive.
Strictly speaking yes, if you are measuring directly across the battery, you are measuring battery power. If you are measuring directly across the load resistor, you are measuring the load resistor power.
We can "cheat" however to obtain the load power without measuring directly across it, as has been outlined already here. It is a "cheat" because the two voltage measurements are effectively in parallel or pseudo-difference mode. As such, this only works for measuring the load power.
The problem here as I see it is perception. I see the circuit as one continuous loop and you seem to see the circuit as two half loops, one CW and one CCW, using the reference point as, well, a reference. To be honest, I do not understand your explanation regarding the relevance ofCW and CCW in your above post. There is also the issue of performing a measurement a certain way because we have no choice, and a certain other way because that is the ideal way.
At any rate, no I do not ever state nor imply that loads compute to a negative power. In fact I state quite clearly in the video that
sources always compute to a negative value.
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 :P
Looking forward to the show 8)
Luc
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.
Quote from: poynt99 on January 20, 2014, 07:54:17 PM
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.
Quote from: gotoluc on January 20, 2014, 09:06:51 PM
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 :P
Looking forward to the show 8)
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.
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.
Quote from: MarkE on January 20, 2014, 09:22:57 PM
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
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.
Quote from: poynt99 on January 20, 2014, 09:31:04 PM
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
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.
Quote from: gotoluc on January 20, 2014, 09:43:44 PM
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.
Quote from: poynt99 on January 20, 2014, 09:49:02 PM
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.
Quote from: MarkE on January 20, 2014, 10:22:56 PM
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.
Quote from: MarkE on January 20, 2014, 10:22:56 PM
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.
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
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.
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.
Quote from: gotoluc on January 21, 2014, 12:30:34 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.
Quote from: poynt99 on January 21, 2014, 09:46:13 AM
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.
Quote from: poynt99 on January 21, 2014, 09:54:24 AM
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
Quote from: gotoluc on January 21, 2014, 11:01:05 AM
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.
Quote from: gotoluc on January 21, 2014, 11:01:05 AM
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.
Good. I think you have a solid enough foundation to decide if a process or the logic of one makes any good sense or not. These videos are aimed more or less at a beginner level of electronics, so they will not go into the degree of detail that others might feel is necessary.
I've been around these forums for quite some time as you know, so I feel I have a good sense for what points need to be stressed in order that important concepts are understood. And for me that is the ultimate goal of these videos, to understand the concepts and be able to apply them in your testing.
Quote
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
There is often more than one way to accomplish a task, as you well know. And often you will find different parties unable to agree upon the "best" or "most standardized" way of performing some task, such as power measurement. Such is the case here with Mark and I.
Again, I have the advantage of being involved in these forums for years, and as such I feel I can get to the root of the problem quickly and efficiently without overloading anyone's learning process with minor, relatively insignificant details.
The bottom line is this: Do we measure a voltage source with the probe's gnd lead on the source's +'ve or -'ve terminal? I have never seen anyone on these forums measure a battery, DC lab supply, or AC source for that matter with the probe's gnd lead on the positive or hot terminal, have you? And would you ever do that? No, I doubt it. I know I would only in some rare case.
So that decision in itself establishes your approach and procedures that follow. And unless someone can convincingly explain why this approach is flawed, I shall continue along these lines with future videos. That is; Pos to Pos, and Neg to Neg wrt to sources.
Quote from: MarkE on January 21, 2014, 11:21:50 AM
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.
Thanks MarkE for making that point clear
Luc
The issue of the proper use of the scope's AC coupled input function has come up before and apparently it is still confusing.
Maybe it would help to know how the scope accomplishes the AC coupled input. Every scope I've seen does this simply by switching in a capacitor in series with the probe input lead, immediately where it enters the scope's pre-amp/attenuator circuit. Old analog scopes like the Tek RM503 use a manual slide switch and a capacitor. Newer DSOs use a software-controlled relay.... and a capacitor. You might be able to hear the relay clicking, when you select AC coupled in the software on a modern DSO; I can hear it easily in the older Link DSO (when the relay doesn't stick, that is.)
The capacitor will block DC and very slow AC oscillations (like below 5 Hz or whatever, determined by the coupling capacitor value) and will only allow the AC component of any signal to pass to the scope's electronics. This has the effect of bringing the _average_ value of the input signal, to the scope channel's "zero voltage reference" line. This could be either a raising up or a lowering down of the peak voltage values, depending on the value and sign of the blocked DC component. This is why the math is screwed up when AC coupled is selected.
AC coupling capacitors and selection switches in the Tek RM503 precision low-frequency analog oscilloscope:
Quote from: TinselKoala on January 21, 2014, 12:48:38 PM
The issue of the proper use of the scope's AC coupled input function has come up before and apparently it is still confusing.
Maybe it would help to know how the scope accomplishes the AC coupled input. Every scope I've seen does this simply by switching in a capacitor in series with the probe input lead, immediately where it enters the scope's pre-amp/attenuator circuit. Old analog scopes like the Tek RM503 use a manual slide switch and a capacitor. Newer DSOs use a software-controlled relay.... and a capacitor. You might be able to hear the relay clicking, when you select AC coupled in the software on a modern DSO; I can hear it easily in the older Link DSO (when the relay doesn't stick, that is.)
The capacitor will block DC and very slow AC oscillations (like below 5 Hz or whatever, determined by the coupling capacitor value) and will only allow the AC component of any signal to pass to the scope's electronics. This has the effect of bringing the _average_ value of the input signal, to the scope channel's "zero voltage reference" line. This could be either a raising up or a lowering down of the peak voltage values, depending on the value and sign of the blocked DC component. This is why the math is screwed up when AC coupled is selected.
AC coupling capacitors and selection switches in the Tek RM503 precision low-frequency analog oscilloscope:
WOW TK!... you are one dedicated man to do all that work on the picture just for this post.
Thank you
Luc
Thanks for opening up the wonderful antique for us to take a peek TK.
Is SP3T as great and dangerous a secret as DPDT?
Aw shucks fellas it were nothing. (I already had the photo, from work done on the scope last year, all I had to do was add the captions.)
I've learned a lot from that old scope. I brought it back from the dead, twice, and calibrated it fully according to the service manual. It has that beautiful blue persistent phosphor CRT which looks really neat with the orange graticule illumination. Keeps the room warm in winter, too.
Anyhow... carry on please, don't mind me.
Quote from: MarkE on January 21, 2014, 07:34:53 PM
Is SP3T as great and dangerous a secret as DPDT?
The really frightening thing is that the switches are DP3T. There is a whole gang of unused contacts, just waiting to cause mischief. The mind boggles!
DP3T may be entering areas of planet level security. Have you considered what might happen if the second set of contacts were used to select between: DC / AC / Gnd of the first set, and: DC / AC / Gnd + series inductor, and DC / AC / Gnd + parallel inductor? Have you considered what might happen if the right kind of cheese were to get involved?