Power Measurement Basics

[poynt99]

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.

[TinselKoala]

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!

[poynt99]

Thanks TK.

[MarkE]

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.

[poynt99]

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.

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.

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.

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).