measuring inductance with a meat-and-potatoes DMM

[Joe Gwinn]

Originally Posted by Mike Sulzer

For a humbucker coil, these effects are large, not small. That is why I have taken the trouble to put together a system that collects enough information so that one can model them. The evidence is in the earlier thread.

Ahh, well, make the measurement. Only measurement will allow us to sort this out.

[Mike Sulzer]

Originally Posted by Joe Gwinn

Ahh, well, make the measurement. Only measurement will allow us to sort this out.

Rather than make a measurement, let's analyze what happens when one first measures the impedance of a coil with some series resistance with the Extech, and then adds some parallel resistance, that is, loss from eddy currents induced in some metal.

The ac resistance R and the inductance L are found from a measurement of the impedance magnitude Z and phase angle phi by these equations:

R = Z/(sqrt(1 + tan(phi)))

and

L = Z/(2*pi*(sqrt((tan^2(phi))/(1 + tan^2(phi))))).

Let us assume that we are measuring at 1 KHz where the inductive reactance is significantly greater than the series resistance in a typical pickup coil. Adding the resistance in parallel with the coil lowers both Z and phi. This lowers the apparent reading of L using the equation above. This is totally fallacious. The inductance has not changed, but the meter reading indicates that it has.

[Joe Gwinn]

Originally Posted by Mike Sulzer

Rather than make a measurement, let's analyze what happens when one first measures the impedance of a coil with some series resistance with the Extech, and then adds some parallel resistance, that is, loss from eddy currents induced in some metal.

The ac resistance R and the inductance L are found from a measurement of the impedance magnitude Z and phase angle phi by these equations:

R = Z/(sqrt(1 + tan(phi)))

and

L = Z/(2*pi*(sqrt((tan^2(phi))/(1 + tan^2(phi))))).

Let us assume that we are measuring at 1 KHz where the inductive reactance is significantly greater than the series resistance in a typical pickup coil. Adding the resistance in parallel with the coil lowers both Z and phi. This lowers the apparent reading of L using the equation above. This is totally fallacious. The inductance has not changed, but the meter reading indicates that it has.

The whole point of measuring is to allow one to sort the theory out, so more theory isn't the answer.

If you don't believe the Extech, resonate an air coil with a capacitor, and observe how the resonant frequency behaves as pieces of brass are brought near. A brass slug is standard way to tune an RF coil.

[Mike Sulzer]

Originally Posted by Joe Gwinn

The whole point of measuring is to allow one to sort the theory out, so more theory isn't the answer.

If you don't believe the Extech, resonate an air coil with a capacitor, and observe how the resonant frequency behaves as pieces of brass are brought near. A brass slug is standard way to tune an RF coil.

Why would that make me believe the Extech? Both these statements are true:
1. A piece of metal near a coil affects the inductance of the circuit.
2. Introducing a loss in a coil due to eddy currents is misinterpreted by the Extech in the situation I described above as a change in inductance.

Statement one cannot be used to disprove two.

Measurement and theory are used together to find a consistent and believable model of the physical situation.

[Joe Gwinn]

Originally Posted by Mike Sulzer

Why would that make me believe the Extech?

It won't. It's an independent way to measure, one that is simpler than an Extech, and known in full detail. One can also use a bridge.

More generally, one way to be sure that some effect is real is to measure it in multiple ways, and see how well the answers agree. In Science, this is called replication of experiments, or just replication.

Both these statements are true:
1. A piece of metal near a coil affects the inductance of the circuit.
2. Introducing a loss in a coil due to eddy currents is misinterpreted by the Extech in the situation I described above as a change in inductance.

Statement one cannot be used to disprove two.

Item 2 is not correct. In fact, the Extech does distinguish added loss from changed inductance, as it measures inductance and AC resistance independently. The Extech directly measures the complex impedance of the component under test, yielding both real and imaginary values, and does not assume that either real or imaginary part is dominant.

As I mentioned before, the Extech gives exactly the same answer as a Maxwell-Wein bridge. Such bridges were the gold standard in impedance measurement for at least a century, until displaced by the rise of digital techniques.

Measurement and theory are used together to find a consistent and believable model of the physical situation.

Agree. All that's missing is the measurement.

[David Schwab]

Originally Posted by Joe Gwinn

In fact, the Extech does distinguish added loss from changed inductance, as it measures inductance and AC resistance independently.

Don't eddy currents increase the AC resistance? That would explain why eddy current loss increases with frequency, which is what (I think) you've been saying all along.

[Mike Sulzer]

Originally Posted by Joe Gwinn

Item 2 is not correct. In fact, the Extech does distinguish added loss from changed inductance, as it measures inductance and AC resistance independently. The Extech directly measures the complex impedance of the component under test, yielding both real and imaginary values, and does not assume that either real or imaginary part is dominant.

Any single measurement of of complex impedance, Extech, wien bridge, op amp circuit, or whatever, can give a correct measurement when the type of loss is known to be series or parallel, but not when it is an unknown mixture of both.

The post above with the equations shows this. If you measure a coil with series resistance, the Extech measures the amplitude and phase of the impedance (or the real and imaginary part, I do not know the details), and then uses a process, involving something like the equations above, to derive the inductance and resistance.

If we add a resistor in parallel with the coil under the conditions described above, the magnitude of the impedance (Z) drops, and the angle drops. The equation (that is correct for finding the inductance when there is only series resistance) incorrectly indicates a decrease in inductance when none has occurred. You can see that the inductance is proportional to Z in the equation. The angle (phi) response is more complicated, but if you examine it, you will see that it also results in a drop in L, although less of one under the stated conditions. Therefore, a drop inductance from the addition of a parallel resistance is indicated, and this is not correct.

Please notice that I am not saying that bringing a piece of metal close to a coil does not cause the inductance of the circuit to be different from the coil alone. I am only saying that you cannot accurately measure this with a single frequency measurement of complex impedance if you have to allow for series loss as well. And this is the case with a pickup coil.

[Joe Gwinn]

Originally Posted by David Schwab

Don't eddy currents increase the AC resistance?

Yes, they do.

That would explain why eddy current loss increases with frequency, which is what (I think) you've been saying all along.

No, the argument is if the inductance also changes (decreases to be specific).

[Joe Gwinn]

Originally Posted by Mike Sulzer

Any single measurement of of complex impedance, Extech, wien bridge, op amp circuit, or whatever, can give a correct measurement when the type of loss is known to be series or parallel, but not when it is an unknown mixture of both.

The measurement is correct in all these cases. It's the interpretation of the measurement that's in dispute.

The post above with the equations shows this. If you measure a coil with series resistance, the Extech measures the amplitude and phase of the impedance (or the real and imaginary part, I do not know the details), and then uses a process, involving something like the equations above, to derive the inductance and resistance.

The fundamental measurement is of complex impedance at the specified frequency. There is a button marked SER/PAL (drawing in manual is wrong) that allows the user to specify which of two equivalent circuits to assume while computing inductance or capacitance and AC resistance.


I found the circuit diagram of the Extech (under a different brand name) on the web. For the record, it measures in the following way:

The 1000 Hz (or 120 Hz) oscillator has two outputs, one at 0 degrees (I) and one at 90 degrees (Q) phase shift. The 0 degrees output is amplified and imposed on the unit under test (UUT), a pickup in this case. Both 0 and 90 degree outputs are fed to a pair of synchronous demodulators, as will be discussed later.

The voltage across the UUT is sensed by one opamp circuit, and the current through the UUT is sensed by another opamp circuit, yielding two voltage signals, one proportional to UUT voltage and the other to UUT current.

These two voltages are fed one at a time to to the pair of synchronous demodulators, yielding a pair of complex numbers.

The first complex number consists of the I (real) and Q (imaginary) values of the UUT voltage, and the second complex number consists of the I and Q values of the UUT current.

The complex impedance (I and Q) is the UUT voltage (I and Q) divided by the UUT current (I and Q).

If we add a resistor in parallel with the coil under the conditions described above, the magnitude of the impedance (Z) drops, and the angle drops. The equation (that is correct for finding the inductance when there is only series resistance) incorrectly indicates a decrease in inductance when none has occurred. You can see that the inductance is proportional to Z in the equation. The angle (phi) response is more complicated, but if you examine it, you will see that it also results in a drop in L, although less of one under the stated conditions. Therefore, a drop inductance from the addition of a parallel resistance is indicated, and this is not correct.

Here is the key misunderstanding. The inductance is not proportional to either the magnitude or to the argument (the angle) of the complex impedance Z, it is instead proportional to the imaginary (quadrature) component of Z alone.

The Extech and the Maxwell-Wein bridge measure the in-phase (follows the signal generator) and quadrature (leading or lagging by 90 degrees) components independently of one another, yielding two signed real numbers. This works over a very wide range of resistance (in phase) and inductance or capacitance (quadrature) in the equivalent circuit.

Most handheld LCR meters measure only the magnitude of Z, and so are useful only for inductors and capacitors with little parasitic resistance. This is why such LCR meters are useless for guitar pickups.

Whenever someone comes up with a possible LCR meter and asks if it will work with pickups, I suggest that they wire a relatively pure multi-henry inductor (such as a choke or audio transformer winding) in series with a 50K pot and try to measure the inductance of the series string while varying the pot setting. If the indicated inductance varies more than a percent or two, the LCR meter will prove useless for pickups. What is being tested is the ability of the meter to accurately and independently measure both I and Q components of the UUT's impedance.

Please notice that I am not saying that bringing a piece of metal close to a coil does not cause the inductance of the circuit to be different from the coil alone. I am only saying that you cannot accurately measure this with a single frequency measurement of complex impedance if you have to allow for series loss as well. And this is the case with a pickup coil.

Actually, as described above, one can measure at a single frequency the inductance or capacitance component while ignoring the resistance component, so long as the resistance doesn't totally swamp the reactance (due to capacitance or inductance).

The Extech will achieve full accuracy if the inductive reactance exceeds the AC resistance by a factor of at least two.

[Mike Sulzer]

Originally Posted by Joe Gwinn

The measurement is correct in all these cases. It's the interpretation of the measurement that's in dispute.

In the case of series and parallel resistance, a correct interpretation is not possible. You have misunderstood my explanation.

Originally Posted by Joe Gwinn

Here is the key misunderstanding.

There is no misunderstanding on my part. Those two equations are derived from the impedance, R + 2*pi*j*L. Why go to the trouble to put this complex equation in the form of two real equations for Z and tan(phi)? Two reasons:
1. More people are familiar with the concepts of amplitude and phase than with complex algebra.
2. Putting them is this form allows one to easily see what the effects are of violations of series only resistance.

In this case one sees that the apparent value of L is a function of two variables, the magnitude of Z, and the phase. Since putting a resistor in parallel with L decreases both variables, and reducing either reduces the apparent value of L, an upper bound on the change in L can be deduced from the change in magnitude only. Since this relationship is one of proportionality, it is a large effect, unless the resistance is significantly larger than the reactance, when the usual square root of the sum of squares makes it small.

Originally Posted by Joe Gwinn

Actually, as described above, one can measure at a single frequency the inductance or capacitance component while ignoring the resistance component, so long as the resistance doesn't totally swamp the reactance (due to capacitance or inductance).

Actually, if you are in the series mode and a parallel resistor is added with a resistance equal to the reactance of the inductor, the apparent (and totally wrong) change in the inductance will be at least just under 30%.

Originally Posted by Joe Gwinn

The Extech will achieve full accuracy if the inductive reactance exceeds the AC resistance by a factor of at least two.

Not in the series mode if there is a parallel resistor present.

[Mike Sulzer]

In summary:

A meter capable of measuring the complex impedance measures two numbers. Characterizing a circuit composed of an inductor either in series or parallel with a resistor requires two numbers. The equations describing these two element circuits can be solved using the complex impedance.

A circuit with an inductor and both series and parallel resistors is in general characterized by three numbers. Therefore it requires at least one number in addition to the two provided by a measurement of complex impedance at a single frequency.

There might be situations where two numbers provide an adequate description. It is up to the person claiming this in a specific case to demonstrate that it is true.

[Joe Gwinn]

Originally Posted by Mike Sulzer

In the case of series and parallel resistance, a correct interpretation is not possible. You have misunderstood my explanation.

OK. We seem to be in a semantic dispute.

There is no misunderstanding on my part. Those two equations are derived from the impedance, R + 2*pi*j*L. Why go to the trouble to put this complex equation in the form of two real equations for Z and tan(phi)? Two reasons:
1. More people are familiar with the concepts of amplitude and phase than with complex algebra.
2. Putting them is this form allows one to easily see what the effects are of violations of series only resistance.

In this case one sees that the apparent value of L is a function of two variables, the magnitude of Z, and the phase. Since putting a resistor in parallel with L decreases both variables, and reducing either reduces the apparent value of L, an upper bound on the change in L can be deduced from the change in magnitude only. Since this relationship is one of proportionality, it is a large effect, unless the resistance is significantly larger than the reactance, when the usual square root of the sum of squares makes it small.


Actually, if you are in the series mode and a parallel resistor is added with a resistance equal to the reactance of the inductor, the apparent (and totally wrong) change in the inductance will be at least just under 30%.


Not in the series mode if there is a parallel resistor present.

I think the core dispute is emerging. I will continue in your Summary posting.

[Joe Gwinn]

Originally Posted by Mike Sulzer

In summary:

A meter capable of measuring the complex impedance measures two numbers. Characterizing a circuit composed of an inductor either in series or parallel with a resistor requires two numbers. The equations describing these two element circuits can be solved using the complex impedance.

A circuit with an inductor and both series and parallel resistors is in general characterized by three numbers. Therefore it requires at least one number in addition to the two provided by a measurement of complex impedance at a single frequency.

Agree. If there are in fact three parameters to be found, two numbers won't do it. The claim was that the parallel resistance was very large, and so had negligible effect on measurement of inductance with series resistance.

There might be situations where two numbers provide an adequate description. It is up to the person claiming this in a specific case to demonstrate that it is true.

This brings us back to the original question, if there is a reduction of inductance seen when a piece of metal is brought near an air coil. The original claim was that the inductance does not change, but the eddy current loading causes the parallel resistance component to increase, thus fooling any instrument that measures a complex impedance at a single frequency, the Extech LCR Meter and the Maxwell-Wein bridge being two such instruments.

Actually, if the inductor is used in a circuit, unless the circuit is pretty fancy, it too will be fooled, so this may be a distinction without a difference.

Anyway, the question is if this parallel resistance effect is in fact real. I therefore propose the following thought experiment:

Perform the experiment with air coil and nearby metal, but with a twist: Use a piece of lead sheet at liquid helium temperature, so the lead becomes a superconductor. Also make the air coil wires of lead, so they also superconduct, and there is no resistance anywhere.

While there certainly are eddy currents in the sheet, there are no eddy-current losses to cause apparent resistances. Will the inductance rise, fall, or remain the same? Why?

[Mike Sulzer]

The original question is whether an inductance meter can be used to measure a pickup inductance. A specific question contained in the the overall question is: if I get different readings at 100 Hz and 1000 Hz, what does that mean?

The effects of eddy currents do need to be included. Remember the model described by these four figures:
http://www.naic.edu/~sulzer/hbAmplitude.png
http://www.naic.edu/~sulzer/hbPhase.png
http://www.naic.edu/~sulzer/hbAmpSD.png
http://www.naic.edu/~sulzer/hbPhaseSD.png
The differences between the measurement and the simple model were resolved by including a frequency-dependent eddy current resistor in series with an inductor to account for the leakage flux.

The errors in using the simple model (no eddy currents) are small at 1 KHz, but they are there. If you measure a lower inductance at 1 KHz than 100 Hz, does this represent some actual decrease in inductance? Not according to the model. No variable inductor needed to be used.

At higher frequencies, say 3Khz, it is clear that a single frequency inductance measurement would be significantly in error. One must measure across a range of frequencies and fit a model.

[Joe Gwinn]

Originally Posted by Mike Sulzer

The original question is whether an inductance meter can be used to measure a pickup inductance. A specific question contained in the the overall question is: if I get different readings at 100 Hz and 1000 Hz, what does that mean?

Well, certain LCR meters are used and work well with pickups. And you will get different readings at different frequencies. This is common experience.

The effects of eddy currents do need to be included. Remember the model described by these four figures:
http://www.naic.edu/~sulzer/hbAmplitude.png
http://www.naic.edu/~sulzer/hbPhase.png
http://www.naic.edu/~sulzer/hbAmpSD.png
http://www.naic.edu/~sulzer/hbPhaseSD.png
The differences between the measurement and the simple model were resolved by including a frequency-dependent eddy current resistor in series with an inductor to account for the leakage flux.

The errors in using the simple model (no eddy currents) are small at 1 KHz, but they are there. If you measure a lower inductance at 1 KHz than 100 Hz, does this represent some actual decrease in inductance? Not according to the model. No variable inductor needed to be used.

At higher frequencies, say 3Khz, it is clear that a single frequency inductance measurement would be significantly in error. One must measure across a range of frequencies and fit a model.

All very nice, but not an answer to the question asked. With superconducting metal, does the inductance rise, fall, or remain the same? And why?

[Mike Sulzer]

Originally Posted by Joe Gwinn

Well, certain LCR meters are used and work well with pickups. And you will get different readings at different frequencies. This is common experience.

Surely you are not saying that the fact that they are used means that they work well? Without any further justification?

The fact that you do get different readings at different frequencies does not imply that that is OK. Poor logic indeed, and completely wrong.

I think in my post yesterday, I was too easy on the Extech. To determine the errors at 1 KHz one should use the inductance in the good model and do the computation based on a series model using the measurement. I will try and get some time to do this later.

Originally Posted by Joe Gwinn

All very nice, but not an answer to the question asked. With superconducting metal, does the inductance rise, fall, or remain the same? And why?

The topic here is pickups, not superconducting inductors. If you wish to show that the one is relevant to the other, please do so. In fact, the answer to that question has little to do with what we are discussing here: it is the resistance that dominates. So eliminating all resistance might reveal a different effect? Your question is just a distraction.

[Joe Gwinn]

Originally Posted by Mike Sulzer

Surely you are not saying that the fact that they [LCR meters] are used means that they work well? Without any further justification?

No, that they are practical, in that their measurements of component parameters predict component behavior in actual circuits. If this were not true, such instruments would not be sold by the million.

The fact that you do get different readings at different frequencies does not imply that that is OK. Poor logic indeed, and completely wrong.

I think in my post yesterday, I was too easy on the Extech. To determine the errors at 1 KHz one should use the inductance in the good model and do the computation based on a series model using the measurement. I will try and get some time to do this later.

The Extech duplicates the measurements of the Maxwell-Wein impedance bridge, which was used for many decades by the national standards labs. They were all illogical and misguided? Who knew?

The topic here is pickups, not superconducting inductors. If you wish to show that the one is relevant to the other, please do so. In fact, the answer to that question has little to do with what we are discussing here: it is the resistance that dominates. So eliminating all resistance might reveal a different effect? Your question is just a distraction.

The claim was that the apparent variation in the inductance of pickups with frequency was not real, instead being caused by eddy current losses appearing as a parallel AC resistance that fooled all instruments that measure complex impedance at a single frequency.

I questioned the claim that eddy current losses in fact appear as a significant parallel resistance, and pointed out that if the conductors (wire and sheet) had zero resistance, we would have eddy currents but no eddy current losses, so it was not clear how eddy currents could cause such resistance increases, so those resistance changes cannot be the cause of the inductance change.


This cannot be a new problem, so I did a little research, and found the answer in the classic textbook, "Classical Electrodynamics", 2nd edition, J.D. Jackson, Wiley 1975. On page 298, the last sentence of the paragraph just below equation (7.77) is the answer (taken from a discussion of eddy currents and skin depth):

"One simple consequence [of eddy currents] is that the high-frequency inductance of circuit elements is somewhat smaller than the low-frequency inductance because of the expulsion of flux from the interior of the conductors."

Note that losses due to eddy currents are not mentioned as a cause, the inductance change being wholly due to alteration in the shape of the magnetic fields caused by flux expulsion.

More generally, it is not clear that any circuit consisting of a small collection of lumped perfect RCL components can adequately model such a distributed physical effect as flux exclusion.

[Mark Hammer]

Ironically, my interest in purchasing the meter (still haven't yet) wass basically to be able to identify unlabelled chokes and inductors cannibalized from other things, and also to measure wah inductors. I figured that since the pickup-maker community tends to make a big deal of inductance, this would be the logical place to ask about meters, but largely from an accuracy perspective (remember the 4% error thing?).

I hadn't realized it would spark that considerable - yet largely civilized - debate that it did. While I have much to learn about inductance measurement, I can tell you this: if you guys can't agree on how to "properly" measure inductance in a pickup, then the specs I see posted as part of the ad copy for a product are about as meaningful to me as the specs on a pair of $10 three-inch computer speakers, sold in Chinatown, that list them as "360W PMP".

Is it ultimately more reliable and informative to the user/buyer to simply list resonant peaks?

[Mike Sulzer]

Originally Posted by Joe Gwinn

No, that they are practical, in that their measurements of component parameters predict component behavior in actual circuits. If this were not true, such instruments would not be sold by the million.

This implies nothing about how well such instruments measure pickups. I am analyzing pickups, not other kinds of components.

Originally Posted by Joe Gwinn

The Extech duplicates the measurements of the Maxwell-Wein impedance bridge, which was used for many decades by the national standards labs. They were all illogical and misguided? Who knew?

I never implied that they were. Humbucker pickups is what I am discussing. You are using sarcasm to cover up the fact that you are constructing a straw man arugument. From Wikipedia:
"A straw man argument is an informal fallacy based on misrepresentation of an opponent's position.[1] To "attack a straw man" is to create the illusion of having refuted a proposition by substituting a superficially similar proposition (the "straw man"), and refuting it, without ever having actually refuted the original position.[1] [2]"

Originally Posted by Joe Gwinn

The claim was that the apparent variation in the inductance of pickups with frequency was not real, instead being caused by eddy current losses appearing as a parallel AC resistance that fooled all instruments that measure complex impedance at a single frequency.

Exactly. The measurements that I have presented here show just that.

Originally Posted by Joe Gwinn

I questioned the claim that eddy current losses in fact appear as a significant parallel resistance, and pointed out that if the conductors (wire and sheet) had zero resistance, we would have eddy currents but no eddy current losses, so it was not clear how eddy currents could cause such resistance increases, so those resistance changes cannot be the cause of the inductance change.

The fact that one can have eddy currents in a lossless situation does not show that, in a situation where there are losses, these losses cannot be responsible for some observed behavior. By the way, I am not saying that these eddy currents cause a resistance increase, but rather a decrease. Without the eddy current effect, the parallel resistance is very large. The parallel resistance becomes significant when it becomes smaller.

Originally Posted by Joe Gwinn

This cannot be a new problem, so I did a little research, and found the answer in the classic textbook, "Classical Electrodynamics", 2nd edition, J.D. Jackson, Wiley 1975. On page 298, the last sentence of the paragraph just below equation (7.77) is the answer (taken from a discussion of eddy currents and skin depth):

"One simple consequence [of eddy currents] is that the high-frequency inductance of circuit elements is somewhat smaller than the low-frequency inductance because of the expulsion of flux from the interior of the conductors."

Note that losses due to eddy currents are not mentioned as a cause, the inductance change being wholly due to alteration in the shape of the magnetic fields caused by flux expulsion.

That the losses due to eddy currents are responsible for the effect that Jackson discusses does not imply that losses due to eddy currents are not important in other situations. For example, a very important effect of eddy currents is heating of transformer cores, causing a waste of energy, and possible deterioration of the device. That is due to losses, not a change in inductance.

Originally Posted by Joe Gwinn

More generally, it is not clear that any circuit consisting of a small collection of lumped perfect RCL components can adequately model such a distributed physical effect as flux exclusion.

Right. But one shows that it is in a particular situation by making the measurements and constructing a model.

[Mike Sulzer]

Originally Posted by Mark Hammer

Ironically, my interest in purchasing the meter (still haven't yet) wass basically to be able to identify unlabelled chokes and inductors cannibalized from other things, and also to measure wah inductors. I figured that since the pickup-maker community tends to make a big deal of inductance, this would be the logical place to ask about meters, but largely from an accuracy perspective (remember the 4% error thing?).

I hadn't realized it would spark that considerable - yet largely civilized - debate that it did. While I have much to learn about inductance measurement, I can tell you this: if you guys can't agree on how to "properly" measure inductance in a pickup, then the specs I see posted as part of the ad copy for a product are about as meaningful to me as the specs on a pair of $10 three-inch computer speakers, sold in Chinatown, that list them as "360W PMP".

Is it ultimately more reliable and informative to the user/buyer to simply list resonant peaks?

If the resonant peaks (and widths) are specified with known values of external capacitance, typical of normal guitar cables, and with known resistances of the controls (for example on "10"), then this is very useful for indicating how the pickup will sound. It will not tell you all the details, of course. But yes, more useful that just stating the inductance, however measured.

[David Schwab]

Originally Posted by Mark Hammer

"360W PMP

PMP=Smoke coming out of box




I've always said that some things are far too complex to model adequately. For example, eddy currents produce their own magnetic field that opposes the field that created them. Surely that has to effect the sound from a pickup, but how would you measure or model such an effect?

[DrStrangelove]

Originally Posted by Mark Hammer

Ironically, my interest in purchasing the meter (still haven't yet) wass basically to be able to identify unlabelled chokes and inductors cannibalized from other things, and also to measure wah inductors.

I hadn't realized it would spark that considerable - yet largely civilized - debate that it did.

But, by God!, it doesn't have to be that way!

-The Immoderator

[Joe Gwinn]

Originally Posted by Mike Sulzer

This implies nothing about how well such instruments measure pickups. I am analyzing pickups, not other kinds of components.

I never implied that they were. Humbucker pickups is what I am discussing.

Humbuckers are not magic, and they don't have their own private laws of physics. Pickups are a form of low-Q inductor.

You are using sarcasm to cover up the fact that you are constructing a straw man argument.

Umm. I answered sarcasm with sarcasm.

The fact that one can have eddy currents in a lossless situation does not show that, in a situation where there are losses, these losses cannot be responsible for some observed behavior. By the way, I am not saying that these eddy currents cause a resistance increase, but rather a decrease. Without the eddy current effect, the parallel resistance is very large. The parallel resistance becomes significant when it becomes smaller.

The series AC resistance does increase quite significantly as a piece of metal is brought near. What controls the balance between series resistance increase and parallel resistance decrease?

That the losses due to eddy currents are responsible for the effect that Jackson discusses does not imply that losses due to eddy currents are not important in other situations. For example, a very important effect of eddy currents is heating of transformer cores, causing a waste of energy, and possible deterioration of the device. That is due to losses, not a change in inductance.

I never could see how the effects of energy loss would appear as a reactive effect, changing inductance or capacitance. Reactive effects are by definition lossless, resistive effects are by definition lossy, and they are independent of one another. In terms of complex impedances, the resistive (lossy) component is the real (in phase) part, while the reactive (inductive and capacitive) component is the imaginary (quadrature) part. What physical mechanism causes transformation of of a lossy effect into a lossless effect?

The quote from Professor Jackson supports exactly this point: flux exclusion changes the magnetic field, where energy is stored, and has no effect on losses. The losses are an independent consequence of the flow of eddy currents in materials with finite resistivity.

The fact that eddy currents cause heating is another example of the conservation of energy - the energy in those eddy currents has to go somewhere. Unless the metal has zero resistance. But this does not bear on the original question: do eddy currents cause the inductance to change?

Right. But one shows that it is in a particular situation by making the measurements and constructing a model.

Well at least we have many models.

The key question is if the parallel resistance varies enough to explain the observed change in inductance with eddy current loading.


Another datapoint. In the radio field, a standard catalog component is a RF coil with movable core made of of a non-ferrous good conductor like brass. As one screws the core deeper into the coil, the inductance is reduced with little reduction in Q. This has been standard practice for decades, and is discussed in many places, such as the ARRL Handbook. Googling on "rf coil brass core" will yield many hits. Inductors with brass cores have been fooling RF oscillators into changing frequency for years.

Interestingly enough, the NBS used a Maxwell bridge tomeasure inductance and resistance changes due to eddy currents: Eddy-current characterization of ... - Google Books

And here is another NIST report: http://ts.nist.gov/MeasurementServic...load/65C-3.pdf. Just above section 9, they mention that inductance varies with frequency due to eddy currents. This in the context of using and calibrating a high-precision Maxwell-Wien bridge. As I mentioned, such bridges were the gold standard for many years, and it seems unlikely that the folk at NIST would use instruments that are easily confused.

[Mike Sulzer]

Originally Posted by Joe Gwinn

The series AC resistance does increase quite significantly as a piece of metal is brought near. What controls the balance between series resistance increase and parallel resistance decrease?

I think you should write "When I use the Extech in the series mode, I measure an apparent increase in the series resistance when I bring a piece of metal near."

We discussed Terman's discussion of eddy currents some time ago. He pointed out that you can represent the effect of eddy currents as either a series or parallel effect. (The transformation equation applies in many situations.) The parallel representation gives the simpler frequency dependence because it is the one closer to a physical model of how mutual impedance effects operate.

To argue that these effects need to be represented by both series and parallel resistors because your meters shows an increase in series R is absurd.

We are making no progress here. This is my last post on this topic.

[Joe Gwinn]

Originally Posted by Mike Sulzer

I think you should write "When I use the Extech in the series mode, I measure an apparent increase in the series resistance when I bring a piece of metal near."

A tautology, but OK.

We discussed Terman's discussion of eddy currents some time ago. He pointed out that you can represent the effect of eddy currents as either a series or parallel effect. (The transformation equation applies in many situations.) The parallel representation gives the simpler frequency dependence because it is the one closer to a physical model of how mutual impedance effects operate.

I had forgotten that reference. Here is the actual posting: Someone having fun with magnets. The Terman quote in that posting says that for low loss, the series term is the more important, and that it varies as the square of the frequency.

To argue that these effects need to be represented by both series and parallel resistors because your meters shows an increase in series R is absurd.

Huh? Where did that come from? The thread has become tangled.

We are making no progress here. This is my last post on this topic.

OK. I'm going to go off and read the relevant section of Terman.