How to measure inductance and Q

[LowNote]

I have noticed companies listing the inductance and "Q" of a pickup. How do I measure a coil for these numbers and what happens to change these numbers?

LowNote

[kayakerca]

I have noticed companies listing the inductance and "Q" of a pickup. How do I measure a coil for these numbers and what happens to change these numbers?

LowNote

To the "how" question.

An LCR meter can be pickup up pretty cheap on ebay that will provide an inductance value. Q can be calculated by formula

Backlight Control LCR RCL Inductance Capacitance Resistance Meter Tester 4070L | eBay

Or you can go the distance and get an LCR meter that calculates both.

LCR200 - Passive Component LCR Meter

[Mike Sulzer]

It is unlikely that an $18 meter will measure the inductance of the pickup coil accurately. A pickup coil is too low in Q for the usual cheap meter to handle. (If this one does, it is a find!) The EXTECH 380193 does the job at a reasonable price.

To the "how" question.

An LCR meter can be pickup up pretty cheap on ebay that will provide an inductance value. Q can be calculated by formula

Backlight Control LCR RCL Inductance Capacitance Resistance Meter Tester 4070L | eBay

Or you can go the distance and get an LCR meter that calculates both.

LCR200 - Passive Component LCR Meter

[kayakerca]

It is unlikely that an $18 meter will measure the inductance of the pickup coil accurately. A pickup coil is too low in Q for the usual cheap meter to handle. (If this one does, it is a find!) The EXTECH 380193 does the job at a reasonable price.

The only thing with the EXTECH 380193 is no DCR. I have an $18 LCR and a EXTECH LCR200. The difference in LCR readings between the two is about 2 decimal places out. Totally insignificant for the purposes of measuring pickups, well, at least for my purposes.

[Mike Sulzer]

The only thing with the EXTECH 380193 is no DCR. I have an $18 LCR and a EXTECH LCR200. The difference in LCR readings between the two is about 2 decimal places out. Totally insignificant for the purposes of measuring pickups, well, at least for my purposes.

That is great! Thanks.

Mike

[mr fab]

I have a cheaper meter, as well as a nice DER EE which I got for a great price.

whilst the cheap one is fine for certain measurements, it measures inductance at 200Hz which is not really useful. the DER EE has a number of frequencies from 100Hz up to 100kHz and is also able to measure DCR and Q

the other thing that annoyed me with the cheap one is the lack of auto power off function, which means I forget to turn it off and end up with flat batteries all the time

it all depends on what you are after and I guess you also get what you pay for. I use the cheaper one how for measuring components and other odd jobs, and use the DER EE for my pickups.

[LowNote]

Thanks for the replies about the meters. I looked at the LCR200 and the EXTECH 380193 , a bit pricy for me since I don't make that many pickups. Now, what determines LCR?. In other wards how do I wind a coil to a specific LCR?

LowNote

[copperheadroads]

Bought the Extech 380193 years ago................. pretty much now I take a reading of Capacitors with it mostly ..........the odd 1 in 50 pickups get a test reading from it ..........

[kayakerca]

Now, what determines LCR?. In other wards how do I wind a coil to a specific LCR?

LowNote

Wind to a certain LCR? Do you mean wind to a certain inductance (henries) level? In my (very limited) experience, it is a combination of how much and what type of wire combined with the level of charge in the magnetics. If you want to measure it, you need an LCR meter.

[kayakerca]

Bought the Extech 380193 years ago................. pretty much now I take a reading of Capacitors with it mostly ..........the odd 1 in 50 pickups get a test reading from it ..........

While it was expensive, I do like the Extech LCR200. Everything electrical I want to measure in a pickup in one meter. Ya, I'm still green enough and anal enough to measure every pickup I wind before and after placing a charge on the Alnico's (whether I understand the measurements or not).

[LowNote]

Thanks kayakerca, that's the information I was looking for. So, tension all so plays a part.

LowNote

[LowNote]

If inductance measurement at 200Hz is useless, what frequency is usable?

You will have to excuse my ignorance in this, it's all new to me. I normally stick a coil on the winder and wind to a certain number of wraps. Now, I want to know how to change the variables and what to expect.

LowNote

[Tony Bones]

Generally speaking, when making inductance measurements you want to stay well below any resonance peak, but choose a frequency high enough that the inductive reactance (the 'influence' of the inductance) is not completely swamped by coil resistance. This is not too hard with guitar pickups. Even though the DC resistance seems high at a few thousand ohms, the inductance is typically a few Henries, which is actually a lot.

The graph below should make things more clear. The curve to look at is marked "X" which, I'll remind you, is the symbol for reactance which is the imaginary part of complex impedance. "R" is ordinary resistance and is the real part. "Z" is the magnitude of the two combined.



The red line is really the result of both inductance and capacitance. Below the resonance peak it's mostly inductive. Inductive reactance is proportional to frequency; that makes a straight line with constant slope. Essentially, the slope is proportional to the inductance. Above the resonance is mostly capacitive. Capacitance (in this graph) is proportional to -1/f, which is curved instead of straight. In the region of 5kHz to 13kHz both capacitance and inductance have a significant influence. At about 8.5kHz they are equal (and opposite) and exactly cancel each other. That's the resonance frequency.

Anyway, You could measure inductance of this pickup anywhere below 4kHz. You could measure self capacitance anywhere above 20kHz. If you try to measure inductance at 20kHz then your meter should report a negative value. Now you know why.

Note that different pickups have different resonant frequencies. If the resonance is at 4kHz, then obviously 4kHz is not a good frequency to measure inductance. Also, Q is a measure of the width of the peak (the region where neither L nor C can be measured.) High Q means a tall sharp peak. Low Q means a low flat peak which pushes the maximum measuring frequency down even more.

TLDR: up to 1kHz is usually safe. 200Hz is probably fine.

[Mike Sulzer]

Generally speaking, when making inductance measurements you want to stay well below any resonance peak, but choose a frequency high enough that the inductive reactance (the 'influence' of the inductance) is not completely swamped by coil resistance...

It is more complicated than that if steel cores are used. The coil inductance is most accurately measured at a frequency high enough to avoid the problem you mentioned, but low enough to avoid errors in the measurement resulting from eddy currents in the steel. Thus you really want to use 120 Hz, or 200 Hz if it is just the coil inductance that you want.

A measurement of impedance at a particular frequency gives two numbers, amplitude and phase, or real and imaginary. However, even at frequencies below where the coil capacitance matters, there are three parameters: series resistance, coil inductance, and a parallel resistance due to the eddy currents. The last can be avoided by going to a low enough frequency, and might not matter in any case depending on the pickup and how accurate you need to be. But sometimes it does matter, and at frequencies where it does the imaginary part does not predict the coil inductance accurately.

Also, even at 20 KHz the capacitance measurement can be affected by eddy current losses. Getting accurate capacitance requires using a range of frequencies in a fitting process.

The included plot is an example of these things. (It is a single coil with steel cores, a humbucker coil.) The dashed gray line is the impedance of the inductance measured at low frequencies extended up in frequency (the straight line you mentioned). The red line, the measured imaginary part, is below this. In order to get a better idea of the eddy current losses and how they affect the apparent inductance, the capacitance is derived by fitting across frequency, and its effect is removed from the impedance. The result is the yellow line. It is well below the dashed gray line above a couple of KHz, and the distance below is an indication of the error that would occur in a simple inductance measurement as a function of frequency.

steelCores.pdf

[Tony Bones]

Thanks Mike! I sometimes have a hard time integrating eddy currents into the whole picture. I don't always remember that, at least to first order, they are like a secondary winding with some resistance.

I'd like to know how the capacitance is derived...