Anyone have a inductance test layout?

[888guitars]

Looking for a way to test the inductance. I have a tone generator. Anyone have the schematic and the formula? Is there a chart of "good sounding" single coils and buckers showing the Henry's ? Or a chart showing a frequency graph of more or less inductance?

[bbsailor]

Looking for a way to test the inductance. I have a tone generator. Anyone have the schematic and the formula? Is there a chart of "good sounding" single coils and buckers showing the Henry's ? Or a chart showing a frequency graph of more or less inductance?

Wire the pickup and a 25K ohm pot in series as a variable resistor. Attach the signal generator across the pickup and the pot. Set your test frequency of around 100 HZ. Use an oscilloscope or meter capable of operating at 100HZ (well away from the pickup resonance point) and put a high signal level through this setup. Try it agin at 1000HZ and calculate both results. Adjust the pot until the voltage at the series junction of the pot and pickup is one half the total applied voltage from your signal generator. In other words, the pot pickup junction has half the applied voltage across both of them. Now remove the pot and measure the resistance of the pot and this pot ohm value now represents the reactance or XL of the pickup. Put the XL value into this formula XL equals 2 pi (6.28) times 100 or 1000 (your test frequency) times L (inductance in Henrys). This should get you close. Add your results into this web link: http://www.bowdenshobbycircuits.info/XLC.htm

If you want to test for the resonance of your pickup use a 500K pot in frequencies above 1000HZ and note your pot resistances at frequencies at 1KHz intervals up to about 10KHz. The highest pot resistance value is near the resonance of your pickup.

Let us know if this helps?

Joseph J. Rogowski

[Mike Sulzer]

Looking for a way to test the inductance. I have a tone generator. Anyone have the schematic and the formula? Is there a chart of "good sounding" single coils and buckers showing the Henry's ? Or a chart showing a frequency graph of more or less inductance?

Or buy a meter such as the Extech frequently discussed on this forum and ignore the generator. You need one that can measure inductance and loss at about 100 Hz, and 1KHz is nice for checking.

[Dave H]

Adjust the pot until the voltage at the series junction of the pot and pickup is one half the total applied voltage from your signal generator.

I think that should be 0.707 of the applied voltage not half.

[rjb]

I think that should be 0.707 of the applied voltage not half.

I agree.
Another way to say it: Adjust until the voltage across the the pot equals the voltage across the pickup- less math that way.
But this method assumes that the pickup's DCR is insignificant- which it isn't at low test frequencies.

Ex) Strat Custom '54 Neck/Middle pickup FDP - Forum
DCR: 5.9 Kohm
L: 2.4 H
X_L @ 100 Hz: L x 2pi x freq_test = 1.5 Kohm; @ 1000 Hz: 15 Kohm
Z_pup @ 100 Hz: sqrt ( 5.9K² + 1.5K² ) = 6.1Kohm; @ 1000Hz = 16Kohm
Note that at 100Hz, the pickup's DCR is almost 4 times greater than it's inductive reactance.

A more accurate method would be to use a 10Kohm/0.5% resistor instead of the pot.
At a given test frequency, measure the voltages across the resistor and across the pickup.
Solve for the current thru the series circuit by dividing the voltage across the resistor by the resistor's value: I = (V_R / R) for R=10Kohm.
Then calculate the voltage across the pup's DCR: V_dcr = I x DCR
Now you have the hypotenuse (V_pup) and one leg (V_dcr) of a triangle; solve for the second leg: V_XL = sqrt ( V_pup² - V_dcr²)
The inductive reactance is then X_L = V_XL/ I.
And the inductance is L = X_L / (2pi x freq_test)

Of course, these calculations assumes insignificant winding capacitance....

Or you could invest in a good LCR meter.

[rjb]

Is there a chart of "good sounding" single coils and buckers showing the Henry's ? Or a chart showing a frequency graph of more or less inductance?

Something like this?
https://www.buildyourguitar.com/reso...emme/table.htm

[888guitars]

Something like this?
https://www.buildyourguitar.com/reso...emme/table.htm

I knew this existed! Thank you!

[J M Fahey]

There is a nice workaround but I should do some searching.

Many multimeters have a capacitance meter.

How do they measure them?
They put the capacitor in series with a relatively small resistor to ground, apply a known frequency to the free end (nothing fancy, it's a plain squarewave ) and measure voltage across the resistor.

What they are actually measuring is capacitor reactance: a larger capacitor will have a lower reactance and drop a higher voltage across the resistor, which is rectified , measured and displayed.

Although it is a voltage, value is displayed as capacitance and in practice works quite well.

A clever guy noticed that inductors also have reactance, and can be measured the same way, simply multimeter makers don't care to supply the proper scale.

The guy offered a "conversion chart" , displayed "uF to Hy" equivalence, and if I don't remember wrong, he checked it by measuring a 1k resistor or thereabouts ... which of course has a known "reactance".

Impossible to find the graph (this must have been 20 years ago or more) but I'll ask mr Google, he might remember something about it.

It was published in Radio Electronics or Pop Electronics and I had a chart photocopy stapled to the wall.

Sort of crude but very useful.

EDIT: can't find it (yet) but this ARRL reactance chart *can* be used for the conversion.



Suppose you connect a 1k resistor to Cx meter terminals and it shows .01uF on the screen ... you look at the chart and see that the inductor showing 1k at that test frequency (which you don't actually need to know) is a 10mH one .

In this case it's not straightforward, you have to go back and forth, the beauty of the chart I mentioned is that it gave you a direct uF>Hy "translation" .

Don't forget both scales went "backwards", a higher "capacitance" value meant a lower inductance one and viceversa, because when you increase frequency, cap reactance goes down but inductor reactance goes up.

I used it basically to check for shorted transformers: DCR did not change much as we all know but inductance was nuked.

[Mike Sulzer]

In order to measure the inductance of a low Q coil such a a guitar pickup, a meter must measure the complex impedance, that is both the reactance and the loss or real part. Another way to put it is that it must measure amplitude and phase. Only a meter designed to do that can give accurate measurements of the inductance of a pickup.

[rjb]

In order to measure the inductance of a low Q coil such a a guitar pickup, a meter must measure the complex impedance, that is both the reactance and the loss or real part. Another way to put it is that it must measure amplitude and phase. Only a meter designed to do that can give accurate measurements of the inductance of a pickup.

I realize that a pickup's AC resistance will vary with frequency (due mainly to eddy current effects?)- but will it differ from the DC resistance enough to render my "Electronics 101 Lab" method useless?

[Mike Sulzer]

I realize that a pickup's AC resistance will vary with frequency (due mainly to eddy current effects?)- but will it differ from the DC resistance enough to render my "Electronics 101 Lab" method useless?

At 100 HZ the dc resistance and the "ac resistance" are close enough to get a very good measure of the inductance.

I think your method can work well, but it sounds a bit tedious. I think it is better to use a recording interface to measure voltage and current very much as you say, but automatically, effectively at many frequencies at once with computer software handling the control and the calculations.

Measuring pickup impedance with an A/D recording interface

Software for performing pickup analysis with a recording interface.

[rjb]

At 100 HZ the dc resistance and the "ac resistance" are close enough to get a very good measure of the inductance.

I think your method can work well, but it sounds a bit tedious. I think it is better to use a recording interface to measure voltage and current very much as you say, but automatically, effectively at many frequencies at once with computer software handling the control and the calculations.

Thanks for the affirmation.
The tedious calculations can be handled with a spreadsheet- just enter test frequency, pickup dcr, and 2 measured voltages.
The fully automated system seems really slick (but overkill for my occasional casual hobbyist use).

[mozz]

Buy a used lcr meter on ebay,you can get a quality unit for a fraction of original cost. Get a scope and signal generator to find resonance, no need to chart it all out.

[rjb]

Buy a used lcr meter on ebay,you can get a quality unit for a fraction of original cost. Get a scope and signal generator to find resonance, no need to chart it all out.

Good advice- assuming OP will have future use for the test equipment, and has the desire to learn how to use it.
But I wouldn't advise buying a scope, say, just to estimate the inductance of one pickup.

[888guitars]

Does the meter provide the 100hz? Is that how it reads inductance? Or do I have to put the signal through the pickup to get the reading?