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Joe Gwinn's Inductance Measurement


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8/10/2005 8:56 AM
Max Weinhall Joe Gwinn's Inductance Measurement
I am new here, plus I am an electronics novice, so please forgive if I show bad manners or laughable ignorance.  
 
I am looking at Joe Gwinn's homebrew test instruments ( http://home.comcast.net/~joegwinn/ ). First of all - THANKS! I will have no problem making the "Simple Hall-Effect Magnetic-Field Meter".  
 
Questions:  
I am keen to try the "Maxwell-Wein Impedance Bridge" for measuring inductance, but I don't have a signal generator. I would like to use a PC soundcard based signal generator - could that be made to work? I think getting the correct frequency is no problem, but what about voltage? Can I just test AC voltage with my DMM and move volume sliders up and down until I'm very close to 1.0V? Or would I need a a specialized measurement tool for that route, like an AC Millivoltmeter?  
 
If that's no good, what about building a simple transistor-based generator like in my personal EE college, a kids' edu-toy breadboard kit/illustrated book? E.g. http://www.redcircuits.com//Page13.htm -Would that do the trick?  
 
I'm mainly want to compare pickups and be able to reproduce the same thing. I have been winding for a few months and already all my friends want copies of some of the serendipitously great pickups I've made. Most of them are assembled from parts of old junker pickups, so I can't just go buy another magnet or bobbin or anything to match. Also I have a cheap Korean ceramic humbucker that I LOVE and I would like to be able to make my own, slightly darker version. So I just need consistency for comparing pickups side by side, not absolute, perfectly-calibrated, team-of-101-scientists type precision measurement.  
 
I have seen the Extech meter that is mentioned on this forum - added it to my Amazon wish list - but I'd like to do this on a super low budget - even lower than $180. Plus I really like making my own tools - gives the hobby an old world luthiery vibe.
 
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8/10/2005 9:10 PM
Joe Gwinn

On 8/10/2005 2:56 PM, Max Weinhall said:  
quote:
"I am new here, plus I am an electronics novice, so please forgive if I show bad manners or laughable ignorance."
Ignorance is OK. Bad manners are another matter....  

 
quote:
"I am looking at Joe Gwinn's homebrew test instruments (http://home.comcast.net/~joegwinn/). First of all - THANKS! I will have no problem making the "Simple Hall-Effect Magnetic-Field Meter"."
This is a robust, easy circuit to build, because the Allegro IC is designed for automobiles, as hostile a market as one could hope for.  

 
[QUOTE]Questions:  
I am keen to try the "Maxwell-Wein Impedance Bridge" for measuring inductance, but I don't have a signal generator. I would like to use a PC soundcard based signal generator - could that be made to work? I think getting the correct frequency is no problem, but what about voltage? Can I just test AC voltage with my DMM and move volume sliders up and down until I'm very close to 1.0V? Or would I need a a specialized measurement tool for that route, like an AC Millivoltmeter? [/QUOTE]The problem is not voltage (one volt rms is sufficient), it's distortion. If the harmonics are significant, at balance (when the fundamental is nulled out), the harmonics will come through and obscure the balance point. A low-distortion drive signal eliminates those interfering harmonics.  
 
I would guess that a PC soundcard can make a sufficiently accurate sine wave, but have never tried it.  

 
quote:
"If that's no good, what about building a simple transistor-based generator like in my personal EE college, a kids' edu-toy breadboard kit/illustrated book? E.g. http://www.redcircuits.com//Page13.htm -Would that do the trick? "
Yes. But I would add a buffer amplifier, so the oscillator isn't too heavily loaded.  
 
The cheap analog signal generators, which have a big mechanical frequency dial and cost a few hundred dollars, will also work.  
 
Or, look for "wein bridge" oscillators.  

 
quote:
"I'm mainly wanting to compare pickups and be able to reproduce the same thing. I have been winding for a few months and already all my friends want copies of some of the serendipitously great pickups I've made. Most of them are assembled from parts of old junker pickups, so I can't just go buy another magnet or bobbin or anything to match. "
It's pretty hard to figure out the recipe for a pickup to match the sound of some other pickup, from electrical measurements.  
,P.  
quote:
"Also I have a cheap Korean ceramic humbucker that I LOVE and I would like to be able to make my own, slightly darker version. So I just need consistency for comparing pickups side by side, not absolute, perfectly-calibrated, team-of-101-scientists type precision measurement. "
Do the same thing, but with a heavier cover or baseplate, so the eddy currents are more significant?  

 
quote:
"I have seen the Extech meter that is mentioned on this forum - added it to my Amazon wish list - but I'd like to do this on a super low budget - even lower than $180. Plus I really like making my own tools - gives the hobby an old world luthiery vibe."
The Maxwell-Wein Bridge is just as accurate as the Extech, but far slower to use. You will need a good DMM, one that measures resistance and capacitance. Frequency is also nice, but not essential.

 
8/13/2005 4:54 PM
Joe Gwinn

An afterthought. The two potentiometers (R1 and R2) need to be either ten-turn types, or to be composite (two pots in series, one 3% the resistance of the other), or it's too hard to adjust to the null manually.  
 
The trick is to get everything nicely nulled (DMM reads less than a millivolt), disconnect the signal source, and then use a DMM to measure tha actual values of resistance and capacitance.
 
8/13/2005 9:33 PM
Dr. Strangelove

Joe Gwinn wrote:
quote:
"I would guess that a PC soundcard can make a sufficiently accurate sine wave, but have never tried it."
It does, although some are better than others. The semi-obsolete Santa Cruz by Turtle Beach/Voyetra is a v.good inexpensive one.  
 
Links to sound card test sites are at:  
<http://www.epanorama.net/links/pc_sound.html#test>  
 
It occurred to me that you could get the inductance of a guitar pickup in a voltage divider by plotting its output from a frequency sweep below its resonance point. Shouldn't be too awfully difficult with a sound card, a couple of cables and a resistor near the DC resistance of the coil.  
 
A freeware sweep generator for sound cards is at:  
<http://www.david-taylor.pwp.blueyonder.co.uk/software/audio.html>  
 
Marchand electronics has a few mpegs for test tones and a frequency sweep over at:  
<http://www.marchandelec.com/sweeps.html>  
 
-drh  
--
 
8/19/2005 10:18 AM
Max Weinhall
Hey thanks for the great info guys. I was away on a very rare vacation since I posted that question, and I only just now ordered the remainder of the parts I need for these gagdets. I'll report back how it works out.
 

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