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previous: Dave Stephens FINAL CONCLUSIONS AND MY MISTAKES -- 7/25/2004 9:18 AM view thread

Using an AC bridge to measure inductance

7/25/2004 11:04 PM
Joe Gwinn
Using an AC bridge to measure inductance
For the measurement of low-Q inductors, like guitar pickups, the standard method for many years was the Maxwell-Wein Bridge, which although a big deal a half-century ago, is easy to implement today. Low-Q inductors appear to flummox many low-cost LCR meters, yielding sometimes wildly erroneous inductance values, and no warning.  
 
A little writeup appears at <http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Instrumentation/bridges.htm>. Much more can be found by googling on '"maxwell bridge" inductance' (without the single quotes). A detailed mathematical analysis can be found at <http://ts.nist.gov/ts/htdocs/230/233/calibrations/Electromagnetic/pubs/65C-3.pdf>, and in many books on electrical measurements.  
 
Today, using a sinewave oscillator and a digital voltmeter (that can measure drsistance and capacitance), once can in theory meaasure inductance to a few percent. Be aware that the measured inductance will vary with frequency, most likely due to eddy currents in the polepieces, and to stray capacitance in the coil, so one cannot really measure the inductance of a pickup to anything like that accuracy. But, this is a very stable and reliable way to measure inductance.  
 
Note that the oscillator waveform must have low distortion because at balance the 1-KHz fundamental is punched out, leaving the remaining harmonics to interfere with finding the exact null point.  
 
In the tests I did some time ago, the minimum measured inductance was seen at about 1,500 Hz. Other pickups may have a minimum at some other frequency. A test frequency of about 1,000 Hz seems best, and this is a traditional test frequency, although the exact frequency isn't critical because balance in the Maxwell Bridge doesn't depend on frequency.