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previous: Jeff G. I wouldn't mind seeing the math beh... -- 10/16/2004 2:04 PM view thread

Re: How Is Inductance of Pickup Measured?

10/16/2004 3:28 PM
Joe Gwinn
Re: How Is Inductance of Pickup Measured?
I've found the Maxwell Bridge to be the best approach, but it requires a very low-distortion (0.01% or less) oscillator to get accurate measurements. (The usual $500 function generators won't do, but one can build suitable oscillators from parts for not much money.) Aside from the good oscillator, all that's needed is a good DMM, one that will measure tenths of a millivolt (to find the null), resistance, and capacitance; and a calculator (to solve the bridge equations, which isn't as hard as it sounds).  
 
Many DMMs will also measure frequency, which is useful because inductance of iron-core inducturs always varies with frequency. The Maxwell Bridge balance conditions are independent of frequency, a great convenience. This process yields both the inductance and the effective AC resistance (which exceeds the DC resistance).  
 
I haven't tested the issue yet, but my theory is that the difference between AC and DC resistance is the energy absorbed by eddy currents in the magnets, iron poles, baseplates, metal covers, etc.  
 
As you would expect, the ratio of AC to DC resistance grows as the frequency rises. The ~#44 wire is too fine for eddy currents in the wire strands to have much effect at audio frequencies, or to be detectable in this setup.  
 
At 1 KHz, it's fairly easy to balance the bridge, but at 100 Hz it can be pretty slow going, because the DC resistance is far larger than the inductive reactance at that frequency. It took me some experimentation and math to find the best component values for the bridge, to make balancing by adjusting the two pots in alternation reasonably easy.