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How to measure small inductors: I recently tried to measure the unlabelled inductors in my speaker crossover to replace them with bake-sealed air inductors. If you don't want to change the balance of the crossover, it is important to roughly keep the resistances of the inductors. I used a 4 1/2 digit multimeter to determine the original resistances. Measuring the DC resistances which were in the range of 0.15 - 0.5 R was much easier than with a 3 1/2 standard instrument. I did use lab type 4 mm banana plugs and 1.5 mm2 leads. Bringing the probe end plugs down on a piece of wire with a little force actually made the reading go down to 0.00 or 0.01 R which is something you don't achieve with el-cheapo test leads. The instrument has a 200 mH inductance range. The leads have to be inserted into the capacitor test slots. Measuring inductors of about 1 mH or more was no problem. Readings of all these coils were within 5% of a standard value, usually a little low. I had the impression that the reading depended on the polarity of the leads but that turned out to be a contact resistance problem. One small inductor which did have the cryptic label "1871" had readings of 0.11 to 0.14 mH. Knowing also its resistance value, I guessed this to be a 0.15 mH inductor. However, when the now 5% inductor arrived, it measured 0.10 to 0.11. So I connected a capacitor in parallel and connected this resonant circuit to the calibrator of a scope and ground while feeding the live pole also to one of the display channels. I first used a 2u2 capacitor which shorted the fundamental 2 kHz square wave. There were some spikes on the transition. They turned out to be below 100 ns while I would have expected something on the order of 200 us and were still there once I had disconnected the large capacacitor. So I was probably looking at the parasitic capacitance of the coil. Next I connected a 680 pF capacitor. Now I could see a nicely damped oscillation with a time constant below 1 us. Connecting a second 680 pF capacitor roughly stretched the time constant by a factor of sqrt(2), so the capacitor was sufficiently big to swamp the parasitic capacitance. Just to be sure, I did my comparison of the unknown and the new coil with a 2n2 capacitor. I noted the amplitude of the first two peaks and the time of the first four zero crossings. The values for the 0.15 mH were about 11% low, so my unknown original coil could have been a 0.18 mH inductor with a pretty big tolerance or some non-standard value. Estimating the number of turns on my inductor, I guessed that adding 10 turns would be roughly correct. I was lucky as after adding these, the two inductors measured the same within about 1%. Actually, if the value of the cap is known, one could compute the inductance. If the oscillation decays within 2-3 full periods, the formula for a damped oscillator should be used. The Q can be calculated easily by looking at the decay behavior. |