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Inductance measuring adapter for multimeter 2012-05-25

For those of us without an actual inductance meter, this project is an easy-to-build way to use an ordinary multimeter (digital or analog) to measure the inductance of coils from about 3 microhenries to 7 millihenries, with an accuracy of about 10%, plenty close enough for most DIY projects.

The inductance tester converts the inductance value of the coil being tested to a voltage which can be measured by a voltmeter. Although a digital multimeter (DMM) will give more accurate results, an ordinary analog volt-ohmmeter (VOM) can be used.

I took this circuit from this page on the QRP Homebrew site and made some modifications to it, described below.

The Circuit

The tester uses 4 NAND gates. The first gate, IC1A, is configured as a simple RC square-wave oscillator, producing about 60KHz on the low range and 6KHz on the high range. The square-wave output is buffered by IC1B and applied to a differentiator formed by R4 and the inductor under test. The stream of spikes produced at pin 9 decay at a rate proportional to the time constant of R3 and the unknown inductor. Because R4 is a constant value, the decay time is directly proportional to the value of the unknown inductor. IC1C squares up the positive-going spikes, producing a stream of negative-going pulses at pin 8 whose width is proportional to the value of the unknown inductor.

The negative spikes are inverted by IC1D (pin 11) and integrated by R5 and C3 to produce a steady DC voltage at the positive meter terminal. The resulting voltage is proportional to the value of the unknown inductor and the frequency of the oscillator. R1 and R2 are used to calibrate the unit by setting a frequency that produces a DC voltage corresponding to the unknown inductance. D1 provides a 0.7 volt constant voltage source that is scaled by R8 to produce a small offset reference voltage for zeroing the meter on the low range.

I added the trimmer R4 to the circuit, to allow some leeway in adjusting the inductance range. You can use this trimmer to raise or lower the range to accommodate the coils you measure. I also added the "calibrate" switch because I'm lazy and didn't want to have to hunt for a piece of wire to short the input connectors to calibrate the adapter.

Since the circuit is powered by a battery, no capacitors are needed on either side of the 5-volt voltage regulator.


I managed to squeeze everything into a mint tin. I put most of the circuit on one piece of perfboard, and the three trimmers (hi/low and zero) on another small board. The test connectors are standard 5-way binding posts (to accept both wire leads and banana plugs), and I used two tip jacks for the meter connection, to allow standard meter probe tips to be easily connected.

The connector on the left is an external power connector, so I can power the adapter from my 3-to-9-volt boost converter in case I don't have a 9-volt battery and want to use 2 AA cells instead.


The adapter must first be zeroed. If you've included the "calibrate" switch, close it, or just short the inductor terminals with a piece of wire. With the adapter powered and a meter connected and set to a low range (200 or 2000mV), adjust the control for zero voltage.

Next, you need to calibrate both ranges. You'll need an inductor of a known value. Anything at or above about 100uH will work. Connect the inductor to the binding posts, connect the meter, and adjust the trimmer for the range being adjusted for the correct reading.

On the low range, the adapter gives 1mv for each 1uH. On the high range, 1mv corresponds to 10uH (or 0.01mH).

Once adjusted, I found the adapter to be extremely accurate and reliable, measuring several coils of known inductance. The results are plenty good enough for most electronics hobbyist's needs.
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