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Circuit to Find value of inductor

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Advanced Member level 5
Jun 13, 2021
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Please can you offer any improvements to the attached method of finding an inductor value?
We dont have a (working) inductance meter. We also dont have a signal generator.

BTW, the scope is on DC coupled, definetely_not AC coupled. Scope probe is also on x1.

Apologies, I had a post on this approx 6 months ago, though fail to re-discover it now.


  • Inductor value measurement.pdf
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Simple RL and a scooe coupld work. Switch ON and measure tau.

Or LC and a scope and measure resonance frequency.

A soundcard to generate sinewave and a scope ... and a RL. Measure the voktages across R and L. Even a DVM should do.

Some test methods are better for low values, others for high values, some are more suitable for high current others for low current, also for high and low frequency...

Thus it would be interesting what inductors you want to test.

One way of doing it - but it would cost more than an inductance meter and risk damaging the inductor.

You don't mention the range of of inductance you want to measure but a simpler method is to feed AC from a signal generator through a variable resistor. Connect one scope input across the generator and across the inductor. Adjust the resistor so you get half the amplitude across the inductor then R = XL so you can do some math based on the frequency to find L.

I would buy a £10 LCR meter!

Astable multivibrator based on one Inductor creating oscillations. Frequency is partially determined by Henry value. Frequency also has to do with Ampere level through inductor as well as L/R time constant.

inductor based astable multivibrator 2 NPN 1uH 30 MHz.png
Another oscillator. Surprisingly simple, easy to get operating. Inductor & resistor bridged between two invert-gates. Frequency based on inductor value. Tried in hardware and it works.

Similar arrangement is feasible using two op amps.

inductor-based oscillator (LR between two invert-gates).png
BradtheRad that looks great, And also i think with ferrite inductors, the fact that the inductor current is very low wont matter?

with ferrite inductors, the fact that the inductor current is very low wont matter?

Since a real invert-gate IC (4069) is low power, I put sufficient R value in the simulation to reflect that. In hardware I vary the resistance, down to very low ohm values. This does not draw high current from the 4069 IC, since its output stage can't provide more than a few mA. Oscillations do continue.

To handle greater power, there is the full H-bridge. Oscillations arise from the inductor value. Transistors are cross-biased.

The zener diodes and capacitors are a contrivance in the simulation for reducing inductive spikes.
Sinewaves are obtainable by substituting a capacitor for the zeners. This creates an LC tank circuit.

inductor load self-oscillating H-bridge 100uH 3kHz.png

also building a Colpitts osc with known C values - if you can measure the freq of oscillation - you can calc the L


We want something pretty rough, cheap and simple, to measure inductance from 1uH to 2.2mH......the following from farnell are £40 and £100 respectively

...way too much
As such, what do you make of the attached, its pretty noddy-ish.....nice for me....see any gremlins?
LTspice and jpeg as attached


  • Inductor measure 555.jpg
    Inductor measure 555.jpg
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  • 555 inductor
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