# Measuring change in inductance

I have a coil whose inductance will be changing continuously. I would like build a circuit to monitor the change in inductance continuously. Can I just use a RCL circuit and give it a sine wave of f Hz, and measure the voltage across the coil?

• If the math works, you can use it. – Ignacio Vazquez-Abrams May 8 '15 at 16:38
• Approximately what is your inductance range? That is: what is the minimum and maximum inductance values that you expect? – Dwayne Reid May 8 '15 at 19:47

It might be easier to build the inductor into a colpitts oscillator and measure the frequency variations with indutance change.

The circuit above is very easy to get working and I have used it plenty of times in oscillators and VCOs.

Of course you can buy Ti's inductance to digital convertor: -

But where's the fun in that?

• +(int)(PI/3) for where's the fun in that? ... that's a real hobbyist's credo! – vaxquis May 8 '15 at 23:04

Yes, though that will (in general) lead to a nonlinear response.

You'll get a peak near: $\omega_0 = \frac {1}{\sqrt{LC}}$ (how near depends on the Q)

There are plenty of ways of measuring an inductance- making the inductor part of an LC oscillator (for example, resonating with a film or NP0 capacitor) and measuring the frequency is a common method for approximate measurements. A circuit developed by AADE, which is LM311-based, is available in unauthorized knock-off form in many hobbyist inductance meters and kits.

Also widely used are RLC bridges (AC bridges are the standard way of making precision measurements).

With inductors there is often a need to measure the real portion of the impedance as well- core loss and/or series resistance loss. That can be done by demodulating and low-pass filtering the quadrature components of the current through the (imperfect) inductor using the applied voltage as the reference. An ideal inductor would have no in-phase component at all.

Whichever method you use (oscillator or bridge), you will have to keep the test frequency well above the rate at which the inductance changes if you expect it to follow accurately.

• You beat me to it. I was going to suggest the AADE oscillator circuit because it works so well. – Dwayne Reid May 8 '15 at 19:46

You can, but the usual way is to use the inductor as one of the frequency setting components in a oscillator, then measure the change in frequency. This will give you better sensitivity at the same noise level on the signals.

This oscillator method is how most metal detectors work.