# What's the best/easiest way to measure an unkown inductance without an RLC meter?

Is there a decent way to measure inductance accurately using an oscope and a function generator? The best method I can find is to build a tank circuit and sweep the frequency until the highest voltage appears. Then use the formula below to solve:

$$f = \dfrac{1}{2\pi\sqrt{LC}}$$

It seems like there must be an easier way!

I've used a two terminal oscillator, with the inductor in parallel with a suitable capacitor, with a scope or counter to measure the oscillation frequency. I once checked an inductor on a very expensive inductance meter at work, and the values were identical. The source-coupled oscillator using two FETs is ideal for this application, or the LM311:

• now THAT'S a trick I'm going to use next time! – superkittens Mar 22 '13 at 6:14
• is there any place to find more explaining for the working principles of this circuit? – iMohaned Aug 16 '16 at 22:43
• You need to read the LM311 data sheet. Ask if there is anything you do not understand - create a new question. – Leon Heller Aug 17 '16 at 8:41
• What is the point to use R5 when its in parallel to R18? – MrHetii Aug 8 '17 at 10:56
• I wonder how that happened! Leaving it off should be OK. – Leon Heller Aug 8 '17 at 17:07

The sweep and oscillator methods are both decent ways but, you need to consider the value of the inductor's parasitic self-capacitance in many cases. You should also consider what errors might be incurred if the Q of the tuned circuit is low. More about that at the bottom but for now I'm assuming you can create a high-Q resonant circuit from an unknown L and known C.

Use $Fn = \dfrac{1}{2\pi\sqrt{LC}}$ to "extract" the inductance value - the L value you calculate is based on "known capacitance" that parallel-resonates the circuit at frequency Fn - this capacitor has to have an accurately known value. This gives you the first estimate.

Add another "known" capacitor in parallel and you get a new lower frequency. You may find that if you recalculate the inductance based on the new circuit, it will be slightly different to before and this is due to the inductor's parasitic capacitance offsetting the known capacitors by a few percent.

You now have enough numbers to calculate the precise inductance value. You also have enough information to calculate its self capacitance and therefore its self-resonant frequency (SRF). Do math now!

As a final check, run the inductor (with no added capacitors) at its SRF and see if the component resonates at what was predicted.

In most cases this will tally. However, if you are dealing with small values of inductance (say < 100nH) the parasitics involved will be of the same order as any measurement probes etc.. Then you'll need specialist equipment to resolve these problems I would say.

Low Q circuits will also incur an error. The "damped" resonant frequency will reduce as Q factor reduces and this means the $\dfrac{1}{2\pi\sqrt{LC}}$ formula will progressively become more inaccurate. Here's a wiki picture that explains: -

Note that this graph works for mechanically resonant situations or electrically resonant circuits.

If you look at the blue line on the graph you'll see this is where the resonant peak moves as damping increases. It can produce significant errors and be aware of this. Adding the extra cap to give a better chance of calculating the real inductance value (as I mentioned above) will also increase the circuit's "damping" so care MUST be taken when trying to calculate inductance when the "resonance" peak is not very strong.

I commonly measure inductance of power chokes by charging a capacitor to a fixed voltage, then momentarily applying that voltage to the choke. Observe the current through the choke with a scope, and the slope and voltage give you the inductance.

$$V=L\frac{di}{dt}\\ L = V\frac{dt}{di}$$

So you'd need a scope, some means of measuring the current (a shunt resistor should do), a capacitor, some means of charging the capacitor, and a switch that can safely short the capacitor with the inductor. Start slow, of course; depending on the size of your inductor, you could easily destroy it by putting too much voltage or too much capacitance on it. A switch capable of opening the contact (and handling the inevitable inductive kick) might be preferable, so you can be sure you don't dump all the energy in the cap straight into heating the choke.

John Becker had a constructional project where he built a PIC LCF meter. He used the following circuit to obtain oscillation. He used the 4011 Nand gate but one can also try using an inverting Buffer (74LS04 etc ) instead of the Nand Gate. I tried the HEF40106 but that did not work well at all.

The standard formula applies:

So the series capacitance C in this instance is 10nF. VR2 is there to ensure that the oscillation starts reliably and remains stable through its operation. The L1 inductor provides a minimum inductance which one can subtract to get the unknown value of L.

• Anthem - Hi, Can you give a full reference (including link to a webpage) for the original version of that diagram which you adapted? I think it came from (as you said) PIC LCF project by John Becker, Everyday Practical Electronics, February 2004, page 93, published by Wimborne Publishing Ltd. If you didn't get that image from a usable online link, then (unless you have a more accurate reference) we can use that as the citation. – SamGibson Mar 5 '18 at 23:25
• Hi Sam. That is where it came from. I have the magazine. A preview can be done here - yumpu.com/en/document/view/8382299/pic-lcf-meterpdf. But I see that there are some problems with Cx measurement part of that meter. See the link electro-tech-online.com/threads/problem-with-lcf-meter.91744/… – Anthem Mar 7 '18 at 6:30
• There is a gentleman who used various other Schmitt Triggered NAND gates on that same project, where he changed the calculations to suite the frequency changes that resulted. See calatron.me.uk/calatronweb/Electronics_Hobby/… – Anthem Mar 7 '18 at 6:53