# A Lesson In Inductance

This is all kind of Schrodinger’s cat, but it turns out not all 6uH is made equally. 6uH can be 6uH, 6uH can be 12uH and 6uH can be a capacitor and the only thing different between all these 6uH’s is the frequency they have been measured at.

So tonight i got well and truly schooled on inductance and why my bandpass filter was looking rather bonkers. I built it with the right capacitors, i wound inductors with the right number of turns for the required inductance i assembled a really nice looking filter. But when it came to measuring the bandpass it was orders of magnitude wrong.

This is how my filter should have looked, well, at least something like that.

This was the bonkers mess I was measuring in the Bode Analyser.

So after much discussion with a much smarter man than me, and working out everything i was doing wrong, I ended up with this testy jig, 200R in series with the inductor, feeding one side with the signal generator while measuring both the input voltage and the voltage across the inductor. Changing frequency until my output voltage is 50% of the input and then using this following formula to calculate the inductance. L= R*sqrt(3)/(2*pi*f) that at least gets me in the ball park, and it turns out I was orders of magnitude off with the inductors i wound and were most likely acting like capacitors at 20mhz where my filter was peaking.

## 2 thoughts on “A Lesson In Inductance”

1. Hi Robert,

I like the bode analyser you are using. It looks very useful.

I have never tried your approach. I will certainly keep it in mind when I want to check any of those ferrite based inductors you find in switching power supplies before I use them as a decoupling choke. Sweep the frequency from AF to RF and see what actually happens.

I use a different approach to measuring inductors which would appear to be as easy as your method. It works from a few nH upwards. And you can measure Q as well. ( http://vk6tt.blogspot.com.au/2017/01/measuring-small-inductors.html )

Regards
Richard

1. robertpowell says:

Hi Richard, I am not sure yet what I am seeing is real or an artifact of poor measurement, I wound 6uH, 39 turns on a T50-6, which is about right for that al, measured with my LCR meter and its close enough to 6uH, I measured on the jig and it comes out at 14.5uh. So i grabbed a known inductor out the box, 10uH, it measured 12uH on my LCR meter, near enough on a 10% component, its measuring 28.8uH on my jig. Well I worked out the problem I was measuring things wrong. Will make a new post soon with the updates. HIHI

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