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Old 5th May 2018, 9:54 pm   #21
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Default Re: Precision inductance tester

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Originally Posted by ms660 View Post
Does anyone know if inductance varies with frequency?
Yes, if it has an iron core.
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Old 5th May 2018, 9:58 pm   #22
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Default Re: Precision inductance tester

If it's a single-layer evenly spaced inductor you can get a reasonable value (about 1%) with nothing more than a tape-measure or rule, using Nagaoka's formula. But you may need to use linear extrapolation (depending on the values given in the table you use) for the figures in your formula.

Wheeler's formula is slightly less accurate but still workable for practical purposes. If the value you get is on the edge, you need a different inductor anyway. Unless you're not designing a rigidly-fixed one, of course.
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Old 5th May 2018, 10:33 pm   #23
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Default Re: Precision inductance tester

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If it's a single-layer evenly spaced inductor you can get a reasonable value (about 1%) with nothing more than a tape-measure or rule, using Nagaoka's formula.
This is something I learned from my construction of the circuit I posted: for a largish air-cored inductor, yes, Nagaoka is incredibly prescriptive. But I had to go through the process.

And to get there, I calibrated the tester with commercial small axial inductors in the sub 10uH->1mH and found them to be rather scattered around their marked tolerances.

When you talk of a tape measure, I imagine rather larger inductors as in tank coils of high-powered commercial transmitters. yes?
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Old 5th May 2018, 10:57 pm   #24
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Default Re: Precision inductance tester

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When you talk of a tape measure, I imagine rather larger inductors as in tank coils of high-powered commercial transmitters. yes?
Yes indeedy. And even taking an average radius on curved profiles yields an accurate result. I've used a Vernier caliper and ordinary steel rule (and magnifying glass! ) on smaller coils of about 30uH.

I've used the 'three voltage' Cosine Rule method many times but never above 10kHz. I have a Peak Atlas LCR bridge of my own and it's a pretty useful device and compares well to a HP4192A or Tinsley bridge.
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Old 6th May 2018, 1:27 am   #25
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Default Re: Precision inductance tester

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When you {russell_w_b} talk of a tape measure, I imagine rather larger inductors as in tank coils of high-powered commercial transmitters. yes?
Either that, or he is simply remembering the rules: Stick to Webers, Teslas and express all distances, however short, in metres; and that way, at least you won't make the maths any harder than necessary
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Old 6th May 2018, 9:47 am   #26
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Default Re: Precision inductance tester

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'... Stick to Webers, Teslas and express all distances, however short, in metres; and that way, at least you won't make the maths any harder than necessary'
Oh, yes! In some books it's inches, others centimetres, others metres... Been there, done that when it comes to different units for the same thing.

As for other formulaic things I get involved with, I rolled my own spreadsheets to take the onerousness out of such things. It's fun to do and educational from the perspective of actually understanding how the maths applies to the problem as well as the mechanics of spreadsheets themselves, but it is vitally essential to check each step on the way. No doubt those whizz-kids au-fait with low-level programming could come up with something flashier, but spreadsheets work fine for me.

A bit like apprentices and tradesmen making their own tools.
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Old 6th May 2018, 12:24 pm   #27
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Default Re: Precision inductance tester

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Originally Posted by russell_w_b View Post

Oh, yes! In some books it's inches, others centimetres, others metres... Been there, done that when it comes to different units for the same thing.

As for other formulaic things I get involved with, I rolled my own spreadsheets to take the onerousness out of such things.
I like the idea of a home-rolled spreadsheet: slightly OT but I just wish they'd leave Excel alone and not keep tweaking it!

I work in metric units only now. I don't find it at all difficult to convert between millimetres and metres or the other way round.
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Old 6th May 2018, 2:02 pm   #28
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Default Re: Precision inductance tester

Random slightly on topic thing... For the excel users out there, if you use this cell format it'll display in engineering notation:

##0.00E+0

Very handy
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Old 6th May 2018, 3:06 pm   #29
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Default Re: Precision inductance tester

That’s great! Neat tip, thanks !
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