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Help in designing an variable freq. inductance meter.

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mr_monster

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After I got disappointed from my impedance meter (for measuring inductors) I decided to try and build one on my own. I was encouraged by the ability to measure inductance (actually calculate) accurately using a simple LC tank. The problem with this method is that a very large range variable capacitor is not available and discrete capacitor matrix will be somewhat limiting and overly complex.

The parameters I want to have at the end are: inductance, impedance (real and j), Q. As far as I see I only need the inductance and impedance to calculate all the other parameters. The question being asked is what method can be used to measure these at variable freq. ?
 

A VNA will do it, but these are expensive. There is a project called 'N2PK VNA' (which I've also been meaning to start but never have time. I might start it during the Xmas holidays).
However, even that is not particularly cheap.. expect to spend 2-3 hundred $ for the parts, or about £250 in the UK : (
You could also investigate the AD8302. If you find a good impedance meter circuit I would be interested. The N2PK is a big commitment (maybe a whole week dedicated to
soldering, testing, troubleshooting.. that's why although I look forward to it, I've never got round to it : (
 

discrete capacitor matrix

Do you mean something like a 4 or 5 position range, each with a different capacitor? This could work with #1 below.

1.

Connect a known capacitor so that you have an LC tank loop. See what ringing frequency it gives after an electrical jolt of any kind.

This will work regardless what is the ohmic resistance in the loop. You can calculate the henry value from the frequency and capacitor value.

Keep in mind capacitor values often have a wide tolerance. You'll want a capacitor whose value you know within a few percent.

2.

See what time constant results after you hook up a low ohm resistor across the coil's leads. Then send current through for a moment and then disconnect the supply. The current takes about five time constants to drop to 1 percent of initial current. The inductive time constant is L/R.

To use this method you must take into account the ohmic resistance in the windings.

3.

There ought to be a way to use phase shifting as a way to calculate henry value. I'm not sure just what setup will do this. This method will require taking into account the ohmic resistance of the windings.
 

VNAs are expensive and more than that they are usually for RF freq., I need something to measure the various attributes of the inductor at audio freq.
As I said measuring the inductance (or reactive part) is pretty easy using an LC tank or a simple resonator using two transistors, a resistor and a cap together with the DUT. I actually got very accurate results using this method and if I'll build it using SMD components I will get even better results by minimizing parasitic parallel capacitance. But even this way the accuracy is within 3% from what measured by the impedance meter which is very accurate up to 700Hz, from some reason passed that things are going off track.

Since I know the reactance I need one more parameter to solve all the rest, I will be guessing impedance should be the easiest one to calculate accurately since it will involve measuring voltage which is probably the most precise parameter I can measure using a scope or my Fluke bench-top DMM.
 

It does look like an interesting ic. And the data sheet shows that it is possible to extend it to measure lower impedances, which I guess
you will need.
I've never seen a cicuit with this device, but I think it may do the job.

- - - Updated - - -

It does look like an interesting ic. And the data sheet shows that it is possible to extend it to measure lower impedances, which I guess
you will need.
I've never seen a cicuit with this device, but I think it may do the job.
 

I will talk with ADI today and ask them about it. Check out the evaluation board. I don't think you can get a complete impedance meter for a better price!!! $60 with software and everything!!!
 

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