inductance measurement

Drive a N-MOSFET with a pulse generator (or 555 circuit or PIC). A variable voltage source connected to your inductor under test provides current through the inductor. A current probe or sense resistor and oscilloscope monitors current through the inductor. A flyback diode allows current flow to continue and decay when the MOSFET is off. A 50 resistor from gate to ground terminates pulse gen. Trigger scope with pulse. Start with low pulse width and low supply voltage. Increase either and watch current ramp. Use V =3D L di/dt to calculate L. Increase pulse width and/or voltage and observe saturation effects. Much more useful than using LCR meter.

You might also look into the 'ring down' method of inductance measurement.

Paul Mathews

It's normal to get different inductance measurements at different frequencies. Pure inductive reactance increases linearly with frequency (wL) but real coils have stray capacitance. This forms a parallel-resonant circuit which behaves like a capacitor above its self-resonant frequency.

Reactance is maximum at resonance. Below resonance, the reactance is inductive; but much higher than it would be for a pure or ideal inductor. Consequently, measured inductance typically increases with frequency. You are seeing the opposite. I wonder if the core is saturating when you measure it at 1 KHz.

I use one of these

and one of these

the latter measures complex impedances at any frequency from 1 to 200 MHz.

It could be thrown off by inter-winding capacitance.

What will the transformer's operating frequency be? The tester that works in that neighborhood will be best. If you are working with 60 Hz components, a couple of grand that will get you into the MHz/Ghz region is worthless.

--
Paul Hovnanian	paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.

No, you're doing something wrong. Maybe reading the scales incorrectly. I would say with a 1khz reference, that meter can not go very low. I really don't think you're getting a permeability issue differences here. then, there's always the remote possibility the 1 Khz meter is not correct.

--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

  SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
  THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

The one I use has already been recommended here.

I got it because Win Hill said it was good, and he was right. A proper impedance bridge offering a broad range of testing frequencies would be better, but they are more expensive.

The problem with your inductor might be its parallel capacitance - and at 100kHz parallel capacitance is often a problem for anything wound with multi-layer windings - but if you were looking at an iron cored transformer, you could also be seeing the effect of the shorted turn in the core.

At low frequencies the resistance around the current path through the core will be higher than the inductive impedance around the same path, but as the frequency rises, the inductive impedance will rise in the same proportion, while the resistive impedance remains the same, and the measured inductance of the coil as a whole will decrease.

-- Bill Sloman, Nijmegen

I do like the Neil's meter, but it measures at 100 to 500kHz, and is therefore NOT at all suited for measuring ac transformer primary inductance, etc. It's perfectly fine for measuring leakage inductance, however. And hams like it for measuring RF coils. As for Jamie's 100kHz planar transformer, it may have high winding capacitance, which will reduce the apparent inductance reading. But as for 21x, whew, I don't see that!

I've been having bad luck with inductance measuring so far, first the cheap meter from ebay arrived with several large cracks in the case, and the other one I bought off ebay doesn't work in the lowest inductance range properly since I soldered some terminals to it, as it only had two slots for sticking component leads into it, probably melted a component off the PCB I think.

The

(linked from Jean-Yves in the "advise on ESR meter project" thread)

That LCR meter looks pretty nice, it is the only one I saw that so far that has user selectable test frequencies. Its maximum frequency is

15.625kHz, would this most likely give an inductance measurement within 10% as a meter that measured at 100kHz? That is all I am hoping for at this point, it would be a good improvement! :)

cheers, Jamie

I just wiped this up....

382Kb screen capture Single chip 555 based power inductor test.

This circuit can be used to measure inductance. Based on v(t) = L di/dt.

It's a crappy circuit but demonstrates the basic idea. With improvement, it can be practical.

Ideally... (neglecting a dozen variables that cause error.) dt ...measure pulse width (555 OUT) on a scope. di is fixed by the circuit v(t) is the supply

Solve for L

It is possible to extract L while rejecting parasitics Cp and Rs.

D from BC

I've the French version and it runs to 25kHz. Really nice kit. As I've lots of equipment and stuff to hand, I checked (all Ferrites) a 22uH wound toroid, a 220uH 8Amp power inductor and a big E core transformer with

1mH primary and 2 secondaries. All checked out OK within 2%, both on the LCR meter and one similar to the AADE unit. Inductors measured over a range of 100Hz to 500kHz. The E core transformer on a VNA showed self resonance at 1.4MHz. Looks like you've some kind of core material problem or a faulty meter somewhere. If the planar has a steel core then forget it, take a normal meter reading and multiply by at least ten :).