# inductance measurement

Discussion in 'Electronic Design' started by Jamie Morken, Dec 21, 2007.

1. ### Jamie MorkenGuest

Hi all,

I have an inductance meter that measures at 1kHz and am getting an
measurement that is 21x higher than another meter that measures at
100kHz (which I don't have access to). Is this typical for the
inductance to change that much depending on the frequency?

The inductor is the primary of a 100kHz planar transformer.

Any recommendations on a good inductor tester? The current one I
have was \$25 on ebay

cheers,
Jamie

2. ### Paul MathewsGuest

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 = 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

3. ### Andrew HolmeGuest

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 http://www.aade.com/lcmeter.htm

and one of these http://www.arraysolutions.com/Products/AIM4170.htm

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

4. ### Paul Hovnanian P.E.Guest

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.

5. ### JamieGuest

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"

6. ### Guest

The one I use has already been recommended here.

http://www.aade.com/lcmeter.htm

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.

7. ### WinfieldGuest

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!

8. ### Jamie MorkenGuest

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 http://www.m3electronix.com/featureslcr.html
(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

9. ### D from BCGuest

I just wiped this up....

http://www.members.shaw.ca/chainsaw/SED/crudeLtest.jpg
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

10. ### john jardineGuest

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 .

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