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inductance measurement

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

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  1. Jamie Morken

    Jamie Morken Guest

    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 Mathews

    Paul Mathews Guest

    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 Holme

    Andrew Holme Guest

    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. 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. Jamie

    Jamie Guest

    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. Winfield

    Winfield Guest

    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 Morken

    Jamie Morken Guest

    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 BC

    D from BC Guest

    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 jardine

    john jardine Guest

    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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