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Simple way to measure inductance?

Discussion in 'Electronic Basics' started by Abstract Dissonance, Feb 5, 2006.

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  1. I have some inductors with unknown inductance and I'm looking for a very
    simple but reliable method to measure its value(doesn't have to be precise
    or accurate but just get me in the neighborhood).

    I was thinking that I could simply using a constant current source to drive
    the inductor and measure the voltage drop across it then use the formula

    L = V/I/(2*Pi*f)

    to compute L.

    This works in theory but is it pratical? I was thinking of the mains to
    generate the frequency and about 1mA of current or so... the current acts as
    a scaling factor that can be used to for small or large inductors... and I
    suppose one could throw in a resistor to "fine tunning" but I'm mainly want
    something simple.

    Would this work? or is there an easier method(this seems like the easiest
    but who knows ;)?

  2. Guest

    60 cycles is NOT Very Practical, Unless there Very Large Inductors.
  3. JeffM

    JeffM Guest

  4. Not sure why this should matter? Does it have to do with the non-ideal
    characteristic of the inductor at those frequencies?

  5. JeffM

    JeffM Guest

  6. um... and how is that any different than what I did? Also where does it talk
    about real world issues that are involved?

  7. JeffM

    JeffM Guest

    [Resonance technique]
    You were talking about using line frequency
    As Gary said, that will have limited utility.

    You also mentioned (completely out of left field)
    a constant current source (a DC construct).
    There is an assumption
    that the "Q" (quality factor) of the coils isn't complete crap.
    Your initial request WAS for a ballpark number.
    If you want to spend some $$
    and quantify the parts more completely:
  8. David Harmon

    David Harmon Guest

    On Sat, 4 Feb 2006 23:03:09 -0600 in sci.electronics.basics,
    What is the inductive reactance of the inductor you are likely to be
    measuring at 60Hz? At 60kHz?
  9. Bob Monsen

    Bob Monsen Guest

    If your inductor is 1uF, then the impedance is

    1uF*2*PI*60 = 377u ohms.

    So, if you put a 1mA RMS current through it, you'll get 377nV across the

    A better way is to measure the resonant frequency that your mystery
    inductor produces when resonated with a known capacitor. You pluck it
    somehow, then measure the frequency of the resulting oscillations. You
    generally need a spectrum analyzer or an oscilloscope to do this.

    You could also buy one of these:

    It uses a microcontroller to measure the frequency. I have one, and it
    works pretty well.

    Bob Monsen

    "I am a strong advocate for free thought on all subjects, yet it
    appears to me (whether rightly or wrongly) that direct arguments
    against christianity & theism produce hardly any effect on the public;
    & freedom of thought is best promoted by the gradual illumination of
    men's minds, which follow from the advance of science. It has,
    therefore, been always my object to avoid writing on religion, & I
    have confined myself to science. I may, however, have been unduly
    biassed by the pain which it would give some members of my family, if
    I aided in any way direct attacks on religion"
    -- Charles Darwin
  10. I have no idea. I have some inductors I ripped off some old junk stuff a
    long time ago and they don't have any markings on them and have some
    "sleeve" on them. I guess it has about 50 wraps of about 18awg type of wire
    in a toroidal form with, I guess, a ceramic(or whatever) or iron core... I
    also have some other smaller inductors about th size of a nickel or so.

  11. what I ment was a constant current AC source(in that it delievers a sin wave
    with constant rms value(and not just some arbitrary current wave)).
    heh, but the sites you gave don't talk about measuring an inductor but just
    the mathematics behind it. Theoretically I could use any frequency and the
    simple configuration I gave using the math I gave(which I guess you

    This doesn't mean that it will even get me in an order of a magnituide in
    the real world and thats why I asked if it was good enough cause I can't
    look at an inductor and say its about x henries so I need something that can
    give me some idea and is going to work over a large range. Maybe a wein
    bridge type of method is good but I don't have any precision components to
    make it with and I really don't need something with more than 2-3
    significant digits(atleast for now).

    It as already been pointed out that using a low frequency can throw off the
    result but I need to know more about this and why.

  12. uH? isn't that kinda small?
    Yeah... I noticed that in simulating some results I can get very wide swings
    of voltage depending on the inductance and frequency... I can use anywhere
    from 1mA to 1A though but it doesn't help much(but a factor of a 1000 can
    kill my circuit if my inductance happens to be larger than what I
    expect(since with 1A I can get upwards of 1000V or something(don't remember
    the specifics now but had 1000V show up on the volt meter);/)

    naw... I don't need to spend that kinda money on something I probably won't
    use much and that I might be able to make on my own(to a good enough
    approximation to satisfy my needs). I'd rather spend the money on the
    components and try to make my own and screw up but learn something.

    I have seen some schematics on some pwm type of inductor meters that seems
    relatively easy to do. If I recall correctly it uses a uC or some other
    method to generate a square wave at some frequency and passes it through the
    inductor(or some circuit with it in there) and then determines its
    inductance from that circuit(I think it uses the resonance method you spoke

  13. Mike

    Mike Guest

    The simplest and most reliable method kinda depends on what test
    equipment you have avaiable.
    Scope? Signal Gem? DMM?

  14. Rich Webb

    Rich Webb Guest

    I put together one of those LC meters also and agree with Bob that
    they're a pretty good value for a home shop.

    For one-off measurements, if you have a signal generator, frequency
    counter, and o-scope you can use a simple bandpass layout to get the
    value of the inductor. Vary the frequency to peak the output; at the
    peak Zc = Zi.

    ___ ___
    Input| \_------|___|-----------. measure here
    here |___/ |
    | |
    | |
    --- C|
    --- C|
    | C|
    | |
    (created by AACircuit v1.28 beta 10/06/04

  15. Guest

    You could build a colpitts oscillator and measure the output frequency.
  16. John Fields

    John Fields Guest

    Using a series-resonant circuit, you'll get a better peak if you do
    it like this:

    | |
    [L] [DET]
    | |

    Where the impedance of DET at resonance is >> than Xl at the
    frequency of interest,

    Or a suck-out if you do it like this:

    | |
    [C] |
    | |
    | [DET]
    | |
    [L] |
    | |

    Where the impedance of the detector becomes less critical.
  17. kell

    kell Guest

    If you use 60 Hz to measure a small coil, the inductive reactance will
    be so small that the resistance in the wire of the coil will swamp it.
    Been there done that.
    You can build an inductance meter
  18. kell

    kell Guest

    If you use 60 Hz to measure a small coil, the inductive reactance will
    be so small that the resistance in the wire of the coil will swamp it.
    Been there done that.
    You can build an inductance meter
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