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LF coil inductance?

Discussion in 'Electronic Design' started by Robert Sterns, Sep 5, 2004.

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  1. How can I determine the inductance of a coil at frequencies below
    10Hz?

    It seems most hobbiest meters apply a higher frequency.

    Robert
     
  2. Pooh Bear

    Pooh Bear Guest

    10 Hz is impracticably low for most puroposes.

    Why do you think the inductance will change significantly with frequency
    ?


    Graham
     
  3. Tim Wescott

    Tim Wescott Guest

    10Hz, or 10kHz? 10Hz is _very_ low unless you're working on high-end audio.

    In any case, if you can safely assume that the coil's impedance will be
    largely a result of it's inductance do the following: connect the coil
    in series with a resistor and apply a sine wave at your chosen
    frequency. Measure the AC voltage across the resistor, and across the
    inductor. The impedance of the coil at that frequency will be:

    |Zl| = R * |Vl|/|Vr|.

    Assuming that the coil is purely inductive then Xl = |Zl|, and the
    inductance of the coil is Xl/(2*pi*f).

    If you _can't_ assume that the coil is purely inductive, but you _do_
    have an oscilloscope, then see if you can measure Vl and Vr
    simultaneously to get both amplitude and phase information. In that
    case you can express them as vector quantities and get a vector
    impedance. The real part of this impedance will then be the series
    equivalent resistance of the coil, and the inductance will be the series
    equivalent inductance, at your frequency of interest.
     
  4. Tim Wescott

    Tim Wescott Guest

    Inductance wouldn't change with frequency, but effective inductance
    would. I could see measuring inductance at 10Hz either because I'm
    working on high-end tube audio and I'm concerned about performance down
    there, or because I'm working at some higher frequency and I want to
    eliminate the effects of some suspected parasitic that shows as a
    parallel impedance.
     
  5. Jim Thompson

    Jim Thompson Guest

    Isn't "high-end tube audio" an oxymoron ?:)

    And be sure you wire up your speakers using parallel-plate
    transmission lines ;-)

    ...Jim Thompson
     
  6. The inductance would have to be very large in order for the reactance to be
    appreciable at 10 Hz. What are you measuring the inductance of?
     
  7. I think we can assume that anything that has appreciable inductive reactance
    at 10 Hz will also have appreciable resistance.
     
  8. Tim Wescott

    Tim Wescott Guest

    We know your prejudices against vacuum-mode FETs, you've already stated
    them. I was speaking of high $$ = high end.
     
  9. Depends on whether you are talking performance or price.
     
  10. Robert Baer

    Robert Baer Guest

    The lower the frequency, the less effect any parasitics like stray
    capacitance and lead inductance have.
    The only "parasitic" element is the DC resistance, which has an effect
    at all reasonable frequencies.
    You are getting into an area that is untenable.
     
  11. James Meyer

    James Meyer Guest

    You failed to warn readers to use **exactly** the same length of
    transmission line for every connection in order to preserve proper phase
    relationships.

    Jim (the other one) Meyer
     
  12. Dave VanHorn

    Dave VanHorn Guest

    How can I determine the inductance of a coil at frequencies below
    Assuming you know it's resistance, then you should be able to build a
    divider against a known resistor, feed it with a known amplitude at the test
    frequency, and measure amplitude and phase shift.
     
  13. Tim Wescott

    Tim Wescott Guest

    Right. So using a lower frequency will reduce these effects, just like
    I said.
    Right. So you can't go too low in frequency below the design range of
    the inductor or you'll just be measuring the resistance. Hopefully the
    OP knows this.
    Oh? How?
     
  14. ASCII's not the best medium for reproducing formulas in. What exactly
    are the quantities

    "Zl" and "Vl" supposed to refer to?

    Thanks,

    p.
     
  15. I read in sci.electronics.design that Paul Burridge

    No, but with care, it is possible to write quite complex expressions
    that can still be deciphered.
    Now this is a much more prevalent problem. People don't define their
    symbols. Sometimes, you even find five or six defined and then see in
    the equations a symbol k or x that isn't defined, so the whole thing is
    useless.
     
  16. James Meyer

    James Meyer Guest

    Perhaps not too useless...

    The symbol k is usually taken as a scalar serving as a proportionality
    constant. It's normally derived by experimental methods and not really
    important to the understanding of the mechanism that the equation is trying to
    describe.

    And as for x...

    The symbol x is usually reserved for an unknown quantity. If it could
    be defined, then it wouldn't be unknown, would it?

    Jim
     
  17. Tim Wescott

    Tim Wescott Guest

    Zl would either be the first complex impedance, or the complex impedance
    of the coil. Vl refers to voltage, same thing. In this case one would
    assume that it's an 'L' and they're referring to the coil.

    When I was taking computer programming oh so long ago at age 15 one of
    the girls in the class was confused by the model 33 teletype's lack of
    lower-case letters, because she had been taught in typing class to
    _always_ ignore the '1' key, and use a lower case 'L' instead. She
    tried typing numbers with an upper case 'L', but that didn't work.

    It was a measure of my ignorance of girls that rather than commiserating
    with her (she was very good-looking) I laughed instead.
     
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