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Oscillator with an inductance with nonzero resistance

  1. Mar 2, 2008 #1
    I need to measure ONLY a change in inductance of a loop (long wire) which has got resistance about 30-200 ohms.
    I have tried to build the oscillator according to this schematic : http://ironbark.bendigo.latrobe.edu.au/~rice/lc/
    but the problem is it works well with an inductance with almost zero resistance only.
    If I connect my loop, it behave wrongly. I guess because of the energy disappation?

    Therefore, I need to modify that circuit, or use another another one. Unfortunately, I havent found anything about how to make an oscillator with an resistive inductor.

    I read about using a negative resistance circuit but I have no clue how to employ that.
    I have also heard about using a transformer (perhaps it is called tapped inductors?).

    To summarize my approach:
    I only need to detect a change in inductance of a wire loop.
    I prefer to have an oscillator with TTL output and to measure therefore the frequency (in a microcontroller)

    May I ask you for some hints on that?
    Thanks in advance,
    Best regards Meereck
    Last edited: Mar 2, 2008
  2. jcsd
  3. Mar 3, 2008 #2


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    Staff: Mentor

    What is the range of inductance that you are trying to measure, in addition to the 30-200 Ohms of real resistance?

    I would probably approach it by driving a signal from a known source impedance, and measuring the in-phase and quadrature components of the divided AC voltage waveform. That will give you values for both the real resistance and the reactive inductive impedance.
  4. Mar 3, 2008 #3
    thanks for a reply.
    The inductance will be about 100-300 microH, its pure resistance will be 30-200Ohm.
    >>measuring the in-phase and quadrature components of the divided AC voltage waveform.
    Right, but this will probably be quite difficult to measure by a common microcontroller such as a PIC. Would you have any supporting circuit for that?

    cheers M.
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