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Inductance Measurement

Discussion in 'Electronic Basics' started by Larry Kirkland, Jan 28, 2005.

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  1. I have a General Radio 1650A Impedance Bridge. It has two positions for
    measuring unknown inductance, Lp (parallel inductance) and Ls (series
    inductance). The book is not clear as to the differences. I can get
    significant differences when measuring the same coil on the two settings.

    The book says that Ls equals Lp within 1% when the Q of the coil is
    greater than 10. The coils I'm measuring have a Q of less than 10 at
    the bridge frequency of 1 Khz (2 or 3). Which scale should I use? What
    am I missing here?

    Thanks
    Larry
     
  2. Parallel resistance is normally caused by core losses. Air core coils
    generally have their losses dominated by the wire series resistance.
    You should be able to compare the DC resistance to Rs and see if they
    agree fairly closely. If so, then the series resistance approximation
    is the way to go. If there is a big discrepancy between DC resistance
    and Rs then parallel loss will probably give a better measure of
    inductance.
     
  3. Fred Abse

    Fred Abse Guest

    Any combination of reactance (inductive or capacitive) and resistance may
    be represented as either a perfect reactance and resistance in series, or
    a perfect reactance and resistance in parallel. The values of both
    reactance and resistance in each case are *not* equal. The values of
    reactance in each case become close when Q>10.

    With a Q between 2 and 3, the values of series and parallel reactance,
    hence inductance will differ significantly.

    Which value to use? It all depends... In the case of small, air-cored
    coils, it is usual to use the series case. With big, iron-cored coils, the
    parallel case is usual.

    Google for "hay maxwell bridge" - plenty of information.

    Also try googling for "series parallel impedance conversion"

    Omit the quotes in both cases.
     
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