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Inductive resistors

Discussion in 'Hobby Electronics' started by Phil Allison, Jun 25, 2011.

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  1. Phil Allison

    Phil Allison Guest

    ** Hi all,

    the self inductance of WW resistors has been a hot topic here - as makers
    rarely specify how much inductance their products have nor publish impedance
    curves.

    For most WW resistors over a few hundred ohms, inductance issues rarely
    arise - but for ones under 100 ohms and in the larger wattage ratings, it
    can be wise to check.

    One resistor is this category is sold by WES Components under the code "
    100W8 " and is a 100 watt ( free air rated ) tubular ceramic type, wound
    with a 2.3 mm flat strip conductor. The wind diameter is 28 mm and the
    length from beginning to end of the wind is 110mm. There are 31 turns of
    strip.

    The WES catalogue claims they are " non -inductive" but this is complete
    nonsense.

    They make useful dummy loads for audio amplifiers and if submerged in water
    will happily dissipate up to 1000 watts - being a hollow tube helps a lot
    with water cooling.

    Using an on-line calculator ( see below) gives an estimated inductance of
    6.0 uH.

    http://www.daycounter.com/Calculators/Air-Core-Inductor-Calculator.phtml

    But as there are significant ( 1.3 mm ) gaps between each turn on the
    resistor, the inductance value ought to be less than the calculated one.
    However, the opposite proved to be the case as the resistors show an
    inductance of 9.4 uH (with a 1uF cap series resonance occurred at 52 kHz).

    The reason is rather obvious, when you do one more simple test on the
    resistor.

    Anyone know ?


    ..... Phil
     
  2. Phil Allison

    Phil Allison Guest

    "Bruce Varley"

    ** Nice, white ceramic tube - as I said above.




    ..... Phil
     
  3. Franc Zabkar

    Franc Zabkar Guest

    That should be easy to test. Just unwind the 31 turns and stretch them
    out in a straight line. Then repeat the measurement. I'd be extremely
    surprised if that was the reason, though.

    - Franc Zabkar
     
  4. Phil Allison

    Phil Allison Guest

    "TonyS"
    ** Good guess.

    The strip is highly magnetic, probably a varierty of lower grade stainless
    steel that is magnetic.

    If you imagine a copper wire coated in steel - the idea that it increases
    the inductance is obvious.



    .... Phil
     
  5. Phil Allison

    Phil Allison Guest

    "kreed"
    Strange claim, as even a straight piece of wire is inductive (though
    not very).

    ** Nothing strange at all.

    There are several ways to wind wire on a tubular former that cancel out all
    inductance.

    http://en.wikipedia.org/wiki/Resistor#Wirewound


    ..... Phil
     
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