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Inductor's Parasitic DC Resistance Question

Discussion in 'Electronic Basics' started by JLD, Jul 22, 2004.

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  1. JLD

    JLD Guest

    Hello all. This is my first post.

    I was wondering about the DC resistance of inductors. If you take an
    inductor in hand and measure the resistance there will be some
    resistance, somewhere around 10 - 30 ohms for a typical 100mH for
    example. What I want to know is what is the bigger "cause" of that
    resistance. Is it the resistance of the actual wire or is it something
    else? Maybe cross coupled E/U fields? Does the core of the inductor
    affect this DC resistance? Is this a result of the manufacturing of
    the inductor (parasitic R)?
    I know that an inductor in a circuit under steady state acts as a
    short, so is that resistance is still there? Also if that resistance
    is still there why is there such a small voltage drop across it?

    tia

    JJ
     
  2. John Larkin

    John Larkin Guest

    It's just the copper (wire) resistance. A bigger 100 mH inductor,
    wound with fatter wire, would have less resistance.
    Yup. It's not a perfect short.

    It follows Ohm's law. If the DC drop is low, it's because the DC
    current is low.

    Imagine your inductor as a perfect inductor with a resistor in series
    with it. That's pretty much how it will behave.

    John
     
  3. If measured with an ohm meter, you are seeing just the wire
    resistance. It would be just the same if you unwound the inductor and
    remeasured the wire resistance.

    At operating frequency, the effective wire resistance goes up, because
    each half cycle, the magnetic field reversal crowds the current to the
    outside part of the wire. The higher the frequency, the less time the
    magnetic field has ot penetrate the wire, to let the current density
    in the wire core approach that at the surface. So at high enough
    frequency, the wore acts as if it were a tube.
    http://www.fact-index.com/s/sk/skin_effect.html
    There are also exaggerations of this effect when turns are wrapped
    around a gap in the core, where the core field fringes out through the
    winding.

    The core also consumes energy and may circulate a significant amount
    of current that looks like a loaded secondary with the winding acting
    as a primary. This looks from the ends of the winding like a
    resistance in parallel with the inductor.

    And we haven't even started into the effects of capacitance.
     
  4. andy

    andy Guest

    afaik, it's just the resistance of the wire. for a DC signal, that's all
    it could be, because there is no varying electromagnetic field to create
    any other effects.
     
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