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Inductance at high frequencies

  1. Nov 13, 2011 #1
    Hi All,
    I have a component that at 1 Khz has 100 mH Inductance and 100 Ω resistance.
    I know that at higher frequencies which I will need to run the circuit, these values wont be the same anymore.
    Is there any way, mathematically, to figure out what those values would be at higher frequencies? Unfortunately the datasheet doesnt say the device behavior at higher frequencies.

    My last option would be to measure those values experimentally but it would be time consuming, and since the circuit would run at different frequencies it would make my project more complex by having to look up the values of that component and calculating the rest of circuit inputs for proper outputs output values.

    Any help and advice would be appreciated.
  2. jcsd
  3. Nov 13, 2011 #2


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    Science Advisor

    The reactance of that inductor at 1000 Hz will be 628 ohms.

    You can work this out with the formula
    Reactance = 2 * π * F * L
    where F= frequency in Hz and L = Inductance in Henries.

    So, you could work out the reactance at some other frequency and this would change although the inductance itself might not change. If the inductor was wound on an iron core, then it would change inductance with frequency.

    The resistance will change with frequency due to an effect called "skin effect".
    Basically, this means that AC current will travel in just the outside of the wire, not the whole wire, so this makes the resistance of the wire seem higher than it would at some lower frequency, or at DC.
    This effect gets greater with increasing frequencies.
  4. Nov 13, 2011 #3
    For one, what frequency range you'll be working with. If it is like 10KHz, you can just trust it. But if you are using it at 100MHz, it is a different world.

    There is no easy way to determine the inductance short of experiment and measure. It is complicated that if you are designing for a product, even if you characterized one inductor, that don't mean they are not going to change in the future production. You really need to find one that specified for the frequencies you are working with.

    To look at it in more detail, the resistance of the inductor will not change with frequency( within reason before the conductance kick in). The inductance should not change with frequency on the first pass. BUT the major factor is the interwinding capacitance. There is no easy way to measure as it is a distributed value. The capacitance will cause a resonance at certain frequency and it will become a complicated circuit instead of a simple inductor.

    Bottom line, find one that specifies at the frequency you are working at.
  5. Nov 13, 2011 #4
    One tends to use components appropriate to the frequency range he is working in. This means that inductors, capacitors or resistors that would be used at audio frequencies wouldn't be used at radio frequencies.

    For inductors in particular, at higher frequencies, the interwinding capacitive reactance can become equal to or less than the inductive reactance. If they are equal the inductor becomes parallel resonant and at higher frequencies the inductor becomes a capacitor.
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