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Curious inductor question

Discussion in 'General Electronics Discussion' started by pgib8, Jun 17, 2016.

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  1. pgib8

    pgib8

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    Jul 26, 2015
    Inductors present a higher resistance to higher frequencies.

    I'm sending a sine wave through an inductor measuring input and output. Strangely the voltage at the output increases with frequency rather than get smaller. The key for this to work is with no load. Even a 30K resistor will stop this effect.

    Years ago I heard about the Ferranti effect with long transmission lines at no or minimal load. Does this apply here too or is there something different going on?
     

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  2. duke37

    duke37

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    Jan 9, 2011
    You cannot get a pure inductor, it will always include capacitance due to the adjacent turns. This results in a tuned circuit which will resonate at some frequency. A 30k resistor may well damp this resonance so that the effect you see is eliminated. Wave winding can be used to reduce the capacitance of RF coils.

    Care has to be employed when designing power supplies that what is intended to be a low pass filter does not raise certain frequencies due to non ideal components.
     
    (*steve*) likes this.
  3. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    Change the freq and see what happens
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    No load? How do you know the output increases with frequency? If measuring the output with an oscilloscope there is still a load, usually about one megohm shunted with twelve picofarads or so of capacitance. This will form a resonant circuit with your inductor. More information, please, before wandering off into Ferranti Effect Land, which is mainly a concern for power transmission over long distances. Did you have a few kilometers of coax cable attached to your inductor and measured at the far end of the cable?
     
  5. Ratch

    Ratch

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    Mar 10, 2013
    No, inductors present a higher reactance to higher frequencies. Reactance and resistance are two different phenomena.

    Ratch
     
    hevans1944 likes this.
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