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Relationship between Frequency, Inductance and Resistance

Discussion in 'General Electronics Discussion' started by abuhafss, Oct 31, 2014.

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  1. abuhafss


    Aug 3, 2010

    Kindly have a look at this article. It describes a cheap and easy method of finding inductance of a coil.

    My question is how do we get the relationship:

    Inductance = 50 x Time period of one Cycle


    Inductance = 50 / Frequency of the Oscillation

    A detailed explanation or an article/tutorial link shall be appreciated.

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Here's the complete article as a single image:

    EDN Cheap easy inductance tester uses few components.png

    I don't know the answer to your question. I guess the 50 in the formula relates to the values of RL and RR in the schematic.
  3. abuhafss


    Aug 3, 2010
    Thanks for your response.

    I already have that complete article. Actually I requested a detailed answer to my question.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    I know. I was posting the article to make it easy for others to see what you're asking about, without having to go to the site and assemble the article text and the schematic, which are in separate locations on the EDN site. And I told you that I don't know the answer to your question.
  5. BobK


    Jan 5, 2010
    Nice little circuit.

    I LTSPICEd it with 2n3904's.

    For a 10u inductor I get 10.75uH from the formula.
    For a 100u inductor I get 101.5
    For a 1u inductor I get 1.55uH

    So it looks like it works pretty good down to 10u and is off by 50% at 1u.

    With RF transistors it would presumably do better at the low end.

    I think I will make one of these.

    Edit: with BF199 I get 1.24 for the 1uH inductor and 10.3 for the 10uH inductor.

    Last edited: Oct 31, 2014
  6. hevans1944

    hevans1944 Hop - AC8NS

    Jun 21, 2012
    The author of the article stated, "The frequency of oscillation depends on the L/R time constant comprising the inductance under test and resistors RL and RR." I don't think using precision resistors would help much with accuracy. In a reply to the article, someone noted that the frequency depends on supply voltage as well as the inductance value. In looking at Figure 2, the period of a full cycle appears to about 1.8 μsec for a 100 μH inductor. If you round that up to 2 μsec and multiply by 50, sure enough you get 100 μH, assuming the fudge factor, 50, has units of henrys.

    I wouldn't throw away my LCR meter just yet, but this does look like a quick way to sort through a pile of junk inductors, especially if you are looking for some that have similar values of inductance.

    Attached Files:

    Last edited: Oct 31, 2014
    KrisBlueNZ likes this.
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