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Relationship between Frequency, Inductance and Resistance

abuhafss

Aug 3, 2010
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Hi

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

or

Inductance = 50 / Frequency of the Oscillation

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

Thanks.
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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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.
 

abuhafss

Aug 3, 2010
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Thanks for your response.

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

KrisBlueNZ

Sadly passed away in 2015
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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.
 

BobK

Jan 5, 2010
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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.

Bob
 
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hevans1944

Hop - AC8NS
Jun 21, 2012
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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.
 

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