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"self resonant frequency" of an inductor

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Posted on November 25, 2014 at 07:09:53
Lew
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Location: Bethesda, Maryland
Joined: December 11, 2000
 I was searching for some small size, low value, low current inductors on Mouser, when I noticed that they give a spec for "self resonant frequency" of each inductor. It surprised me to see that parts with very similar current and inductance ratings can vary widely in their self resonant frequency. For example, as low as 1kHz and as high as >1MHz, for two different products rated at ~100mH/50 to 100mA. My instinct would be to choose an inductor whose self resonant frequency is well above the audio range, e.g., 1kHz would seem to be unacceptable for audio. Is this spec indeed something to think about, or does it not matter much? Thanks.

 

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RE: "self resonant frequency" of an inductor, posted on November 25, 2014 at 07:40:14
Eli Duttman
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Posts: 10455
Location: Monroe Township, NJ
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 The self resonant freq. DOES matter. Your instinct is correct.

The turns of wire that make up the choke cause a capacitance to be in parallel with the inductance. Remember, at the resonant freq., a parallel LC setup presents an "infinite" impedance.


Eli D.

 

Thanks very much..., posted on November 25, 2014 at 07:46:33
Lew
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Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 The inherent capacitance of an inductor, would it be in parallel with any external added capacitance, in an LC or CLC filter, for example? If so, and since the external capacitance is likely to be many orders of magnitude higher than the self-capacitance of the inductor, would that not alter the actual resonant frequency, greatly? (But I can see that the capacitance of an inductor may not be considered in parallel with external C.)

 

RE: "self resonant frequency" of an inductor, posted on November 25, 2014 at 12:45:44
Tom Bavis
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Joined: May 25, 2007
 1 KHz doesn't sound physically possible - to resonate 100 mH at 1 KHz would require a winding capacitance of .25 uF. 100 pF might be possible, depending on size, and would resonate it at 50 KHz.

 

RE: Thanks very much..., posted on November 25, 2014 at 14:51:19
Steve O
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 The internal capacitance of an inductor isn't influenced by external capacitance to any great extent. Also, above the self resonant freq. the inductor begins to behave as if it's a capacitor which could be a factor in overall filter performance.

 

Thanks for the further clarification..., posted on November 25, 2014 at 20:09:04
Lew
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Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 So, above 1kHz, the one inductor I found on Mouser with 1 kHz self-resonant frequency would begin to behave as a capacitor that does not block DC. It's capacitative in that its impedance goes down with increasing frequency above 1 kHz, rather than up, as one would expect from an inductor. (This is well described in Morgan Jones' books; I must have banked the knowledge in some shady area of my brain.)

 

RE: "self resonant frequency" of an inductor, posted on November 26, 2014 at 05:35:45
cpotl
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Posts: 1002
Location: Texas
Joined: December 6, 2009
 "1 KHz doesn't sound physically possible - to resonate 100 mH at 1 KHz would require a winding capacitance of .25 uF. 100 pF might be possible, depending on size, and would resonate it at 50 KHz."

Agreed. Something doesn't sound right. It is not plausible that the self capacitance of the winding in a 100mH inductor could be .25uF, and yet that is what it would take to be resonant at 1 KHz. A typo in the specs sounds likely.

Chris

 

RE: Thanks for the further clarification..., posted on November 26, 2014 at 05:54:21
Steve O
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 Actually, to a simple approximation, it is capacitance in parallel with the series resistance of the inductor, thus the ability conduct DC. Also take note of comments above regarding that 1KHz spec. 1KHz isn't really plausible for a 100mH inductor so must be a typo. Certainly bears further investigation.

 

RE: "self resonant frequency" of an inductor, posted on November 27, 2014 at 21:14:51
Lew
Audiophile

Posts: 10912
Location: Bethesda, Maryland
Joined: December 11, 2000
 was actually some value slightly lower than 100mH but not much. The low quoted self resonant frequency is exactly why i asked my question. the 100 mH inductors i did buy have resonant frquency at 0.9 MHz. if there is interest i can go back on Mouser website and identify the actual part. i think it was 86 mH value.

 

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