[TowerTalk] Determine inductance of a Torrid with N Turns

[Jim Brown]

Hi Chuck,

That core is #31 material, which is designed for use to wind common mode 
chokes. An effective common mode choke has a low-Q parallel resonant at 
frequencies where we want to use it. The resonance is between its 
inductance and its "stray" (some say "parasitic") capacitance. The 
resistive component is coupled from the core in series with the 
inductance.  The value of C is so dependent on winding style and the 
value of mu has a relatively wide tolerance that I've never looked at 
the equations you cite for this application. What matters is that R and 
L increase proportionally with N squared and C increases proportionally 
with N, where N is the number of turns. The circuit Q of a choke wound 
on #31 cores to resonate at HF is typically around 0.4 to 0.5. This very 
low Q makes the resonance quite broad, and the dimensional resonance of 
the core adds another octave of bandwidth on the low end when the 
resonance is below about 4 MHz.

Lots of detail about this in k9yc.com/RFI-Ham.pdf

If you want a ferrite core for use as an inductor, other materials are a 
better choice.

73, Jim K9YC


On 10/3/2017 8:46 AM, Chuck Gooden wrote:
> Could someone please tell me how I can determine the inductance of N 
> Turns of wire on a Fair-rite #2631803802 (2.4"x1.4"x0.5") Torid?
>
> I am trying to use the formula on page 140 for the 2017 Fair-Rite 
> catalog.  I think my trouble is with calculating the value for C1.   I 
> believe this will be a ratio of the cross sectional area of the torid 
> core and the circumference of the torid (l).
>
> I have looked at other inductance equations on the net and I am also 
> confused why the Fair-Rite uses 4 * PI, the other equations use 2 * PI.
>
> I am working in the metric values given in the Fair-Rite catalog even 
> though I gave the dimensions in inches above,  and I am using a value 
> of 1500 for Ui.
>
> Thanks,
>
> Chuck - K9LC
>
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