[TCML] Dr. Resonance inductors - 1/16th of an inch?

bartb bartb at classictesla.com
Sun Mar 8 10:31:09 MST 2009

```Hi David,

David Dean wrote:
> So it is the wrong tool for the job. Still something wrong with that
> javascript
> though, does not work right with the proper formula either.
The Proline javascript is using the following equation Wheeler
approximation.

L = (d² * n²)/(18d+40*l)
where:
L is inductance in micro Henrys,
d coil diameter in inches,
l  is coil length in inches, and
n is number of turns.

It's obvious from the result and script that Proline has determined "d"
to be the average diameter.

http://www.pronine.ca/multind.htm
For a 2"D x 5"L coil of 5H using 24 awg, they will show 11731 turns and
49.97 layers (their 24 Awg table uses .0213" wire diameter).

That's what's going on with the Proline Calculator. The question is, are
they correct to use the average diameter here? Wheeler approximations
would show 4H for the above (so 20% error). What Proline has done is
taken the Wheeler equation for an single air core solenoid, L = (d² *
n²)/(18d+40*l), and changed d to indicate the average diameter of the
coil for a multilayer coil. Sort of a hack job I think, but you can
understand what was being attempted here.

Probably best to not rely on the Proline calculator and simply stick
with Wheeler approximations.

Take care,
Bart
```