Vernall wrote:

Alberto,

I think you'll find that each Amidon formula is "different" by scaling
factor only.  Inductance is still "turns squared" and scaling coefficients
and turn reference numbers are selected to arrive at convenient values in
millihenries for ferrites (generally high Al values) and microhenries for
powdered iron (generally low Al values).

Bob,
   I do not agree with you, sorry... from an Amidon document, in the chapter
regarding iron-powder toroids :

               N = 100 * SQRT(uH / AL)

     where :
       N      = number of turns
       uH    = wanted inductance in uH
       AL   = uH for 100 turns

Where in the chapter of ferrite toroids the formula is given as :

               N = 1000 * SQRT(mH / AL)

     where :

       N      = number of turns
       mH    = wanted inductance in mH
       AL    = mH for 1000 turns

Mathematically speaking, they are not equivalent. And also in practice,
they aren't too. I know, as I tried to compute the number of turns needed
on a T37-2 core to obtain 2.5 uH of inductance. That core has a tabulated
AL of 40.

Applying the first formula, the result is :  100 * SQRT(2.5 / 40) = 25 turns

Applying the second one, the result is : 1000 * SQRT(0.0025 / 40) = 7.9 turns

I wound 8 turns on such core, and measured 0.28 uH, in good accord with the
theoretical value (computed with the first formula) of 0.26 uH.

The original spreadsheet by WA1ION used the second formula for both kind of
cores, that's why I spotted the error, when trying to wind my 2.5 uH inductor, 
and
ending up with a 0.28 uH one....

73  Alberto  I2PHD

P.S. Yes Alan and others, the spreadsheet is done with Excel 2000, sorry...
I will check if there the possibility to convert it to a previous format.