Message-ID: <3D563604.BF5139FF@usa.net>
Date: Sun, 11 Aug 2002 12:01:40 +0200
From: "Alberto di Bene" <dibene@usa.net>
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To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: An Excel sheet to play with toroids
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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.