Date: Sun, 11 Jan 2004 13:07:20 +0100
Message-ID: <200401111207.i0BC7KQ16752@mailgate5.cinetic.de>
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From: "Hans-Albrecht Haffa" <dk8nd@web.de>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re:3C90  Al value?
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Dear Dave, 

dont calculate the AL-value, try to get out the relative permeability of the material.
Your formula: 
L=n2*AL

my formula:

L=u*u°*n2*Afe/lfe

lfe is the length of the magnetic path.
Permeability constant of the vacuum: uo=4*Pi*1E-7 (V*s/A*m)
Afe: magnetic cross section in basic unit m2, i.e. 1mm2=1E-6 m2

If You know the relative permeability, than the frequency rage and the losses may be estimated. All magnetic materials with a similar permeability of Valvo -  Ferroxcube  - Philips and competitors are similar. I never could measure any practical difference in using N27/N67/FI322/FI324 using cores for switch mode power supplies.  It is easy to reduce magnetic losses in the core, but nearly impossible to optimize winding copper. Wire thicknes (wire, Litz or copper foil) frequency, and current and of course temperature class ( 135 °C, 155°C or even 180°C) of insulation will never harmonize on 136kHz. 
Please note: 

If You cannot reduce current density in the wire anymore, as well as reduicing magnetization (B) of the core, than oe must magnify the surface of the core, th omprove cooling.  Two (2) toroids stacked hav twice teh cross section, but not thwice the surface...
More upon request.

55, Hans-Albrecht
DK 8 ND


rsgb_lf_group@blacksheep.org schrieb am 11.01.04 10:11:54:

Hello Group.

After a 4 month wait I've received a batch of 58mm cores claiming to be 3C90, but according to the catalogue they have the wrong colour code. These are white (pos 3C11) not blue/white for 3C90.

The way to confirm is to calculate the Al value and compare to the data on 3C90 3C85 3C11 etc materials. Unfortunately 3C90 is not listed in my old data book.

I've measured the inductance of a 21 turn winding at 1.43mH. 
Using  L = Al x N squared, this suggests an Al value of 3249.

Is this a good figure for 3C90?  It does seem far too low for 3C11 (5000+)
Perhaps they've changed the colour code again?

73

David    G0MRF


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