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Dear David, LF Group,

My experience is that for toroids, low-level measurement of AL is fairly reliable, allthough it does have quite a large tolerance. As far as I can see, according to the Ferroxcube data, the 58mm dia cores are the TL 58/41/18 core size - Assuming this is the correct core size, the web site gives AL for these in 3C90 material as 2890nH/t +/-25%, so 3249nH/t is quite feasible. The 3C11 material in the same core size has AL = 5400nH/t +/-25%, which is not consistent with your measurement. But the colour code for 3C90 is given as "ultramarine/white", while 3C11 is just plain white, so it looks like they forgot the blue paint.

The permeability of ferrites does change greatly with applied H, temperature, f, etc, but for an inductance measurement at room temp with small signals at low frequency (eg. with an audio bridge or similar), this is usually not a significant issue, especially with large core sizes. 3C90 has a nominal initial permeability of 2300, against 2000 for 3C85, so winding inductances will be similar for both materials. 3C90 should have slightly lower loss. The 3C11 material has higher core losses, and a much lower curie temperature than 3C90, so would not be very suitable for a PA.

----- Original Message -----

From: G0MRF@aol.com

To: rsgb_lf_group@blacksheep.org

Sent: Sunday, January 11, 2004 9:07 AM

Subject: LF: 3C90 Al value?

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