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Date: Tue, 11 Jun 2002 13:07:09 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: RE: LF: DJ
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Dear Andy, Mal, LF Group,

At 11:32 11/06/2002 +0100, you wrote:
>A T200 core does intuitively seem a bit small, but the theory says it should
>work

Well, up to the point that it won't saturate - but that is just the first 
hurdle. I was slightly amazed to find the micrometals catalogue at work in 
my filing cabinet, containing core loss data for -2 mix amongst other 
things. There are 2 versions of the T200 toroid, one is 14mm thick, the 
other is 25mm. Lets assume Mal is using the big thick one. The area of this 
is 2.32cm^2, using Andy's figures, the flux density works out to 50mT or 
500 gauss. At 136k this results in core loss of 0.44W/cm^3, and with a 
volume of 30cm^3 the total core loss is about 13W. The thinner core is 
about double this. By itself, the 13W will result in a temperature rise of 
about 50 or so degrees C - the resistive loss in the windings will add at 
least a few watts and another 20deg C or so to this. By the time you have 
added some more temperature rise due to being in a box and the insulating 
effect of the PCB, the actual operating temperature will be way over 100deg C.

I think stacking 2 of these cores would give a more reasonable temperature 
rise - but the power dissipation would still be more than the original 
air-cored coil - take your pick!

Cheers, Jim Moritz
73 de M0BMU