Envelope-to: dave@picks.force9.co.uk
Delivery-date: Fri, 03 Feb 2006 14:49:58 +0000
From: "james moritz" <j.r.moritz@herts.ac.uk>
To: rsgb_lf_group@blacksheep.org
Date: Fri, 3 Feb 2006 14:48:15 -0000
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Subject: LF: RE: Keeping the smoke in
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Dear Warren, LF Group,

Sorry to hear about your problem. The transformer core seems more than
adequate for the power level you are using - the flux density in the core at
1.2kW should be about 40mT, which is very low, so core size is more than
ample; certainly I am using smaller cores at the same power level without
heating problems. At that flux density, magnetic losses in the core would be
something like 5 or 6W, according to the Fair-Rite graphs for 77 material.
You might lose a similar amount in the winding, making about 12W loss
overall. I guess if this was thermally very well insulated it could get very
hot - this might happen if the box has very low thermal conductivity, and
there are other heat-producing components inside as well. 

What mystifies me is why high voltages should also be present around the
transformer. 1.2kW into 50R is about 245V RMS, which is nothing in corona
terms. The low impedance secondary should only have 46V across it. A high
voltage between primary and secondary windings could result from having the
transformer connected into the loop at a high potential point, but it isn't
obvious to me why that would be done. This could also result in increased
core losses due to dielectric loss in the ferrite.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Warren K2ORS/WD2XGJ
Sent: 02 February 2006 22:41
To: rsgb lf reflector
Cc: LWCA LF Reflector
Subject: LF: Keeping the smoke in

Hello the list,

      I've heard that the secret to keeping things going in electronics is
to make sure you don't let the smoke out (that's what makes it all work
apparently).

....
    I guess I need a bigger core than the 3 stacked FT-290-77s any
suggestions out there?

--
73 Warren K2ORS/WD2XGJ
FN42hi
http://www.w4dex.com/wd2xgj.htm