"Rich Grise" wrote in message
news
On Thu, 12 Apr 2007 17:49:49 +0000, SparkyGuy wrote:
Mu-metal probably won't help.

My understanding is that it should be high-nickel content metal, such as
is
mu, and that without that content magnetic fields aren't much effected by
metals.

A copper shorting strap around the
transformer would. You can make that out of sheet copper, or even bare
wire and a lot of solder.

Or does solder serve as the poor-man's nickel?

Sounds like DIY mu-copper :-) You say "strap" which I interpret as not
full-height of the transformer. Should it be less-than the height of the
lams? Or is taller better?

It goes right on the outside of the lams, and around the windings, in
the same direction as the windings, like this:
http://www.germes-online.com/direct/...ransformer.jpg

It acts as a shorted turn; it has something to do with "leakage
inductance".

Cheers!
Rich


That transformer in the picture is a ferrite cored one from a switch-mode
power supply, and is designed to run at a frequency of around 100kHz. A
copper band around such a tranny, is a whole different kettle of fish to
putting one around a low frequency power transformer. Some such trannies
have a copper shield right around the windings, inside the lamination limbs,
but if I recall correctly, this is more for electrostatic shielding
purposes, than magnetic. The first thing I would try, before getting all
complicated with mumetal shields, is to rotate the tranny through 90 degrees
in the horizontal, and if that doesn't work, doing the same in the vertical,
or any combination. I've found that this will often reduce or even eliminate
stray field pickup by adjacent circuitry. The tranny mounting holes may even
still line up in the best case, but even worst case, it's only a case of
drill 4 more, or make up a right-angle bracket.

Arfa