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In Reply to: RE: Lead foil to block transformer fields.... posted by blah on April 13, 2008 at 18:26:54
Be very careful about real mu-metal and transformers.
There are two different basic alloys suitable for magnetic shields:
Nickel alloys (such as mu metal, a trademark)
Iron alloys (slightly fancy steels).
Note that magnetic shields do not absorb fields, they reroute the magnetic lines through the shield material and away from the victim.
The problem is that the concentrated field lines in the thin magnetic material give a high flux density ("B field").
Roughly, the resulting field outside the metal is that high B field divided by the permeability of the metal.
Mu metal has a relatively low saturation B field, but extremely high permeability. If you keep the internal field below saturation, it works beautifully.
However, if you do the math (not handwaving), you may well find that the leakage flux from your transformer divided by the small cross-section area of the shield will saturate mu metal. In that case, you want a first layer of iron alloy (cold-rolled steel may be enough), and use a second layer (spaced away from the iron by a thin air, paper, or plastic gap) of mu metal to further decrease the leakage.
Copper shields are often used to shield AC magnetic fields, which is a separate section of the shielding textbook. Ferrous shields work down to DC.
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