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When reverse engineering a common mode choke from a line filter I found a wrap of very thin metallic foil inside the plastic housing of the toroid core. A little research on magnetic materials indicated to me, that the core material has to be some kind of permalloy because the L/N² value is exceptionally high, about 40uH/N². And then I asked myself the following question: wouldn't it make sense to build the core from solid permalloy instead of the thin foil? After all the only benefit of the multilayer construction I see is the reduction of eddy current losses. Why would I want to reduce them in a common mode choke instead of exploiting them?

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    \$\begingroup\$ Could the foil have been some sort of electrostatic shield? Was it actually metal or just shiny plastic? Do you have a photo of the item in question? \$\endgroup\$ – Andrew Morton Mar 28 '17 at 18:44
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A solid conducting core is effectively a shorted third winding. This will inevitably mean that the inductance of each regular winding is dramatically reduced and this will seriously affect the ability of the choke to perform as one would expect it to.

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    \$\begingroup\$ I agree unless the foil is insulated and open. In that case it might be some kind of electrostatic shield only. \$\endgroup\$ – Decapod Mar 28 '17 at 19:13

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