Coil Shielding Choices - John Dunn, Consultant, Ambertec, P.E., P.C.

The choice of which conductive material to place near to or far away from a coil can affect that coil's electrical behavior. The effect on a particular coil of wire of placing a non-ferrous metal (a twenty-five cent coin in this case), placing nothing at all and a placing small piece of ferrous metal is illustrated as follows: 

The coil under test was of seven hundred turns on a nylon bobbin with the coil's outside diameter being approximately 0.6 inch. The coil was hooked up as the "coil under test" to the dual resonance test fixture previously described, where that test fixture uses an automatic gain control to keep the oscillator output amplitude at one volt peak.

For each of the four test capacitances within the fixture and for the presence and absence of the indicated materials near the coil, dual resonance data was taken with the results as shown.

The non-ferrous material caused the exhibited inductance to be less than the free-air value while the ferrous material did the opposite, it caused the exhibited inductance to be greater than the free-air value.

Comments

[Lotfi]

Hi there,

Thanks for the demostration. I think, it is because of the eddy currents which are induced in the coin and who create a magnetic flux in the opposite way of the original magnetic flux created by the coil. The resultant magnetic flux is the superposition of both and finally is less than the original magnetic flux.

Magnetic_Flux=Inductance*Current
==> for the same current in the coil:

Inductance = Magnetic_Flux / Current.

That means that, for the same current in the coil, the inductance decreases if the magneic flux magnetic flux decreases too.

[John Dunn]

That is precisely correct, Lofti.

I had left that out of the posting.

[Prem Sobel]

Ever hear of mumetal? :)

[John Dunn]

Of course, but µ-metal can be costly and it has to be carefully handled to get the best performance. These sheets of steel and aluminum gave satisfactory results, we utterly rugged and were very cheap. Their effects on the shielded transformer windings simply needed to be taken into account.