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I admit that I have already made this question in physics SE, but after a month of no answers, I realized engineering might be better suited to everyday applications.

I intend to use (Qi) wireless charging for a project of mine. It uses induction coils to transfer power wirelessly.

In my project, between the coils there will be a 1.5mm (0.04 inch) sheet of brass. The first question I had is if this would work. I tried using a 3mm copper sheet that I have, and sure enough the answer is negative, the sheet absorbs the magnetic field.

But what if I made holes in the sheet? Would some of the field penetrate then? Would it be proportional to the size and "density" of the holes? Or would the conductor still absorb the fields?

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  • \$\begingroup\$ Why will there be a sheet of brass? That seems kind of strange and oddly specific. If you're able to put holes in it, surely you can just replace it with something elses? \$\endgroup\$
    – DKNguyen
    Commented Mar 23, 2021 at 7:07
  • \$\begingroup\$ Well, it's not a sheet, it will be a 3d printed shape actually. Just keeping the question minimal. Being specific isn't strange, it is a large jewellery project. \$\endgroup\$
    – George Menoutis
    Commented Mar 23, 2021 at 7:09
  • \$\begingroup\$ You're 3D printing brass? That's some 3D printer you have. I assume you mean something else. You might have to live with copper coloured paint on plastic. \$\endgroup\$
    – DKNguyen
    Commented Mar 23, 2021 at 7:09
  • \$\begingroup\$ @GeorgeMenoutis Eddy currents will likely oppose your goals. You could cut long slots instead of holes and this might force the electrons, motivated by the non-Coulomb electric field, to have to travel a lot further around and depending on frequency this might have some value. Hard to say without a lot more specifics in hand. \$\endgroup\$
    – jonk
    Commented Mar 23, 2021 at 7:09
  • \$\begingroup\$ @DKNguyen Nah, I will be using online services, I don't have a car-valued 3d printer :) \$\endgroup\$
    – George Menoutis
    Commented Mar 23, 2021 at 7:10

2 Answers 2

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It won't.

If you have any conductive material between your wireless charger and phone, the magnetic field will take the easiest path and go to the metal sheet.

Power will be lost by eddies current (and ultimately heat) within that conductive material.

Holes won't help.

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From what I understand, the Qi charging system operates between two planar coils at a frequency of the order of 100-200kHz or so. So what you really have is a pair of coupled coils.

If you place a sheet of brass between them, the powered coil will induce eddy currents just like you want in the receiver coil, only in the brass sheet they will act as a shorted turn and dissipate power and/or detune the coupled circuits. Now, I have never disassembled a Qi charger, but I imagine that the charger plate is simply a round planar coil, and if that is the case, the eddy currents will circulate around the centre just like the currents in the source / receiver coils.

What will probably work is to cut slots to interrupt the eddy currents. Make radial cuts in the brass sheet, you can probably make a neat radial pattern to be part of the jewellery design. I'd also ensure that there are several cuts that go from the centre completely to the edge (meandering cuts are fine). Your goal is to make sure that you cannot go completely around the centre without encountering a cut.

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  • \$\begingroup\$ Interesting, and very enlightening in terms of science! However, for the practicality, it seems that swapping brass for another material will probably by less undesirable than having these kind of patterns. \$\endgroup\$
    – George Menoutis
    Commented Mar 23, 2021 at 9:52
  • \$\begingroup\$ I like this answer. It would at least have a chance of working if you did this. \$\endgroup\$
    – user57037
    Commented Mar 24, 2021 at 5:31
  • \$\begingroup\$ @GeorgeMenoutis don't use carbon fiber, either! LOL. \$\endgroup\$
    – user57037
    Commented Mar 24, 2021 at 5:31

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