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Every time I hear the description of a wireless charging system it goes like this: "remember a transformer? It's the same here - one coil is in the base and another is in the device and when you put the device onto the base the transformer gets assembled and starts transmitting power".

One problem with this description is there's no magnetic core in the system. AFAIK each transformer has a magnetic core in its design. A magnetic core is a huge set of steel plates that goes though both coils and this set of plates in fact conducts energy - the alternating current in the primary coil induces alternating electromagnetic field in the core and that field induces current in the secondary coil.

So looks like the core is a key component and a transformer can't possibly work without a core. Yet I don't see a magnetic core in a wireless charging system.

How does it work without a magnetic core?

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  • \$\begingroup\$ Some inductive chargers, especially in electric toothbrushes, have a ferrite core as part of the charger. \$\endgroup\$ – drxzcl Apr 21 '11 at 14:12
  • \$\begingroup\$ @Ranieri: Okay, but th core won't pass through the other coil located in the brush will it? \$\endgroup\$ – sharptooth Apr 21 '11 at 14:15
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    \$\begingroup\$ Yes, actually it does. Let me see if i can dig up a picture: braunelectrictoothbrush.org/images/page3/4.jpg That little plastic nub houses the ferrite core. It sticks into the toothbrush acting both as a transformer core and as an anchor. \$\endgroup\$ – drxzcl Apr 21 '11 at 14:17
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A metal core is advisable because it concentrates the magnetic field. This makes magnetic coupling between coils more effective, but you can do it without the core too, especially if both coils are tightly coupled (mechanically close to each other). If you don't have a core and the two coils are some distance apart, part of the generated field will pass outside of the other coil.
(In a transformer the outside metal "shortcuts" the field, increasing it. Without this outside metal the field would fill a large volume around the core.)

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the alternating current in the primary coil induces alternating electromagnetic field in the core and that field induces current in the secondary coil

Yes, but "in the core" is not necessary. A coil creates a magnetic field all around it, whether there's a core there or not. The core just confines the magnetic field so that less of it "leaks", which makes the transformer more efficient.

You can also make the transmission more efficient by putting a capacitor in series with both coils so that they resonate at the same frequency: http://en.wikipedia.org/wiki/Resonant_inductive_coupling

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A good example of electromagnetic induction without a metal core is a passive RFID tag, which has no battery. Instead, its draws power from the reader, which sends out electromagnetic waves that induce a current in the tag's antenna. The distance between the tag and the reader can be several inches. Obviously at this distance, very little power can be transmitted, but it illustrates the principle.

enter image description here

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All that's required for a transformer is that the magnetic fields of the two coils are linked. Iron cores make this more effective at power and audio frequencies, but at radio frequencies (more than a few 100kHz) cores actually become a liability.

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