Moderator - The unviable title should read: Inducing some power for a LED across 5 cm (2") air gap ? Please edit.
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Hi all.
Looking for guidance and suggestions to transfer 10mW of power across a ~5 cm (2") air gap to turn on a LED in a sealed container.

The type of coupled coils, the convenient frequency, the oscillator, calculation of air gap losses. The 'receiver' coil feeds just a bridge rectifier to power the led.

A led is on a turning platform inside a sealed ~10cm (4") glass dome and needs to be turned on wirelessly energized from the outside. Solar cells inside cannot be used. The power 'emitter' coil would be centered under the dome, the power 'receiving' coil centered on the turning platform.

Weight of the 'receiving' coil is to be kept to minimum possible, should yield 20mA at 4V.

 

This should be doable. The air gap will cause a big loss, but I think you should still be able to get a few mW.

I picture a pair of pancake coils, maybe 3" in diameter (i.e. as big as possible given the project constraints), one powered below the enclosure and one inside. They should be on the same plane, so that the magnetic field created by one is driven through the other. You'll need to oscillate the transmitter field. A 555 monostable multivibrator circuit driving a MOSFET, which in turn switches current to the coil, should be adequate. There are surely more elegant ideas for efficiently driving a "high" current inductor, but perhaps efficiency is not a concern?

 

Thanks. Correct, efficiency is not a concern. The optimal frequency and windings are mostly my concern, using few turns of thicker wire or the opposite. And the coil cores, iron or ferrite or no core...

 

Certainly doable. Years ago I saw demonstrated an incandescent-bulb table lamp (reasonably bright, but wattage unknown) powered inductively through a ~2cm thick wooden table-top. IIRC the Tx and Rx coils were on standard 50Hz mains transformer E-cores. The Tx voltage/frequency was 240AC/50Hz.

 

This is the best write up I have seen on how to make one.
http://www.discovercircuits.com/dc-mag/Issue-1/issue-1-exciter-circuit.htm
Mouser has a receiver coil that looks nice. I think you would wind your own transmitter coil. Enjoy!

 

The optimal frequency and windings are mostly my concern, using few turns of thicker wire or the opposite. And the coil cores, iron or ferrite or no core...

"Optimal" makes this a challenge, but here are my hunches. The receiver coil will not carry a high current or voltage, so more windings of a thin (high gauge) wire makes sense. Maybe 100 loops or more of 28 gauge. If the wire gets much thinner, it becomes hard to handle.

The field generated by the emitter coil, on the other hand, is proportional to both the current in the wire (so you want a fat wire to carry as large a current as you can handle) AND the number of turns. Thin wire gives more turns but adds resistance. I'd look at 18-20 gauge or so. If you have to buy wire, and want to save money, you might compromise and get 22-24 gauge for both coils. Just guesses.

I don't think a core will help much, since its length will be small compared to the total field line length. A long core with a small air gap would be different, but you have the opposite.