Is it in any way feasible, in a small form, to power a 12v light with a power source or transition source 3" away without any physical connection?

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    \$\begingroup\$ A big flashlight and a solar cell? \$\endgroup\$ – PlasmaHH Apr 24 '15 at 8:04
  • \$\begingroup\$ A blower/fan with a "windmill" on the other side? \$\endgroup\$ – Paul Apr 24 '15 at 8:28

A few watts of power can be magnetically transferred a few inches using resonating coils. Not hard to do but if you expect a compact solution that is inexpensive you will be disappointed.

It works on the same principle as commonly used RFID readers: -

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It's simpler of course because only power is transmitted and there is no need for data systems at either end. If you need to transmit a lot more power at a greater distance then sturdier coils are needed that have sub-milli-ohm losses: -

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The basic principle is that a parallel resonant circuit (with extremely low losses), when driven with an alternating supply at the resonant frequency, will circulate several amps to many tens (or even hundreds) of amps between coil and tuning capacitor, This is the power transmit end.

The receiving end has the same arrangement i.e. it is tuned to match the resonant frequency and, even though it might only receive a small fraction of the magnetic flux produced, the resonance of the receive coil can still produce a substantial voltage that is enough to drive real power (watts to tens of watts) into a load: -

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  • \$\begingroup\$ What sort of efficiencies can be achieved in theory/practice? \$\endgroup\$ – CL22 Apr 24 '15 at 9:13
  • \$\begingroup\$ @jodes you would be lucky to approach 50% as a rough estimate. If you developed a power oscillator that produced a sq wave and carefully interfaced it to the tuned circuit you might get a tad more. \$\endgroup\$ – Andy aka Apr 24 '15 at 14:21

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