Contactless Electrical Current Transfer?

ferralis (Not an EE) asks: "Recently I've come up with a design for a very fun toy (to be unveiled later if I'm successful). What's missing is a means to send electrical power over a distance of five to ten centimeters (2-4 inches). I've done some research (mostly online) and have found extremely limited information. Even my beloved Google has forsaken me, and even my pleadings to eldritch information deities such as AltaVista have gone unrewarded. Can anyone help?" "The way I see it, to do this a person needs merely set up a high-frequency electrical field using a larger coil (primary) and a similar but smaller coil (secondary) can be placed within it, creating an air-core transformer. Unfortunately I can't find the math or even anecdotes about what happens when the secondary is off-center, or there is more than one secondary introduced... and I am not looking to build a Tesla coil here. I can imagine that many toys could be built using such a system, and one would think the knowledge would be well known and readily available, but apparently it is not. For this application, efficiency is -not- an issue."

Comments

The coil thing should work.

[rasteri]

I have an electric toothbrush that charges wirelessly, I assume by using coils. Try dismantling one of those?

Found this on Google

[hords]

Does this help?

or this?

[hords]

or this?

Go to WPT

[gtrubetskoy]

If you happen to be in Spain this summer, stop by the Wireless Power Transmission Conference.

Re:I've thought of this before too

[gtrubetskoy]

The reason wireless electricity is a problem is that as distance increases linearly, power drops off exponentially

No that's not the reason, just pick a medium that can be focused. I'm sure you can transmit power wirelessly using microwave or laser very long distances. (Consider that nearly all of the energy we use on Earth has been "transmitted wirelessly" from the Sun).

The problem is safety - if anything or anyone happens to be in the path of such a transmission, they get fried.

Here's a link about using microwave to transmit power from the moon.

Short-range power transmission

[Animats]

This sounds like the stuff we see in sci.inventors. "I have this great valuable idea, but am too clueless to make it work and too clueless to read a few books about the technology".

Anyway.

Short-range inductive power transmission works reasonably well. It's commonly used to recharge electric shavers and toothbrushes. Considerable power can be transferred this way. The GM EV1 electric car used an inductive charger, where a flat "paddle" containing a coil was inserted into a rectangular slot in the car.

Efficiency improves with frequency. The EV1 charger ran at 400KHz or so. But you have to take precautions not to become an RF emitter, and get FCC type approval. If you stay with 60Hz, that's usually not a problem.

Coil area helps. If you can use large diameter coils, bigger than the air gap between them, it will probably work.

If you don't need much power but want directionality, one interesting option might be to have a bright light aimed at a solar cell. You'll be lucky to get 1% efficiency. If that's enough, you're done. It's safe.

If you need very little power but have room for a physically large antenna, you might be able to build something that runs off ambient RF fields. Just make a big flat coil, wire it to a diode, and see what comes out. The output will vary enormously depending on how close you are to a transmitter. If you're lucky, you might be able to power a clock.