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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."
I have an electric toothbrush that charges wirelessly, I assume by using coils. Try dismantling one of those?
Does the person using this toy have to live past the first use? :->
Does this help?
use lightning. i've seen it travel much further than you require.
You could mix up some chemicals, use your electric power source to put some current into them to cause a complex electrochemical reaction. Call this wireless energy reaction "charging", for example. Then when you want to power your toy, just put the chemicals into a sealed metal container in the toy (such as a "cell"). If you need more power, you could even use a group or "battery" of these "cells" to increase the voltage available to your toy. Then when you need more power, simply remove the chemical containers from the toy, "charge" them again, and put them back in.
--- You shall know the truth, and the truth shall make you mad- Neal (not Cowboy) Boortz
or this?
A piece of copper wire of about 10 cm should do the trick.
Unfortunately this is one of those questions like you get on sci.electronics.* which doesn't include the basics like how much power is required; at what voltage and current levels, AC or DC, what frequency, can "it" run unconnected (e.g. on battery) and if so for how long (application-wise, not technology-wise), etc. etc.
Barring nice engineer-friendly technical specs, at least outline more specifically what you're trying to do, at least in vague terms, would be more helpful. Starting out by saying it's a product idea (rather than just some hobby thing) was probably mistake if you're paranoid about competitors.
Without some minimal specifications of this sort, absolutely any answer you get will either be hopelessly vague, utterly useless or simply a troll.
If you happen to be in Spain this summer, stop by the Wireless Power Transmission Conference.
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.
There are few resources for contactless power supplies because they are so darn inefficent that there's almost always a better solution.
Since you didn't give us any information on how much power you actually need (a few milliwatts I hope...) then all I can tell you is to avoid air coils. You'll get almost no practical energy transfer. Make the smaller coil fit inside the larger coil, and put a suitable core inside the smaller coil.
Alternately, if you want two flat faces facing each other, get two large cheap speakers. Remove the cone and coils from the magnet assembly. You may need to remove the magnets themselves and replace them with another ferromagnetic material. Place new coils where the old speaker coils were (wrapped around the core inside the assembly). Face them to each other and put low voltage AC on one side.
There are transformer books which will give you the information you need to accomplish this. It's hard to give you better information than that, though, without knowing the power requirements of your device.
If this PDF treatment on the subject doesn't help, then you probably don't have enough knowledge to correctly design one and you ought to simply start toying around with different designs until you find a suitable match. If/when you mass manufacture the device you'll want to pass it by a real engineer who can redesign it for you. Pay attention to the references in the paper for more information.
-Adam
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.
Squirt a stream of water across the four inch gap, aimed at a small Pelton wheel spinning a generator on the receiving end.