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Science Technology

Contactless Electrical Current Transfer? 73

Posted by Cliff from the keep-your-hands-and-feet-away dept.
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."
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Contactless Electrical Current Transfer? More

Contactless Electrical Current Transfer?

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  • The coil thing should work. (Score:3, Informative)

    by rasteri ( 634956 ) on Friday April 16, 2004 @09:20PM (#8888346) Journal
    I have an electric toothbrush that charges wirelessly, I assume by using coils. Try dismantling one of those?
    • I have a shaver that does this too, it sits in holder and charges.

      However he says he needs to do it over a distance of 2 to 4 inches, and there won't be enough coupling between the coils to achieve enough voltage to charge the battery.

      I'm not saying that it is impossible, but rather exotic electronics means expensive electronics, and most toys are very price sensitive.

      Anyway, that was Tesla's great dream, to transmit power.
      • Man if you can't find a way to do it on Google, it probably can't be done.

        As I regularly tell one of my friends : you have used too many pronouns / variables in that sentence in order to come to a conclusion / make any sense. That said, describe in tight detail what exactly you want to do and maybe we can help you figure out a way to do it.

        I want to make something something and have it use some power to do something something and it will be cool if I can figure this part out. Surprisingly, given all tho
        • Man if you can't find a way to do it on Google, it probably can't be done.

          Idiot. That assumes someone bothered to write a web page about air core transformer theory. It is dangerous to assume that all human knowledge is on the web, and that google has indexed it.

          I did some transformer theory stuff when i was studying for my HND. Go buy to a library and borrow a book on Electrical Engineering. Its how people did research before the friggin net came along
      • Well, there's an easy and cheap way to do this I've played around with, I've only used this to light up lightbulbs, and it's a bit dangerous, but it'll go alot further than 4 inches.

        You'll need to get the magnetron out of a microwave (higher wattage is better, but danger increases with power) and build a wave guide horn for it, then simply point it at what you want to power, of course you might want to do something like build a faraday cage around the whole damn unit...

        Enjoy, and please don't kill yoursel

        • You'll need to get the magnetron out of a microwave (higher wattage is better, but danger increases with power) and build a wave guide horn for it, then simply point it at what you want to power, of course you might want to do something like build a faraday cage around the whole damn unit...

          You're insane. Microwave oven innards are extremely dangerous. Best case scenario is cataracts; worst case scenario is cooked liver or electrocution.

          Besides, unless you got the impedance of the waveguide just right, y

          • Not the first time I've been accused of that...

            However I digress, you can do this safely, it simply requires knowledge of microwave radation and exactly what you can and cannot do safely.

            Now I'm gonna go press my head against the front of my microwave and watch some food cook for a while.

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

      I'd imagine so. This technique is also used to recharge the batteries in some pacemakers. It's just a non-conventional transformer.

      But the problem he has is the distance. The charger for the pacemaker, toothbrush, electric shaver, etc. can be brought very close to the device being recharged: he wishes to charge something at a distance. The holder will also be aligning the coils very closely.

      Here'

      • I'm aware of an inductive transfer system used in car steering wheels, of all places. The clock-spring wires used to connect to the rotating part of the wheel had some difficulties, so at least one air-bag manufacturer went wireless to power the air-bag electronics and communicate the command data to the bag and the status data back to the stationary part.

        ferralis asks about distance, but doesn't mention the size of the objects on either side. This is crucial if you are trying to use magnetic coupling, be

  • A question (Score:5, Funny)

    by DAldredge ( 2353 ) <SlashdotEmail@GMail.Com> on Friday April 16, 2004 @09:21PM (#8888352) Journal
    Does the person using this toy have to live past the first use? :->

  • I've thought of this before too (Score:3, Funny)

    by Toxygen ( 738180 ) on Friday April 16, 2004 @09:21PM (#8888357) Journal
    A wireless power cord.

    I mean, we've got wireless everything already, and all those radio waves and GHz signals are basically a form of energy streaming through the air, so why couldn't we do it with electricity?

    Think of the bliss a wireless power bar would bring.

    Patent pending.
    • Wireless transmission of energy was Nikola Tesla's [pitt.edu] dream, and he pulled it off at short distances. Too bad he died in 1943, and can't post to this thread. If he had found funding and support, we might only have to put up energy antennas to catch electricity, but then the power company couldn't send us a bill each month. That's why he never found enough funding his reasearch, and people called him crazy.
      • The reason wireless electricity is a problem is that as distance increases linearly, power drops off exponentially
      • Mr. Tesla is one of the more underrated scientists of the 20th century. From his coil to his steam turbine (which goes fast enough to cause it to break apart under centrifical (or is it centrifugal? I can never remember the difference) force much like spinning a CD with a drill/Dremel. Speaking of which, the poster should check out Sam Barros' site [powerlabs.org], which shows his past experiments (most/all of which include technical information) You are bound to find something helpful/entertaining in there (like fun th

        • Mr. Tesla is one of the more underrated scientists of the 20th century. From his coil to his steam turbine (which goes fast enough to cause it to break apart under centrifical (or is it centrifugal? I can never remember the difference) force

          A great many turbines will break apart if they lose their loads and overspeed, standard reaction-type steam turbines and many water turbines among them. The virtue of the Tesla turbine is that its pieces are very simple, its vice is that it is woefully inefficient com

    • Re:I've thought of this before too (Score:1, Funny)

      by Anonymous Coward
      Think of the bliss a wireless power bar would bring.

      Eternal bliss, as you step between it and your home network and wind up arcing 110 volts.

      Well maybe not eternal bliss, if you have the evil bit set.
    • Here [thinkgeek.com]

  • Found this on Google (Score:4, Informative)

    by hords ( 619030 ) on Friday April 16, 2004 @09:23PM (#8888373)
    Does this [vahleinc.com] help?
  • I believe what you are trying to do is induction. A current in one coil should produce an opposite current in another coil. For best effect, put an iron core inside, which should make the magnetic field stronger. Or something like that. Can't remember much else from physics 2.
    • Oh, yeah, thats right... *change* in current will produce the opposite current. Which is one reason why alternating current is nice, you can build transformers fairly easy.

      • Re:Induction (Score:3, Informative)

        by Micro$will ( 592938 )
        I think the key is the frequency used. While standard wall mount 50-60 Hz transformers have to big and bulky, the ones that run from 20K - 40KHz in switching power supplies can be much smaller. Combine this with some sort of ferrous antennas and you may be able to "transmit" power over some distance, perhaps even unidirectionally.

        Another way would be an infrared laser and a solar cell, but I don't think you can get much power out of it.

  • easy (Score:5, Funny)

    by pizza_milkshake ( 580452 ) on Friday April 16, 2004 @09:31PM (#8888431)
    use lightning. i've seen it travel much further than you require.

  • One solution... (Score:5, Funny)

    by wronskyMan ( 676763 ) on Friday April 16, 2004 @09:31PM (#8888434)
    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.
    • Oh come on, at least try a suggestion that could be accomplished with current technology. Your fancy schemes are obviously science fiction.

    • Re:One solution... (Score:3, Informative)

      by netringer ( 319831 )
      I apologize in advance for following up your joke with a serious post.

      Considering that the "ZipZap" RC mincars use something like this, I would guess that it's a viable solution. I'm amazed that the clone I bought had the car, motor, steering, rechargeable battery, radio control transmitter and in-car receiver, etc, and with manufacturing and trans-Pacific shipping it got ito my hands for $6. Chinese labor is cheap!

      • I apologize in advance for following up your joke with a serious post.

        Apology accepted :-)

        IIRC the tiny RC cars use a large capacitance capacitor to store their energy, which is why they can be charged quickly by the controllers.

        • Re:One solution... (Score:3, Interesting)

          by JabberWokky ( 19442 )
          Yes, they (or at least the one I took apart) use the exact same block unit that's used in the disposable flash cameras. One of which, when disassembled and charged, arced in a white flash and blew a chunk out of one of my keys... and caused a friend in the studio to leap back, and he's a professional high-current electrician. Heh.

          (I'm proud of that run on up above, yes I am).

          --
          Evan

        • i was pretty sure they used a little NiCad......
      • Being able to damn near own and work to death your slaves/labor helps lower prices also.

        So does dumping your toxic waste untreated into the environment.
  • Isn't the standard practice for wireless power transfer to use a horde of small, highly skilled mice working smoothly together, each transporting a few columbs of electrons?

    What the hell are you people learning in schools these days? Are your teachers clowns?

  • Well... (Score:4, Funny)

    by Chilles ( 79797 ) on Friday April 16, 2004 @09:51PM (#8888581)
    A piece of copper wire of about 10 cm should do the trick.
  • Let me guess... it's some variation on the "Jump To Conclusions" mat that shocks the user in midair if he is en-route to a dangerous conclusion.

  • Wouldn't it be better if the application was more thoroughly describe or at least the purpose for sending power over that distance? It doesn't take a genius to see that's pretty much what a solar power cell and a flashlight does.

  • Details? (Score:4, Insightful)

    by JGski ( 537049 ) on Friday April 16, 2004 @10:17PM (#8888714) Journal
    Hmmm.

    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.

    • Re:Details? (Score:3, Informative)

      by ferralis ( 736358 )
      Very sorry, was knocked offline over the weekend.

      Details:

      Type: DC (at the end, I'm assuming high-freq AC in transit)

      Voltage: Between 3 and 10 volts, the curcuite is pretty flexible.

      Current: In the milliamp category, basically just charging one tiny NiMH battery in a waterproof enclosure that I'd prefer for ease of use/durability to have no insertion points.

      Thanks!

  • rectenna (Score:3, Interesting)

    by jayrtfm ( 148260 ) <jslash@sophontCOFFEE.com minus caffeine> on Friday April 16, 2004 @10:20PM (#8888730) Homepage Journal
    radio waves in the UHF frequency for the power source, and a rectenna could do the trick

    google [google.com]

    • Too bad the microwaves fry everything between the emitor and receiver. No way this would pass any kind of radiation safety tests for home use
      • Microwaves don't "fry" anything any more than any other wavelength that is absorbed by the thing in question. Actually 2.4GHz waves are several orders of magnitude less energetic than light for example.

        Power is the key, sure, 1000W microwave oven magnetron will damn certainly fry (well, boil) everything, but try hopping in front of 1000W laser for example and come back to tell that visible light doesn't fry you equally well.

  • Go to WPT (Score:4, Informative)

    by gtrubetskoy ( 734033 ) * on Friday April 16, 2004 @10:44PM (#8888856)
    If you happen to be in Spain this summer, stop by the Wireless Power Transmission Conference [congrex.nl].
  • although its obviouslly a much smaller current, wacom tablets wirelessly power their tablet pens, look into that--also the name passes me now but theres a charger pad that charges devices just by putting them on the pad with a thing on the battery, if i remember ill add a response.

  • You didn't state how much power you need. (Score:4, Interesting)

    by stienman ( 51024 ) <adavis&ubasics,com> on Friday April 16, 2004 @11:27PM (#8889078) Homepage Journal
    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 [microbeam.ch] 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
  • Beware that for pretty nearly any method of transferring power you can think of, you will be unable to achieve the FCC's Class B certification for consumer electronics. The device is going to wind up emitting a significant amount of energy in the radio spectrum, and even if you were able to focus them almost entirely in the vertical direction (I'm assuming you're trying to feed power to something you're levitating), the Feds will frown on it.
    • Use a small solar cell in the receiver, and a diffused semiconductor laser (you may need to use orange or yellow to get enough energy into the solar cell) to beam power over to it.
    • Built a little catapult that is wound back by a stepper, and tosses a metal ball into a catcher.
      The catcher runs a generator. Now all you need is
      an infinite supply of metal balls.

      Okay, no metals balls? Use one of atmospheric gas
      molecules, alpha particles, or photons.

      (1) Atmospheric gases: Use a motor on the power
      source to drive a fan. Use a fan on the power sink
      to drive a generator.

      (2) Alpha particles: Nevermind. This is a non-starter unless you can do thermionic power.

      (3) Photons laser on the pow
  • go to a library and get any decent high school or college text book.

    Or if you are too lazy, pay some college physics student a couple bucks and they'll get you the info you need in no time.

    I hate to make the same point that many have made before, but aren't there any better ask /. questions that could be posted.
  • This sounds simply like an electric transformer. A slightly more efficient way to do it over large (and even larger) distances is using a Tesla coil. These are simply just transformers where the primary is run at some frequency in the order of kilohertz and the secondary doesn't necessarily have to be wound tightly around the primary.

    True wireless electrical current transfer occurs as a spark (think lightning). That is current flowing through whatever meduim between highly positively charged and negati
    • Of course lightning transfers current with out a wire in the conventional sense, but it doesn't go thru empty space. Once the cloud-cloud or cloud-ground potential difference is great enough, you get some ions and a nice toasty ribbon of plasma that the current can flow down. Sure, it's in the vapor state, but lighting does have "wires."

      I know that this is a case of pedantic nit-picking coming from an physics ignoramus, but maybe it points ot a way to get this guy's job done. If you get the right frequen
  • You can throw them a lot further than 10 centimeters (about 4 inches for you metrically challenged).
  • I am not entirely certain how it is done but using magnetisim it is possible, it is commonly use in medical places where power needs to be transferred to somthng inside the body. Asking a doctor you know that does surgury might reveal some answers.

  • Short-range power transmission (Score:4, Informative)

    by Animats ( 122034 ) on Saturday April 17, 2004 @01:23PM (#8892203) Homepage
    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.

  • Do you mind if things get wet? (Score:3, Funny)

    by ankhank ( 756164 ) on Sunday April 18, 2004 @02:30AM (#8895863) Journal
    Squirt a stream of water across the four inch gap, aimed at a small Pelton wheel spinning a generator on the receiving end.

  • - try searching for "contactless smartcards" .. i don't know much details about how is done, but these devices use contactless transmited power ... also see ISO14443 & ISO15693

    - check Mifare & NFC technologies from Philips and Felica from Sony

    - as a side note, my electric toothbrush(philips jordan) has a docking/charging station wich does its job without any metalic contact - it's just a plastic-on-plastic touch there, i remember being a bit boggled by the device when i first saw it :). Again

  • It sounds like you're using induction to do the power transfer. Calculate from first principles [gsu.edu], most likely the Biot-Savart [gsu.edu] law directly, since your normal solenoid inductance [gsu.edu] calculation [gsu.edu] makes assumptions that aren't valid in your case. You will have to prepare and evaluate some integrals based on your off-axis geometry.

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