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Wireless energy transfer coil

  1. Sep 3, 2009 #1
    I've found this video on the web:
    and id like to be able to build one myself.

    But i don't really know how to start with this nor how it works.
    -The 8 shaped coil is really 8 shaped or just 2 coils bound together?
    -how do you make the second coil (the one with the light) resonate in the same frequency?
    -what voltage do you put into the primary coil to make it work?

    I've looked on the net but can't find anything helpful.
    Please help me or atleast push me in the right direction.
  2. jcsd
  3. Sep 4, 2009 #2


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    If he is just using low frequency signal, like AC from the wall, then you do not really need to worry too much about antenna shape and design. Anything you make of reasonable size will be a "small" antenna. Steven Best has done a lot of papers on small wire antennas and the last few papers I read seemed to confirm that the radiation properties were fairly independent of the wire antenna form, it mainly changed the impedance characteristics and efficiency.

    So just loops would work fine. It appears though that the "figure 8" loop is probably two opposing loop antennas. A loop antenna is an approximation of a magnetic dipole antenna, so only one loop (or a coil of loops) would be fine. The number of coils and increasing the area of the loop would improve the radiation properties. However, he states in the description that the figure 8 is meant to confine the radiated field. If we place two dipoles close to eachother and operate them in opposite modes, the fields in the far-field will quickly die off. So it is my guess that he is simply looking to run two loop antennas whose currents run in opposite directions, the result being that the fields are very localized. You could do this by running the wire in a figure 8, the cross point causes the direction of the current flow to switch. A similar idea is used in a folded wire dipole of one wavelength in length (except it is meant to improve the radiation and not stiffle it). Just make sure to insulate the cross-point, it is not meant as a short between the wires.

    The receiving antenna uses much smaller loops, so he has boosted the gain by winding many loops and inserting an iron core. As to what voltage on the transmitting antenna, *shrug*. Who knows, it all depends on how strong of a signal you need to receive, I would just be reasonable about it. You do need to have an AC signal, you might be able to just take the signal off of the house mains but step it down in voltage. Something on the order of a volt or so would be a starting point. A typical LED only needs 0.7 V to turn on, so it would be a matter of slowly ramping up the input voltage to your transmitting antenna until you met that requirement but I do not think you would want to overdo it.

    EDIT: There is a better video: http://www.youtube.com/watch?v=SSgo_N-5JOg&NR=1
    You can see that he is working in the low MHz range and you can more easily see that he has added a capacitor and an inductor (I guess the wire coil can provide the necessary inductance) as a tuning circuit. What I mentioned above, just making loops and pulling the AC off of mains could work but it would be very very VERY inefficient and may not be able to work in actual practice. Both videos probably used MHz signals and matching networks to achieve a high enough received signal to power the LEDs.
    Last edited: Sep 4, 2009
  4. Sep 4, 2009 #3
    Ok that did clear up a lot of things, the only thing that still confuses me is what they/you mean by the frequency. You do mean the freq of the voltage right?
    because from what i've learned in physics till now did not have anything about frequencies, just voltages.

    Also i see that this coil lacked a metal core, this makes it work less good then it would with one or am i mistaken?

    Also is it more practical to use an round coil or an square one?
    Since from what i understood the 8-shaped coil just weakened the signal and i want an as far range as possible. That would probably just require more windings but i thought that asking can't hurt.

    http://img524.imageshack.us/img524/4570/dsc00678n.jpg [Broken]
    Ive made a coil thats 21X11Cm. Its made from the coper wire that I got out of an small electric motor, it's 14 windings thick.
    I pluged it into my pc (via USB) and it didnt do much, just heated up a bit and moved when i put a magnet near it.
    Should i plug it into my wall?
    Last edited by a moderator: May 4, 2017
  5. Sep 4, 2009 #4


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    No, I would not plug it directly into the wall, not only is sticking metal things into outlets a rather unpleasant experience, you would be shorting out the outlet which under the best results would only trip your breaker. In addition, the ohmic heating of the current off of your mains would also be unpleasant to handle. You need an AC signal if you are going to have an antenna of any form. The size of the loop does matter if you wish to have a moderately efficient antenna. In actuality, a wire antenna that is much smaller than the applied wavelength will still radiate but very inefficiently. It will all be a question of just how much power you will need to deliver to your load and from what distance. In addition, the videos included a matching LC circuit to help tune the resonant frequency of their antennas so when in Rome...

    Here is a cheap function generator. Get a free sine wave generator program that will create a sound at a desired frequency. Get a stereo jack plug and take the ground and one of the lineouts and hook this up to the terminals of your antenna. You may want to put on a 50 or 100 ohm resistor to regulate the current flow so that you do not attempt to draw too much from the stereo jack although soundcards are used to having low impedance loads. Then you can have your soundcard generate a 15 KHz sine wave that will be your AC signal (you could do 20 KHz if you want but you may want a buffer region in case you have a really crappy soundcard). You could then use an antenna textbook to see how to come up with the necessary matching circuits and such for your transmitting and receiving loops to optimize your antenna link. Most antenna texts will only work in the far-field in their equations so you cannot use their equations to estimate the power delivered but the matching network and antenna impedance stuff will be fine. If you want to test a simple receiving antenna, just put a simple headphone as your load, you should be able to listen for the received signal since it is in the audible range (although you may want to go down to at most 10 KHz since our hearing degrades rapidly from 20 KHz over the years, maybe even 1 KHz). I'm not sure what would be more sensitive to the received signal, an LED or headphone.
  6. Sep 4, 2009 #5
    Ok i got a jack but what wires do I attach?
    Its got a red+copper and a blue+copper wire
    I suppose that the copper is the ground?

    I also found a led and more wire (altho this is thinner) and made a small coil around a plastic cap.
    I dont know if the windings matter but its way more then 14, i think close to 50
    http://img83.imageshack.us/img83/1829/dsc00681.jpg [Broken]

    And the idea is that i connect the stereo jack to the coil and just change my freq till it lights up?
    Last edited by a moderator: May 4, 2017
  7. Sep 4, 2009 #6
  8. Sep 4, 2009 #7
    could you give me the pdf by any chance?
    and I'm not in the USA so i can't buy the function generator.
  9. Sep 4, 2009 #8
    I suspect that the two transmitting loops are in a push-pull configuration and are being driven by a tuneable signal generator. The pickup coil seems to be wound around a rectangular ferrite stick like used in AM radios. The number of turns may be optimized for this demonstration. There appears to be two LED lamps in series with two two-lead capacitors. The signal generator, probably in the 500 kHz to 1000 kHz range, was tuned to maximize the received signal. A typical signal generator should be capable of putting out about 5 volts at 50 ohms (100 mA) into the coil.
  10. Sep 9, 2009 #9
    I made a round coil and put about 30V on it (in our school lab). The coil just got really hot and the other coil only got 0.1V at the highest spike.
    How do I get more?
  11. Sep 9, 2009 #10


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    What was the frequency?
  12. Sep 10, 2009 #11
    I dont know, probably 50Hz
  13. Sep 12, 2009 #12


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    You could do several things.

    1. Increase the number of turns in the antennas.
    2. Add a ferrite core to increase the permeability in the middle of the loops.
    3. Add a matching network for the desired frequencies (should add capacitors since a loop antenna would be inductive).
    4. Change the frequency and/or size of the loop to be either resonant or more efficient.

    The last two are going to get you the real appreciable gains.
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