Theory behind evanescent wave coupling, aka wireless power

[Alan Yates] is building a persistence of vision display and needs a way to transfer power from the stationary base to the spinning circuitry. He’s decided to go with wireless energy transfer and he’s sharing all of his research and experiment data from the development process. It comes in two forms, the written version we just linked to, and a 37 minute video which is embedded after the break. If you liked some of the inductive energy transmission devices we’ve featured in the past, [Alan’s] video will fill you in on the why’s and how’s by using a combination of illustrative schematic examples and measurements on test coils that he built.

[via Hacked Gadgets and EEVblog]

30 thoughts on “Theory behind evanescent wave coupling, aka wireless power

  1. Those experimenting with this technology need to keep an important consideration in mind. The energy being dealt with is magnetic in nature; unlike radio waves, (whose intensity falls off as the inverse of the square of the distance), the field intensity here declines as the inverse of the CUBE of the distance. So, as an example, doubling the distance between transmitter and receiver necessitates transmitting 8 times as much power to obtain the same amount of received power. For this reason, power transmission over long distances is problematic. Just something to keep in mind…

  2. @Alan Yates:
    in your particular setup, you already had movement. why didn’t you just put magnets on the static side and make the rotating disk something like a dynamo, but “inversed”?

  3. I’ve been meaning to do one of these setups for my wireless mouse. I mean, I bought the thing because the cord on my previous mouse picked up an intermittent short, not because I wanted to be able to use if across the room from my computer. Should be able to get good efficiency going through just the thickness of the mousepad and the plastic shell.

  4. Wow! Very Interesting!
    This is an episode of Tha Avengers
    The Positive Negative Man (22) 1967

    A scientist is blown through a wall by a man directing high voltage electricity through a metal tip on his finger and the contents of his safe are fried beyond recognition. Steed and Emma discover that the destroyed documents related to an abandoned scheme to develop broadcastable electricity, but when they contact the other scientists involved in the project, they soon begin suffering similar fates. Throughout The Avengers you’ll find science fiction proposals that seemed far-fetched at the time but which later came to some sort of fruition, and scriptwriter Tony Williamson’s proposal for broadcastable electricity must have seemed a particularly unlikely candidate back in 1967, despite being proposed as long ago as 1905 by Serbian inventor Nikola Telsa (a name that should be familiar to all Fallout 3 fans). Yet just last year Eric Giler demonstrated his company’s first successful experiments with – you’ve guessed it – wireless electricity, so I’d keep your eyes peeled for a man in heavy make-up with a metal tip on his intex finger. An involving mixture of intrigue, espionage thriller and science-fiction, in which man-shaped holes in walls and windows remind us of series 5’s comic book influence, and rubber soles and galoshes prove to be the world’s safest footwear.

  5. I think it is stupid. Ever seen a transformer? That is what it is. No reason to ask questions about it, it has a technical history of more than a century. This is the same crap as unit energy engines, and it has no use whatsoever. First you expose yourself to varying magnetic fields that screw up your DNA and nervous system, second, it is wastefull.

  6. I spent a lot of time working on this myself in the past. It is the basis for LF RFID communications.

    I made a 125kHz square wave generator and fed it into a 125kHz resonant circuit, the inductor was a coil that also acts as antenna.

    The “tag” part of the circuit was also just a coil and capacitor tuned to resonate at 125kHz. It was possible to use a diode to half wave rectify the tag and use the transferred energy to light an LED.

    By shorting out the inductor on the tag side (I was just using a push button), you could modulate the amplitude of the resonating voltage on the transmitter side. An envelope detector can be used to detect the push button being pressed.

    I had planned to make an oscillator in the tag side which would modulate the transmitter side on its own (emulating the push button but at regular pulses). This could have then been used as a tuned / coded sensor.

  7. My guess on why to use wireless power transmission as apposed to generating power from the rotation:

    The frequency is much higher for wireless power transmission, thus a much more compact capacitor can be used to smooth the current.

    Also, using wireless power transmission, the rotation speed of the POV device can be changed without changes to the circuit.

    Also, it’s more interesting.

  8. This looks cool. I bought the Qi wireless power dev kit from Texas Instruments last week, and it has what seems to me to be an absurdly complicated circuit. Lots and lots of components, to the point where using it might double the part count on one of our boards. I don’t know if all that is necessary as I haven’t gone over the datasheet yet, but I sure hope the chip doesn’t need all those components! Clearly it wouldn’t have to.

  9. a few people on 4HV are working on this, take a look :)

    the decay with distance effect can be somewhat mitigated by additional tuned resonators between the transmitter and receiver.
    Still doesen’t solve the underlying problem but it can somewhat improve efficiency.

    Interestingly a microwave collimator based on metamaterials might allow longer distances, the basic structure of these is relatively simple.
    (works with cantennas too!)


  10. A transformer _is_ wireless energy transmission. Just with a much better medium than air.

    Using a generator is a terrible idea. It would be hugely annoying to try and regulate the voltage at the spinning end, since the EMF would depend on the rotation speed. Not to mention, if the load varied (say, an LED turns off) it changes the back EMF, which means that the resisting force on whatever’s driving the spinning bit changes.
    That is to say, the LED turns on, and the thing would spin slower. Might screw up the effect.

    He could improve efficiency by adding an iron core through the two sets of coils, but it’s probably not worth it.

    While I doubt that he’ll keep it long enough for this to be an issue, using an inductive power system is the most reliable, longest lasting solution. Contacts wear out, and batteries die, but those coils will last pretty much forever.

    ledstart, I hope you never walk near a microwave! or a fluorescent light, or a computer, or a power line or a radio station or an electric radiator.. so on and so forth.

  11. There is a difference between RF and ‘wireless’ transmission, because RF is Electro magentic waves, while ‘wireless’ uses a varying magnetic field.

    I just think that if you take your basic physics textbook you can exactly predict what your coils will do, nothing to it, it has limited potential (can be calculated) and it is a waste of smart minds imho.

  12. @ledstart I thought I was missing something. I guess they rip the inductance chapters out nowadays while standing on their desks shouting “Oh Captain My Captain” or some such. I think it had to be repackaged and re-termed for to avoid patent payment on a number of similar methods of power transfer. Powermat might not make so much if they had to use 13 pre-ordained patents. My two cents…

  13. @ledstart and blue carbuncle
    Before you two get all in a tizzy about this I should inform you that these two phenomena (transformers and what’s going on here), while related, are not quite the same. A transformer is not using resonant effects to induce a current in the secondary, where this is. See for more info on this method. You can actually find resonant transformers in the high-voltage power supplies used for cold cathode tubes in the back of LCD screens :)

    I messed around with this a while ago as well, and ended up building a device that works pretty good. Again, nothing new from what the submitted article contains, but worth checking out if you are interested:

  14. @RandomNut Yeah, I’m glad someone mentioned this. While an evanescent wave is not used specifically to study fluorescent particles, that is one of it’s more common uses. An evanescent wave is a projection of the electric field into a medium that arises opposite the point an intense optical wave experiencing total internal reflection. The title of this article is crazy wrong, and probably just chosen because “it sounds cool.”

    However, this hack is very nice. Keep up the good work, just watch the terms you sling around.

  15. I’m confused with all the random hostility and declarations of “oh this is useless”. Someone is trying to illustrate an interesting project and explain some basic physics. What is useless about that?

    Why don’t you folks go back to making movies about your cat or whatever it is you do other than crapping all over someone’s work.

  16. @eresonance thanks for the link :) That cleared it up a good bit. I thought everyone was going crazy since we have all of these tech folks that have built power supplies and have “rang out” cat5 at one or more times during our lives lol. The resonance I understand from building synths and from audio but it seems to lose me in between the “this is what audio does and this is what the power does” in resonance. Guess I’ll do some more reading :) I use a VERY simple circuit that I have written about on here before that just snacks on any radio signal it finds. Actually it is a very simple circuit replicated 300 times lol so it is a little bulky to fit inside of a mouse lol.

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