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Power transfer with LEDs

  1. May 25, 2005 #1
    Numbers

    Having "googled" into this thread, I was hoping you could help with my own "LED lighting" question. I have a cost-sensitive instrument application where I need to supply roughly 50mW to an assembly rotating on a motor shaft, about 6mm from the nearest support.

    I wondered if I could use a solar cell to do the job, but I have begun to wonder if I can get a better power transfer by using two (or more) identical LEDs, one as a transmitter, the other as a receiver. My thoughts were that the (approximately) monochromatic light sources might couple power more efficiently than by using white light sources and standard solar cells, I am replacing a very small, elegant (hideously expensive) slip ring assembly, and I have considered electromagnetic coupling, but the elegance of light power is very attractive.

    Does anyone have any handle on the likely power tranfer efficiency of my proposed link, before I try and test it ? Given that red LEDs can push out 50lm/watt , and given I know I cannot expect 1/50 watts/lm, what would one expect to see ?

    Any comments would be much appreciated.

    Thanks

    Steve
     
  2. jcsd
  3. May 25, 2005 #2

    chroot

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    Steve,

    You likely don't want to use two identical diodes; a laser diode would make a fine optical power source on the transmitting side, and you'd get more of that power back on the other side with, say, a PIN photodiode.

    - Warren
     
  4. May 25, 2005 #3

    brewnog

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    May I ask what your application is?
    How fast is your shaft rotating? How big is it? Are you doing away with your slip-ring purely because of cost?

    The light-transfer idea is nice (I'm thinking of Podracers' energy beams :smile:), but I doubt you'd get anywhere near the coupling you need.
     
  5. May 25, 2005 #4

    chroot

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    Hmm, I just realized that a 50mW laser diode would be very dangerous.... maybe just stick with an LED and a light pipe. :smile:

    - Warren
     
  6. May 25, 2005 #5
    yes what is your rotation speed ??
    and have you considered putting coils on the shaft, rotating inside a few fixed magnets?
    generating your own power for this application..
     
  7. May 25, 2005 #6

    This is what I would do. Even with a slow rotating shaft you could generate a fair amount of power with the right magnet/coil assembly.
     
  8. May 26, 2005 #7
    Thanks for comments so far

    Thanks for all your comments so far - maximum shaft speed is 0.3 RPM, so direct generation is not a go-er.

    The trouble with laser sources is the beam is very small and that makes coupling the light to the pickups very difficult. My current thought is to use the mighty Luxeon Lumiled to power the thing, and six leds as pickups.

    Anyway, I am going to do a couple of basic experiments today with some HE yellow LEDS (Toshiba S4E38XX if I remember correctly.

    The rest of the rig incidentally is ticking away on the bench next to me. We are measuring capacitance of two overlapping plates, and reporting the result back.

    Once again, thanks for the comments. I will report on my results later on !

    Steve
     
  9. May 26, 2005 #8
    Dumb idea.

    Well, for future reference that was a very dumb idea. There is no power available at all. With three yellow diodes in series, there is quite respectable voltage (4V ), but the output impedance is in the order of 100s of KiloOhms.

    Transformer coupling, here we come.
    Still, I might burst the datastream through an optical link.

    Steve
     
  10. May 26, 2005 #9
    If you intend to use transformer coupling and your data rate isn't very high you can make the data link (in both directions) via the same transformer by switching capacitive load in parallel to primary or secondary winding, and sensing the V/I phase change on the other side.

    What's avout the solar cell for energy transfer? Did you find somethimg suitable?
     
  11. May 26, 2005 #10

    chroot

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    Steve,

    As I said, don't use normal LEDs on the receiver. Use PIN photodiodes that are intended for the purpose.

    - Warren
     
  12. May 26, 2005 #11
    Thanks Warren

    Thanks for the reminder Warren - I used what I had, but when I get my hands on some PIN photodiodes, I'll try again. Any suggestions for sources of 'em ?

    Steve
     
  13. May 26, 2005 #12

    Now I'll have to source some PIN diodes and try again after Warren's comments. The phase sensing idea is pretty cute. I'll have to think about that too.
    Thanks again
    Steve
     
  14. May 26, 2005 #13
    steveastrouk ,
    50 mW ok .. at 4V gives 12.5 mA i think that a bright light shining on some solar cells mounted on the motor shaft should give you what you want..
     
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