Power transmission over 1 wire?

Discussion in 'General Electronics Chat' started by THE_RB, Jul 20, 2014.

  1. THE_RB

    Thread Starter AAC Fanatic!

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    Hi, does anyone have suggestions for small scale power transmission over 1 wire, with no ground?

    Imagine a small helium balloon, tethered to ground using a fine single conductor. The balloon has camera and other sensors and needs a few mA average. Probably under 10mA at 3.3v (33mW).

    Obviously the goal is to have the balloon electronics powered over the tether conductor, and have it permanently aloft.

    So the receiving power converter needs to be both small and light.

    The ground based power transmitter can be larger and does not need to be efficient.

    Any suggestions will be much appreciated. :)
     
  2. Alec_t

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    Off the top of my head, with no research, I wonder if applying high frequency AC to the wire and relying on atmospheric capacitance to ground to complete the circuit would work?

    Edit: Quick search indicates Earth's atmospheric 'parallel-plate' capacitance between ground and ionosphere at ~70km is ~710uF (not verified). Assuming a linear dependence on altitude that means ~10uF per km, or 1uF at 100m. However, the top of a tether is a point (not a layer around the planet), so would capacitance to ground from 100m be less than 1uF?

    Edit2: Capacitance of a 1 sqm 'plate' at 100m above Earth is (1 x 1 x 8.85 x 10^-12)/100 F = 8.85 x 10^-2 pF. So any realistic ballon size will have too small a capacitance. That knocks the idea on the head :(.
     
    Last edited: Jul 20, 2014
  3. kubeek

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    That seems like the only possible idea. But the inductance of the long wire will screw things up, so you might need to find a sweetspot which will be kinda tricky to do since you wont have access to the top of the baloon to do any mesurement. You could set up a baterry powered RF device that will measure the incoming power and use it to tune the frequency.
    You willl most likely want to have as big conductive area up there as you can get to maximize the capacitance. Beware of lightning and static buildup due to wind friction etc., that might ruin your day.
     
  4. BeerBelly

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    Solar power would be the obvious way to go. Radio transmission up the wire would be easy enough if you are licensed to transmit.
     
  5. KMoffett

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    How high? Why not two wires? One bare with appropriate tensile strength and one light weight, insulated wire for the return. Also, allows both power and date communication.

    Or, is this just your usual inquisitive "can it be done" approach? :)

    Ken
     
  6. wayneh

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    Just wondering how much current you could harvest from the naturally occurring voltage between elevation and ground. Might be plenty, but I can't find a decent reference on how much current is available. Everything talks about voltage, which is ~100V/m.

    Never mind. I found a good reference, and the current available is only 1-2pA/m2. Just a few orders of magnitude less than practical!

    You'd get more by rectifying ambient RF.
     
    Last edited: Jul 20, 2014
  7. RichardO

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    I think I would use a pair of very fine magnet wires rather than a single wire. Send up high voltage -- say, 50 volts -- so the I*R drop is not a problem. A small buck regulator should not weigh too much.

    Some side notes... How much lift do you have available? At my altitude (5280 feet) the lift is greatly reduced. They don't fly blimps this high. Also, I have heard that "balloon gas" is diluted helium so you need to track down full strength helium for maximum lift.
     
  8. sirch2

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    I remember many years ago an electrical surplus place selling reels of guided missile wire, about 2000m on a reel IIRC. Don't know if it still available.
     
  9. Lestraveled

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    The TOW missile receives its guidance commands on a pair of wires that trail out the rear of the missile. You may be able find some at surplus dealers or recyclers. I believe the bobbin holds >3000 meters of wire.
     
  10. Lestraveled

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    With medium grade solar cells you get about 1 watt per 10 square inches. You could get the power you need (.033W) from less than 1 sq. inch of solar cells. Operating at night would be a problem. Although, at night, you could have a larger solar cell facing downward and illuminate it from the ground.
     
  11. alfacliff

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    there is a low loss microwave transmission line (one wire) look up "g "line. dosnt radiate much at all.
     
  12. Lestraveled

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    RB this is a very interesting project....

    In order to send power on one wire you will have to induce current on the wire. As Alec_t pointed out, the capacitance is very small. This suggests that you use very high frequency, very high voltage, or both.

    The problem with high frequency is that the current approaches zero at the end of the dipole. If you could have your electronics lower on the tether than the balloon, you could use a standing wave effect to be at the max current point on your antenna/tether. Because the end point (balloon) determines the minimum current point and your electronics is a fixed length below it, means that you would use a fixed frequency to energize it.

    Mark
     
  13. Lestraveled

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    This is what I am talking about.
     
  14. kubeek

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    If you can get the frequency up to some hundreds of MHz, then the 1/4 wavelength point will be just a few meters below the balloon, so that should not be a big problem.
    The bigger problem I see that these all are more or less assumptions, and the reality might be very much different.
     
  15. Lestraveled

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    1/4 wavelength at 27 MHz is 9 feet.
     
  16. THE_RB

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    I didn't know I had a "usual" hypothetical approach. :)

    The reason for only one wire was to eliminate the need for insulation to reduce wire weight and increase payload.


    As a guy who normally suggests simple alternatives to posters, I'm kicking myself for not thinking of this. :eek: Flexible lightweight solar film cells would produce an excess of power even when taking into account the weight of some thin LiPo batteries (which I was considering anyway). And it means the cabling can be nonconductive and lighter too. Thanks! :D

    Although it might still be worth running a single conductor conductive cable for the data back down.

    Thanks everyone for the suggestions, but I think the solar idea wins out by far.

    And yep it's still a hypothetical design at this point. ;)
     
  17. wayneh

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    FWIW, a tethered balloon might have a decent wind exposure also. A windmill sounds more complicated than a solar cell but might be an option to get power over more hours of the day. And you could get windspeed telemetry data as well.
     
  18. THE_RB

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    Thanks!

    Fortunately ebay is full of TINY dc motors for quadcopter use, low RPM (so they generate power at lowish revs) and very light weight. That's not a bad idea at all. :)
     
  19. alfacliff

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    THE_RB likes this.
  20. wayneh

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    I'd be very curious to hear any results if you happen to experiment with those.
     
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