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Building an LC Circuit powered by 60Hz AC Current

  1. Feb 12, 2013 #1
    Hey guys,

    So I'm working on this project, and I'm trying to make two coils of different diameters to be tuned to the same resonance frequency. I got the equation ω=1/(LC)^0.5 for the frequency. My thinking was that the frequency of the wall power shouldn't matter because the resonance frequency of the primary (larger coil) would be in much higher, and would not be dependent on the wall frequency. Any ideas? Thoughts?

    - Berkh
  2. jcsd
  3. Feb 12, 2013 #2


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    Both coils can be tuned to the same resonant frequency irregardless of what your power supply is actually putting out. It's just that if your power supply is putting out a different frequency then you will have much more impedance than if the resonance matched the power supply frequency.
  4. Feb 13, 2013 #3
    Can you describe in words what your formula means?

    Also, 'two coils' is a transformer.
  5. Feb 13, 2013 #4
    Thank you for responding,

    So should I use the w formula and plug in the capacitor for a desired frequency? The primary coil I have has an inductance of 640uH and the capacitors I have range from 0.1uF to 5uF. Can I use these values to define the frequency myself? Basically, the frequency of the power supply isn't factored in that equation?
  6. Feb 13, 2013 #5
    The equation says the the frequency w is equal to the inverse square root of the inductance times the capacitance of the circuit. Yes, this is like a transformer but the goal is to transfer power at a distance greater than in a transformer.
  7. Feb 13, 2013 #6


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    You seem to be hoping to achieve the dreaded Wireless Power Transmission. Apart from in your toothbrush charger, it isn't done at 60Hz but at much higher frequencies because of the crippling losses across even the smallest distance.
    Read around this topic as much as you can before you even think of picking up a soldering iron. Have you seen what is being manufactured (possibly even released)? A specification exists, I believe, for so-called wireless charging, in which your phone etc. sits right on top of a pad or table top. If any larger distances were feasible, I think we could rely on some enterprising company to have such equipment out there on the market. It would be the Holy Grail for personal electronics but it hasn't been found yet.
    I know I'm being negative about this but I really have serious doubts about the worth of spending much time or effort on it. I have seen YouTube movies of guys' garages full of glowing discharge tubes, powered 'wirelessly' but they never give values for the actual Power that's been transmitted.
    I reckon you'd stand a better chance trying to manufacture your own transistors.
  8. Feb 13, 2013 #7
    The standard formula for resonance is (2 PI * sqr rt (L in henry's * C in farads)) Inverse

    I can't type the formula in regular math symbols so I have to make do with this awful notation but using the numbers of 640 micro henry's and the mid range of the capacitance value of 2 microfarads, I come up with a frequency of about 175 Kilohertz so you are already thinking in terms of the right frequency range. If you make the second coil smaller you just add the number of turns to equal the same number of Henry's, so if it is half the diameter you can still come up with the same inductance. Just google the inductance formula for turns giving X amount of Henry's.
  9. Feb 13, 2013 #8

    Yeah, i've looked a lot into this and there are companies that make this type of power transfer for buses at a range of ~2ft. I know the limitations of this, but I've already built the coils and in it for the long haul.


    I was actually looking into changing the capacitance to alter the frequency. My plan was to put varying capacitors in parallel to achieve equal capacity without ordering a custom one.
  10. Feb 13, 2013 #9
  11. Feb 13, 2013 #10


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    That's interesting - do you have a reference / link? Are you sure they use 60Hz? I have seen a website describing a system using 60kHz.
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