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Basic questions about inductive coupling

  1. Aug 24, 2008 #1
    I've been fascinated by the idea of inductive coupling for a long time, so this weekend I decided it was finally time to try to learn a little something about it. My knowledge of this stuff from before this weekend is pretty much limited to what I still remember from my physics AP class that I took in the late 90s, so there's a pretty good chance I've made some kind of obvious mistake somewhere along the line. Go easy on me.

    I feel like I've gotten this experiment to the point where the general information on the dozens of query sites that I've checked isn't useful to me anymore, and I need some specific information about my own project.

    I was initially inspired by this site: http://bea.st/sight/levitation/

    I read the hell out of his instructions for the inductive coupling portion of this thing, and I went and got the following materials:

    A step-down transformer with an output of 12.6V 1.2A AC
    22-gauge copper wire
    Two round ferromagnetic cores, one 3.5" in diameter and the other 1" in diameter. The 1" one is an iron pipe coupler, and the 3.5" one is much thinner, and could be one of the mistakes I made in this experiment. It is ferromagnetic, but it's thin.
    Capacitors totalling 24nF and 12nF. I got those numbers as follows.

    Using the web site above, I decided to use the same frequency he used. Using


    I found that for F=320 and C=.000000024, L=10.307

    Using the BASIC program at http://www.epanorama.net/documents/components/coils.html , I found that for a 1" coil diameter of 22 gauge coil, this would mean 21 turns of coil. This was the coil I used the 24nF capacitance for. Then using the same equations, I got

    F=320, C=.000000012, and L=20.61385

    Which for my 3.5" coil, meant 11 turns of coil. This was the coil I used the 12nF capacitance with.

    I wound the coils and added the capacitors, then hooked up the power to the larger coil and some LEDs to the smaller coil. I put the smaller coil inside the larger coil and plugged the transistor in.

    Nothing happened.

    I used my multimeter to test it, and there's definitely power coming out of the transformer. However, no power is being transmitted to the smaller coil.

    Have I made some sort of obvious mistake? Is it just a matter of tuning the frequencies more precisely, with more capacitors? If so, how should I approach that? Do I need a better ferromagnetic base for one or both of the coils?

    On a more basic level, how can I test to see if there's even an electromagnetic field being created by the larger coil? Maybe I've messed this up in an extremely fundamental way.

    Also, the guy at http://bea.st/sight/levitation/ used a signal generator. I have a 555 timing chip and the capacitors/resistors I would need to make it produce a 254 kHz frequency, but I haven't added that yet, because I confess that I don't understand if/why it's necessary. Also, does a 555 chip require DC power? I have googled this extensively but I don't have the vocabulary to understand if I'm getting a definitive answer about this. On the one hand, it looks to me like a 555 chip may require DC power. On the other hand, as far as I can tell it takes AC power to create the type of electromagnetic field I need for this experiment, and the site I reference says that such a chip can be used for inductive coupling. So I don't know.

    Do I need to convert this experiment to DC, on the idea that the timer chip will add the cycles that I need to get the field I want? Can I put the chip in between my AC transformer and my coil? How much power can a 555 chip take? Would my transformer fry it? Or, on the other hand, do I need more power for what I'm doing?

    Thanks for any advice anyone can give me. If you don't want to answer all the questions above, if you could just point me in a new direction or give me some new equations to study, that would be great.

  2. jcsd
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