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  1. #1
    la vache fantôme phantomcow2's Avatar
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    How would I be able to detect resonance?

    In a series capacitance - inductance circuit? I'm told I can use an oscilloscope. BUt what will I be looking for?
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  2. #2
    '05 NUEser EJ123's Avatar
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    Something very peculiar.

  3. #3
    Senior Member mezza's Avatar
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    Ummmmmm. Bananas, you'll be looking for Bananas...

    To be honest all I read was 'blah blah blah blah blah? blah blah blah blah'.
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  4. #4
    Trans-Urban Velocommando ax0n's Avatar
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    Sounds like a run-of-the-mill tank circuit.

    Step 1) Turn on O-Scope. I would use a real one that can take HV. Not some computer stuff or sound-card software thing.

    Step 2) Connect the test probes in parallel with the circuit.

    Step 3) Take the readings. You know how to use an O-Scope, right?
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  5. #5
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    At the resonant frequency the current flow will be highest. To measure the current you'd need to connect the scope in series. Vary the frequency to find the highest current flow. You should be able to calculate the resonant frequency near enough before starting, if you already know the inductance and capacitance.

  6. #6
    2-Cyl, 1/2 HP @ 90 RPM slvoid's Avatar
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    Quote Originally Posted by tspoon View Post
    You should be able to calculate the resonant frequency near enough before starting, if you already know the inductance and capacitance.
    That would require some kind of math...

  7. #7
    1/2 man,1/2 bear,1/2 pig ManBearPig's Avatar
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    Or if something breaks. That's how you know. The Tacoma Narrows bridge being a prime example. I also demonstrated the phenomenon as a freshman engineer when I inadvertently broke a $6000 window in my high-rise dorm with a well-placed $10 engraver.
    ...

  8. #8
    '05 NUEser EJ123's Avatar
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    Yo, towles, shoot me up.

  9. #9
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    Dude, like, isn't resonance like that major sticky ****, like dude we detected and smoked that major resonance.
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  10. #10
    2-Cyl, 1/2 HP @ 90 RPM slvoid's Avatar
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    Quote Originally Posted by Towlie View Post
    Or if something breaks. That's how you know. The Tacoma Narrows bridge being a prime example. I also demonstrated the phenomenon as a freshman engineer when I inadvertently broke a $6000 window in my high-rise dorm with a well-placed $10 engraver.
    I believe the bridge was some kind of self reinforcing resonance where the system itself kept making the amplitude greater and greater. Something that normally resonates would be like, a flute. And I've never seen a flute get destroyed by an overly enthusiastic orchestra.

  11. #11
    Senior Member bitingduck's Avatar
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    Quote Originally Posted by slvoid View Post
    I believe the bridge was some kind of self reinforcing resonance where the system itself kept making the amplitude greater and greater.
    that would require a power source in the bridge.

    The wind drove the resonance and the amplitude of the oscillations exceeded the bridge's ability to flex without breaking.

    Back to the OP's question-- if it's a simple LC circuit you should just be able to calculate it. If you need it very precisely then you need to be able to drive the circuit with a frequency generator and watch the response on the scope while you sweep the frequency. When you see the maximum amplitude, you read off the period from the scope (unless you have a really precise generator) or from a frequency counter.
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  12. #12
    Videre non videri
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    Quote Originally Posted by slvoid View Post
    That would require some kind of math...
    For a simple circuit, it's a simple calculation. Couldn't remember it off the top of my head, but an Excel sheet I made a few years back says:

    Frequence (Hz) = 1/(2*Pi*SQRT(F*H))

    F = Capacitance (Farad)
    H = Inductance (Henry)

  13. #13
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    Quote Originally Posted by slvoid View Post
    I believe the bridge was some kind of self reinforcing resonance where the system itself kept making the amplitude greater and greater. Something that normally resonates would be like, a flute. And I've never seen a flute get destroyed by an overly enthusiastic orchestra.
    IIRC, the Tacoma Narrows bridge experienced Von Kármán vortices. They form downwind of an object, and release from the surface in an alternating/oscillating way. That set the bridge in motion, and it just happened to be at the resonant frequency of the bridge, AFAIK.

    http://en.wikipedia.org/wiki/Von_K%C..._vortex_street

    Here's a nice aerial shot of such a vortex street, coming off an island:

  14. #14
    la vache fantôme phantomcow2's Avatar
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    I don't know the inductance of the coil, or else I would just calculate it. I soldered up a special frequency generator, so I plan to use that. It has 3 different ranges, and 1080 degrees of potentiometer adjustment, so I can fine tune it pretty well.
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  15. #15
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    You can calculate the inductance of the coil as well. You just need to know the length of the coil, its radius and the number of turns.

  16. #16
    la vache fantôme phantomcow2's Avatar
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    I could, but I don't trust my homemade and selfwound coil enough for consistency. My multimeter does have a capacitance measurement feature, but I couldn't get it to work. I think it has limits on the capacitance it can measure or something.
    I was able to measure stand alone ceramic capacitors, but nothing else.
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  17. #17
    Senior Member bitingduck's Avatar
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    For a simple coil it's remarkably easy to do an accurate inductance calculation from the geometry.
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  18. #18
    la vache fantôme phantomcow2's Avatar
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    Yes, but my coils are going to be bifilar wound. I believe that makes a difference
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