# Foo - How would I be able to detect resonance?

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phantomcow2
10-14-07, 08:36 PM
In a series capacitance - inductance circuit? I'm told I can use an oscilloscope. BUt what will I be looking for?

EJ123
10-14-07, 08:38 PM
Something very peculiar.

mezza
10-14-07, 08:39 PM
Ummmmmm. Bananas, you'll be looking for Bananas...

To be honest all I read was 'blah blah blah blah blah? blah blah blah blah'.

ax0n
10-14-07, 08:48 PM
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?

tspoon
10-14-07, 09:06 PM
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.

slvoid
10-14-07, 09:09 PM
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...

ManBearPig
10-14-07, 09:11 PM
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.

EJ123
10-14-07, 09:19 PM
Yo, towles, shoot me up.

Krink
10-14-07, 09:33 PM
Dude, like, isn't resonance like that major sticky ****, like dude we detected and smoked that major resonance.

slvoid
10-14-07, 09:51 PM
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.

bitingduck
10-14-07, 10:20 PM
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.

CdCf
10-15-07, 03:52 AM
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))

H = Inductance (Henry)

CdCf
10-15-07, 03:58 AM
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%C3%A1rm%C3%A1n_vortex_street

Here's a nice aerial shot of such a vortex street, coming off an island:
http://hsu.as.ntu.edu.tw/cyber_course_II/chapter7/cloud/von-karman-vortex.jpg

phantomcow2
10-15-07, 02:48 PM
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.

CdCf
10-15-07, 03:03 PM
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.

phantomcow2
10-15-07, 06:14 PM
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.

bitingduck
10-15-07, 06:44 PM
For a simple coil it's remarkably easy to do an accurate inductance calculation from the geometry.

phantomcow2
10-15-07, 06:49 PM
Yes, but my coils are going to be bifilar wound. I believe that makes a difference