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RLC Circuit lab equipment problem

  1. Mar 2, 2017 #1
    Hello all,

    I am a lab technician for a university and I am having some problem with some lab equipment. We are doing an RLC circuit lab with a frequency generator, oscilloscope, decade resistance box, capacitance box, and inductor. The equipment is connected in series, except for the oscilloscope. The oscilloscope channel 1 is connected to the frequency generator for the whole system and channel 2 is connected to the resistance box first. The resistance box is at 100 ohms, the capictance box is at 1 microFarad and the inductor is 0.24H.

    Now when I turn on all of the equipment I get two sine waves on the oscilloscope. What is supposed to happen is that as I increase the frequency the sine waves go in and out of resonance. The amplitudes will change sizes and so will the wavelengths. I have resonance at about 1000Hz.

    However, for some reason this does not always happen. Sometimes when everything is set up correctly, the sine waves start in resonance and never go out of resonance as I increase the frequency. The wavelengths get longer, but the amplitudes never changes. Today, I had them in resonance, but the amplitude was very low across the resistor giving me a voltage of 0.019V, which didn't make sense. I checked my connections and then turned everything on again and now I cannot get the channels out of resonance, however the voltage reading is better. I have tried different oscilloscopes, frequency generators, resistance boxes, and connections and it does not make a difference. I still get the same results.

    Does anyone know what may be causing this? I am desperate to get this to work by tomorrow.
  2. jcsd
  3. Mar 2, 2017 #2


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    Funny, I would expect a resonance at ##f = {1\over 2\pi\sqrt{LC}} = 325 ## Hz ?

    This the circuit ?

    Last edited: Mar 2, 2017
  4. Mar 2, 2017 #3
    Yes that is how it is set up.

    Sorry, I meant 0.1microF, not 1 microFarad.
    Last edited: Mar 2, 2017
  5. Mar 2, 2017 #4
    I calculate a resonant frequency of 1255 Hz. Can you check you capacitor or substitute another one?
  6. Mar 3, 2017 #5


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    And how do you do that ? 1030 ?
  7. Mar 3, 2017 #6

    Mark Harder

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    What do you mean by 'not always'? Apparently, you're repeating the demonstrations, but are you using the same components, exactly the same components? As a former experimentalist, I have learned the hard way that tiny changes in a setup can give rise to very large differences in behavior. Somebody else here suggested testing the capacitor. As capacitors age, their dielectrics can change and with that, their electric characteristics can change. This is especially true of electrolytic caps. If you do change your components, it's a good idea to change them one at a time if you can. It may seem like you're building an apparatus, but as soon as you start troubleshooting, you're experimenting. Follow the rules of experimentation. Change only one variable at a time, otherwise you don't know which one is responsible for any observable changes. Test your measuring equipment, then use only it to perform your experiments. How do you know that both channels of your 'scope are working identically? If you swap channels, are your results the same (they should be). Don't assume you know what sorts of things are important and which are not.
    I once was doing some biochemical experiments with synthetic DNA, which was ordered from a supplier and arrived freeze dried in brown glass bottles. Then the supplier switched to little polyethylene tubes. My results were no longer reproducible. I tried everything until I had to admit to myself that the only variable that could account for the change was the packaging of the DNA- plastic vs. glass. I was able to order some more in the old glass packaging. The original behavior returned, until I cleaned up the DNA. Specifically, I removed any metal ions from the samples from the glass bottles and voila, the treated brown-bottle DNA that was metal-free no longer behaved like it did without metal removal. Turns out that glass contains metal ions that can be leached out in small quantities and brown glass was especially contaminated with iron and other heavy elements. The whole effect I was investigating was dependent on the binding of metals to dissolved DNA, which is an avid binder of metals, especially under my experimental conditions. This was the basis of a significant paper I wrote, which reported new information about the effect of bound metals on the conformation of left-handed DNA. BTW, if you rely on glass stills to distill your water, you will have small amounts of calcium, magnesium and perhaps other metals in your water. Stills made of pure silica glass work better but are very expensive. This is why de-ionized water is often used instead of distilled stuff for sensitive lab experiments.
  8. Mar 3, 2017 #7
    I have tried different capacitors and inductors, but I would also get the same results. I went home and slept on it. When I came back in this morning I tried it again and everything worked, but I changed the order of the resistor, capacitor, and inductor. Which is odd because I never had to do that before. Most of the equipment I use is from the 60s, but it usually works fine. I am thinking it had something to do with the ground so I paid close attention to the wiring this morning as well. I had the resistor followed by the capactior followed by the inductor because that was how the lab was written and how my colleague showed me two years ago. Now I switched the capacitor and inductor to follow the schematic posted and it works.

    Lesson learned: Don't assemble a lab when you have a splitting headache.

    Thank you for all of your responses, all your comments helped me out in thinking some things through.
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