LC circ

Discussion in 'The Projects Forum' started by zogorean, Sep 15, 2013.

  1. zogorean

    Thread Starter New Member

    Jan 12, 2013
    11
    0
    Hi all.

    I am having trouble with LC Circuits. It is my understanding, and correct me if I'm wrong, that RLC Circuits include the resistor in order for the oscillation to die eventually, while LC Circuits can in theory and in perfect lossless conditions live forever. It is also my understanding that you can use any capacitor and any inductance/coil to make an oscillation, and the size of the components determine the oscillation frequency. As I read it, puting a capacitor in parallel with a coil should result in a oscillation that would eventually die out after a long period of time (due to the resistance in the wires etc).

    I have been trying to create a LC Circuit, but don't seem to create this long oscillation, but rather a small spike, then silence... I've tried different sized inductors and capacitors, but no success.

    Using LT Spice I've designed a Circuit that seems to die slowly over time but in real life, I can't reproduce this.

    I'm measuring with an oscillator where I connect the probe at one inductor leg and the ground to the other. The setting is to measure DC.

    Can someone help a noob?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,522
    2,369
    An LC parallel circuit will 'ring' at its resonant frequency if hit with a suitable pulse, adding a parallel resistor decreases the Q of the circuit and widens the frequency range and also tends to dampen or decrease the 'ring'.
    Max.
     
  3. WBahn

    Moderator

    Mar 31, 2012
    17,732
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    While the frequency of oscillation is related to the product of L and C, the Q of the oscillator is related to their ratio.

    Also, in your simulation you are not modelling the parasitic resistance, particularly of the inductor. Take an ohmmeter and measure your inductor's DC resistance and then put a resistor of that size in series with your inductor in your simulation. This isn't the whole story, but it's a step in the right direction. See how much that impacts your simulation.
     
  4. zogorean

    Thread Starter New Member

    Jan 12, 2013
    11
    0
    Thanks for your input all.

    WBahn you're right, when I inserted the resistance value for the inductor in the model, it was damped. I didn't take that in consideration when I created the model.
     
  5. adam555

    Active Member

    Aug 17, 2013
    858
    39
    I've been going around that question too for a few days; while I see the resonance going forever with virtual components in simulations, it will only oscillate a couple of times in real life.

    I found that even though in theory any coil would do, in real life the bigger the inductor, the best it will work.
     
  6. WBahn

    Moderator

    Mar 31, 2012
    17,732
    4,789
    Good.

    Simulators are wonderful and valuable tools, but they offer only crude approximations to the real world. In many cases, even the crudest approximation using ideal components is more than good enough. But at other times we need to incorporate at least the most significant non-ideal elements in order to get a result that is even in the ballpark. Learning when is when can really only be done via the kind of experience you are getting now -- comparing simulations to real world and then adjusting the sims to get close. The real world lets you see the shortcomings of the sims and the sims lets you explore which elements of the real world are the ones that need to be paid attention.
     
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