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Capacitor loop with no resistance ?

  1. Feb 8, 2012 #1
    "Capacitor loop with no resistance" ?

    In Circuit Simulator at http://falstad.com/circuit/ I created a circuit attached below which refuses to run because "capacitor loop with no resistance". To me it seems that there is no problem with this circuit and that the two capacitors will simply act as an equivalent capacitor of capacitors in parallel. They would charge up and that would be the end of that. What am I missing? The reason I'm posting here is that I assume there is that this is due to my lack of understanding of the physics of the situation rather than some bug.

    Attached Files:

    Last edited: Feb 8, 2012
  2. jcsd
  3. Feb 8, 2012 #2


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    Re: "Capacitor loop with no resistance" ?

    There's nothing "wrong" with your circuit. Your two 10uF capacitors in parallel are equivalent to a 20uF capacitor.

    I guess the problem is that the simulator only works for LRC circuits. It's not a general purpose simulator like SPICE.
  4. Feb 8, 2012 #3
    Re: "Capacitor loop with no resistance" ?

    This is a limitation of the simulator - it's saying there is a path that current can go, which is from one capacitor to another, and then back to the first, never encountering any resistance. That messes up the calculations. In real life that never happens, because everything has a very small resistance, even wires.

    This is resolved by adding a very small resistor to either one of the capacitors - you can make it 0.001 ohms, which is the resistance of half an inch of wire AWG26.
  5. Feb 8, 2012 #4
    Re: "Capacitor loop with no resistance" ?

    It has its limitations, but it is general enough to simulate the internals of the 741:


    Attached Files:

    • a.bmp
      File size:
      270.1 KB
  6. Feb 8, 2012 #5
    Re: "Capacitor loop with no resistance" ?

    You are probably violating some conditions of the simulation. For example if one of the capacitors has an initial voltage different from the other then this is not a valid circuit and simulation is not possible unless a resistance is inserted as indicated.
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