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DE explaining simple RC circuit

  1. Dec 2, 2009 #1
    I'm trying to wrap me head around what should be a very simple system, but I just can't manage it. I know there is no closed form for this problem (ignoring a voltage step like is shown in the diagram below, imagine it is a arbitrary voltage supply, or even more exactly, that R2 changes over time), and that to solve it I need to convolve equations (though I would hope I could do a pretty good numerical approximation too).


    What I'm trying to do is get the equation for dV/dt in terms of Vcmd, R1, R2 and C

    So I know
    Itot = Ires + Icap
    Itot = V/R2 + C * dV/dt

    I also know V = Vcmd - Itot*R1
    Itot = (Vcmd-V)/R1


    (Vcmd-V)/R1 = V/R2 + C * dV/dt
    rearranging gives
    C*dV/dt = (Vcmd-V)/R1 - V/R

    I like that, because it means as R1 drops to zero, dV/dt approaches infinity (which is what I would expect, i.e. V should approach Vcmd instantly) BUT if R1 did drop to zero, and V = Vcmd, dV/dt should be zero, right? But that equation says otherwise. Likewise, as R1 approaches infinity, I would have thought dV/dt would approach zero.

    Am I doing something wrong here? Making an invalid assumption?

    Attached Files:

  2. jcsd
  3. Dec 2, 2009 #2


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    Staff: Mentor

    Not sure why you think there is no closed form solution -- looks like your equations are just fine. The only issue is the infinity you think you are getting with R1 --> 0. You cannot use an ideal voltage source with zero output impedance and an ideal capacitor together in series with a voltage step. The non-real zero output impedance of the voltage source is what is giving you the non-real infinity.
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