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Nodal Analysis

  1. Oct 2, 2009 #1
    1. The problem statement, all variables and given/known data
    I have to solve for V0 , using nodal analysis. I am getting different answers in matlab and PSPICE. The circuit is below
    2j4eel3.jpg


    2 supernodes ; or 1 big supernode Here are my nodal equations:

    v1-v3 = 12
    v3 - v2 = 6

    (v3)/6k + (v1)/4k - 6/k = 0

    Matlab output is here:

    Code (Text):
    A=[1,0,-1;0,-1,1;.004,0,.006]
    b=[12;6;.006]
    format long
    inv(A)*b

    A =

        1.0000         0   -1.0000
             0   -1.0000    1.0000
        0.0040         0    0.0060


    b =

       12.0000
        6.0000
        0.0060


    ans =

       7.799999999999999
     -10.200000000000001
      -4.200000000000001
    PSPICE Schematic:
    20poeas.jpg

    So confused!!!!



    2. Relevant equations
    I = V/R


    3. The attempt at a solution
    I have gotten -4.2 from matlab and 7.2 from pspice.
     
  2. jcsd
  3. Oct 2, 2009 #2
    Try grounding the node connected to 2 voltage sources. This will simplify the analysis by only having to solve for a single node voltage.
     
  4. Oct 3, 2009 #3
    I have solved this by hand and I can tell you that your equations, and pspice are correct.

    I just wonder about what it is you're 'saying' by that matlab code, I will have to look at it in more depth.
     
  5. Nov 6, 2009 #4
    Clearly the Matlab answer is wrong since it does not satisfy the III equation:

    (v3)/6k + (v1)/4k = -4.2/6k + 7.8/4k = -.7/k + 1.9/k =1.2/k != 6/k

    On closer inspection, you will see that the conversion from
    (v3)/6k + (v1)/4k - 6/k = 0
    to
    A=[1,0,-1;0,-1,1;.004,0,.006] is erroneous -
    1/4k != .004, 1/6k != .006

    I suppose you already figured it out by now.. anyhow it may help someone who reads the thread.

    Cheers,
    Abha
     
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