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Find the thevenin equivalent circuit (just need my values checked )

  1. Feb 28, 2013 #1
    find the thevenin equivalent circuit (just need my values checked!!)

    1. The problem statement, all variables and given/known data
    find the thevenin equivalent circuit (using open/short circuit method is how i did it):
    5lq58j.png


    2. Relevant equations
    n/a


    3. The attempt at a solution
    I just need to verify that my V(th)=V(oc) value is right:

    Vc/4 + Vc/6 + (Vc-2)/3 - 4 = 0
    Vc = Voc=Vth=6.22V


    i am pretty sure i did it right..however, when i build my circuit in the software my class uses (multisim) i get a different voltage and current reading when i correct it like the picture above (with point B above the 2V source). i have to connect to point B is BELOW the 2V source, like this, to get the rite reading:
    2gt28eg.png

    i know why this is. when i connect it to point B above the 2V source, i get a reading of 4.22V with the software...but i built the circuit like it looks in the diagram. and when i did it below the voltage source, i just add 2V to that value. so my actual question is, is the thevenin voltage 4.22V or 6.22V? and did i set up my circuit in the software wrong somehow?
     
    Last edited: Feb 28, 2013
  2. jcsd
  3. Feb 28, 2013 #2

    gneill

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    Staff: Mentor

    Re: find the thevenin equivalent circuit (just need my values checked

    If everything behind the a and b terminals was hidden inside a black box, what would you determine the open circuit voltage to be with your meter?
     
  4. Feb 28, 2013 #3
    Re: find the thevenin equivalent circuit (just need my values checked

    huh?? so you're saying i should only measure open circuit voltage where a and b are in the diagram? i got 4.22V when i did that, but it disagrees with my node voltage calculation.
     
  5. Feb 28, 2013 #4

    gneill

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    Re: find the thevenin equivalent circuit (just need my values checked

    Your node potential was calculated with respect to the reference node, which is NOT not node b :wink:
     
  6. Feb 28, 2013 #5
    Re: find the thevenin equivalent circuit (just need my values checked

    sorry but i dont understand. the only node i labeled was Vc. are you saying i did my node voltage thing wrong? that i should only look at what is between terminals a and b and ignore the 2V source entirely?
     
    Last edited: Feb 28, 2013
  7. Feb 28, 2013 #6

    gneill

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    Re: find the thevenin equivalent circuit (just need my values checked

    No, you did your node potential calculations fine. You chose a reference node and found the potential of the node Vc with respect to that reference node. But the voltage you're interested in is the potential difference between terminals a and b.

    For that you should find the potential at terminal b with respect to the reference node and take the difference: Vc - Vb, as you found out by subtracting the 2V potential at node b from Vc.
     
  8. Feb 28, 2013 #7
    Re: find the thevenin equivalent circuit (just need my values checked

    ah i see..so working my way from the ground node to the right path, I find there's a 2V increase when it reaches terminal b, because of the voltage source. so I take the voltage at a and subtract the voltage at b:

    6.22V-2V = 4.22V.

    is that right?
     
  9. Feb 28, 2013 #8

    gneill

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    Staff: Mentor

    Re: find the thevenin equivalent circuit (just need my values checked

    Yes, that's right :smile:
     
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