Thevenin Equivalent

Discussion in 'Homework Help' started by Hitman6267, May 5, 2010.

  1. Hitman6267

    Thread Starter Member

    Apr 6, 2010
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    4R2C2Vdiamond.jpg

    Before the switch closes at t = 0, the circuit is in steady state. R1=R3=3 kΩ,R2=R4=10 kΩ,C1=8 μF ,C2=9 μF. What is the value of the time constant τ1in ms that governs the behavior of v1 as a function of t for t>0? (Hint: Find the Thevenin equivalent for t>0.)

    To calculate the thevenin equivalent I short circuit the voltage sources, but what does this short-circuit by-pass ?
     
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The "trick" with this circuit is to realize that once the switch closes the two halves of the circuit have no interaction either side of the center line formed with the switch closed and the sources thus connected in series.

    So with the switch closed, the time variable current / voltage behavior of right hand side elements comprising R3, R4 & C1 has no relationship to the behavior of the parameters of elements on the the LHS - R1, R2 & C2.

    You need only determine the initial condition on C1, which is pretty obvious once you get your head around it. Then consider only the right hand side of the circuit - i.e. discard R1, R2 & C2 from the analysis.
     
  3. Hitman6267

    Thread Starter Member

    Apr 6, 2010
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    It worked if I consider that the only resistors that are left are R3 and R4 on the right.
    But I can't imagine how that can happen unless the branch that has the sources intersects the line it's going through. I assumed it's going over it. If they intersect shouldn't they have to put a node between the intersecting branches ?

    I added the node I'm talking about it in red.
    4R2C2Vdiamond.jpg
     
  4. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    This image looks you redrew it. Can you take a picture of the actual problem and post it?
     
  5. Hitman6267

    Thread Starter Member

    Apr 6, 2010
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    No, I just added the red dot using paint. It is the problem like it I got it.
     
  6. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    The circuit as drawn is ambiguous. Can you ask your instructor about it?

    The solution will be quite different, depending on whether the center two lines are connected or not. I think it will be substantially more difficult if they are not connected.

    If you can't ask, which way do you intend to proceed; connected or not?
     
  7. Hitman6267

    Thread Starter Member

    Apr 6, 2010
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    My friend considered that they were connected so I solved it based on that assumption and it worked. I was just wondering if I'm the only one unsure if they're connected or not. I'm planning on asking my professor on it, I just needed to confirm that I'm right in not be certain. Thank you
     
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