# Thevenin Equivalent

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

1. ### Hitman6267 Thread Starter Member

Apr 6, 2010
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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
5,448
782
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
82
0
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 ?

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
82
0
No, I just added the red dot using paint. It is the problem like it I got it.

Oct 9, 2007
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311