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Thevenin with dependent voltage source

  • Thread starter savageqm
  • Start date
  • #1
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Homework Statement


Find Rthev as seen by terminal (a,b)

http://cid-3c099006138591f8.office.live.com/self.aspx/Pictures/IMG^_0337.JPG [Broken]

Homework Equations



Can somebody point me to the right method to solve this problem. I know how to solve when there is a dependent source, but the dependent voltage source is throwing me off. I tried nodal analysis and supernode but I can't get the right answer.

The Attempt at a Solution


I used nodal analysis and supernode. I applied a 1A to the A,b connector.

Homework Statement





Homework Equations





The Attempt at a Solution


Homework Statement





Homework Equations





The Attempt at a Solution

 
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Answers and Replies

  • #2
You usually inject 1A to get RTH. Since I don't see any independent sources, I'd say that the open circuit voltage is 0V.
 
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  • #3
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I am including the 50 ohms.
 
  • #4
Ok. Sorry for changing things up so much. I just realized what the question was.

After injection, the one next to your terminal pair will definitely be bypassed. I'd also say that the one coming off of the dependent source will also be bypassed and will give you a voltage divider. I'm not sure what the polarity of vx is, so you'll have to be careful about that.
 
  • #5
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so I can get rid of the first 50 and the 25 next to the 3vx?

can mesh analysis be applied to this?
 
  • #6
The Electrician
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Show the equations you derived for nodal analysis so we can see where you went wrong.

I used nodal analysis and got 10 ohms for Rth.
 
  • #7
I worked out the node voltages and got 0V. For some reason I was thinking short circuit current. That would knock out that resistor, not a current source, sorry...been a long day/night/day.


[PLAIN]http://www.fission-systems.com/images/physics/supernode.png [Broken]
 
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  • #8
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I come up with this nodal analysis, which is wrong because I know the answer is 10ohms like somebody just mention.

http://cid-3c099006138591f8.office.live.com/self.aspx/Pictures/IMG^_0339.JPG [Broken]

I guess my problem is setting it correctly. Any ideas where am going wrong?
 
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  • #9
vela
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Which circuit is the correct one, the one in your original post or the one in your latest post? They're not the same.


EDIT: Like The Electrician, I found the Thevenin resistance for the original circuit is indeed 10 ohms, so you should start over. It looks like you tried to combine one of the 25-ohm resistors with the 50-ohm resistor, but you can't as they are neither in series nor parallel.
 
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  • #10
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The first post is the original problem, my last post was me trying some source transformation to be able to solved the problem.

I still just trying to solve the original post.
 
  • #11
The Electrician
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I come up with this nodal analysis, which is wrong because I know the answer is 10ohms like somebody just mention.

http://cid-3c099006138591f8.office.live.com/self.aspx/Pictures/IMG^_0339.JPG [Broken]

I guess my problem is setting it correctly. Any ideas where am going wrong?
You can't rearrange your resistors to have 75 ohms in parallel with 25 ohms from the left end of the source to ground. That's not a permissible operation. Try again.
 
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  • #12
Why not use a 1V or 1A test source?
 
  • #13
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I always use a 1A test source, but am just out of idea of how to setup the equation. The one I posted is the one I always get back to.

I don't know if I should use Mesh Analysis to calculate the current or Nodal analysis, but then that would mean I would have a supernode which I think am writing wrong.
 
  • #14
vela
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The only equations you've posted are in the image you linked to in post 8. As both The Electrician and I pointed out, you can't combine the resistors the way you did, so any equations you derive from that circuit won't yield the correct results.
 
  • #15
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i have a question like this one, so i try to solve this question. i also found Req=10 ohm but Vth=0. Is the Vth 0? I mean when we draw the equivalent circuit, do we set Vth=0 and Req=0?
Thanks
 

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