1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Thevenin's Equivalent

  1. Sep 8, 2014 #1
    1. The problem statement, all variables and given/known data
    Find the Thevenin equivalent circuit with respect to terminals a, b


    2. Relevant equations
    i1+i2+i3..=0
    v1+v2+v3..=0
    i=v/R


    3. The attempt at a solution
    I set a short circuit across 72V, and calculated that Rth is 6Ω. My first question is, how come I can't always do this? I get problems where I have to calculate Rth a completely different way. How can I tell when this is acceptable to do?

    Now I'm using the node voltage method to calculate Vth

    So I did part of my equation, which is:
    (v1-72)/5+v1/20+(v1-Vth)/8=0
    (Vth-v1)/8-...
    But I'm not sure what I use next
     

    Attached Files:

    • th.png
      th.png
      File size:
      2 KB
      Views:
      62
  2. jcsd
  3. Sep 8, 2014 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    Hm ... offhand, I don't remember this ever being an issue unless there is some kind of weird controlled source. Can you give an example of such a problem?
     
  4. Sep 8, 2014 #3
    I meant ones with controlled sources, I don't know how to solve those. Also ones like this.

    It works if I pretend the 10kΩ isn't part of the circuit, but I'm not sure why that is...
     

    Attached Files:

    • exa.png
      exa.png
      File size:
      2.7 KB
      Views:
      54
  5. Sep 8, 2014 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    If you pretend that the 10K is not part of the circuit, you cannot possibly get the right Rth, so I suggest your book has a wrong answer or something else is misleading you.
     
  6. Sep 8, 2014 #5

    gneill

    User Avatar

    Staff: Mentor

    You need to include the 12Ω branch for the Vth node equation.
     
  7. Sep 8, 2014 #6
    Here's what my book says for Rth, they calculated 12||20 +2.5, and ended up with 5kΩ
     

    Attached Files:

  8. Sep 9, 2014 #7

    phinds

    User Avatar
    Gold Member
    2016 Award

    Exactly. Why did this make you think they ignored the 10K resistor? They obviously did not
     
  9. Sep 9, 2014 #8
    Why is it not in the equation though? It is because it's parallel to a zero resistor?
     
  10. Sep 9, 2014 #9

    phinds

    User Avatar
    Gold Member
    2016 Award

    Reread post #4. Forget what the book says. Do it right and you'll get the right answer (which is NOT the answer they got, 'cause they didn't do it right).

    Why would you think it is parallel with a zero ohm resistor? WHAT zero ohm resistor?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Thevenin's Equivalent
  1. Thevenin Equivalent (Replies: 3)

  2. Thevenin Equivalent (Replies: 3)

  3. Thevenine Equivalents (Replies: 6)

  4. Thevenin equivalent (Replies: 2)

  5. Thevenin equivalent (Replies: 4)

Loading...