Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help in finding R Thevenien?

  1. Nov 22, 2009 #1
    The question is find R Thevenien using Voltage test source across terminals a and b of the following circuit:

    http://img255.imageshack.us/img255/8660/rthevenen.png [Broken]

    I did it by adding a 20 test voltage source and used mesh to get the following 4 equations of the 4 meshes ( I deactivated the 480 V independent source to be able to use test voltage method)

    M1:6(x-t) + 40(x-y)+4x=0
    M2: 40(y-x)+20=0
    M3: 80(z-t) - 20q +2z-20=0
    M4:4t + 8(t-z) +6(t-x)=0

    I R thevenen to be 11.55 but actually the correct answer is 12 ohms.

    Where is my mistake.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Nov 23, 2009 #2

    The Electrician

    User Avatar
    Gold Member

    I don't see 4 meshes; I only see 3 meshes.

    You need to explain in more detail what you are doing. What are x and y? I don't see anything labeled x and y on your schematic. You should tell me what they are, and don't expect me to figure it out for myself.

    I would have used the nodal method, and select the node labeled b to be the reference (ground).

    Then you can inject a test current of 1 amp into node a, and the voltage there will be equal to the Thevenin resistance (setting the 480 volt source to zero, of course).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook