Total resistance

Discussion in 'General Electronics Chat' started by Dritech, Jan 25, 2015.

  1. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Hi all,

    I am trying to calculate the total resistance at point X of the attached circuit.

    I'm I correct if I calculate it as below:

    R2 // R4 = ANS1

    ANS1 // R1 = ANS2

    ANS2 + R2 = ANS3

    ANS3 // R5 = FINAL ANSWER

    I know this should be easy, but I'm getting confused.
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    To find the equiv. Thevenin source resistance at X, Set the 5V node to 0V (Ground it). That eliminates R5 from the problem.

    Note that you have two R2s, so rename the upper R2 to be R3. Now you have (R3//R1) in series with R2, or R3*R1/(R3+R1) + R2 = 10K*10K/(10K+10K) + 10K = 15K = Ru. That is the effective resistance looking up from node X.

    Looking down from node X, you have Rd = 20K, which is in parallel with Ru = 15K, above. So Rx = Ru//Rd = Ru*Rd/(Ru+Rd) = 15K*20K/(15K+20K) = 300K^2/35K = 8571.

    Note that I can use LTSpice to find the Thevenin Equiv of this circuit by connecting a current source to node X, and then sweeping the load current from 0μA to 100μA. Note that the Thevenin Equiv no-load output voltage occurs at I(I1)=0, which is shown at Cursor1 to be 1.42857V. The Thevenin Equiv output resistance of the network is the slope of the load line as the load current sweeps from 0μA to 100μA, which is computed automatically as the Difference between Cursor1 and Cursor2. The Slope is 8571.43 (Ω), which agrees with what I calculated algebraically, above...

    Note that R5 has no effect on the Thevenin Equiv...

    131.gif
     
    Last edited: Jan 25, 2015
    Dritech likes this.
  3. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks :)
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Here is what I get:
     
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  5. Dritech

    Thread Starter Well-Known Member

    Sep 21, 2011
    756
    5
    Thanks a lot for your help :) That is still Thevenin's theorem right?
     
  6. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    It is a step along the way to find the Thev. Equiv. open-circuit output voltage. As part of finding that, you need to know the voltage at Vout.
    I used Kirchoff to sum the currents into the middle node, and went from there...
     
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