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Help with Thevenin Theorem

  1. Jan 17, 2015 #1
    1. The problem statement, all variables and given/known data

    The circuit looks like this: http://i.imgur.com/QtgiCdR.png

    R1= 10 Ω
    R2= 8 Ω
    R3= 3 Ω
    R4= 5 Ω
    R5= 2 Ω

    V= 15 V

    I'm trying the find the current in R1 by using Thevenin's Theorem.

    2. Relevant equations

    Basic Circuit Analysis Formulas

    3. The attempt at a solution

    So I calculated the voltages at the points where R1 is removed. 15V on the left side and 90/101V on the right side (confirmed by LTspice). Then I tried to calculate the Thevenin resistance which (once the voltage source is shorted) is the R2 || R3 || (R4 + R5) which turns out to be 168/101Ω. So putting everything together now, Vth = 15 - 90/101 and Rth = 168/101 and Rl = 10, I get that the current in R1 is 1.2096A but according to LTspice, it should be 1.22003A. What am I doing wrong?
  2. jcsd
  3. Jan 17, 2015 #2


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    2017 Award

    Staff: Mentor

    Rounding error?
    With Kirchhoff's laws, I get 270/73 V and 1635/584 V for the voltages at the two points (relative to negative source connection, first value at resistors 2/3/4) and 1425/1168 A which is about 1.220034 A as current through R1.
  4. Jan 17, 2015 #3


    User Avatar

    Staff: Mentor

    Check your evaluation of Rth. Note that R4 and R5 are not in series in this case, since their junction feeds the load terminal.
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