# Help with Thevenin Theorem

1. Jan 17, 2015

### whoareyou

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. Jan 17, 2015

### 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.

3. Jan 17, 2015

### 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.