## PHYS345 Electricity and Electronics

Quiz 3 Solution: Thévenin Generator

Construct a Thévenin generator to determine the voltage across the 5 ohm resistor shown?

1.   Find RTH by killing the sources.

RTH = 1 ohm + 2 ohm || 3 ohm.
RTH = 2.2 ohm.

2.   Find Voc by circuit analysis.

Since we are looking for the open-circuit voltage, I would recommend nodal analysis. If we solve for V1 we will be most of the way to the complete solution since Voc = V1 + 2 V. The 2 V comes from the voltage drop across the 1 ohm resistor that has 2 A current through it from the current source.

KCL at node 1: (V1-2)/2 + V1/3 -2 = 0.
3(V1-2) +2V1 - 12 = 0
5V1 = 18
V1 = 3.6 V
Voc = 5.6 V

3.   Use Thévenin generator to complete circuit analysis.

The voltage across the 5 ohm resistor is (5.6 V) (5.0 ohm) / 7.2 ohm) = 3.9 V.

Thanks go to Eddie Knotts for reporting an error in way the current source was killed in the original version of the figure in Part 1.

"http://www.physics.udel.edu/~watson/phys345/quiz/03soln.html"
Last updated Oct. 31, 1999.
Copyright George Watson, Univ. of Delaware, 1999.