Suppose \$R_{1}\$ and \$R_{2}\$ are replaced by resistors of unknown values which are different from those assumed in part (a).

Using a variable external voltage source, a student applies \$+1.5V\$ between terminals a and b (a is positive relative to b) and measures a current \$i_{0}\$ of \$-0.1A\$. When the external voltage is \$+3V\$, the external voltage source generates power and \$i_{0}\$ is \$+0.2A\$.

Recalculate the Thevenin voltage and resistance in this case, assuming all sources are ideal.


I am not sure how to approach this.

  • \$\begingroup\$ Use mesh analysis ( loop currents) or nodal analysis to find R1 and R2 and proceed in usual fashion. \$\endgroup\$ – Plutonium smuggler Nov 26 '14 at 0:12
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    \$\begingroup\$ @Plutoniumsmuggler, I don't think that's the desired approach here. It will work, of course, but there is a far simpler method that Olin and I have in mind and, further, is the most likely object of this exercise. \$\endgroup\$ – Alfred Centauri Nov 26 '14 at 1:54
  • \$\begingroup\$ @AlfredCentauri . I think you mean we already assume a Thevenin circuit and a voltage source in place of Load resistance ; then solve 2 equations for two unknowns . Right ? \$\endgroup\$ – Plutonium smuggler Nov 26 '14 at 11:20

Think of a black box that you are told is a Thevenin source. You take two different pairs of current,voltage measurments. How would you solve that?

Do you need to know if the inside is made of several sources with a mesh of resistors, or single voltage source with series resistance? How would you tell the difference between these two cases?

Hint: much of the details of this problem are a distraction to see if you fall for getting lost in the minutia instead of thinking about the whole problem.


You can think of this as an algebra problem in disguise. Remember making equations to describe lines on a graph? You've been given two points on a voltage vs. current graph for a linear circuit...


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