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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.

Ckt

I am not sure how to approach this.

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  • \$\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
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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.

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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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