Electronic – Order of circuit decomposition to find thevenin circuit

circuit analysisthevenin

In the circuit below, Vs=18V, R1=100Ω, R2=100Ω, R3=100Ω, and R4=100Ω.! Determine RTh for the Thévenin equivalent circuit.

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I know to find the thevenin resistance, all independent voltage sources are converted to shorts and current sources are converted to open circuits.

In the diagram above, if the circuit is if the voltage source is replaced with a short, then R1 // R2. My confusion is in part II of the diagram. I thought in part II of the simplified circuit, it doesn't matter how the final circuit is decomposed–but apparently it does. When I simply for the circuit as I did in green, I get the correct solution, but in the the blue ink, its incorrect… why?

In part II, I thought R3 and R4 are in series and (R1//R2) is in series with R3. Why is (R1//R2) //(R3+R4) incorrect?

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Best Answer

You can't consider R3 and R4 in series because of the terminal there on the node between R3 and R4. In order to be in series, the components have to have the same current path. With no load connected between the terminals, they would be in series, but that rather defeats the purpose of a source. So the correct Req would be ((R1 // R2) + R3) // R4.

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