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I am looking at an example in my text regarding Thevenin circuits and when they set the voltage source on the most left hand side to zero and solve for the equivalent resistance they get the following:

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However when they revert back to the original circuit to solve for the current the equivalent resistance is found as follows:

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My question is why Req is not equal to Rt if they are both solving for the equivalent resistance of the circuit. I thought 15 and 30 would be in series for both cases but its only in series for the second case. Can someone help me understand this? Thank you very much!

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Both calculations are correct, they just calculate different things.

The first resistance is the impedance seen by the load, so it is the output impedance of the circuit, the Thevenin equivalent resistance when the system is replaced by single voltage source and single series resistor.

The second resistance is the equivalent load the 32V supply sees if all the resistors are replaced with one.

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  • \$\begingroup\$ Hi, could you please elaborate a little more on why the 15 and 30 ohm resistors are in parallel for the first case and in series for the second? \$\endgroup\$
    – programmer25
    Oct 24, 2021 at 8:58
  • \$\begingroup\$ The 15 ohm resistor is not parallel with anything, it is always in series. Depending on if you look at the circuit from left to right or from right to left, you get the two different formulas. In the other way the 15 ohm is in series with 5 ohms, and in the other way it is in series with 30 ohms. \$\endgroup\$
    – Justme
    Oct 24, 2021 at 9:32

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