# Source transformation brings out a different result compared to other methods

High schooler here that just recently found out about source transformations. I wanted to try it out on a previous circuit we did using the good ol' KCL/KVL/Node/Mesh methods, but somehow, it gave me a different result. The circuit on the top is the original set up, with the voltage measured at R3 being -3.75V (correct answer). However, doing a source transformation around V1 and R2 results in the voltage at R3 changing to 24.5V!

I am stumped and may need an explanation on why the voltage changed when the source transformation technique was done properly. Or perhaps my technique was actually done improperly?

In the first figure, there is one node between V1 and R2 that is connected to another element in the circuit. In such conditions, the source transformation is not applicable, the circuit in the second figure is not identical to the first one. If you try converting I5 and R5 back to the voltage source form, you will see that the nodes would be connected differently compared to the first figure.

You should recognize that I1 (10mA) and R1 (10k) are irrelevant. Why? because V1 dominates.

Try your transformation circuit again, leaving out I3 (10mA) and R4 (10k).

Although there are right answers I would like to explain why these two circuits are not equivalent so you can avoid these mistakes later on.

Two one-port circuits are equivalent, when for a given V we get the same I and vice versa (i.e. same V-I characteristics) like between A and B in the following:

You are applying this to a two-port circuit which is not right. For a two-port circuit like the following:

The equivalent circuit must have the same characteristics on both ports. Which is not true in your case.

• Thank you for this! We just zoomed past the topic of source transformation, so my teacher didn't really teach us the fact that it had to have no three-port nodes to make sure the transformation was equivalent Feb 18, 2021 at 7:52