Finding the Thevenin equivalent by zeroing

[Jelsborg]

Homework Statement

Determine Thevenin equivalent for the given circuit

Relevant Equations

Basic zeroing and shorting

I am asked to find the Thevenin equivalent for the circuit above. I already got the equivalent voltage by node analysis on the nodes B and A3.

What I'm trying to do is to zero the 10V-source and then calculate the equivalent resistance between A3 and GND.

Whatever expression I come up with, doesn't seem to fit my simulation value, and I guess it's because I'm missing some key point, and the problem seems to be the $R_{22}-R_{19}-R_{21}$ combination, which I am not sure how relates to the other resistances.
To me, R23 is in parallel with R22+R24, but it is not clear to me how this relates to R21 and R19.
Can someone guide me in the right direction?

 

[vela]

When you replace the 10-V source with a short, note that R21 and R24 are now in parallel.

 

[Jelsborg]

When you replace the 10-V source with a short, note that R21 and R24 are now in parallel.

Since the voltage and thus the current over $R_{20}$ is then 0, right?
Moreover, is this reasoning correct:

R21 and R24 are in parallel with the series of R19 and R22, which is then in turn in parallel with R23?

 

[vela]

Since the voltage and thus the current over $R_{20}$ is then 0, right?

Right. You can remove R20 because it's shorted out.

Moreover, is this reasoning correct:

R21 and R24 are in parallel with the series of R19 and R22, which is then in turn in parallel with R23?

Not quite. R19 and R22 aren't in series. Resistors are only in series if the current through one has to go through the other.

 

[Jelsborg]

Right. You can remove R20 because it's shorted out.Not quite. R19 and R22 aren't in series. Resistors are only in series if the current through one has to go through the other.

Then I don't really get how to evaluate the R22-R19 connection. They both meet at the A3 node, which also meets R23, while the other end of R23 meets the node connecting R24 and R21. So I'm trying to convert the whole R24-R21-R19-R22 into one resistor which I can then consider as parallel to R23 - or maybe this is the wrong strategy?

 

[vela]

Redrawing the circuit is often helpful. The one on the left is with R20 removed and the source shorted out. The circuit on the right is the same circuit, but it's easier to see how to deal with it.

 

[Jelsborg]

Redrawing the circuit is often helpful. The one on the left is with R20 removed and the source shorted out. The circuit on the right is the same circuit, but it's easier to see how to deal with it.

View attachment 250472

Clever redrawing - I'm getting the same as my simulated value out of this circuit.
Thank you very much!