Simple Thévenin equivalent?

June 14, 2010 7:36 PM

What is the Thévenin equivalent of this circuit?

I have this circuit. I am required to find the Thévenin equivalent voltage and impedence of this circuit as seen by the capacitor C. An algebraic expression is required.

I arrived at the solutions Vth = (R3*Vi)/(R2 + R3 + R4) and Rth = R3(R2 + R4)/(R2 + R3 + R4), but my friend has 1/(1/R1 + 1/(R1 + R4)) as the solution.

Are either of us right? Many thanks for any light Mefites can shed on the matter!

posted by PuGZ to Technology (6 answers total)

You both agree on V_Th? I believe you are right on R_Th: R₁ is shorted by the voltage source (replaced by a short to compute R_Th), so it doesn't come into account.
posted by Monday, stony Monday at 8:21 PM on June 14, 2010

Best answer: (And I can confirm to the best of my abilities that you are completely right; you can check your answer by computing I_Th and checking that V_Th = R_ThI_Th (see).
posted by Monday, stony Monday at 8:31 PM on June 14, 2010

Best answer: I agree with Monday, stony Monday-- your R_th appears to be correct. Noting KCL around the outside loop, your expression for the current through R3 and subsequent expression for the open-circuit voltage at the output nodes appears correct as well.
posted by thack3r at 10:57 PM on June 14, 2010

Response by poster: Thanks a lot, guys/girls. Upon closer inspection (warranted by your responses!), we agreed that my answer appears to be correct.

The uncertainty arose from my friend's insistence that the professor had given the other solution, but it turns out that they had made a mistake in copying it down. :-)
posted by PuGZ at 11:06 PM on June 14, 2010

So, this was Homeworkfilter?
posted by 6550 at 6:06 AM on June 15, 2010

I am not a big fan of homework filter as it often is a way of avoiding doing the work yourself. After someone has completed the work, and especially when it is relatively complex like a Thevenin analysis, then having it checked here seems appropriate. I recently had some trouble trying to help my son with his physics analyzing a circuit. I felt better after asking an electrical engineer to help us and even he had trouble. Thevenin provided an answer, but he had not been taught that so we had to brute force a loop analysis. Both my son, the EE and I were doing it right from the beginning, but the math was voluminous enough that we all made some errors along the way. Having an answer from AskMe would have been helpful to check for those errors and I think that would be an appropriate use of AskMe. I used SPICE instead. Anyway I am glad PuGZ got his answer here.
posted by caddis at 12:57 PM on June 15, 2010