3
\$\begingroup\$

I'm trying to find the theveinin equivalent of this RC circuit but am a little confused. Since R2 and C are in parrallel, how does this factor into finding the thevenin equivalent resistance?

Is it still \$R_{th} = \frac{R_2}{R_1+R_2}\$ or is there something more subtle here I'm missing?

Does the capacitance remain the same here too?enter image description here

\$\endgroup\$
  • \$\begingroup\$ That's the unitless voltage divider ratio, treating the bottom node as the reference. Not \$R_{TH}\$. \$\endgroup\$ – jonk Apr 18 '17 at 19:20
1
\$\begingroup\$

The Thevenin equivalent is found at a pair of nodes. What it is connect to doesn't change. In this case, the black dots are probably causing confusion. If you want the Thevenin equivalent made up of the source, R1, and R2 not including C then C doesn't change. The pair of nodes are either end of R2, between R2 and C. If you are only interested in DC then the capacitor doesn't matter much.

If you are interested in the frequency dependent Thevenin equivalent then C will make a difference. You still have the option of not including it in the circuit and get a Thevenin equivalent of the source and resistors. Then the the circuit will look like the one on the right. If you include the C in the equivalent, you would end up with a source and a complex impedance without C across the output.

\$\endgroup\$
0
\$\begingroup\$

You're on the right track, but your formula is wrong. R1 and R2 are in parallel, so you need to use the formula for combining parallel resistors to get the Thevenin resistance.

\$\endgroup\$
  • \$\begingroup\$ Shouldn't this be a comment? \$\endgroup\$ – Enric Blanco Apr 18 '17 at 19:58
  • 1
    \$\begingroup\$ I don't think so. It's an answer to the question. We try not to be too verbose when helping people with homework. \$\endgroup\$ – Adam Haun Apr 18 '17 at 20:36
0
\$\begingroup\$

No. Your question is on the system's capacitance at it's input port, the thevenin capacitance. 'C' is just a part of that!

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.