finding Thevenin equivalent resistance across capacitor terminals for RC circuits

Discussion in 'General Electronics Chat' started by PG1995, Oct 27, 2011.

  1. PG1995

    Thread Starter Well-Known Member

    Apr 15, 2011

    While solving problems related to RC circuit, I'm having problem finding Thevenin equivalent resistance across the terminals of a capacitor. Please have a look on the attachment. It has my question there. Please help me with it. Thanks a lot.
    Last edited: Oct 27, 2011
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    hey, only took me a page and a half for 4 different Thevenin equivalents (TE) to work this out, and thus I don't trust my work until I do it again on a clean sheet and get the same answers. But here it is and you can get the general scheme I used.

    First I split the circuit up at A&B into 3 pieces: Vs R1 R2, C, and R3 R4 R5. Leave C alone and solve the left & right side TE's 1&2.

    Now jam the left & right side TE's 1&2 together and find THAT resultant TE3.

    You now have a nice single source, single R and single C. Solve for how C charges Vcap.

    Now go back to the right side TE and break it up to look back from R4. Note here the C cap is the source and we treat it like any other source: we short it for the analysis.

    That TE gives you how the Io changes given the cap voltage Vcap. But we already found Vcap, so it is a simple substitution to find the current.
    PG1995 likes this.