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 Active 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.