The switch S has been closed for a long time, and the it is opened. I want to calculate the time constant for the RC circuit below. I don't know how to tackle this problem, is the time constant for an RC circuit always RC, no matter how the circuit looks? If yes, is R and C the equivalent resistans and capacitance one gets when reducing the circuit to one containing a resistor and capacitor in series?

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    \$\begingroup\$ When the switch is open, the circuit is just the capacitor in parallel to (R1 + R2) || (R3 + R4). (Oh well, in this case "in parallel" is the same as "in series") \$\endgroup\$
    – Eugene Sh.
    Commented Mar 11, 2021 at 18:26
  • \$\begingroup\$ @EugeneSh. thanks for the comment. What do you mean with parallel being the same as in series? \$\endgroup\$
    – Pi314
    Commented Mar 11, 2021 at 18:30
  • \$\begingroup\$ If you take just one resistor and one capacitor and connect them into a closed circuit - how would they be connected, in parallel or in series? \$\endgroup\$
    – Eugene Sh.
    Commented Mar 11, 2021 at 18:33
  • \$\begingroup\$ I think @EugeneSh. meant R1//R2 = Ra on the left and R3//R4 = Rb on the right then T= (Ra+Rb)C \$\endgroup\$ Commented Mar 11, 2021 at 18:46
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 If the switch is open R1 is in series with R2 (on the top) and R3 is i series with R4 (on the bottom) \$\endgroup\$
    – Eugene Sh.
    Commented Mar 11, 2021 at 18:48

1 Answer 1


Apply Thevenin you will get a voltage source applied to a capacitor through a resistor and then the time constant is trivial. You just need to know where to apply Thevenin.

  • \$\begingroup\$ Thevenin theorem is not a part of my course unfortunately, is there any other way to do it? \$\endgroup\$
    – Pi314
    Commented Mar 11, 2021 at 19:18
  • \$\begingroup\$ @Pi314 Not that Thevenin is really needed here, but I am surprised it is not part of your course. It should be a part of the very first electrical circuits course as a basic circuit analysis tool. \$\endgroup\$
    – Eugene Sh.
    Commented Mar 11, 2021 at 19:22

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