# Calculate the time constant for a RC circuit

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? • 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") – Eugene Sh. Mar 11 at 18:26
• @EugeneSh. thanks for the comment. What do you mean with parallel being the same as in series? – Pi314 Mar 11 at 18:30
• 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? – Eugene Sh. Mar 11 at 18:33
• I think @EugeneSh. meant R1//R2 = Ra on the left and R3//R4 = Rb on the right then T= (Ra+Rb)C – Tony Stewart EE75 Mar 11 at 18:46
• @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) – Eugene Sh. Mar 11 at 18:48

## 1 Answer

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.

• Thevenin theorem is not a part of my course unfortunately, is there any other way to do it? – Pi314 Mar 11 at 19:18
• @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. – Eugene Sh. Mar 11 at 19:22