# 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") Commented Mar 11, 2021 at 18:26
• @EugeneSh. thanks for the comment. What do you mean with parallel being the same as in series? Commented Mar 11, 2021 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? Commented Mar 11, 2021 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 Commented Mar 11, 2021 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) Commented Mar 11, 2021 at 18:48