What is the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel?

The time constant (τ) of an RC circuit is calculated by multiplying the total equivalent resistance (R) by the total equivalent capacitance (C). First, you must determine the equivalent values for the components in parallel. When capacitors are wired in parallel, their values add together. Two 220-microfarad capacitors in parallel yield a total capacitance of 440 microfarads (0.000440 Farads). When resistors are in parallel, their equivalent resistance decreases. Two 1-megohm resistors in parallel yield a total resistance of 0.5 megohms (500,000 Ohms). Multiplying the total resistance by the total capacitance gives: 500,000 Ohms × 0.000440 Farads = 220 seconds. If you incorrectly apply the parallel rules, you might get 55 or 110 seconds.