RC Time Constant

- Falling Voltage Graph -

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e_c= E_T[e^-(T/t)]

The fall time for a resistor, capacitor combination is shown in the graphic above. 1 Time constant [TC] equal R x C. Two TC's equals 2 x[RC], and so on. The Time constant is the time it would take for the potential difference across the capacitor to decrease to zero voltage. The capacitance voltage rises at an exponential rate. Keep in mind, the voltage over the capacitor rises at the same rate. The Capacitor Rising voltage is shown on it's own page. Additional Data which can be derived from the graph:
0.2 time constant equals 20% amplitude.
0.7 time constant equals 50% amplitude.
Also keep in mind that normal IC logic uses 10% and 90% as nominal rise time values. That would be 10% of final value and 90% of final voltage value. So in some cases you may need to worry about the 90 to 100% rise time.
Definition of values for the equation above:
T = Time
t = Time Constant
e_c = Voltage over the capacitor at any instant
E_T = Voltage applied to the circuit

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