# RC Time Constant

- Falling Voltage Graph - RC Fall Time Constant

ec= ET[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
ec = Voltage over the capacitor at any instant
ET = Voltage applied to the circuit

The graph is meant to show the voltage decay in a capacitor over time.
However what it does not show is the effect a complex circuit might have on the decay.
That is a capacitor discharging through an inductive resistor.
The graph assumes a normalized capacitor discharging through a perfect resistor.
The graph also does not account for any leakage in the capacitor.
Leakage would also account for a small drop in voltage and error in the graph, assuming the time constant was long.

 Logic Slew Rate Trace Termination Ground, Power Planes Passive Design IC Package Type Definitions

Modified 3/05/12