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Time constant
Intermediate
Time for the capacitor to reach about 63% of final voltage.
τ = RCτ = RC (seconds) is the time for the voltage to change by 63% of its final change. Larger R or C means slower change.
1 MΩ × 1 μF = 1 s.
After one τ it's not finished; wait 5τ.
In more detail
The time constant τ = RC is the time for an RC circuit to reach about 63% of its final value during charging, or fall to about 37% during discharging.
Real-world uses
- Designers choose RC values to set filter cut-off frequencies.
- Watchdog timers and blinking LEDs use RC time constants.
- Engineers use the time constant RC to predict how quickly a sensor circuit responds to a change.
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Frequently asked questions
How long does a capacitor take to charge?
About five time constants to be essentially fully charged.
What units does RC have?
Seconds, since ohms times farads equal seconds.
What is the significance of 63 percent?
After one time constant the capacitor has charged to about 63 percent of the final voltage, a convenient reference point.
Related topics in Current Electricity & Circuits
Delta–star conversion · RC circuit charging · RC circuit discharging · Current in RC circuit · Capacitor networks · Diodes