Charging and Discharging an RC Circuit

The time constant explained.

When a capacitor charges through a resistor, the voltage rises quickly at first and then more slowly, following an exponential curve. The product of resistance and capacitance is the time constant, and it sets the pace of the whole process.

After one time constant the capacitor has reached about 63 percent of its final voltage, and after five it is within one percent. Discharging is the mirror image: after one time constant, about 37 percent of the voltage remains.

• Larger R or C means slower charging.
• Five time constants is treated as fully charged.
• Timers and filters rely on this behaviour.

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