How long does a capacitor take to discharge?
Through a resistor, the voltage falls by 63.2% each time constant: V(t) = V0 x e^(−t/RC). To find the time to a target voltage, use t = RC x ln(V0 / V). A 100 µF capacitor at 12 V through 10 kΩ (τ = 1 s) reaches 1 V after 2.48 s, and half its voltage after 0.69 seconds.
Large capacitors in power supplies and camera flashes can hold a dangerous charge for a long time. Discharge them through a suitably rated resistor and confirm with a meter. The capacitor charging time calculator covers the other direction.
t = RC x ln(V0 / V)
Capacitor discharge by time constant
Voltage left after each multiple of RC.
| Time | Voltage left | From 12 V |
|---|---|---|
| 1 τ | 36.8% | 4.41 V |
| 2 τ | 13.5% | 1.62 V |
| 3 τ | 5% | 0.6 V |
| 4 τ | 1.8% | 0.22 V |
| 5 τ | 0.7% | 0.08 V |