Capacitor Discharge Time Calculator
How long a capacitor takes to discharge through a resistor to a safe voltage, the voltage left after any time, and the energy it held.
Results
Time to reach the safe voltage
97.73
s
Time constant
47.000
s
Voltage after that time
211.28
V
Energy stored at the start
37.600
J
The dashed line marks the value you entered.
What this tool does
A capacitor discharging through a resistor loses about 63 % of its voltage every time constant RC, so it never quite reaches zero but gets close quickly: after five time constants less than 1 % is left. Power supplies and inverters fit bleeder resistors for exactly this, but they can fail open — always measure before touching, and use an insulated discharge tool on large capacitors. The safe voltage is your choice; touch voltages of around 50 V AC or 120 V DC are commonly quoted limits.
Formula
V(t) = V₀ e^(−t ÷ RC) · t_safe = RC × ln(V₀ ÷ V_safe)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
cu | Capacitance | µF |
v0 | Starting voltage | V |
rk | Discharge (bleeder) resistance | kΩ |
vs | Voltage considered safe | V |
t | Time after disconnecting | s |
TS | Time to reach the safe voltage | s |
TC | Time constant | s |
VT | Voltage after that time | V |
E0 | Energy stored at the start | J |
Worked example
- Capacitance470 µF
- Starting voltage400 V
- Discharge (bleeder) resistance100 kΩ
- Voltage considered safe50 V
- Time after disconnecting30 s
- Time to reach the safe voltage97.73 s
- Time constant47.000 s
- Voltage after that time211.28 V
- Energy stored at the start37.600 J
Limitations
- Electrical installations are governed by national wiring rules. Cable sizing also depends on installation method, grouping, ambient temperature and protection devices, which this calculator does not evaluate.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.