Capacitor Safe-Discharge Time
t = −R · C · ln(Vsafe / V0)
Calculator
Result
—
Formula
t = −R × C × ln(Vsafe / V0)
Description
A bleeder resistor discharges a capacitor exponentially, and this gives the time for the voltage to fall from its initial value to a safe touch level. It is essential for safety design in power supplies and high-voltage equipment: the resistor must drain the stored charge to under ~50 V within a required time (often 1–5 seconds) after power is removed.
Variables
- t — Time to reach the safe voltage (s)
- R — Bleeder/discharge resistance (Ω)
- C — Capacitance (F)
- Vsafe — Target safe voltage (V)
- V0 — Initial charged voltage (V)
Practical Notes
Safety standards (e.g. IEC) often require discharge to ≤ 50 V within 1 s (or 5 s for larger equipment). Smaller R discharges faster but wastes more standby power; size it to meet the time limit with margin.
Related Concepts
All Safety / Protection formulas → Fuse I²t Rating Touch Voltage / Body Current Hi-Pot Test Voltage MOV Surge Energy GFCI Leakage Current Circuit Breaker Fault Power Capacitor Safe-Discharge Time
Need more features?
Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.
Try the App