Safety
Capacitor discharge: time, energy, bleeder resistor
How long does it take until a charged capacitor is safe? The tool calculates energy, time constant, peak current and peak power in the resistor, and the largest resistor that meets a target time.
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Abbreviations and symbols on this page (6)
- τ
- Electrical time constant L ÷ R: after this time the current has reached 63 % of its final value.
- SELV, 60 V
- Safety extra-low voltage. 60 V DC is the limit named in power supply standards below which a charged capacitor is considered safe to touch (note in the TI datasheet UCC28630).
- E = ½ C U²
- Energy stored in the capacitor in joules. Double the voltage = four times the energy.
- Bleeder resistor
- Resistor permanently connected in parallel with the capacitor that discharges it automatically after switch-off.
- C
- Capacitance of a capacitor, unit farad (F). Indicates how much charge it stores per volt.
- pF, nF, µF
- Pico-, nano-, microfarad: 1 nF = 1000 pF, 1 µF = 1000 nF.
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µF
V
kΩ
60 V (safety extra-low voltage)
s
| Stored energy | 8 J |
| Time constant τ | 1 s |
| Time to below 60 V | 1.897 s |
| After 5 τ (≈ 0.7 % residual voltage) | 5 s |
| Peak current | 40 mA |
| Peak power in the resistor | 16 W |
| Resistor for 5 s to 60 V | ≤ 26.36 kΩ |
Above 60 V DC a charged capacitor is dangerous: do not touch, discharge first and measure again.
Discharge curve U(t) with target voltage; play shows the progression.
Formula
E = ½ C U0². τ = R C. U(t) = U0 · e^(−t ÷ τ). Time to U_target: t = τ · ln(U0 ÷ U_target). Peak current U0 ÷ R, peak power U0² ÷ R. Largest resistor for time t: R = t ÷ (C · ln(U0 ÷ U_target)).
Example
100 µF at 400 V stores 8 J. Through 10 kΩ (τ = 1 s) the voltage is below 60 V after 1.9 s, and at 2.7 V after 5 τ = 5 s. Peak current 40 mA, peak power 16 W, which the resistor must withstand briefly.
Notes
- Limits differ by standard: 60 V as the safety extra-low voltage limit (power supply standards, TI note), 50 V within 1 min (NFPA 70, Art. 460, USA), 75 V within 3 min (IEC 60831, power capacitors). Check the applicable standard for your own installation.
- After discharging, check with a meter and short the terminals; some capacitors recharge slightly afterwards (dielectric absorption).
- The resistor needs a voltage rating for U0 and must withstand the energy; several resistors in series share the voltage.
- High-voltage and high-energy capacitors can be lethal, even small-looking ones.
A calculation aid for planning. The result does not replace a measurement on your own build.