Capacitor Discharge Calculator
t = RC ln(V0/Vs). 1000 µF, 400 V → 60 V through 10 kΩ is 18.97 s.
Calculator
Formula
Discharge time
t = RC ln(V0 / Vs) Resistor from time
R = t / (C ln(V0 / Vs)) Variables
| Symbol | Meaning | Unit | Typical range | Precision | Default |
|---|---|---|---|---|---|
| mode | Solve | — | — | 1 sig | time-from-r |
| C | Capacitance | F | ≥ 0 pF | 4 sig | 1 mF |
| V0 | Initial voltage | V | ≥ 0 V | 4 sig | 400 V |
| Vs | Safe voltage (default 60 V, IEC 62368-1 ES1 DC reference) | V | ≥ 0 V IEC 62368-1 ES1 DC reference — not a hard legal limit | 4 sig | 60 V |
| R | Discharge resistor | Ω | ≥ 0 mΩ | 4 sig | 10 kΩ |
| t | Discharge time | s | — | 4 sig | — |
| I0 | Initial current | A | — | 4 sig | — |
| P0 | Initial pulse power | W | — | 4 sig | — |
| E | Stored energy | J | — | 4 sig | — |
Worked example
1000 µF, 400 V → 60 V ES1 reference, 10 kΩ
| Input | Value |
|---|---|
| mode · Solve | time-from-r |
| C · Capacitance | 1 mF |
| V0 · Initial voltage | 400 V |
| Vs · Safe voltage (default 60 V, IEC 62368-1 ES1 DC reference) | 60 V |
| R · Discharge resistor | 10 kΩ |
| Output | Value |
|---|---|
| t · Discharge time | 18.97 s |
| P0 · Initial pulse power | 16 W |
| I0 · Initial current | 40 mA |
| E · Stored energy | 80 J |
1000 µF, 400 V → 50 V, 10 kΩ
| Input | Value |
|---|---|
| mode · Solve | time-from-r |
| C · Capacitance | 1 mF |
| V0 · Initial voltage | 400 V |
| Vs · Safe voltage (default 60 V, IEC 62368-1 ES1 DC reference) | 50 V |
| R · Discharge resistor | 10 kΩ |
| Output | Value |
|---|---|
| t · Discharge time | 20.79 s |
| P0 · Initial pulse power | 16 W |
| I0 · Initial current | 40 mA |
| E · Stored energy | 80 J |
470 µF, 350 V → 36 V in 10 s
| Input | Value |
|---|---|
| mode · Solve | r-from-time |
| C · Capacitance | 470 µF |
| V0 · Initial voltage | 350 V |
| Vs · Safe voltage (default 60 V, IEC 62368-1 ES1 DC reference) | 36 V |
| Output | Value |
|---|---|
| R · Discharge resistor | 9.355 kΩ |
| P0 · Initial pulse power | 13.09 W |
| E · Stored energy | 28.79 J |
How it works
A charged electrolytic can still bite after the supply is off. The exponential is simple; the part that is easy to get wrong is calling a convenient voltage a legal limit.
Enter C, V0, Vsafe and either R or a target time. The initial current and pulse power are shown so the resistor can be sized for the dump, not only for DC.
Pitfalls
- Treating 60 V as a legal limit. It is labelled as an IEC 62368-1 ES1 DC reference only.
- Sizing the resistor for DC wattage when the first millisecond is a pulse of V0² / R.
- Vsafe ≤ 0 would take infinite time — the exponential never reaches zero.
FAQ
Why is the default 60 V?
It is the IEC 62368-1 ES1 DC figure used here as a reference, not a hard legal limit. Change Vsafe for your procedure.
How is the time calculated?
t = RC ln(V0 / Vs) from V(t) = V0 e^(−t/RC).
What if V0 is already below Vsafe?
Time is 0. No discharge is required for that reference.
Does this include a MOSFET dump?
No. It is a single resistor. Peak current and energy still apply to whatever switch you use.
Related
Sources
