Part of Analogue and filter calculators
Capacitor Discharge Calculator
Calculate capacitor voltage decay, time to a threshold, and discharge time constant for a first-order RC discharge path.
Inputs
Choose charge or discharge mode, select what to calculate, then enter the other four values.
Engineering notation is supported: 10k, 100n, 1m, 47u.
Results
After one time constant, a charging capacitor reaches about 63.2% of its final voltage; a discharging capacitor falls to about 36.8% of its initial voltage.
Use capacitor discharge timing for bleed resistors, reset delays, hold-up decay, and safe-voltage checks
A capacitor does not instantly become harmless when power is removed. This calculator uses the first-order RC discharge model to estimate remaining voltage, time to a threshold, and the discharge time constant.
Bleed resistor sizing
Estimate how long a capacitor remains above a target voltage after a discharge path is applied.
Power-off decay
Check hold-up, reset, enable, and sequencing behaviour when stored charge decays through a load.
Safety screening
Compare remaining voltage with a chosen threshold, then check stored energy and resistor stress separately.
Equations and model
The page uses the ideal first-order RC discharge equation. The input form also supports charge mode because the same shared RC engine is used across the Phase 1 calculator family.
Discharge voltage
The ideal capacitor voltage after discharging through an effective resistance for time t.
Time to threshold
Use this to estimate how long a capacitor remains above a safe or logic threshold voltage.
Time constant
The characteristic time scale for the exponential discharge.
Vinitial - Initial capacitor voltage
Unit: volts (V)
The capacitor voltage at the start of the discharge interval.
Vc(t) - Voltage after time t
Unit: volts (V)
The remaining capacitor voltage after the selected elapsed time.
R - Discharge resistance
Unit: ohms (Ω)
The effective resistance in the discharge path, including bleed resistors, switch resistance, and any parallel paths.
C - Capacitance
Unit: farads (F)
The capacitance being discharged. Use effective capacitance when voltage bias or tolerance matters.
Worked example
The numbers below use the same exponential discharge relationship as the shared RC time-constant calculator.
Design question: A 100 µF capacitor charged to 12 V discharges through 10 kΩ. What voltage remains after 5 s?
Inputs: R = 10 kΩ, C = 100 µF, t = 5 s, initial voltage = 12 V.
Time constant: τ = R × C = 10 kΩ × 100 µF = 1 s.
Voltage after 5τ: Vc = 12 V × e⁻⁵ ≈ 80.9 mV.
Practical meaning: the capacitor voltage is low, but energy, safety threshold, and resistor pulse stress still need separate review.
Assumptions and limitations
Ideal RC path
The model assumes one effective resistance and capacitance during the discharge interval. Switching, leakage, and nonlinear loads can shift the result.
Energy is separate
A low voltage target does not by itself confirm stored-energy safety, discharge current, or resistor pulse capability.
Tolerance can dominate
Capacitance tolerance, voltage bias, bleed-resistor tolerance, temperature, and leakage can materially change real discharge time.
Related calculators and next checks
Follow the next check based on whether the capacitor is being used for timing, energy storage, filtering, or discharge safety.
RC time constant calculator
Use the general charge/discharge solver when you need both charging and discharging modes.
Capacitor energy calculator
Check stored energy and charge when discharge safety, hold-up, or spark risk matters.
Resistor power calculator
Check the steady-state or pulse dissipation in a bleed or discharge resistor.
RC low-pass filter calculator
Use for the frequency-domain version of the same resistor-capacitor pair.
Analogue and filter hub
Follow related RC, timing, coupling, filter, and analogue workflows.
Engineering reference
Equations, assumptions, and design guidance
Solves the ideal first-order RC charge or discharge equation for resistance, capacitance, time, capacitor voltage, or reference voltage.
Equations and variables
tau = R * CVc(t) = Vfinal * (1 - exp(-t / tau))Vc(t) = Vinitial * exp(-t / tau)- R
- Resistance (ohm)
- C
- Capacitance (F)
- t
- Elapsed time (s)
- Vc
- Capacitor voltage (V)
Assumptions and limitations
Assumptions
- The source and switch are ideal.
- The capacitor is linear over the voltage range.
- Initial and final voltages match the selected charge/discharge model.
Limitations
- ESR, leakage, dielectric absorption, source resistance, and comparator thresholds are not modelled.
Worked example and design use
10 kOhm and 100 nF charge
Inputs: R = 10 kOhm, C = 100 nF, t = 1 ms, Vfinal = 5 V
Outputs: tau = 1 ms, Vc is about 3.16 V after one tau
Design guidance
- Use worst-case R and C tolerances for timing limits.
- Account for input thresholds and leakage in reset and delay circuits.