Capacitor Discharge Calculator (RC)
Enter your RC values to calculate voltage, current, charge, and energy during capacitor discharge.
What this discharging capacitor calculator does
This tool models the classic RC discharge curve for a capacitor connected across a resistor. In a discharge event, stored electrical energy in the capacitor is converted mostly into heat through the resistor, and both voltage and current decay exponentially over time.
The calculator returns key outputs at a chosen time: capacitor voltage, resistor current magnitude, remaining charge, stored energy, and percent of the initial voltage left. If you enter a target voltage, it can also estimate how long it takes to decay to that level.
Core equations used
Current magnitude: I(t) = (V0/R)e-t/(RC)
Charge: Q(t) = C · V(t)
Energy: E(t) = ½ C V(t)2
Time constant: τ = R · C
The time constant τ (tau) is the most important RC parameter. After one time constant, voltage falls to about 36.8% of its initial value. After about 5τ, the capacitor is effectively discharged for many practical applications.
How to use the calculator
1) Enter initial voltage
Set the starting capacitor voltage before discharge begins.
2) Enter resistance and capacitance
Use real component values and choose units carefully (Ω, kΩ, MΩ and F, mF, µF, nF, pF). The calculator internally converts everything to SI units.
3) Enter elapsed time
Provide the moment where you want values evaluated (for example 2 seconds after connecting the resistor).
4) Optional target voltage
If you need the time required to drop to a specific voltage threshold, enter that value in the target field.
Example scenario
Suppose you have a 100 µF capacitor charged to 12 V, discharging through a 10 kΩ resistor. The time constant is:
- τ = RC = 10,000 × 100×10-6 = 1 second
At t = 2 s, the voltage is:
- V(2) = 12e-2 ≈ 1.62 V
So after two seconds, only around 13.5% of the initial voltage remains.
Practical engineering notes
- Electrolytic capacitors can have tolerance and leakage effects that alter real discharge behavior.
- Resistor tolerance directly affects tau (±1%, ±5%, etc.).
- Parasitic resistance (wiring, meter input resistance) changes total effective R.
- Large capacitors can store hazardous energy—always discharge safely with an appropriate resistor.
Frequently asked questions
Why doesn’t the voltage reach exactly zero?
Exponential decay approaches zero asymptotically. In theory, exact zero requires infinite time. In practice, engineers choose a threshold (for example 1% or 0.1% of initial voltage).
What happens if resistance is very small?
Current can spike high at t=0. That may exceed resistor power ratings, wiring limits, or capacitor ripple/current ratings. Always verify safe operating conditions.
Can I use this for charging too?
This specific calculator is for discharge. Charging uses a related but different equation: Vcharge(t) = Vsupply(1 - e-t/RC).