Capacitor Charging Calculator
Generated infographic and interface snapshot for Capacitor Charging Calculator
From τ to time-to-threshold — RC math without the ln() key hunt.
Free Capacitor Charging Calculator Online — Big Das
Every decoupling network, debounce filter, timing circuit, and soft-start ramp is a resistor charging a capacitor. The Big Das Capacitor Charging Calculator solves both directions of the exponential — voltage at a given time, and the time to reach a given voltage — for charging and discharging alike, plus the time constant, peak current, and stored energy ½CV². A live curve shows exactly where your observation point sits over five time constants.
Enter R and C with unit selectors (Ω/kΩ/MΩ, pF/nF/µF) and everything recomputes as you type.
What Is RC Charging?
When a voltage step is applied to a resistor in series with a discharged capacitor, the capacitor voltage does not jump — it follows an exponential approach toward the source. The speed of that approach is set by a single number, the time constant τ = R × C:
After 1τ the capacitor reaches 63.2% of the source (or falls to 36.8% when discharging).
After 5τ it is within 0.7% — for practical purposes, fully charged.
The current starts at its peak, Vs/R, and decays with the same τ.
Discharge is the mirror image: the initial voltage decays exponentially through the same τ.
How to Use the Capacitor Charging Calculator
- Choose Charge or Discharge mode.
- *Enter the source voltage
- Vs — or the initial capacitor voltage V₀ for discharge.
- Enter R and C with their unit selectors.
- Query ① — enter an observation time (µs/ms/s) to get V(t).
- Query ② — enter a target voltage to get the time to reach it.
- Read the panel — τ, peak current, energy at V(t), and the full-charge energy, all live, with a 0–5τ SVG curve marking your time point.
The Formulas Used
Time constant: τ = R × C
Charging: V(t) = Vs × (1 − e^(−t/τ))
Discharging: V(t) = V₀ × e^(−t/τ)
Time to voltage: t = −τ × ln(1 − V/Vs) (charge)
t = τ × ln(V₀/V) (discharge)
Peak current: I₀ = Vs / R (decays as e^(−t/τ))
Stored energy: E = ½ × C × V²
Fully charged: t ≈ 5τ (99.3%)
All values are converted to ohms, farads, volts, and seconds before evaluation.
Worked Example
A 5 V logic supply charging a 100 µF capacitor through 10 kΩ:
τ = 10,000 × 0.0001 = 1 second
V(1 s) = 5 × (1 − e^(−1)) = 3.16 V — exactly the 63.2% point
Time to 3.16 V: t = −1 × ln(1 − 3.16/5) = 1.00 s, confirming τ
Peak current = 5 / 10,000 = 0.5 mA — safe for any GPIO
Max stored energy = ½ × 100 µF × 5² = 1.25 mJ
Full charge ≈ 5τ = *5 seconds
The curve highlights 1τ at the 63.2% line so these landmark values are visible at a glance.
Common Use Cases
- Timing circuits — how long until a comparator or Schmitt trigger trips.
- Debounce filters — pick R and C so switch bounce dies inside one τ.
- Power-on-reset delays — guarantee the supervisor holds reset long enough.
- Capacitor dropper / hold-up — discharge mode answers "how long does my rail survive a brown-out?"
- Camera flashes and pulsers — size C for the energy a pulse needs.
Frequently Asked Questions
Why does the capacitor never quite reach the source voltage?
The exponential only approaches asymptotically — mathematically it never arrives. In practice the difference drops below measurement noise after 5τ (99.3%), which is why "fully charged = 5τ" is the engineering convention. The calculator flags asking for a target voltage ≥ Vs for this reason.
The initial current is Vs/R — is that dangerous?
It can be. A large capacitor charged directly from a low-impedance supply can demand huge peak current, welding relay contacts or tripping protections. Inrush-limiting resistors, NTC thermistors, or soft-start circuits exist precisely to tame I₀.
Can I use this for a
Thevenin source driving a capacitor?
Yes — use the open-circuit voltage as Vs and the Thevenin resistance as R. A voltage divider feeding a capacitor is exactly this case.
Why is resistor tolerance relevant?
A 10% resistor and a ±20% electrolytic capacitor move τ by up to 30% combined. For precision timing use film capacitors and 1% resistors, or measure the built circuit.
What happens in discharge mode if my target voltage is above V₀?
Nothing physical — a discharging capacitor starts at V₀ and only falls, so the target must be below it. The tool shows an inline warning instead of a nonsense negative time.