Capacitor Charge & Discharge Calculator

How the capacitor charge & discharge solver works

Enter the starting voltage, the target voltage, and the supply voltage, then fill in two of resistance, capacitance, and time — leave the third blank and the calculator solves for it. It works from the standard RC exponential relationships below.

Charge or discharge is chosen automatically

If the target voltage is above the starting voltage, the circuit is charging toward the supply voltage. If the target is below the start, it is discharging toward zero. The target must be physically reachable: you cannot charge past the supply voltage, and a passive RC discharge approaches — but never exactly reaches — 0 V.

Equations

τ = R × C

Charging: V(t) = Vₛ − (Vₛ − V₀) × e^−t/τ

Discharging: V(t) = V₀ × e^−t/τ

Solving any of the three unknowns comes from rearranging t = R·C·ln(1/ratio), where the ratio is the fraction of the transition still remaining at the target voltage.

Peak current, power, and energy

Current is largest the instant the switch closes: I₀ = ΔV / R, where ΔV is the full voltage across the resistor at t = 0. The resistor's peak dissipation is I₀²·R, and the energy stored in the capacitor at the target voltage is ½·C·V².

Worked example

Charging from 2 V toward a 10 V supply, targeting 5 V, with R = 1 Ω and C = 100 nF: the ratio remaining is (10 − 5)/(10 − 2) = 0.625, so t = 1 Ω × 100 nF × ln(1/0.625) ≈ 47 ns. Peak current is (10 − 2)/1 Ω = 8 A and peak resistor power is 8² × 1 = 64 W.

Frequently asked questions

Why can't I reach the supply voltage exactly?

The charging curve approaches the supply voltage asymptotically, getting ever closer but never equalling it in finite time — so a target equal to or above the supply has no finite solution.

Which value should I leave blank?

Leave blank whichever of resistance, capacitance, or time you want to find. Fill in the other two and all the derived results update instantly.