Capacitor Constant Current Calculator
Calculate capacitor voltage change, current, capacitance, or time for constant-current charge and discharge.
About constant-current capacitor charging
I = C × (Vfinal − Vstart) / t
Vfinal = Vstart + I × t / C
Driven by a constant current source, a capacitor's voltage changeslinearly with time (unlike the exponential curve of RC charging). The sign of the current sets the direction: positive charges the capacitor up, negative discharges it. This is the basis of current-source ramp generators and coulomb-counting.
How it works
Current is the rate at which charge flows, and a capacitor stores charge as Q = C·V. Feed it a fixed current and charge piles on at a steady rate, so the voltage rises in a straight line — the defining relationship I = C·(dV/dt). That's the key difference from charging through a resistor: with a resistor the current tapers off as the capacitor fills, giving the familiar exponential curve, but a true current source keeps pushing the same current regardless of voltage, so the ramp stays perfectly straight until the source runs out of headroom.
Worked example
Charge a 10 µF capacitor with 1 mA for 20 ms. The voltage change is ΔV = I·t / C = (0.001 × 0.020) / 10×10⁻⁶ = 2 V. Starting from 0 V it reaches 2 V; the charge moved is Q = C·ΔV = 20 µC. Halve the capacitor to 5 µF and the same current makes the voltage climb twice as fast, reaching 4 V in the same 20 ms.
Where it's used
Linear ramps drive sawtooth and triangle-wave generators, dual-slope ADCs, and timing circuits that need a straight edge rather than an RC curve. Coulomb-counting fuel gauges use the same Q = I·t idea in reverse to track how much charge has left a battery. For the exponential resistor-charged case instead, see thecapacitor charge & discharge calculator.
The current source has limits
A perfectly constant current gives a perfectly straight voltage ramp, and that is what this calculates. Real current sources have a compliance voltage — a ceiling beyond which they can no longer hold the current — so the ramp is linear only until the capacitor voltage approaches it, then it flattens. Check that your source's compliance exceeds the final voltage you are aiming for, and remember that capacitor tolerance and leakage apply here exactly as they do toRC charging.