Constant-Current Capacitor Charge Time

Q: How do you calculate Constant-Current Capacitor Charge Time?

Use the formula t = C × ΔV / I. When a capacitor is charged with a constant current source (rather than through a resistor), the voltage rises linearly with time. Enter the values you know in the calculator above and it solves for the remaining variable.

Q: What is the formula for Constant-Current Capacitor Charge Time?

The Constant-Current Capacitor Charge Time formula is t = C × ΔV / I, where t—Charge time (s); C—Capacitance (F); ΔV—Voltage change across the capacitor (V); I—Constant charging current (A).

t = C × ΔV / I

Quick answer

This calculator solves t = C × ΔV / I for Charge Time. With Capacitance 100 µF, Voltage Change 12 V, Charging Current 20 mA, the result is 60 ms. It solves in 4 directions, so you can enter the values you have and get the one you need.

Calculator

Capacitance

Voltage Change

Charging Current

Charge Time (ms)

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Formula

t = C × ΔV / I

Description

When a capacitor is charged with a constant current source (rather than through a resistor), the voltage rises linearly with time. This is the fundamental relationship Q = C×V differentiated with respect to time: I = C × dV/dt, rearranged to find the time to charge from one voltage to another. Constant-current charging is used in current-source LED drivers, ramp generators for ADCs, timing circuits, and battery charging (CC phase of CC/CV charging). The linear voltage ramp makes this technique ideal for precision timing applications.

Variables

t — Charge time (s)
C — Capacitance (F)
ΔV — Voltage change across the capacitor (V)
I — Constant charging current (A)

Practical Notes

This assumes a perfectly constant current source. In practice, current sources have finite output impedance and compliance voltage limits. The charge time is independent of the starting voltage — charging from 0 V to 1 V takes the same time as charging from 4 V to 5 V with the same current and capacitance.

Symbols and units

Symbol	Meaning	Default unit	Accepted units
t	Charge Time	ms	ps, ns, µs, ms, s, min, h
C	Capacitance	µF	pF, nF, µF, mF, F
dV	Voltage Change	V	pV, nV, µV, mV, V, kV
I	Charging Current	mA	pA, nA, µA, mA, A, kA

Step by step

1. What you know: Capacitance 100 µF, Voltage Change 12 V, Charging Current 20 mA
2. The relation: t = C × ΔV / I
3. Substitute: C = 100 µF, dV = 12 V, I = 20 mA
4. Result: Charge Time = 60 ms

Related quantities from the same inputs

P = V × I = 0.24 W

Worked examples

Minimum range

Given: Capacitance 100 nF, Voltage Change 0.33 V, Charging Current 100 µA

Result: Charge Time = 0.33 ms

Low range

Given: Capacitance 1 µF, Voltage Change 3.3 V, Charging Current 1 mA

Result: Charge Time = 3.3 ms

Typical

Given: Capacitance 100 µF, Voltage Change 12 V, Charging Current 20 mA

Result: Charge Time = 60 ms

High range

Given: Capacitance 1 mF, Voltage Change 0.23 kV, Charging Current 0.5 A

Result: Charge Time = 0.46 s

Maximum range

Given: Capacitance 10 mF, Voltage Change 2.3 kV, Charging Current 5 A

Result: Charge Time = 4.6 s

Solving for each variable

Solve for Charge Time

Capacitance 100 nF, Voltage Change 0.33 V, Charging Current 100 µA → 0.33 ms

Solve for Capacitance

Charge Time 100 µs, Voltage Change 0.33 V, Charging Current 100 µA → 30.3 nF

Solve for Voltage Change

Charge Time 100 µs, Capacitance 100 nF, Charging Current 100 µA → 100 mV

Solve for Charging Current

Charge Time 100 µs, Capacitance 100 nF, Voltage Change 0.33 V → 0.33 mA

How Charge Time changes with Capacitance

Capacitance (input)	Charge Time (result)
10 µF	6 ms
25 µF	15 ms
50 µF	30 ms
75 µF	45 ms
100 µF	60 ms
150 µF	90 ms
0.2 mF	120 ms
0.4 mF	0.24 s
0.7 mF	0.42 s
1 mF	0.6 s

The same result in other units

Unit	Value
µs	60000
ms	60
s	0.06
min	0.001

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Frequently asked questions

How do you calculate Constant-Current Capacitor Charge Time?

Use t = C × ΔV / I. With Capacitance 100 µF, Voltage Change 12 V, Charging Current 20 mA, Charge Time works out to 60 ms.

What is the formula for Constant-Current Capacitor Charge Time?

The formula is t = C × ΔV / I. When a capacitor is charged with a constant current source (rather than through a resistor), the voltage rises linearly with time.

How do you find Capacitance when you know the other values?

Rearrange for Capacitance: with Charge Time 100 µs, Voltage Change 0.33 V, Charging Current 100 µA, Capacitance is 30.3 nF.

How do you find Voltage Change when you know the other values?

Rearrange for Voltage Change: with Charge Time 100 µs, Capacitance 100 nF, Charging Current 100 µA, Voltage Change is 100 mV.

What units does Constant-Current Capacitor Charge Time use?

Charge Time is shown in µs, ms, s, min. Values are held in SI base units internally, so you can mix prefixes without converting by hand.

Related Concepts

All Capacitors formulas →

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