RC Time Constant Calculator Formula
Understand the math behind the rc time constant calculator. Each variable explained with a worked example.
Formulas Used
Time Constant (τ)
tau = resistance * capacitance * 1e-6Time Constant (ms)
tau_ms = resistance * capacitance * 1e-6 * 1000Full Charge Time (5τ)
time_5tau = 5 * resistance * capacitance * 1e-6Variables
| Variable | Description | Default |
|---|---|---|
resistance | Resistance(Ω) | 10000 |
capacitance | Capacitance(μF) | 100 |
How It Works
RC Time Constant
The time constant characterizes how quickly a capacitor charges or discharges through a resistor.
Formula
τ = R * C
After one time constant, the capacitor reaches about 63.2% of its final value. After 5τ, it is over 99% charged.
Worked Example
A 10 kΩ resistor with a 100 μF capacitor.
- 01τ = R * C
- 02τ = 10000 * 100e-6
- 03τ = 1.0 s
- 045τ = 5.0 s (99.3% charged)
Frequently Asked Questions
What does one time constant mean physically?
After one time constant, the voltage across the capacitor has reached 63.2% of the supply voltage during charging, or dropped to 36.8% during discharging.
Why is 5 time constants considered fully charged?
After 5τ, the capacitor has reached 99.3% of its final value. The remaining 0.7% is negligible for most applications.
Where are RC circuits used?
RC circuits are used in filters, timing circuits, smoothing power supplies, debouncing switches, and many signal processing applications.
Learn More
Guide
How to Calculate Capacitance
Understand how to calculate capacitance, read capacitor markings, combine capacitors in series and parallel, and use capacitance in timing and filter circuits.
Ready to run the numbers?
Open RC Time Constant Calculator