RC Circuit Time Constant Calculator
An RC circuit's time constant τ = RC determines how quickly a capacitor charges or discharges through a resistor. After one time constant, the capacitor reaches about 63.2% of the supply voltage. This calculator shows the time constant and capacitor voltage at key intervals.
The RC time constant (τ = RC) is the fundamental parameter governing the transient response of a resistor-capacitor circuit. During charging, the capacitor voltage follows the exponential curve V(t) = V0(1 - e-t/τ), asymptotically approaching the supply voltage. During discharging, it follows V(t) = V0e-t/τ.
Key milestones: after 1τ the capacitor reaches 63.2% of its final value; after 3τ it reaches 95.0%; and after 5τ it reaches 99.3%, which is considered "fully charged" in most practical applications. The rule of thumb is that an RC circuit settles to its final state within 5 time constants.
RC circuits are ubiquitous in electronics. They serve as low-pass and high-pass filters, smoothing power supply ripple, debouncing switches, creating time delays, and coupling AC signals between amplifier stages. The cutoff frequency of an RC filter is fc = 1/(2πRC), directly related to the time constant.