Capacitor Charge / Discharge Time
Calculate RC time constant, charge time to 63% and 99%, half-life, and voltage at 1t and 2t for capacitor charging and discharging circuits.
Results
What is it?
An RC circuit charges exponentially: V(t) = Vsupply � (1 - e^(-t/t)), where t = R � C is the time constant. After 1t, the capacitor is at 63.2% of full charge. After 5t it is at 99.3% (considered fully charged for most purposes). The same exponential applies during discharge toward 0V.
How to use
Enter the resistance in kO and capacitance in �F. The time constant t, time to 63% and 99% charge, and voltages at 1t and 2t are calculated. To find t for a target charge time T at a percentage P: t = -T / ln(1-P).
Example scenario
10kO resistor, 100�F capacitor, 9V supply: t = 10,000 � 0.0001 = 1000 ms = 1 second. After 1s: 63.2% ? 5.69V. After 5s: 99.3% ? 8.94V (effectively full). This RC time is commonly used in simple power-on delay circuits.
Pro tip
RC circuits form simple low-pass filters with cutoff frequency fc = 1/(2pRC). At fc, signal amplitude is -3 dB (70.7% of input). Use this for debouncing buttons: add 10kO + 100nF to a GPIO input to filter glitches. For precision timing, use a 555 timer or crystal oscillator � RC tolerances (�10�20%) make RC timing imprecise.