Time a capacitor's charge
An RC pair charges toward the rail on an exponential curve — τ tells you how fast.
Tools used in this guide
The job
Power-on delays, debounce filters, and timing circuits all lean on the RC curve: voltage rises 63.2 % in one τ, 95 % in three, and is called 'charged' at 5τ (99.3 %). Knowing τ answers 'how long until the input reads high'.
Resistance and capacitance
Enter R and C — τ = R×C — then either ask what % it has reached at a time, or the time to reach a chosen %. Discharge mirrors the same curve downwards.
Worked example
1 kΩ × 100 µF gives τ = 0.1 s and 5τ = 0.5 s: at 0.1 s the cap sits at 63.2 %, at 0.3 s about 95 %. A 10 ms RC delay needs R×C = 0.01 — say 10 kΩ and 1 µF.
Where it stops
Ideal components only — electrolytic leakage, ESR, and the load the cap drives all shift the real curve. For precision timing use a comparator and measure, don't calculate blind.