RC Time Constant

The RC time constant, \u03c4 = RC, sets the speed at which a capacitor charges or discharges through a resistor. It comes from the fact that the resistor limits current while the capacitor's voltage changes gradually, so the current gets smaller as the capacitor fills up. After one time constant, a charging capacitor has reached about 63% of its final voltage, and a discharging capacitor has fallen to about 37% of its starting voltage.

Use this when you need to estimate how quickly a capacitor-based circuit responds, such as in delay timers, filter circuits, sensor smoothing, or power-up/power-down behavior. It is also useful for choosing resistor and capacitor values to get a desired charge or discharge speed.

• Forgetting to convert k\u03a9, \u03bcF, or nF into base SI units before multiplying
• Thinking \u03c4 is the time to fully charge the capacitor instead of about 63% of the final value
• Using the same formula for every circuit without checking whether the resistor really sets the capacitor's charge/discharge path