# A-001-001: Time Constants

## Capacitors#

Capacitors are measured in Farads ($\si\farad$). Commonly in amateur radio, you will see $\si{\micro\farad}$, $\si{\nano\farad}$ and $\si{\pico\farad}$.

$$T = RC$$

where $T$ is the time constant of the circuit, $R$ is the resistance in series, and $C$ is the capacitance.

The initial potential of a capacitor is 0. It takes 5 time constants to charge or discharge a capacitor. A capacitor charges at 63.2% each time constant, and discharges at 36.8% each time constant.

Time ConstantsCharge %Discharge %
1$63.2%$$36.8% 286.5%$$13.5%$
3$95%$$5% 498.2%$$1.8%$
5$99.3%$$0.7% (note that for the exam, you only need to know the first two time constants) ## Inductors# Inductors are measured in Henries (\si\henry). Commonly in amateur radio, you will see \si{\milli\henry} or \si{\micro\henry}. The initial resistance of an inductor is infinite, and it decreases as the inductors charges.$$T = \frac{L}{R}$$where T is the time constant of the circuit, L is the inductance, and R is the resistance. Similar to capacitors, it takes 5 time constants to charge or discharge an inductor. Time ConstantsCharge %Discharge % 163.2%$$36.8%$
2$86.5%$$13.5% 395%$$5%$
4$98.2%$$1.8% 599.3%$$0.7%$

(note that for the exam, you only need to know the first two time constants)