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 Constants | Charge % | Discharge % |
|---|---|---|
| 1 | $63.2%$ | $36.8%$ |
| 2 | $86.5%$ | $13.5%$ |
| 3 | $95%$ | $5%$ |
| 4 | $98.2%$ | $1.8%$ |
| 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 Constants | Charge % | Discharge % |
|---|---|---|
| 1 | $63.2%$ | $36.8%$ |
| 2 | $86.5%$ | $13.5%$ |
| 3 | $95%$ | $5%$ |
| 4 | $98.2%$ | $1.8%$ |
| 5 | $99.3%$ | $0.7%$ |
(note that for the exam, you only need to know the first two time constants)