# 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)