Hi,

I am learning about caps and how to measure

their charging/discharging time, esp. in a DC

circuit where there is a cap in serie with a resistor.

The RC constant is r * c and is the time a cap gets

charged at ~ 63 % its final value. Multiplying it

by 5 would give an approximation of the charing

time.

With a circuit of my own, I measured the discharing

time of differents RC configurations in order to

verify the above approximations. Here they are:

For a ceramic cap, 100 pico Farads:

resistor(ohms) | | measured time (usecs) | | 5 * RC constant |

100k | 50 | 50 |

200k | 400 | 100 |

235k | 432 | 162 |

470k | 512 | 235 |

For a ceramic cap, 100000 pico Farads:

resistor(ohms) | | measured time (usecs) | | 5 * RC constant |

3.9k | 1856 | 1950 |

10k | 4416 | 5000 |

175k | 85200 | 87500 |

470k | 199300 | 235000 |

While the second table seems coherent (except for the last value),

the measured values in the first table greatly differ from the expected

ones. My conclusion: as capacity decreases and resistor value increases,

the measured time and the expected one diverge more and more.

Do you have any clue about that? Caps rating are not accurate as they

decrease? A friend of mine once told me high resistors are capacitive,

can it explain the divergence?

Thanks for helping,

Regards,

Fabien.