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.