[sdiy] Capacitor value checking

[Scott Bernardi]

Actually, rise/fall time is a good way to measure the cap. Take a known resistor value and make an RC circuit.
Measure the voltage across the cap, and put a square wave into it (0v to some value, slow enough that the
output "reaches" the end states).   On the "decaying" side, it will decay to 37% of its value in one time
constant = RC.
So if you measure the time t that it decays to 37% (1.84v for a 5v step), C = t/R.
Another way would be to input a sine wave into the same circuit, and measure the frequency where the amplitude
decreases to .707 times its peak value.  At this frequency, f = 1/(2*PI*RC). or C = 1/(2*PI*R*f).

John Lamb wrote:

> Is there some way to approximate the value of a cap using a scope?  I don't have a cap checker and
> cannot find one for cheap, and I need to sort these caps before my head blows up.  :/  Perhaps I could rig
> it to display the rise/fall time and read it from that?  Any help is appreciated
>
> Thanks in advance
> John Lamb

--
Scott Bernardi
sbernardi at home.net