[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

RE: capacitance of homemade caps

Original poster: "Steve Conner" <steve.conner@xxxxxxxxxxx>

>I've tried using  function generator and an o-scope. the function generator
>doesn't have enough voltage to charge the caps.

It's been said before- you don't need a high voltage because a capacitor
will have the same capacitance with 0.001 volt in it as it does with 1,000

There are several ways of measuring a capacitor with the equipment you have.
The one I prefer is to connect it to a known inductor (what do you mean you
don't have one!) and find the resonant frequency using a sine wave from the
generator. Then work out the capacitance using the formula

Another way is to feed a square wave into the capacitor through a known
resistor R. Connect your scope across the capacitor. Start at a low
frequency and turn the frequency up. The wave will turn from almost square
to sort of curvy, and start to get smaller (from top to bottom)

Keep going until the peak-to-peak size of the wave you see has shrunk to
about two-thirds of what it was at low frequency. Then the period is equal
to the time constant: t=RC=1/f and hence C=1/(f*R) -- where f is the
frequency you read off the signal generator dial. I think.. maybe there
should be a pi in there somewhere :-/

Steve Conner