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*To*: tesla@xxxxxxxxxx*Subject*: RE: capacitance of homemade caps*From*: "Tesla list" <tesla@xxxxxxxxxx>*Date*: Thu, 07 Apr 2005 17:23:35 -0600*Delivered-to*: chip@pupman.com*Delivered-to*: tesla@pupman.com*Old-return-path*: <teslalist@twfpowerelectronics.com>*Resent-date*: Thu, 7 Apr 2005 17:27:25 -0600 (MDT)*Resent-from*: tesla@xxxxxxxxxx*Resent-message-id*: <dqEpPC.A.wRH.SHcVCB@poodle>*Resent-sender*: tesla-request@xxxxxxxxxx

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 volts.

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 fres=1/(2*pi*sqrt(L*C))

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

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