# RE: capacitance of homemade caps

• To: tesla@xxxxxxxxxx
• Subject: RE: capacitance of homemade caps
• From: "Tesla list" <tesla@xxxxxxxxxx>
• Date: Thu, 07 Apr 2005 17:23:35 -0600
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• Resent-date: Thu, 7 Apr 2005 17:27:25 -0600 (MDT)
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`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`