# Re: capacitance of homemade caps

• To: tesla@xxxxxxxxxx
• Subject: Re: capacitance of homemade caps
• From: "Tesla list" <tesla@xxxxxxxxxx>
• Date: Wed, 06 Apr 2005 17:52:19 -0600
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• Delivered-to: tesla@pupman.com
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• Resent-date: Wed, 6 Apr 2005 17:54:29 -0600 (MDT)
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`Original poster: "Christoph Bohr" <cb@xxxxxxxxxxxxxxx>`

`Hello Steven.`

```> I can't do anything
> with an o-scope to measure it becaus the caps require high voltages to
> charge them at all.```

```Huh? The capatcitance is in no way related to
the charging voltage, low voltage tests will be OK.```

> Maybe if I used a DC power supply, like a computer
> moniter, and set it up with a spark gap, I may be able charge it longer
> than .008 seconds(half the period of a 60Hz waveform), and I could time how
> long it takes (approximately) for the spark to break out.
If you knew the exact voltage and current capability of the monitor HV supply this
could theoreticaly work, but probably won't due to ( at least ) two factors:

1st:The uncharged cap will seem like a short to your monitor HV supply and kill the
diodes
in the multiplyer. You would have to limit your current to protect the supply.

2nd: You first have to know the exact voltage at which your gap fires. A static gap
can
be made to fire pretty reliable at a predetermined voltage, depending on the shape of
the electrodes...

Of yourse you can still calculate the capacitance based on the well known formula,
but unfortunately I do not have any specs of your cap.

If you have a signal generator you could even use a known L in parallel with your cap
and measure where it resonates, then you could calculate C from F and L....

```You could even take you cap to any TV-technician, who can easily measure it
faster than you can say capacitance ;-)```

`And now to the only advice in this mail that makes any sense at all:`

There are very cheap DMM out there that can do the job for you and
probably even more. Accurate enough for most basic coiling needs.
So I suggest you save your time and money and get yourself a DMM that can measure C.

`best regards`

`Christoph`