RE- "6 Pack" Cap question
Subject: RE- "6 Pack" Cap question
Date: Mon, 09 Jun 1997 13:35:00 GMT
From: robert.michaels-at-online.sme-dot-org (Robert Michaels)
Organization: Society of Manufacturing Engineers
T> From: Pete Demoreuille <pbd-at-cybernex-dot-net>
T>How do you measure the capacitance of salt water caps, and the
T>approximate breakdown voltage of the arrays?
T>I had an idea if the breakdown was too low, and you don't want
T>to put alot in series, could you first coat the outside of the
T>bottle with a coat of polurethane, then apply the foil?
T>has anyone tried different materials other than aluminum/tin foil?
T>how about anything other than salt water?
T>Any help is greatly appreciated!
The capacitance is measured with -- ahem! -- a capacitance
meter (or bridge). Radio Shack stores sell capacitance-
measuring meters which are good enough.
Lacking such a meter, place a 100K-ohm resistor in series
with the capacitor. Connect the series pair across a
high-ish voltage ac source -- ideally the transformer to
be used in the Tesla coil, but 120-ac will work if you
have sensitive instruments:
Measure the current thru the series pair and
the voltage drop across the capacitor.
From Ohm's Law, calculate the impedance of the capacitor:
Z = E/I.
Although there is a =dc= resistance component in this
value for Z, it can be neglected for practical work. In
which event take Z = to the capacitive reactance Xc.
Then, from Xc = 1/(2pifC), calculate C (in Farads)
where pi = 3.14159...; f = frequency (60 Hz.).
- - - - - - -
There is no non-destructive way to measure the breakdown
voltage of a capacitor - AFAIK:
Construct a series pair, as above. Select a resistor value
according to the current range of the ammeter to be used
Place an milliammeter in series with the capacitor. Apply
a variable high-voltage supply across the resistor-capacitor-
ammeter string. Use a voltmeter to measure the voltage
being applied across the string.
Gradually increase the voltage, carefully watching the
milliammeter. The current will gradually increase with
voltage but when breakdown is reached, the current will
immediately jump to what the limiting resistor allows.
For casual at-home work, apply the capacitor
at no more than 50% of the breakdown voltage
in actual Tesla-coil use.
Working at full capacity,
in -- Detroit, USA