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Re: Capacitor Testing

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Winston,

You ask a complex question with many answers ;-)

Three ratings really concern us.  Peak voltage, peak current, and RMS current.

The peak voltage is almost always printed on the capacitor so QED.

Peak current is harder.  If the cap is rated for high current, pulse duty,
high dV/dT, etc. it can probably handle the short high current pulses
without blowing out the internal connections.

RMS current is the ability of the capacitor to run without overheating and
melting down.  That depends on the value and dissipation factor.  No
"cheap" capacitance meter will be able to measure the small dissipation
factors we deal with.  If you or perhaps friends work in the electronics
industry, asked around to see if anyone can use one of the fancy HP are SRI
meters.  The Agilent 4288A would be nice in that it could measure the
dissipation at exactly the frequencies and harmonics you need.  Of course,
$10,197.00 latter, would you care...

The first, and easiest way, to determine these numbers is to look in the
data sheets for these caps.  Many or on-line now so with a little digging
you may find the numbers you need.  If they are common "we" may know too.

I have a known good/bad list of caps at:


I have used a 350kHz CW power supply to run say three amps through a cap
and a thermal couple to measure the temperature rise.  That takes a fairly
high power signal source that can tolerate high reflected power.  You also
need to do some fiddling to get it to "tune".  You also have to be able to
accurately measure the current which is a bit hard at that power and frequency.

Another possibility is to hook up a NST-gap-primary coil circuit to make
sort of a cap tester.  Basically a primary circuit designed to test your
caps.  Since there is no secondary, you do not need to worry with tuning or
anything.  However, that does require some calculation and thought so as to
quantify the test and determine how the results will apply to an actual
coil.  The voltage on the caps could easily be varied to control the
"stress" on the caps. 

A standard function generator and scope "may" possibly work if they are
really good.  Basically you would have to copy the function of a high
dollar capacitor tester and search for the tiny effects of dissipation.
Perhaps look for a tiny phase angle deviation or perhaps some LC circuit
could be set up were the dissipation factor of the caps would be obvious.

It may be easier and cheaper to just get Geek Group caps and not worry with
it too ;-)

See the data at:


So that will get you started.  It's not an "easy" one hour project but
almost an area of Tesla coil science.  One could probably spend years
working in this area...



At 04:37 PM 5/14/2002 -0700, you wrote:
>Hi All.
>	Are there any "easy" methods of testing unknown caps for TC
>suitability?  Something like a high power function generator that could
>push a given amount of current through a capacitor.  Maybe amplify the
>output of a small generator with a couple of HV MOSFETs and an MOT or
>two?  Since crummy caps seem "attracted" to me, I'd like a way to test
>them without smoking up the neighborhood :-0.
>	Is this worth the time of day, or am I just an idiot? ;-)  Comments
>Winston K.
>PS- Can I measure how much energy a cap dissipates using a "normal"
>function generator, and an o' scope?