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Re: Advice needed on capacitor tests: update: BIG resonance!
On Thu, 1 Apr 1999, Tesla List wrote:
> Original Poster: Alex Crow <user-at-alexcrow.clara-dot-net>
>
> Dear all,
>
> I will add to this after another test....
>
> I hooked up my cap across my NST again, but this time put my multimeter on
> the output of the NST. I turned up the voltage, and at only 1 or 2 percent
> of line voltage I was getting about 600-700 volts on the meter!! I tried
> it without the cap and all I was getting was around 80-100 volts on the
> same setting... All I can think of is that the NST windings are resonating
> with my cap at 60Hz and causing this voltage rise of about 6 to 8 times!!
> Instead of shunts, these things have a small gap in the core which provides
> enough leakeage inductance to limit the current.
>From what I was told, you really shouldn't be testing a cap that way. It
could blow your cap and your transformer. The resonant rise you are seeing
is quite real, and as you discovered, quite large. This is a desired
effect for getting "more power" from your transformer durring coil runs.
The down side, is that if the coil is not properly tuned you can blow your
components with the voltage rise. Test and break in your caps on your coil
with a varriac set low and your spark gap VERY close. I broke my rolled
poly caps in with the gap set at .030" and the varriac set to deliver
about 4V to the transformer, just enough to fire the gap. Use many short
runs on the coil to break it in and then tune it up. Don't increase power
until you are sure you have the coil in tune. And increase it SLOWLY, by
adding more gap distance (more gap elements in a static series like mine)
and run the variac just high enough to jump the gap. And put that cap in
oil. It sounds like it's working fine to me. But if you keep doing this
you could destroy it.
Travis