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Re: [TCML] DC pulse capacitor help
Shelby L. wrote:
Hello, I am new here and am currently building a Tesla coil using 2 15kv
60ma NST's. It will have a rotary spark gap but my problem is what type of
capacitor to use. Currently I found a Pulse capacitor on ebay (
http://cgi.ebay.com/High-Voltage-Pulse-Capacitor-50kVDC-043uF-tesla-coil-/280562398405?pt=LH_DefaultDomain_0&hash=item4152d274c5
its
a GE DC pulse capacitor rated at 50kv with one HV bushing on the top model
#17L 9PH) and I am wondering if this will work for my configuration. I am
aware it should be used with DC but read that you can feed these capacitors
AC as long as it's lower than the rated voltage. I also notice If I do use
this capacitor, the case will be live also because the other terminal is
connected to the case and one on the bushing. So, any advice is greatly
appreciated as this is going to be my first big Tesla coil. Thank you again
Hello Shelby,
Welcome to Tesla Coiling - it can be a very addictive hobby!
These GE pulse caps use a metalized polyester (Mylar) film-oil
dielectric system. Although rated for 180 PPS operation in DC Pulse
Forming Networks, they are not designed for high-Q oscillatory discharge
circuits such as Tesla Coils. However, you CAN use them on your Tesla
Coil for moderate run times, but they will not perform as well as
polypropylene caps since the dielectric system is 25 - 30 times as
lossy. The metalized film is also more resistive (lossier) than pulse
caps that use film-foil construction. The combination of dielectric and
resistive losses mean that a significant portion of each "bang" will end
up heating the innards of your tank cap instead of incinerating air at
the top of your coil.
The good news is that, because of the large thermal mass of these caps,
you should be able to run for many minutes before capacitor heating
becomes a problem as long as you keep each run cycle short (say less
than 1-2 minutes). The interior of the internal capacitor rolls become
much hotter than the rest of the cap, so you need to give the cap 2-4
minutes of "rest" between runs to help dissipate the internal heat
through the rest of the capacitor and case. Monitor case temperature at
the end of each rest period and also look for any signs of bulging
between runs as well. Capacitor bulging means that the dielectric system
is becoming overstressed by corona. liberating gases from the dielectric
fluid and increasing internal pressure. A bulging cap will generally
fail in the near future.
You may want to consider using a self-healing polypropylene MMC instead
for better performance, reliability, longer-term cost.
Bert
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