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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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