Re: exploding capacitors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Tesla list wrote:
>Original poster: "Steve Zeitler" <zeitler-at-verizon-dot-net>
>I'm reading all these postings about caps blowing up. Being the keeper of 
>a medium size coil in a commercial setting, naturally i don't want this to 
>happen to me (caps blowing up)
>The latest post mentions an asyncronous rotary gap being used  causing the 
>failure. I've not heard of this before and the coil I mention has one. Is 
>this bad and why?
>Also, the "cap" in this coil consists of 4 .1uf 30kv plastic cap brand 2 
>in series on each side of the primary giving .025uf  at 120kv. An old Ham 
>radio manual I have shows using resistors in parallel with caps when they 
>are hooked in series to equalize the voltage each cap sees. I'm not doing 
>this right now should I? I'm running 11-12kv after ballasting and I'm in 
>the process of upgrading to 2 potential transformers instead of 2 homemade 
>ones. The voltage is about the same but the delivered current is higher. 
>This allows me to run the rotary gap much faster ...more PPS. comments?
>                                                    Steve Z
>                                                     Philly
>
>.

Steve,

An async gap can cause unexpectedly high voltages to develop across the the 
HV transformer and tank cap under certain operating conditions. This 
particular problem can easily be alleviated by making sure you connect a 
properly set static spark gap (similar to a safety gap) directly across the 
async gap, and that you also use a front end "Terry filter" and safety gaps.

Plastic Capacitors makes both Polypropylene and Mylar capacitors, and they 
normally recommend using type BNZ for Tesla Coil applications. These 
phenolic-cased caps have been known to fail (and may begin leaking oil), 
but I have not heard of any violent failures. Which model are you using in 
your system?

Although connecting resistors across the capacitor chain will help bleed 
off any residual voltages after the system is shut down, they'll have 
virtually no influence on equalizing the voltages developed across the 
chain. For pulse applications, voltage sharing is governed entirely by the 
relative values of the capacitors in the chain. Since your caps are all 
approximately equal and of the same type (i.e., similar ESR), the voltage 
sharing should be approximately equal.

Equalizing resistors are often necessary for proper voltage sharing in 
steady state DC/filter applications due to variations in capacitor leakage 
current. For your "equidrive" tank configuration, I'd recommend adding them 
to prevent a possible shock hazard since otherwise the equidrive 
configuration can leave the primary winding somewhat  "hot" long after the 
system has been shut down.

I think I remember seeing your system while visiting my daughter in Philly 
a couple of years back - if I remember correctly there was also a Marx 
Generator nearby...?

Good luck!

-- Bert --