Re: [TCML] DC pulse capacitor help

[Gary Lau <glau1024@xxxxxxxxx>]

In addition to Bert's concern about the Mylar dielectric, I would be very
wary of the bent insulator and terminal screws.  While the cap may test just
fine on a capacitance meter, there's no telling if the terminal insulation
has been compromized..Even it the vendor accepts returns, shipping such a
heavy beast back and forth won't be cheap.  Better to invest in a guaranteed
MMC.

Regards, Gary Lau
MA, USA

On Sat, Oct 30, 2010 at 12:01 PM, Bert Hickman <bert.hickman@xxxxxxxxxx>wrote:

> 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
> --
> ********************************************************************
> We specialize in UNIQUE items: coins shrunk by ultra-strong magnetic
> fields, Captured Lightning Lichtenberg figure sculptures, and scarce
> technical Books. Please visit us at http://www.capturedlightning.com
> ********************************************************************
>
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla