Re: [TCML] DC pulse capacitor help

["Shelby L." <shelbylope@xxxxxxxxx>]

OK, Thanks for the help so far. I have decided not to go with the GE cap
because of safety issues, efficiency and also because of bulk. Now I was
also looking at some maxwell pulse capacitors from the same seller (
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=280568809498&category=4662&_trksid=p5197.c0.m619&_trkparms=1876%3A1)
and am wondering if these might be a bit better to use, as they are also
slightly cheaper than the GE capacitor. Would I still be able to use these
with my 15kv transformer as they are rated at a much lower 35kv DC? Also I
was also looking at some chinese capacitors that claim to be .01uF @ 20kv (
http://cgi.ebay.com/0-01uF-20KV-High-Voltage-Capacitor-HV-Tesla-Coil-Ham-/360313485656?pt=LH_DefaultDomain_0&hash=item53e45b7958)
what is your opinion on these or should I just stay away from them and get
some expensive Cornell caps. Thanks again for the advice and cant wait to
get this thing up and running!!

On Sun, Oct 31, 2010 at 9:55 AM, Gary Lau <glau1024@xxxxxxxxx> wrote:

> 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
> > --
> > ********************************************************************
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-- 
Shelby
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