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Re: [TCML] DC pulse capacitor help



Concerning the maxwells, I also have some of these and they also contain "bubbles" inside, so I would say it is normal for this type.

----- Original Message ----- From: "Shelby L." <shelbylope@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Sunday, October 31, 2010 6:40 PM
Subject: Re: [TCML] DC pulse capacitor help


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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