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MMC capacitor for sgtc

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bvongunten

Sun Nov 16 2008, 07:04PM

Registered Member #1813 Joined: Sun Nov 16 2008, 06:28PM
Location:
Posts: 6

Hey all!
This is my first post here!
I have a question about selecting capacitors for my coil.
I used the capacitor calculator at this site:

and calculated a required capacitance for my two neon sign transformers of 0.018 uF.
My two transformers are 7000V 25mA models which I plan to run in parallel. I actually have four of these transformers, but I am going to just start with two...

So I need roughly 0.02 uF of capacitance. I searched ebay and found the following model:

for which I think 12$ apiece seems like a reasonable price. I would order two of these and wire them in series for a capacitance of 0.02uF and a voltage rating of 20Kv. The advantage is, that if I wanted to later, I could simply order another two, double my capacitance, and use all of my transformers! These capacitors are made by CJE. Its a name that I havent heard before; when most people talk about MMCs they seem only to talk about cornell dubilier or doorknob style caps. Both of these are really expensive! The ones I found seem to have similar ratings, if not better, than the CD models... Am I missing something? Does anyone think that these two capacitors wired in series with a couple of bleed resistors would be a bad idea or would just fail quickly or whatever?

Thanks for input!
Brian

Arcstarter

Sun Nov 16 2008, 07:35PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I have used those CJE caps, they where something like .02uf and 15kv. I used them with a 6kv 30ma nst, and they worked great, but they failed in maybe 5 minutes, and i could see it arcing inside. It let out a small puff of smoke. To me they looked so great, i payed 13 bucks and it had a great rating, but if it looks too good to be true, it surely is.

I think that the only capacitors you should use are the ones that have been proven in tesla coil use, like CDE (the 942c is best but Anon01 used 940c on his pig power tesla coil and they are working ok, but they are supposed to fail at the leads where they connect to the plate) or wima, and some panasonic type was used on terry's(fritz i think it was) tesla coil.

bvongunten

Sun Nov 16 2008, 10:01PM

Registered Member #1813 Joined: Sun Nov 16 2008, 06:28PM
Location:
Posts: 6

Thanks for the warning!

Thats why I asked... Glad I didn't waste my money!

Ill look around for those models you mentioned.

As in most things in life you get what you pay for. However, I did want to make sure that models like the Cornell-Dubillier 942C20P15K-F are not just being artificially inflated in price to take advantage of HV hobbyists. I guess that you are really paying for a cap that can take some punishment!

Cheers,
Brian

teslacoolguy

Sun Nov 16 2008, 10:30PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

The GE 42L series capacitors have also proven themselvs in tesla coil duty.

bvongunten

Mon Nov 17 2008, 04:11PM

Registered Member #1813 Joined: Sun Nov 16 2008, 06:28PM
Location:
Posts: 6

Could someone tell me if it would be alright to use black colored PE drain piping for a secondary form? I got a brand new piece today that was extra, cut off, from the job site I am working at from a plumber. It is 11 cm in diameter and about a meter long... it seems to me that it would be perfect for a form. Perhaps I could dry it out on the radiator for a few days and then varnish it, just to be safe?

I ask because I have never seen any coils online formed around black drain pipe and I doubt that is a coincidence...

cheers,
Brian

Arcstarter

Mon Nov 17 2008, 04:17PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

bvongunten wrote ...

Could someone tell me if it would be alright to use black colored PE drain piping for a secondary form? I got a brand new piece today that was extra, cut off, from the job site I am working at from a plumber. It is 11 cm in diameter and about a meter long... it seems to me that it would be perfect for a form. Perhaps I could dry it out on the radiator for a few days and then varnish it, just to be safe?

I ask because I have never seen any coils online formed around black drain pipe and I doubt that is a coincidence...

cheers,
Brian

No, you can not use black drain pipe, or at least not gray and i doubt black would be any better. It is slightly conductive, i think it has carbon or something in it. Of course, i cannot be sure, but make sure you don't wind it because if it is slightly conductive, it will just basically short the coil, and eat the insulation on the wire(if it does short) and then you have wasted money on wire. I got a 6 inch by 26 inch pvc pipe for just 11 bucks at Ace hardware.

bvongunten

Mon Nov 17 2008, 04:18PM

Registered Member #1813 Joined: Sun Nov 16 2008, 06:28PM
Location:
Posts: 6

Found something on this forum stating that black ABS plastic is a no-go, but I am fairly sure that the drain pipe used here is polyethelyne... Which isnt really the same thing... The black could still come from carbon though, I dont know? Anyone with experience with black PE piping?

coillah

Mon Nov 17 2008, 07:54PM

Registered Member #1517 Joined: Wed Jun 04 2008, 06:55AM
Location: Chico CA
Posts: 304

I have heard just that,

black pipe = carbon

which makes sense. I have had no experience using it though.

Now I am talking about it, I wonder how true that statement could be...

but why use the black if it runs the risk of messing up your coil?

Andyman

Mon Nov 17 2008, 08:58PM

Registered Member #1083 Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256

Arcstarter wrote ...

No, you can not use black drain pipe, or at least not gray and i doubt black would be any better. It is slightly conductive, i think it has carbon or something in it. Of course, i cannot be sure, but make sure you don't wind it because if it is slightly conductive, it will just basically short the coil, and eat the insulation on the wire(if it does short) and then you have wasted money on wire. I got a 6 inch by 26 inch pvc pipe for just 11 bucks at Ace hardware.

I use gray pvc electrical conduit for my 4.5 inch coil, and it hasn't failed me yet, after many hours of use. You just have to be sure to sand it down really well, and be sure to seal it as best you can. It's used for underground powelines, so...

bvongunten wrote ...

As in most things in life you get what you pay for. However, I did want to make sure that models like the Cornell-Dubillier 942C20P15K-F are not just being artificially inflated in price to take advantage of HV hobbyists. I guess that you are really paying for a cap that can take some punishment!

Indeed! I tell you, those cde caps can take a HUGE beating! They are worth EVERY penny! And doorknob caps are pretty good too, if you get the right ones. With doorknobs, however, you need the build the bank to about 3 times the voltage of the transformer. The CD caps will work even when the bank is the same voltage as the transformer! Well, it will shorten their life, but they still work!

bvongunten

Mon Nov 17 2008, 09:57PM

Registered Member #1813 Joined: Sun Nov 16 2008, 06:28PM
Location:
Posts: 6

See thats where my whole concern comes from... In the US and indeed much of the world black drain pipe would mean ABS loaded with carbon. On the other hand, the tube im talking about is used also here for burying high voltage lines underground, so... i dunno. I could always mess around with the insulation tester at work...

The thing is, i know that PVC isnt exactly expensive, but the PE tubing is FREE. One of my favorite words...

It looks like i'm going to be investing a good 150Â£ on caps, a line filter and some magnet wire... I just dont want to keep racking up expenses that I can avoid. That said, I do want the coil to function optimally and run at a reasonable efficiency... And it would be a shame to drop 20 bucks worth of magnet wire onto a junk coil to save 10 bucks. Have to ponder it some.

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