Re: [TCML] Help choosing tank caps for dual MOT

[jhowson4@xxxxxxxxxxx]

I have not heard that the paper caps could be used, I never really thought of it as the oil was the dielectric and the paper merely a spacer , guess you learn something new everyday. 

With regard to poly caps. 
Honestly, I have never had an issue with my poly caps and I abuse the heck out of them. I originally designed them for use with obits 10Kv maximium of 1 maybe 1.2 Kva, then I put 10 orbits on them for 2.3Kva. But recently I have been running them for 2-3 minutes at a time with a PT maxed out at 3Kva ~16.8 Kv. These have a 84 mil thickness 3" spacing around the edges, a tap comming off of every turn, and no oil. I built them junior year of high school for the total cost of maybe 30 bucks (I got a good deal on 6 mil rolls when a local store went out of business). 
For me at that age this was my perfect solution for my experimentation. The fact that I have gotten 5 years of use out of the little buggers makes the 5 or 6 hours I spent making them totally worth the effort. I would never be able to do what I have done to these caps with a professionally built unit, because i would not want to risk breaking a professionally built cap. 

So my opinion is that for those of us who are using relatively small power levels, are on a college/high school budget, and building forever evolving coils that we are constantly swapping parts out of and making adjustment and improvements to as we learn; a rolled poly cap can be perfect. I personally have not yet had a failure, maybe i am just lucky, maybe i did something to reduce the chance of a failture. Whatever it is, hears to hopin that when they do finally bite the dust (as all pulse capacitors do) I am in a position to get the good quality professional ones. Until that time, I can still have my fun and in the process get some pretty decent sparks. =D 

Thanks, 
John "Jay" Howson IV 

----- Original Message -----
From: "Jim Lux" <jimlux@xxxxxxxxxxxxx> 
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> 
Sent: Thursday, September 1, 2011 10:39:24 AM 
Subject: Re: [TCML] Help choosing tank caps for dual MOT 

On 9/1/11 6:23 AM, jhowson4@xxxxxxxxxxx wrote: 
> Just to throw my 2 cents in here, 
> 
> A quick Google search yielded the discovery that option 1 uses paper 
> as a dielectric, not ideal for tesla use. 
> 

Not necessarily... while polypropylene film with extended foil 
electrodes is preferred, oil/paper has a long history of successful use. 
The dielectric is oil, which is low loss (the paper is basically a 
spacer). What it really depends on is the electrode design. paper/oil 
DC filter caps, for instance, have fairly high inductance. 



> option 2 i could find no information on, but be wary because people 
> on ebay love to put Tesla in the description for things, regardless 
> of its actual purpose or not. 
> 
> option 3 well yea its gonna work, i cant really say much about which 
> size would be better though. 
> 
> 
> but might i present option 4 A home made poly cap could be made for 
> your voltages relatively easily and cheaply. I have been doing alot 
> of digging into the poly caps information as I am having the same 
> problem sourcing a capacitor. I am assuming that you are seriesing 
> the 2 mots and doubling them? Running the numbers threw my little 
> excel calculator I made. and using the advice that your capacitor 
> should be moderately over rated so it does not cook itself. 

rolled poly caps are a lot of work: you're basically trading time and 
reliability for money. There's a non-zero chance that first one you 
build will have some crippling flaw or have short life, because it's 
simple in concept and tricky in execution. Do you have a vacuum pump? 
A big clean surface to lay the layers of poly out on? Do you have a 
source for the poly, and for the foil? 



> 
> you could make a .1uf 9600V capacitor using 135ft of the 6mil poly 
> which could easily cost you 50-60 for the plastic. that of course is 
> excluding the costs of aluminium for etc etc. over all the price 
> would be almost 60-80 depending on how fancy you want it. The 
> resulting cap would be rated for ~40kv plenty of head room and be 
> pretty bullet proof when running off your mot supply. in my opinion. 
> You could even put it under oil if you were feeling ambitious but at 
> the lower voltages it should not be necessary. 

Running a rolled poly cap without oil impregnation is asking for short 
life. You will, even at low voltages, have corona at the edges of the 
foil plates, which will degrade the plastic slowly but surely. 
Eventually, under the mechanical stresses of the capacitor, the plastic 
goes away, and you'll get breakdown. I guess, without oil, you at least 
don't get the burning oil leaking out.. just smoldering melted plastic. 

Almost 15 years ago or so, the rolled poly started to be less popular, 
and there was some interesting failure analysis done on these caps by 
dedicated list members. There's just a lot of things that can go wrong. 




> 
> I personally have used the home brew caps for the past 6 years with 
> out failture, 2 are open air, 1 is under oil. the air units run off a 
> obit supply (10kv) and I have had about 10 obits in parallel at one 
> time or another. I personally love the home brew caps, but other 
> people seem to have trouble with them. 


It's like carbon fiber composites: there's a lot of factors that go into 
success/failure so it's unpredictable in small manufacturing lots.. Some 
people roll tighter than others (I'm not sure which would actually be 
better). Some have different run duty cycles (which changes heating and 
such). Some have different LC ratios and system Q (which changes di/dt 
and voltages) 



These days, there's three basic approaches that are fairly low risk: 
1) beerbottle salt water (in various forms) - heavy, can be lossy, real 
cheap, not real durable in general. A variant is the plastic party cup cap. 

2) MMC - labor intensive, expensive for big capacitance 

3) commercial caps - very tricky if you're buying surplus 

There are other approaches which might be useful, in a particular situation: 
1) ceramic HV caps - some work well, some don't, if you have a cheap 
source, go for it. 

2) home built foil caps - rolled poly, some work, some don't, labor 
intensive, not all that cheap if you have to buy materials 

3) all manner of flat plate capacitors - foil/glass is lossy, but, hey, 
it's worked for people in the past. foil/plastic baggies/plastic 
sheets,etc If you have a cheap source for the materials (free) and lots 
of time, it can be worth trying. 

Pretty much anything you can imagine has been tried and reported on the 
list, so a search using the materials you're thinking of will probably 
turn up someone's practical experience. 
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