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Registered Member #2360
Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14
Hello, all i am glad to be on this forum, i have read the threads for a while now and and never decided to join, i prefer to spend my free time in the garage not in front of the computer . I hope to learn a lot from you guys and also contribute something myself. Any who now for my problem. I have built a couple coils before but all where smaller, i am bulding my largest coil now 15kv 60ma (and if all goes well upgraded to 120ma). I have everything close to being built except for the capcitor bank. For the bank i chose some USSR surpluss ceramic caps rating 30kv 470pf (same as these guys but higher rating When testing everything ran fine the gap was firing nicely, then gaps stopped after 20sec and all could hear was the hum of the transfomer and smoke rose up from the bank. Now the reason i chose the caps is price i don't have a lot of money for this hobby. Is this an isolated issue, or was purchasing 40 something year old USSR a dumb move and i should scrap the bank. If so what would you guys recomend, granted none of these will get the .0106uf right on but i can't aford a huge MMC
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
First of all, welcome aboard .
Also, when giving a link, just paste the URL. It will turn into a little green arrow... I say this because your links do not work. I just copied and pasted.
Doorknob capacitors, while having ratings that sound sufficient, are not good for Tesla coils. I am not an expert with them, but i assure you they are no good. Unfortunately...
Making Tesla coils is defiantly not cheap. Cheap capacitors *do not work*. Never get those 'CJE' capacitors. They die very quick. I bought some quite a long while ago, think two in series for 40kv and the right capacitance (these where a bit different) would not die. In the first ten or twenty seconds they smoked. This was with a 12/60 NST.
Also, do *not* use 10nf. that is resonant size. It will resonate, making it pull huge currents and it will make the voltage rise. 17nf is better suited for you NST and a static gap. For a synchronous rotary spark gap, around 25nf is suggested.
Even though CDE cost quite a bit, that is the only way to go. There are other kinds that work well that cost less, but they have lower capacitance or voltage, or both, making you need more, making the cost about as much as the CDE. So either way, it is likely you would have to spend a few hundred dollars to have an efficient reliable coil.
EDIT: Yea, i forgot, those capacitance values were for 60hz, if you live in 50hz land, use about 20nf for a static gap and 30nf for a sync rotary gap...
Registered Member #2360
Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14
17nf? I punched in my specs on tesla cad and wintesla and got the 10.07nf. I am not planning on changing the coil 120ma till later. I also updated the links. This was the old bank till it croaked.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Reading this, I sorta have to reply because there isn't a clear understanding of what's going on, nothing personal, I just wanted to clear a few things up here.
1. The picture of the cap bank shows some misunderstanding of construction. You really ought to utilize your threaded rod concept there to really tie the bus together. That is a good start because it will really carry the current well, rather then the weak GTO wire. Remember you are trying to minimize several factors here, ohmic heating and inductance. Every inch of wire here takes away from primary inductance, which means that you are dissipating power in your leads rather then into your primary, critical loss! So tidy up the leads, double them up if possible, or use heavier wire and shorter lengths. This pewny stuff isn't ment to carry a couple hundred amps during ringdown.
2. I am not a total expert with doorknob caps, but I have a huge leg-up on most of the SGTC builders around because I DO USE THEM. After reading the reply, NOTHING PERSONAL, I felt that I had to say something because there is a lack of understanding here.
Doorknobs work great if you use the proper doorknobs and proper setup. If you don't use the right parts for the job, of course they fail, its what you could call operator error. So lets look at the failure mode:
A. Ringdown in your 15KVAC system will transfer energy from cap to inductor, stressing the system to 2x Vpk, meaning your system will stress the caps with a 30KVAC oscillation, that's why you need them rated so high. In this case 40KVDC would be a better choice if you can find them.
B. Because the caps are rated in DC, the AC rating on the caps would be .707 X DC rating at best, or 21KVAC, and maybe 32KVAC impulse, but that's a guestimate and would vary by design and mfg.
C. IF these are MICA, which they probably are, THEY DIE QUICK. I used MICA on one of my smaller coils, and you will see these in the picture, they are old SPRAGUE 4000pF transmitting caps. MICA do NOT work in pulse applications, they just can't take the abuse.
D. Do use 1.4 to 1.5 x Resonant. That comment is fine. I did the research on that one. Its around here somewhere.
E. SrTiO doorknobs are champions! their uniform dielectric is tough and robust compared to anything on the market, of course at a price. These are the ones you are looking for. Made by TDK, they are expensive, durable, and hopefully you find some with a 40KVDC rating, but they are hard to come by. I would recommend these if you are wanting to build a decade for SGTC research. OTHERWISE just buy the cheapos that everyone else buys, the CDE caps. You will need to use a lot of them to get your voltage rating, but that beats spending a thousand dollars on doorknobs that do the job right, trust me on that one. (the box is somewhere)
I've gone through several caps. I've even use the oil filled ones until they pop. I decided to go with the ceramic doorknobs because I'm doing research and wanted to validate some calculations, which is why I'm not suggesting you buy these things. You will however need to find some CDE's somewhere to get up and running. SrTiO caps in my bank. They work perfectly, do not get warm, total champions. 9x for .018uF @ 40KVDC for 12KVAC 60mA operation. If you want to and can find them for $22 each, that's about $220 out of pocket.
MICA Sprague caps. Values drifted far away from marked values because they are shorted! Die quickly. DO NOT USE!!! Transformer Killers!!!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hello Lad, and welcome the home of High Voltage thinking!
It's my experience, over five years, that ex-USSR parts, including capacitors, are generally first class, and good value.
Generally, during the Cold War, the Russians chose robustness over miniaturization, which are complementary strategies, each having their own strengths and weaknesses.
As for the money issue, which Arc Starter mentioned, whilst many of our members are ingenious in recycling stuff from rubbish tips and dumpsters, no way is this a cheap hobby. I have a way of 'not looking' at my real costs, but would guess them to be at least $10,000 a year, and I'm sure I'm not the worst!
That's why we have so many members who haven't got married yet, or whom have got tired of being married and prefer this instead.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Doorknobs work great if you use the proper doorknobs and proper setup. If you don't use the right parts for the job, of course they fail, its what you could call operator error. So lets look at the failure mode:
Yessir :P.
My point was that they failed, so they obviously are not strontium titanate. In fact, it was a statement made in one of your threads or posts, that led me to believe the mica doorknobs where crap. I remember you talking about that huge ass box of different doorknobs :P.
Here is a question for you. With doorknobs, obviously mica are good for RF and maybe high current (?). But what are teh strontium titanate used for? High frequency still? Or is it lossy at RF?
Registered Member #2360
Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14
Wow thanks for the input guys.
You really ought to utilize your threaded rod concept there to really tie the bus together. That is a good start because it will really carry the current well, rather then the weak GTO wire.
I origanally had this set up but the bolts got very hot i think i could find some better bolts to use with the om meter. However, from the looks of thing i will have to scrap the bank after an autopsy of the caps, Almost half failed. I will take into consideration power loss with next bank though and clean things up.
Never get those 'CJE' capacitors. They die very quick.
*sigh* those where my back up plan, what about the other one i posted a link to?
A. Ringdown in your 15KVAC system will transfer energy from cap to inductor, stressing the system to 2x Vpk, meaning your system will stress the caps with a 30KVAC oscillation, that's why you need them rated so high. In this case 40KVDC would be a better choice if you can find them
The caps i used where listed as pulse application, but for transmitters, i think they might be mica i will take a hammer to a failed on and see what i can find out.
As for the money issue, which Arc Starter mentioned, whilst many of our members are ingenious in recycling stuff from rubbish tips and dumpsters, no way is this a cheap hobby.
This is very true between gas, and saving for college very little money is not avalable for my hobby, and only working fastfood doesn't help either Though there is a company half a mile form my house called Signal Point Sytems thy put up radio and microwave towers, mabey i could ask to take some old hv caps of their hands.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
From what I understand, the SrTiO doorknobs come from dismantled flash lamp pumped lasers, quick discharge systems, and pulse applications, BUT they use DC not AC.
MICA's work well with CW applications. They are DECOUPLING devices, not pulse devices. They do tend to handle high current, but that requires you define high current. The Sprague's i'm showing may handle "high current RF CW" as up to 10A continuous each as a decoupling capacitor in an output stage of a high power amplifier. But remember this is a FAR cry from the hundreds of amps pulsed in the SGTC.
In my simulation of my system I think it was showing about 300A in the tank. Now with 10 doorknobs that's 30A a piece which is probably pushing them too much. I really think its a matter of di/dt through the dielectric causing hot spotting in the mica, since its not totally uniform. It's a Re-constitued mica multilayer dielectric, sorta like laminated sheets.
Unfortunately with this hobby, either the cap falls in your lap or it doesn't, which means the difference between a TC or not. Just keep at it and eventually you'll get the cap you need.
It took me years to get my doorknobs together, and I think the grand total was $3k ballpark for ~67 2nF 40KVDC TDK doorknobs. Don't bother trying to get them in all honesty. I needed something of this type for my number crunching and it's totally un-necessary for your application. Just save up the $400 when you can and get the other caps.
These are some WAAAYYY OOOLLLDDD pictures!!!! had to dig way back .
Registered Member #2360
Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14
Instead of getting those CDE caps, i think i have decided on getting two of these and putting them in series with 70kvdc ratings and being pulse capactiors i think they could handle it, granted the capacitence is a little high, but i have seen these maxwell caps used in large TC with great success. Though if you guys see any big flags let me know.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Well, that would work, but that'd cost two times more than the CDE capacitors, which work just as good if not better than the Maxwell, because they are possibly lower inductance and ESR.
And Hazmatt, 10 amps would qualify as high current. Hehehe...
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Capacitor Woes
. I hope to learn a lot from you guys and also contribute something myself. Any who now for my problem.
This is very true between gas, and saving for college very little money is not avalable for my hobby, and only working fastfood doesn't help either 
Though there is a company half a mile form my house called Signal Point Sytems thy put up radio and microwave towers, mabey i could ask to take some old hv caps of their hands.
