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Need help on getting around some limitations for a Tesla coil

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Lucas

Wed Jul 18 2012, 10:47AM

Registered Member #5691 Joined: Wed Jul 11 2012, 10:19PM
Location: Belo Horizonte, Brazil
Posts: 4

Greetings, everyone.

I'm a physics undergrad here in Brazil, and I'm currently helping a professor in building/improving a Tesla coil.

It's been frustrating for us because the literature we can find on the subject is very focused on the US, UK and Australia, or other "first-world" countries, where a lot of the components are not too hard to come by. You guys talk about caps for MMCs, pulse capacitors, aluminum tape, nylon screws and a bunch of stuff that simply doesn't exist around here. We've looked everywhere, spent hours on the phone, all for nothing. Brazil is very limited in the availability of these sort of special things, and when we do find them, they are incredibly overpriced.

So we're improvising with what we can. After a lot of research and ingenuity, I've managed to circumvent a few of these things, but I'm a bit stuck on a few things.

What we've got so far is:

- NST: we've got two transformers rated at 15 kV and 30 mA, but we'll only use one for the coil. Input is 127 V @ 60 Hz.

- Secondary: Salvaged from an older, failed TC attempt. Winding is 91.5 cm tall, 10 cm wide, wire is about 0.455 mm thick (AWG "25.7").

- Top load: toroid made out of aluminum duct, 12 cm ring diameter, 53 cm total width.

- Primary capacitor: according to our research, the ideal capacitor here would be 5.3 nF, and we should use a larger than resonant of about 8.6 nF. I've rolled two capacitors, but they ended up at 6.4 nF and 14.3 nF, according to our measurements, even though they were only a pair of plates in difference. They have worked just fine in our tests and have endured 5 minute runs with a smaller demonstration coil manufactured by Leybold (just like this one) that I've restored earlier this year.

That's about it so far.

Throwing this data on TeslaMap and other such simulators, I get that the primary coil will need about 15 turns of 1/4" copper tubing with 1 cm spacing and a hole diameter of 20 cm for proper resonance matching.

TeslaMap also tells me my top load needs a bit more capacitance, but apparently it's already pretty close to 1/4 wavelength (98%) as it is, which, hey, is pretty good.

Now, this is where the doubts lurk in.

We're not too sure what effects using the 14.3 nF capacitor will have here. I've read that it can easily blow up the NST. Others say it won't matter much and it'll just not charge to the full potential within each cycle. Some of our doubts:

1. What can we expect from a too large capacitance?
2. How dangerous is it for the rest of the components?
3. How exactly does it affect performance?
4. What value should we really aim for?

If it's absolutely necessary, I could attempt rolling a proper capacitor, but the necessary primary gets wider than we'd hope for this coil. We don't want a 80 cm wide coil, that seems excessive and it won't be able to be transported around easily.

In fact, we've looked at a bunch of coils out there, and they all seem to have pretty tiny primaries in comparison, whereas all the calculations tells us we need a very wide (flat) primary. What's going on here? Where's our error, if any?

Speaking of the primary, it's the last part I need to build here. Are there any other considerations I should take into account? I've found conflicting information regarding the winding direction of the primary and the secondary. Does it matter? When should I consider a conic primary instead of a flat one? I understand you're increasing the coupling, but I'm not sure how to decide on this before actually building it and testing.

The other bit we're worried about is NST protection. I've checked the recommended Terry filter, and I've searched for the high voltage caps necessary and the MOVs and whatnot. Couldn't find them anywhere around here.

Some sources also recommended building a RF choke made out of ferrite core and a few turns of a thick copper wire. We can do that, but I doubt that's enough to protect the NST. Also, every time I see this RF choke being used, the circuit diagram for the coil has the secondary ground connected to the primary, like in this diagram. I haven't found any detailed explanation for this alternative design, and frankly, I can't make any sense out of it. I'd love if anyone could explain what are the differences between the two designs.

What can we do to reasonably protect our NST here? Are there any alternatives? Preferably, something we can mostly build ourselves, as buying things seems like a lost cause.

The other major concern is the spark gap. We're currently working with a set of rounded-tip screws, and it works all right. But we'd like to try something better. I've read that rotary spark gaps are better, but that asynchronous ones are potentially dangerous. Do you guys build your own synchronous rotary spark gaps or you just buy them somewhere? Are there any DIY alternatives superior to a static gap? I'm already working on adding some quenching, but is there anything else?

All in all, as far as our current calculations go (and please, check our math and simulations using the values given above), we're ready to go as it is, and the coil should work reasonably well. But we'd like to check it with more experienced folks before we invest our very limited budget on something that won't work, or that'll most likely trash our equipment.

I hope you guys can help us. Both my professor and I have built our own Tesla coils when we were teenagers (we're actually using his secondary!), and both of those coils were very disappointing. This project has been a bit of a second chance to us, so we'd like to do it right, and as best as we can. I hope that's possible.

Cheers!

LuceÅ¡

Wed Jul 18 2012, 03:49PM

Registered Member #4108 Joined: Sun Sept 25 2011, 11:43PM
Location:
Posts: 149

Hi, I don't have a whole lot of experience with spark gap Tesla coils, but I can give you what I know. pretty much every first coiler makes their own capacitors because they are expensive. you could check ebay. most people ship world wide.

The Rf choke literately chokes out hazardise currents. they just cant pass through. you could also put a safety gap in parallel with the nst. make it slightly bigger than the spark gap. any voltage spikes will discharge through the safety gap.

like I said I don't know a whole about sgtc's, i have only built one, I like vacuum tube tesla coils better because the discharges are awesome, and it doesn't piss off the neighboring apartments!:D

Steve Conner

Wed Jul 18 2012, 10:04PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The most important parts of the Terry filter (IMO) are the resistors. You can leave all the other parts out

A NST can be damaged by a "resonant" capacitor value: the one that resonates with the NST's leakage inductance at the mains frequency. Larger than resonant ("LTR") is preferred, but smaller than resonant is also safe: it just gives less output.

Tesla himself used wine bottles for capacitors, surely they have those in Brazil?

Lucas

Wed Jul 18 2012, 10:21PM

Registered Member #5691 Joined: Wed Jul 11 2012, 10:19PM
Location: Belo Horizonte, Brazil
Posts: 4

Bottle caps are inferior to rolled poly, no? Either way, I feel the capacitors are all right. I'm just worried if the 14.3 nF will explode the NST or something. How worried should I be about how closely the primary cap is from the "ideal" LTR value (1.5 or phi * Cr)
?

Coronafix

Wed Jul 18 2012, 10:33PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

You should parallel the two transformers together to get 15kV @ 60mA. As soon as you switch it on you're gonna wish you had done that to begin with. This of course will change your cap specs. Have you got enough copper for the primary coil?
Your winding ratio for the secondary isn't very good at 9:1, perhaps you should cut it in half. With 2000 turns on the secondary, the resistance is too great for a spark gap coil. It's no suprise he had dissapointing results with it.

Lucas

Wed Jul 18 2012, 10:47PM

Registered Member #5691 Joined: Wed Jul 11 2012, 10:19PM
Location: Belo Horizonte, Brazil
Posts: 4

We thought about cutting the secondary a bit, yes. We tried out some simulations using a 60 cm tall winding, and it seemed a good option. Cutting it in half would make it 45 cm tall, which seems really short.

Won't the proximity to the primary be an issue, especially considering the size of the toroid? Is it OK to simply raise the toroid in that case?

Aesthetically, it would look pretty weird too (@ 60 cm height for the secondary):

Zj3QU

We've been considering dropping this secondary altogether as well and make a new one, with thicker wire and better proportions, but we'd like to see the other options first.

As for the copper for the primary, we'll buy it once we figured out the dimensions and if it'll be conical or flat spiral. We want to minimize weight and size as much as possible, so it is easier to transport and so it won't be too expensive.

Ben Solon

Thu Jul 19 2012, 01:55AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

Some input for the nst/cap config. Put the nst's in parallel. This way a large cap will be charged twice as fast. Also, bellow 10n is really small for output. You want as much energy per "bang" as possible. I hope you are using the spark gap setup where the main gap is parallel with the power source. If so, then use a bigger cap with 2x charging current. To keep the system resonant reduce the primary inductance.

Lucas

Thu Jul 19 2012, 04:10PM

Registered Member #5691 Joined: Wed Jul 11 2012, 10:19PM
Location: Belo Horizonte, Brazil
Posts: 4

We've opted to use a single NST.

M.A.D.

Sat Jul 28 2012, 10:55AM

Registered Member #4052 Joined: Thu Aug 11 2011, 04:43AM
Location: IN ,USA
Posts: 69

With only one NST you could shorten the secondary a lot and not need to worry about arcs to the primary.

I personally believe that low power SGTCs perform better at higher frequency, I could be wrong However, as this is my personal experience.

My SGTC has a 12" high secondary consisting of only 450 turns on a 2" diameter form, with a very small topload, soviet doorknob caps, and one 10KV NST, and it runs at well over 1MHz. I get 10" arcs from this coil and have yet to damage anything, despite a lack of protection for the NST.

Also, since your caps will be one of the least efficient parts of your system you should put as much air across you spark gap as you can to quench it quickly. This combined with low secondary resistance should get more power from the NST to the top load.

Sulaiman

Sun Jul 29 2012, 03:51AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

just a few thoughts;

1) assuming perfect energy transfer
from primary (capacitance) to secondary (capacitance)
the peak secondary voltage will be Vpri(pk) x sqrt(Cpri/Csec)
so if you have a large secondary (coil capacitance) and topload (capacitance)
the peak secondary voltage may be too low for impressive/long sparks

2) if you have a large secondary inductance and capacitance
the resonant frequency will be relatively low
If the primary capacitance is also relatively low then
a relatively large primary inductance will be required.
(by "relative" I mean relative to a 'typical' sgtc)
There is no harm in a high Lpri/Cpri ratio as it puts less stress on the spark gap
as the resonant current will be lower.

3) the 'easiest' way to use a higher primary capacitance would be as noted above
... use both NST in parallel.
This would result in double the primay capacitance (14 nF would be ok)
and half the primary inductance, and 41% higher secondary peak voltage,
or more energy available to the spark/arc.

4) one NST stressed to it's limits will soon require a second replacement NST which will also not last long,
two NST in parallel with a shorter (lower voltage) spark gap would give similar results but last longer.

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