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a few questions

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LAN_403

oucivileng

Mon Dec 22 2008, 02:17AM

Registered Member #1772 Joined: Tue Oct 21 2008, 05:23AM
Location: Athens, OH
Posts: 71

I'm working on finishing up my first coil and I was hoping somebody could help me with a few questions. The answers to some of these questions are probably on an earlier thread but in some cases there were too many search results for me to sort through so bear with me.

I got it up and running and I've gotten about 13 inches out of it so far but I know it could use some improvement. My setup consists of a 4inch diameter secondary with about 1200 turns, a primary (cylindrical) with 19 turns, a bank of beer bottle caps that are set in a tub of saltwater and mineral oil. My spark gap is just piece of tubing with two screws coming out each side. It's all connected to a 15kV 25mA NST.

Prob 1:
When I first made the spark gap I used the pointed ends of two screws. Now I know that the pointed ends are a bad idea. I went and got some carriage bolts, thinking that I could use the round heads for the gap. Now that I've switched to those, the coil will work for just a split second when I plug it in and then quit immediately, regardless of how close the gap is. Anybody know why this is?

Prob 2:
I've been trying to figure out how to tune the primary to the secondary using a signal generator and oscilloscope. The instructions I had said to look for a spike in the voltage on the scope as I changed the frequency. I am of the opinion that I'm looking for a dip in the voltage. Is it the dip or the spike? Also, any advice or suggested references would be helpful. I'm basically learning this as i go.

Prob 3:
Is the saltwater tub a bad idea for the caps? I get a LOT of corona (it almost looks like blue flames up the sides of the bottles) and I have sparks jumping from bottle to bottle. The mineral oil doesn't seem to help much. Would aluminum foil be better? Should I scrap the bottles and make a cap from a ceramic plate or plexiglass or something? I would like to build an MMC but I've gotta pay my tuition and it looks to me like an MMC would be at least 50 bucks that I don't have.

Prob 4:
Has anybody ever tried those stainless steel gazing balls for a top load? They're cheap but it sounds like they have some kind of glaze or something on them.

Thanks a lot! I appreciate any help at all!
Merry Christmas, Happy Hannukah and Kwazy Kwanzaa

LithiumLord

Mon Dec 22 2008, 02:55AM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Prob1:
Your gap was working before because the sharp electrodes create a higher dE/dr, therefore you could use a higher gap distance. Now you have to put the electrodes closer. The short discharge at startup is caused by the NST's startup behaviour.

Prob 2:
You are right, if you connect your scope to the gen output and rif it all to your primary/MMC circuit, in resonance the impedance would be purely active, therefore you are shorting your gen and getting a dip on the peak voltage vs frequency curve. With the secondary base it gets the same.

Prob 3:
Actually I don't know how to help you out on this one - due to the lack of caps I kept working on VTTC and SSTC only. The only thing I can tell - corona loss is bad (well, kinda dumb, also obvious and useless - but that's all I can tell) - try looking for someone's bottlecap projects for a point or just start looking for generic caps (second one would be perfect in case you succeed).

Prob 4:
Well, I tried a steel troid on my smaller secondary for SSTC and didn't like it at all. However you could still try that out, in your case there's much less primary-to-toroid coupling, however I still don't like the idea of magnetic material in a coil. (Hehe I'm an old paranoid and yes, all my toroids are gapped :D) Better start off with a good old airhose - it's cheap, can be found of many sizes and densities - the best choice for a coil if you can't get a spun toroid I guess, also you can choose whatever radius you like.

Merry X-Mas to you as well!

Dylan

Mon Dec 22 2008, 05:11AM

Registered Member #1024 Joined: Sun Sept 23 2007, 10:56AM
Location: Northern NSW, Australia
Posts: 95

Hi, just some advice on an alternative to bottle caps.
Find a local picture framing shop and ask them to keep there failed glass plates for you (most of them throw away heaps of glass that the cut didn't go quite right on), get some alluminium flashing from the hardware store (or even start with kitchen alfoil) and make up something like this:

Then suspend the whole thing in a tub of Bio-diesel (an excellent dielectric) and cheeep.
This was how I started out with tesla coils before going solid state, the glass was free, the alluminium flashing was cheeep, the bio-diesel was cheeep, and the results were great!
There are numerous calculators on the web to work out the plate count and size.

BTW: All my early coils were DC (lots of 1n4007's), with either a static or rotary gap, and I used a simple (but BIG) flyback driver based on a tl494 and 4 parallel IRFP250's driving a 4-pak ignition coil from a ford V8 with outputs in series, worked GREAT! up to 1.5M strikes :)

Anyway, good luck with your first coil, no doubt it will not be your last

==== update ====
Heres a pick of my old rotary gap (model plane motor, chopping board and junk heater element for speed controll), and the 6 pack (not 4 pack) ignition coil I used , the driver has long since been re-cycled

LithiumLord

Mon Dec 22 2008, 03:48PM

Registered Member #1739 Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261

Oh by the way! Do not forget that among cap and spargap only one thing can be inefficient! For example, if you have a decent RSG or static gap with a very good airlow, but your cap has pretty high losses, you still may achieve a quite good level of performance because the quenching is pretty fast and not much energy is wasted in the cap bank. If your gap fails to quench fast enough, but your caps are good, you'll get a pretty low loss level again. But in case both the gap fails to quench fast and caps waste too much energy the things will get pretty bad - with the lack of fast quenching the energy is transferred into your cap bank and back many times per single bang, and the losses in the cap are high so a lot of initial energy is just wasted into heat! So in order to get a better performance you also have to either move onto an RSG or add a good airflow into your present sparkgap.

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