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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Very nice secondary and primary . It does seem like the coupling might be a problem. Usually to get the right coupling the secondary bottom turn is about level with the top of the primary.
The resin block unit does look like a ss nst, but i can't be sure. The ss nst will work for a tesla coil, but you would have to rectify it, and the sparks might not be as long as with a 50/60hz supply.
The large 10/50 nst looks like a definite winner :D.
The capacitor should be around around 18-20nf for larger than resonant. The theoretical spark length for the nst is 38 inches. For the .15uf 2kvdc CDE capacitors, 2 strings of 15 in parallel would be about perfect. It would be 20nf and 30kvdc which is a 10kv margin. (just some FYI)
Registered Member #2038
Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60
The image of the primary and secondary you saw was before i had finalised mounting the two together, they were only just wedged together with little bits of wood. The base now has support for the secondary and it is adjutable in height. What effect does adjusting the height of the secondary have? at the moment the bottom coil is roughly in line with the centre of the primary winding.
the Coil is now complete & working, using the 10/50 nst, the tank cap is made of 3 strings of 10 0.1uf 1.5kv caps giving a total of 30nf 15kv.
i had hoped to fire it up yesterday afternoon with a quick & dirty wiring affair, with the pieces just sat out on the floor, but instead i decided my time would be better spent making the base and keeping the wiring as tidy as possible. I also set the spark gap yesterday afternoon before i connected the rest of the primary circuit, i was somewhat surprised that the gap was so small, Wintesla advised a gap of about 1.2" but i could not get the gap to fire at anything more than about 0.5" so i left it there.
I had a few cables to connect this morning; the HV supply from the nst to the spark gap (which i used RG58 cable) and some ground cables.
once i had driven a 3ft peice of 15mm copper tube in to the ground (i only managed to get about 1.5ft in as the ground around here is terrible, lots of really tough clay and loads of big chunks of building debris) i ran a length of RG58 coax with both the core and shield connected together from the ground point to the base of my secondary & primary.
I'm a little unsure about grounding the NST body, if i understand the wiring of my NST the center tap of the secondary winding is also grounded to the body of the transfomer, so if i were to fix this to ground i would efectively be shorting one half of the secondary, if i remove the link between the center tap and the body and switch on the power there is a lot of arcing somewhere... see the last image for evidence (the big patch on the bottom)! I think the screw that holds the NST support rail was touching the body of the NST and shorting out to ground. anyway for now i have left the nst ungrounded.
i set up a simple 25A RCD to use as a power switch, and switched on the power. nothing much happened, on closer inspection i noticed that there was some arcing from the shield of the RG58 to the wooden base, so i removed the sheilded section and that seemed to cure the problem, but the coil still did not fire. I then noticed that one of the terminals of the MMC was arcing to the wooden base, i snipped off the cable ties that were holding the MMC down and just used a couple off cuts of wood carefully placed away from the live parts, powered on an no more arcing, but the coil still did not fire, through all of this the spark gap did not even hint at firing, i disconected the primary circuit from the spark gap and powered up again, nothing! i moved the spark gap in a little and it worked, re connected everything and powered up... nothing! i i repeated this several times until the spark gap was at about 1/4"... The spark gap fired nicely, and i got about a 1" streamer to a grounded metal rod.
after a little bit of playing around with the primary tap point i managed to get the streamer steady at about 5-6" but i cant seem to get any more out of it. I'm thinking somthing isnt quite right, can anyone suggest what i might try next?
I'm not expecting big streamers but i think i should be able to get quite a bit more than this (WinTesla predicts about 20" but i'm not seeing anything like this)
Here are a few pictures of the coil taken this evening after my inital testing:
Complete SGTC assembly.
Primary Coil.
Top Load.
MMC side view.
Spark Gap side view.
The importance of good insulation!!
I think i have isolated most of the causes of arcing through the wood now, but before long i will give all the wooden parts a liberal coating of polyeurethene varnish to give better insulation.
Unfortunatly its forecast to rain all night and I have nowhere indoors that is suitable for running the coil so i'll have to wait for tomorrow night before i can get any pictures of it all in action.
The resinbock NST actually is a FART branded unit, i realised this when i removed the stickers to clean it up. yesterday when looking at various TC websites i found someone using two of the very same model so i think it will be usable although the 10/50 is my favorite at the moment.
Registered Member #1911
Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165
So, is the ResinBlock NST nearly as tall as the other? Is the body the same type? Also, does the ResinBlock have some kind of switch on the outside of it; maybe it has a pull-string?
Congrats on the coil! I'd love to see some pictures.
Registered Member #2038
Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60
Firstly sorry for any confusion caused by the two threads, the second thread has now been deleted and its contents is now in this thread. I wil now keep all discussion of this TC in this thread.
Plasma Lover wrote ...
So, is the ResinBlock NST nearly as tall as the other? Is the body the same type? Also, does the ResinBlock have some kind of switch on the outside of it; maybe it has a pull-string?
Congrats on the coil! I'd love to see some pictures.
The Resinblock NST is quite different in apearence to the Tunewell, for a strart it is longer and thinner than the more box shaped tunewell, it is also (as the name suggests) made of resin, there is only a metal bracket for fixing atached to it. It has no switches or pullcord atached to it.
I've now posted images of the completed coil in my previous post.
In the other (now deleted) thread Diarrg Wrote ...
The height of the primary in reagrds to the secondary affects coupling, or the measure of the intersection of the magnetic fields of the two coils. The more it intersects, the fewer pulses it needs to transfer all the energy between the primary and secondary. Low coupling will decrease efficiency, while too-high coupling will result in arcing between the primary and secondary. Try punching your values into JavaTC If you set it to adjust the coil for you, it should give you a good height for the primary. I know that raising the primary on mine by a quarter inch increased sparks by a good 3 inches (a 30% increase). will give you more information on coupling and k.
Thanks for that info Diarrg I will download JavaTC and see what it comes up with, hopefully the bad weather will hold off today, there are a few showers forecast but at the moment there is plenty of blue sky!
I'm now begining to wonder if i am losing a lot of power through the wooden parts of the spark gap, as can be seen in the last image i posted there is a certain amount of burning evident between the screws that hold the spark gap to the base. I'm hoping to get some nice ceramic insulators to mount the spark gap on, but it will have to wait for now as i'm skint!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I'm a little unsure about grounding the NST body, if i understand the wiring of my NST the center tap of the secondary winding is also grounded to the body of the transfomer, so if i were to fix this to ground i would efectively be shorting one half of the secondary, if i remove the link between the center tap and the body and switch on the power there is a lot of arcing somewhere
The centre tap of an NST must be grounded. In a 10kV unit, each leg is only good for 5kV or so from ground, and the 10kV is available balanced about ground (not floating) between the two legs. If you removed the centre-ground link, then one leg could easily swing to 10kV, depending on what load is connected to the other one, giving you "a lot of arcing somewhere", not good. If you review your primary schematic, you should find that you only need to ground the NST centre tap. The primary coil, tank cap and spark gap should all be floating with respect to ground.
Registered Member #2038
Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60
Dr. Slack wrote ...
The centre tap of an NST must be grounded. In a 10kV unit, each leg is only good for 5kV or so from ground, and the 10kV is available balanced about ground (not floating) between the two legs. If you removed the centre-ground link, then one leg could easily swing to 10kV, depending on what load is connected to the other one, giving you "a lot of arcing somewhere", not good. If you review your primary schematic, you should find that you only need to ground the NST centre tap. The primary coil, tank cap and spark gap should all be floating with respect to ground.
Ok at the moment i have my coil wired like this:
I borrowed the image from gary lau's site and modified it slightly to show my setup, please ignore the vlaues of the components as they are not relevant to my setup, i also did not mark on the center tap of the NST secondary.
Are you suggesting wiring like this:
Could this possibly be the reason that i am only able to get 6-8" sparks? I've tried just about every turn on the primary turn 4 gives the longest spark, once settled on turn 4 i tried adjusting the height of the secondary coil and this made very little difference.
unfortunatly i dont have a scope to perform a more detailed tuning so everything i do must be by trial and error.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Are you suggesting wiring like this:
Yes, take the max voltage between the two hots, protect the NST by grounding the CT, make everything else work by floating the rest of the primary. Safety gap each hot of the NST to ground.
Your first schematic shows only two terminals, and the NST has 3. Every way of using it like this is bad, some worse than others. Either the NST CT is unconnected and and a hot is connected to ground, causing overvoltage on the other hot, or the CT is grounded and one hot is unused, which is safe, but you are only getting half the possible voltage out of the NST by using only one hot. There's a third possibility of grounding the CT and one hot, this is now running the NST in current limiting, which will mean even lower voltage from the other hot.
Registered Member #2038
Joined: Fri Mar 20 2009, 03:39PM
Location: Bristol Uk
Posts: 60
Cool, well i've altered the wiring now so that the CT and secondary base are grounded and have left the primary floating as you suggest, i then found that my spark gap would fire at a wider seperation (with primary disconected) once it was all hooked back up i found that i had gained at least another couple of inches of spark length.
I havent sorted out a safety gap yet, i'm totaly out of money at the moment and cant find the materials i need at home. I had intended to install a terry filter and safety gap at some stage, i'll now look at doing that as soon as possible.
I'm going to wait untill i can opperate the coil in dark conditions so i can see the dimmer sparks and look for corona and other potential losses and problems that arent visible in daylight.
the last time i ran the coil i also noticed a small spark on the secondary about 2/3 of the way up, i didnt see a streamer to the point, but on inspection i noticed a small burn in the insulation varnish, i think i might give it a couple more coats in the next few days, i dont want to kill my secondary after spending all that time hand winding it!
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
i'm totaly out of money at the moment and cant find the materials i need at home
I'll bet you could find the materials in a bin at the back of the local dry-cleaners, if not in the back of your wardrobe. A bit of bent coat-hanger is enough for a safety gap. If you look up at the lightning protection gaps on overhead power wires, they look like bent wire, albeit a heavier gauge than you'd hang your coat on. It's a good thing to have before you get the tuning right and start flinging really long sparks from your secondary.
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First Tesla Coil
. It does seem like the coupling might be a problem. Usually to get the right coupling the secondary bottom turn is about level with the top of the primary. 
