My very first Tesla Coil (SGTC) / Projects / Forums

Forums

4hv.org :: Forums :: Projects

« Previous topic | Next topic »

My very first Tesla Coil (SGTC)

Move Thread

LAN_403

Sigurthr

Sat Apr 21 2012, 04:38AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

I've been wanting to make a TC since I was a young child (about 1sec after I first saw one). I've finally done it, and it is awesome.

I used a pre-wound secondary, preassembled spark gap, and preassembled coilform assembly from eBay seller Kr_Scott because I am short on tools, space, and funds. I basically rebuilt it from the ground up though, the original primary and capacitor were removed and replaced.

Last week I won the eBay auction from Kr_Scott and ordered all my other parts online. Today, my Coils, Spark Gap, and discharge wand came in the mail, it was time to build! I spent all of my free time building and rebuilding this TC, and I am VERY pleased with the result.

At first I tried it out as it came - as the seller intended it to be... meh. Total meh. 1" corona, no sparks, no streamers, disappointing. This was expected though as the capacitor included was a 1.6nF 20kV ceramic cap inside some PVC tubing. I couldn't find a single NST or OBIT that would be resonant or LTR with that value. I think it is the seller's method of making the TC "safe" and getting around the requirement for a specific NST value.

I removed the stock capacitor and wired up my MMC which is specially designed for my NST. I decided to go with a bleeder resistor after all, once I tested performance with and without to make sure there was no detriment. It is a 20kV 6.5MOhm 5W non-inductive resistor, and I placed it right across the high current buss from the MMC.

I decided not to fully wire in the NST and instead leave the leads flying and use jumpers to connect the power. This forms Safety Disconnect #1. If someone were to walk by and plug it in (won't happen but just IF) nothing would happen, no arcs, no corona, no HV, no caps charging at all.

I began by modeling the Tesla Coil with TeslaMap and JavaTC to see what needed to be done to the heart of the coil. I rewound the primary, using the wire supplied to the specs that TeslaMap and JavaTC suggested; 4.5 turns.

The computer modeling also suggested a topload to bring down the resonant frequency, so I made one. I made it 4" sphere, just as the modeling had suggested, and to my surprise... no real measureable increase in output. It was totally unexpected, but I kept on working.

I then removed the new topload and scrapped my models and remodeled it from the ground up. The new primary would be 2.24 turns and a break out point would be needed (streamers/sparks increase capacitance) to compensate. Done. Hey! the coil is starting to look pretty damn good! I managed 2.5" to 3" corona and sparks to air.

Now for fine tuning. I didn't really know where the issue was, so I started at the spark gap. I set and re-set it often but always being careful to make sure the gap fired without fail with the primary circuit removed - I was NOT going to chance a resonant rise blowing out my MMC! Modeling doesn't realy help in this area, it can tell you the maximum safe gap to test but it can't account for atmospherics, air currents, thermal effects on the gap, etc. I also played around with quenching, but found it to be of more detriment than anything. Cooling the electrodes did help extend run times and keep power levels up though. In the end I set my electrodes to fire at approximately 98% open circuit voltage (with primary circuit removed) and went with either a very gentle breeze for long run times or gentle suction to pull ionized gas out for shorter run times (this gives the best performance when thermal electrode issues are not a concern).

Now I should have been seeing better results than I was at this point, but I figured it was just inexperience and too high of expectations. I started looking for subtle changes between the model and the real thing and I discovered that I had neglected inputting the Primary Lead lengths. I had nearly 26 inches of combined lead length, all the same gauge as the primary. Stray Inductance galore!

Now one thing I noticed also was that the primary got very hot after only 30sec of run time, and the performance suffered. This was the primary wire which came with it. It turned out to be ~14AWG Stranded STEEL wire, the kind they use for bicycle brake lines! NO WONDER!

I replaced the primary with 12AWG solid copper wire (it took every bit I had) and I had to make another jumper for the HV side of the Primary Lead out of 22AWG copper wire + aligator clip (just like I did for the LV side of the Primary Lead / Safey Disconnect #2). This turned out to be a very wise choice, both the 22awg jumper, and the new primary that is.

For the first time I saw actualy STREAMERS! Wiggly beautiful streamers! But wait... what is that... Racing Sparks????!!! NOOOOOOOO....

Coupling coefficient was too high. Two ways to fix this;
1) change to a flat sprial coil.
2) change the intercoil geometry (spacing/diameter).

I went with the second option. I figured, why not see if it can be a simple fix? I pushed the primary down to against the wooden base, the lowest 3/4" is completely below the secondary's first turns, it may be enough, at least try it, right? IT WORKED! Now the primary is 3/4" tall, from 0" to 0.75", and the secondary starts at 1" and goes to 7.5" in height above base. Racing sparks completely gone AND now the streamers are more persistant and LONGER! Ooooooo dancing lightning goodness.

End Result:
Operating Frequency (Fo): 1.1745MHz
Voltage peak @ Topload: 741,000V
Max Streamer Length: 4.5"
Skin Depth at Fo: 2.71mils
Coupling Coefficient: 0.114 (0.124 Recommended) [VERY Close to Ideal]
Primary Peak Current: 1518.9AMPS

-New primary runs cold for even 5 minutes.
-Spark gap does not need quenching, but does benefit from gas removal and cooling.
-NST runs cool and shows no signs of stress.
-MMC is Larger than Resonant, Bleeder Equipped, and only runs at 66% the rated voltage. No detectable temperature increase.
-Secondary runs cool. Interturn capacitance of secondary causes it to retain a charge well after disconnected and shorted. Damn thing makes for a painful but harmless shock.
-The electric field from the TC has doubled if not tripled in size in addition to the increase in streamer length. Now fluorescent tubes light from several feet away.

Pic:

Video:

Sulaiman

Sat Apr 21 2012, 06:35AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Nice first TC, congratulations.

However .....
with that NST you could get 1ft. or more sparks with changes.....
- the spark gap blower needs to be more intense
- use a topload, start with your 4' sphere, even larger would be better
(any conductive object, cooking pots, beer cans ......)
- you will need more turns to match the lower resonant frequency due to the larger topload, multi-strand pvc insulated house wiring cable is ok.
Coper pipe/tubing (e.g. eBay item 180837410862) would allow a flat spiral with easy tuning.
- ideally use JavaTC or similar.
The predicted number of turns is a good start BUT fine tuning is required
- you will know when it's tuned !

Sigurthr

Sat Apr 21 2012, 07:54AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Thanks!

Yeah JavaTC claims I should see up to 22" streamers with this NST. I've been thinking that I'll just make a new tesla coil with a much larger secondary and make a nice dryer duct torroid topload (or snag a big sphere from eBay). The MMC is set to the NST, and the spark gap is universal, so I'll re-use them. Most of the expense is the MMC anyway, especially if I make the secondary instead of buying it. Though I really like the idea of having EasternVoltageResearch wind me a secondary, their prices aren't bad considering wood is expensive out here and I can't get PVC locally. The plywood I bought for this TC cost me $30.

I completely agree about the primary, I'm definitely going to go with copper tubing and use a moveable tap. Trying to tune the primary by snipping wire and unwinding incrementally has been a lot of trouble. This coilform assembly is epoxied together, so I can't even remove the primary form if I wanted to. I can't take the secondary off the base it is on as a result of that as well. I started out with full intent to use a flat spiral when I was planning the purchase and when I received the TC I had to alter plans.

I don't really want to try and alter things too much or put too much work/money into such a tiny secondary that I can't even take off the base. Besides, everything I've read says 510 turns is no where near enough for proper operation (they all say 1000-1500). Adding a spiffy primary and huge topload kinda seems like putting Nitrous and a turbo on a lawnmower powered go cart.

Is there any "rule of thumb" size you would recommend for a sure-fire medium size (less than several feet long) coil? Secondary coilform diameter and winding length would be sufficient, I can figure the rest out from modeling programs. I have no basis for comparisson to tell what to aim for. I was thinking 3.1" dia, 15" winding, 30AWG. That would net nearly 1250 turns, 4.8:1 H:W ratio, could use either an 8" or 10" sphere, and the primary would take about 25ft of tubing. Sound like a decent plan?

Sulaiman

Sat Apr 21 2012, 12:33PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

The size of your secondary should be ok for your NST
larger may be better but it's not necessary.
If/when you get your TC optimised
a long spark from a small coil is much more impressive than
a slightly longer spark from a much bigger coil

Where possible use plastic rather than wood, chopping boards etc.
Eventually due to humidity wood starts to form carbonised tracks.

I would advise low cost approaches to making early Tesla Coils
(your NST, MMC and Secondary are an excellent start)
as you are in the 'learning phase',
and it's amazing how many things there are to learn in a circuit with only

NST,
Spark Gap,
Primary Capacitor (specifications?)
Primary Inductor
Secondary Inductor (specifications?)
Topload

Just six components ... but MANY variables.

Sigurthr

Tue Apr 24 2012, 01:49AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Thanks for all the info and replies, Sulaiman!

Aye, It was not my choice to use wood for the base that the coilforms are on, they came that way. I also would not have epoxied them in place so close to the spark gap.

Just an update: I was finally able to source some 1/4" soft copper tubing locally and grabbed up as much as I could afford (60feet! cost me $72 after tax). I also ordered a nice 3200' HPN enameled 30ga copper magnet wire ($25). Now just to make some more calls to see if I can't track down some PVC pipe and a sheet of plexiglass for the new primary support.

With the new primary and secondary I should be able to truly fine tune everything and get the most out of it. If I decide to move up to a larger 9k/30ma NST later I can just add 6 more 942C20S40k-F caps to go from a 8x3 to a 10x3 which will give me close to the ideal 1.618 x resonant capacitance.

Any tips for making the spark gap a bit quieter or diverting the sound elsewhere? Can't hear myself think with it on, haha.

Forty

Tue Apr 24 2012, 04:40PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

You could enclose the gap except for the air flow in and out. Several inches of pvc pipe on the air in and out might also help to confine it a little better. If that's not enough, you could enclose the enclosure so that you've got two walls with an air gap in between. Should help deaden the sound and at least protect your eyes from the flashes.

The heating and quenching problems (as well as hv insulation) make spark gaps a tricky thing to silence I imagine.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob