AC Tesla Coil 

This is my most complete project to date.  I designed, fabricated, tested, and redesigned an AC Tesla Coil powered by neon sign transformers and operated with a forced-air static gap.  The coil is among the lightest and smallest coils in the hobbyist community that can produce 5 ft arcs.  See the "Component" pages for lots of good pictures, best practices, and test results.  See the Arc Pics page to see what this baby can do!

Construction of the components

My coil is on the right, Beth is in the cage


Add new pic of coil with spun AL toroid and stainless steel sphere topload
Random Thoughts about this design

Model 1, Rev B  (July 2000)

Revision consisted of major upgrades to 4 main coil components

  • Spark Gap was modified for the radial flow of air between the electrodes.  This aids in quenching and cooling.
  • Rolled LDPE caps were replaced with a MMC design (see my capacitor section).  The MMC design is much smaller, weighs about 50lbs less, and doesn't contain messy oil that inevitably leaks all over the place
  • A 110 lb bank of neon sign transformers (NSTs) was removed and the best Franceformer 12KV, 60ma unit was depotted and modified for more power (see my HV PSU section).  The enhanced NST produces 200ma.  Initial tests didn't manage to kill the unit even at this extreme power level.
  • I built a new stand that is really small and lightweight.

click for close-up of bottom end


Model 1, Rev A  (July 99)

Model 1A with the skirt and clean background.  

mod1A_ugly.jpg (104206 bytes)
 Model 1A the day of 
SoCal Teslathon '99
Model 1A typically ran using 3 of the homemade rolled LDPE caps, up to 4 NSTs, and sported a flat primary.  Model 1A could produce 60 inch arcs pretty regularly with the occasional arc out to 66 inches!  I typically ran with larger topload than shown in the pics for the big arcs.  The small toploads were good for making lots of smaller arcs.









Model 1, Original (Oct 1998)


This is my first coil.  I had never even seen a real coil in operation before I built this one.  I managed 32 inch arcs my very 1st night!

Don't build conical shaped primaries.  They will overcouple and are too close to the toroid so with any reasonable performance you coil will constantly strike itself in the primary (that's a bad thing).  Read the section on primaries for more info.










Random Thoughts (July 2001)

The goal of this project was to build the smallest, lightest coil that could produce frequent 5 ft arcs at the SoCal Teslathon 2000. Furthermore, everything except the power controller had to be attached to the coil and it had to be easily transportable.

Secondary is 6" dia SDR PVC wound with 24" of 22AWG.
Primary is .25" CU tubing on .5" centers
Spark gap is the TCBOR type with 7 pipes spaced at .030, total gap is 0.180
Capacitor is 8 strings of 11 caps each of "Terry's" MMC caps, total = 40 nF
Toroid is one of the OWL jobs, X" tube, X" O.D.
Power is supplied via a 12/60NST that was depotted and modified to run at 200+ ma


(1) Toroid Observation:
This toroid is a little too small for maximum spark length. I see 10-20% longer sparks when I add a larger toploads such as a 12" stainless sphere or (borrowed) larger spun AL toroid.
I find The chaotic motion of the arcs traveling around on the perfectly smooth spun toroid more satisfying than the arcs from my previous aluminum ducting.

(2) Gap Observation:
My TCBOR gap is being overpowered and I really need more gap for optimum performance. That said, the small gap may be the only thing that is saving my severly overpowered NST from certain death. I notice that my coil will run great for 30s to several minutes and then start to die down and then surge back to life. I think this is due to spark gap troubles. It seems odd that the pipes never seen to feel hot when I stick my finger in there after a long run.

(3) Bangs per second
I recently used my O-scope and a wire antenna to look at my coil during operation. I noticed that I get about 500-600 bangs per second. 

(4) Strike Rail Evaluation:
Tribal knowledge has it that one shouldn't let arcs from the secondary hit the primary for fear of killing capacitors or damaging one's transformer. This may be true for certain types of capacitors or transformers. That said, I've been running without a strike rail for about a year. I have VERY frequent primary strikes. I haven't noticed any drop in performance or loss of capacitance. Occasionally a strike to the primary will cause an arc between some of the bus work on the "bottom end" of the coil but I haven't had any damage. Has anyone killed a primary capacitor of the commercial type with a primary strike?

(5) Depotted NSTs:
It's best to run them in oil. For experimental reasons, and because I hate oily messes, I decided to run mine dry. I had a very hard time insulating the core so that I didn't get flashovers. My primaries and secondaries quickly get hot enought to sizzle spit. I've had the entire core at about 100C. The transformer does seem to mind. I usually run my NST at around 130-140V and the ammeter bounces from 25-30 amps. I don't have any PFC at the moment. I don't know the real power consumption for lack of a wattmeter.

(6) Protection circuitry:
I don't have any safety gaps. I have one 30KV, 500pf "doorknob" capacitor between each HV bushing of my NST and the core/RF ground. I have one 700 ohm, 150W resistor from each HV bushing of my NST and the spark gap. I haven't had any problems with killing capacitors or the depotted NST despite the aforementioned abuse. The 700 ohm resistors get super hot so I decided to short them out for a recent run at the TCBFW lab. There was no noticable increase in performance and there was no discernable damage to the coil's components. I don't know if I'm getting any protection from this RC filter or not. I would be comfortable building the next system without any protection.

(a) a better spark gap would increase performance
(b) a larger topload increases performance
(c) I'm not sold on the necessity of strike rails, safety gaps, or protection networks on a coil of this design. 

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�2000  Ross Oversteet