Design Notes

( Modifications, Experimentation and Headaches )

4 inch coil running outdoors


I went through a lot getting this coil to work the way it does.  Although things were

quite disappointing at times, I don't regret it because it was a great learning experience.

On this page, I'm going to attempt to explain the problems I ran into in sequence and the

many different things I tried to progress forward.


At first, I was getting nothing from the coil at all.  I put a breakpoint on the toroid. The toroid

I was using at first was made from two 9 inch pizza pans bolted together surrounded by 4 inch

aluminum flex duct. The gaps in the flex duct were filled with Spackle and the whole thing covered

with aluminum tape.  With a breakpoint, I was getting some corona, but even with the tedious tuning

process, I was seeing no improvement.  Using a multimeter, I discovered the aluminum tape I was

using didn't seem to conduct electricity very well. I decided to build another toroid without going

through the hassle of smoothing the surface.


I made a toroid from two 8 inch circles of 1/4 inch plywood.  I covered them with aluminum foil

using spray on glue and surrounded it with a 4 inch aluminum flex duct. Now with the breakpoint,

I was getting 4-5 inch arcs.  I was quite disappointed when no sparks were thrown out after removing

the breakpoint.


I thought about a possible coupling problem.  I figured I'd grind the bottom of the PVC to bring the first

windings more level with the primary coil.  I noticed a huge arc burn on the inside wall of the 4 inch PVC.

Originally, I ran the ground of the secondary through the inside for cosmetic purposes.  The connection to

the heavier gauge wire was also made there.  That connection was made inside the tube, above the table,

instead of below it.  It never occurred to me that the output from the top of the coil would find it's closest

path to ground through the inside of the secondary.  I tried grinding away the carbon mess the arc created,

but after doing so when no performance improvements were noticed, I wound another secondary coil.


With the new secondary, I was getting 10-12 inch arcs with and without a breakpoint.  Additional

tuning made no improvements.


Now I thought perhaps the toroid is too large.  I made new ones with 6 inch centers and 4 inch centers.

Still no improvement.


My original sucker gap was made from copper tubing instead of the PVC I currently use.  The thought was

maybe the copper was causing unwanted resistance.  I rebuilt the spark gap using PVC tubes and also used

brass reducing couplers the made the openings even smaller. 


The modified spark gap actually made an improvement.  I was now getting 19-21 inch arcs with a breakpoint!

But, no matter what else I tried during the tuning process, no improvement was made.


I then tried a tank cap MMC that was closer to the resonant value of the NST.  After attaching an 8.3nF

cap and retuning, things were worse.


I decided to try a bigger MMC.  I figured I had nothing to lose at this point.  I installed a 15nF MMC and

KERAAAAACK.  A nice 34 inch arc!!!


I tuned out to turn 11 on the primary and achieved a 42 inch arc!!! Now we're getting somewhere.


I noticed that even with a breakpoint, arcs were coming out of the toroid in all directions.  The toroid is

too small.  I used two 12 inch deep dish pizza pans bolted together and surrounded it with 4 inch aluminum

flex duct.  Using this toroid resulted in arcs 44 inches long, but there were still stray arcs.


I decided to go a bit crazy and build a giant topload.  The center disk is 24 inches in diameter, surrounded

by 4 inch aluminum flex duct.  We are now producing 50 inch arcs!  But, we still have the stray streamers.


The giant toroid worked great, but didn't look quite right because of it's size.  I made another toroid using

two 12 inch deep dish pizza pans bolted together, but this time, I used 6 inch aluminum flex duct.  This

toroid produced 55 inch arcs.


I then added 2 more 12kV NST's.  I played around with the spacing on the sucker gap.  Each time I widened

the sucker gap, I noticed the NST protection gaps would fire more often.  I widened the protection gaps and

finally found a nice balance between all gaps.


My 4 inch Tesla Coil was now producing 60 inch arcs!  At this point, I considered this project complete.