Specifications


Primary
     .25" copper tubing
     .25" edge-to-edge spacing between turns
     16.25 total turns
     Inner diameter = 6.5" (primary to secondary spacing of 1")
     Outside diameter = 22.5"
     Height from floor = 26.125"
 
Strike Rail
     .25" copper tubing
     Diameter = 24.625"
     Height from floor = 29.25"
 
Secondary
     Diameter = 4.5"
     Wound height = 28"
     H/D ratio = 6.21
     Wire = 22 AWG magnet wire, 1006 turns
     Measured inductance = 17.2 mH
     Measured DC resistance = 19.9 W
     Self-capacitance = 10.45 pF
     Height adjustable +/- 3" from primary plane
 
Topload
     Aluminum flexible duct
     Cord (minor) diameter = 4.5" (after covering with masking tape and aluminum tape)
     Outside (major) diameter = 22.5"
     Topload height (centerline) = 31.125" +/- 3" from primary plane
     Capacitance = 19.2 pF
 
Tank Capacitor
     MMC Design
     Cornell Dubilier .15 mF, 2000 V caps (Geek Caps)
     11 caps per string, 6 strings that can be arranged in several series/parallel configurations
     10 M, 3500 V, bleeder resistors across each cap
     All initial runs with a 12000 V 60mA NST were made with MMC set up for LTR at 20.4 nF.
     Subsequent runs using a 4-MOT supply used a reconfigured MMC set to 81.8 nF.
 
Power Supply
     12000 V, 60 mA neon sign transformer (initial runs) and a 4-MOT supply (~8800 V and ~300mA)
     138 mF power factor correction (NST) and 140 mF power factor correction (MOT)
     Power was controlled using a 0-140 V variac
 
Spark Gap
     Triggered spark gap design, vacuum fan quenched
 
Safety Gap
     .125 tungsten carbide drill blanks mounted in brass stock and brass drawer knobs
 
Filter
     Standard "Terry Fritz" design
 
 

Performance Notes

 
Before firing the coil for the first time, each component was tested and the specifications were run through E-Tesla 6 and JavaTC 9.1 to estimate resonant frequency and tap point. The estimated loaded resonant frequency was 223.71 KHz with a tap point at 8.87 turns. Measurements obtained using a signal generator, oscilloscope, and DMM indicated a resonant frequency of 230KHz.

I opted to use an LTR capacitance of 20.4 nF. On my MMC design, that required configuring the battery cable jumpers so that the two strings on each layer were in series, with the three layers then put in parallel.

The safety gap was set as recommended--to just not fire at top voltage input. The main gap was set at .375".

A breakout point was used for first light. It can be seen in this photo, taken just before the coil was fired for the first time.

The breakout point proved to be completely unnecessary and was soon removed. I began the tuning with the secondary elevated at 1" above the primary plane. The estimated optimal tap point (8.87 turns) was very close to actual optimal tap point. A few adjustments found the best performance at 8.62 turns. I then dropped the secondary down so that the first turn was level with the primary.

I ran the coil for a total of 2 hours and 45 minutes, with some sustained runs over 5 minutes. Maximum distance from a breakout point to a grounded rod was 44.5". Only one arc to the strike rail was noticed. Occasionally the last turn of the primary would arc to the strike rail. The trigger gap worked very well, with very little vacuum needed to quench. Adjustment of the trigger point indicated a definite "sweet spot." The smooth toroid promoted a small number of large streamers.

The fun ended when a run at full power (140 V input) was producing big sparks followed by that sudden and sickening silence. One side of the NST failed (open). It was old and of unknown prior use and abuse--probably a problem with the NST and not the safety set-up. The safety gaps fired occasionally as would be expected. I have a couple of other NSTs, but this experience has convinced me to explore sturdier power supplies. This part appears to be the weak link in this system. All other parts performed flawlessly and with no apparent stress. So, I've begun exploring MOTs as an alternative and some preliminary work has been quite promising.


The new MOT supply was put into service in December 2002 with spectacular results. The tank capacitor was set to 81.8 nF and the primary was tapped at 3.875 turns. A single MOT ballast was used on the primary side of the 4-MOT supply and two 1mF MOT caps in series were used on each secondary leg. The input voltage had to be kept around 90 V. Otherwise the streamers were hitting the ceiling too frequently for my taste. The streamers were noticeably thicker, longer, and louder compared to the NST set-up. Continuous runs over 20 minutes produced no degradation in performance and very little warming of the ballast MOT. The MMC caps did not warm up that I could notice. Like the performance for the NST-based set-up, the trigger gap had a definite sweet spot, but it was much closer to the middle of the available adjustment range. By comparison, the best adjustment for the NST design was near the very end of the available range.