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Re 43" First light! - Dead Transformers :-(
>Original Poster: Tony Lekas <tony-at-lekas-dot-org>
>
>Here are the stats for our first coil. I have several questions.
>
>3-15kv 30 ma NSTs
>0.028 30KV Pulse Discharge Cap - Surplus
>Primary - 8 turns 1/4 fridge tubing, angled up at 15 degrees
>Ground - ~10' heavy copper clad steel rod, #4 stranded wire
>Secondary - 8.4" SDR35 Sewer Pipe, 45" long, 1500 turns #22, 1/4 wave freq
>77.5 Khz
>Corona Shield - 36" across, 7" cross section, flex heating duct and
>aluminium tape
>
>Spark Gap - Single gap set at ~1/2" Made of 1/8" brass fittings. There
>is a flat square plug with a small hole drilled in it facing a round
>plate. A 1/2 horse air compressor is kept busy blowing air through the
>small hole. Lots of force and flow.
>
>After tuning the primary and adjusting the spark gap we achieved 43"
>sparks to a grounded rod. This was with the base of the primary aligned
>with the base of the secondary.
>
>How much better performance should we expect to be able to get from a
>15KV/90ma supply?
>
>We then tried raising the primary to adjust coupling. We seemed to get
>better sparks but we got several strikes from the secondary to the
>primary. We also got one bright arc across the full 45" of the secondary
>from the ground connection at the bottom to where the toroid connected at
>the top.
>
>When we shut the coil down at the end of this session it was working
>fine. However the next day it would not work. It appears that at least
>2 of the NSTs were damaged. We did have a protection network in place
>but it was poorly constructed and "blew itself up". We had been
>operating without it when we got the primary/secondary strikes.
>Impatience :-(
>
>I am looking for suggestions for a good protection system for this coil.
>I will have 4 more NSTs available so I should be able to operate with 4-7
>(120-210ma) depending on how many I can repair. I am considering
>unpotting them and puting them in oil.
>
>I am also looking for general suggestions on solving the self strike
>problem and increasing the performance of the coil without destroying it.
>
>The gap appears to be working reasonably well. We had a scope hooked to
>a CB antenna. The gap was firing once per 1/2 cycle. There was so much
>noise on the scope that it was hard to tell how well it was quenching.
>We will be working on the instrumentation, maybe a current transformer on
>the primary. The coil performed much better with the air on so that
>seemed to help quenching.
>
> Tony
Hi Tony:
43" for first light is not bad at all, I think mine was only 14" or so.
As far as what to expect for a 15KV/90mA power supply, I use a 15/60 on a
4" coil and am now seeing 51".
The 0.5" gap seems slightly wide. I also use a single gap with massive
airflow, and have trouble starting up much above 0.36". As you know,
wider gaps produce correspondingly higher voltages and stresses for the
NST and caps
I have always used good safety gaps, a primary strike rail, and either
L-C-R or R-C low pass filters for protecting the NST, and I STILL blew my
15/60. I unpotted it, not a fun job, and repotted it in melted vasoline,
and haven't had a bit of trouble from it since. I think the asphalt used
to insulate it just doesn't hold up to Tesla Coil service.
I think a primary strike rail should be your first order of business,
followed by safety gaps if you don't already have them, and an R-C low
pass filter (R's in series from NST to gap, C's and safety gaps from NST
terminals to gnd). I don't recommend L's anymore.
The strike rail will further reduce your toroid-to-nearest-target
distance. Strikes may be avoided by using tacks or bumps on the toroid
to direct the streamers in the direction that you want them - outward,
although the toroid by itself should do a fair job of that by itself.
Could there be surface features on the underside of your toroid that are
directing streamers downward? It may also be that you were overcoupled,
suggested by the arc that traveled the length of your secondary.
Regards, Gary Lau
Waltham, MA USA