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[TCML] Pig SISG: Destroying the Indestructible

    Last night I rolled out the Pig SISG again.
    In the interim I had to repair the secondary. All  the screwing around I 
did on the last run had resulted in several strikes from  various point on the 
top half of the secondary to the strike rail. Removing the  strike rail 
solved that problem, but those strikes were very damaging to the  secondary. At 
each point on the secondary I had to remove ten turns of the # 25  wire. Quite a 
"banded" appearance to the 6" secondary now! But it started with  1500 turns, 
so I've got wire to spare. ;) 
    At one point the secondary coating (Dolphs AC-43)  had caught fire, but 
the soot was easily removed with thinner. And at every  damaged point the 
strikes had melted the copper, thus shorting three or four  turns together. So 
towards the end of the last time I ran it, it probably wasn't  as efficient as it 
should've been!
    Last night everything ran pretty darn well! I  remembered to put up the 
heavy sheet steel vertical guard under the primary on  the same side as the 
toroid's breakout point. Previously, arcs from the toroid  had reached under the 
primary and hit something (the primary caps or wiring to  them), and had 
repeatedly vaporized some of the diodes in the DC supply  rectifier bridge. This 
time I had no diode failures, even though I kept the  current setup with two of 
the bridge legs still composed of the remaining  seriesed UF4007's.
    My impression of the coil behavior was that it  needed to run a bit to 
"find its tune". For the first 20 seconds, it would act a  little erratic, with 
shorter streamers that were brief in  duration. The streamer sound was 
reminiscent of lightning, with a  "k-k-k-kik-VRP!"  As it went on, it gradually 
coalesced into a very  steady metallic steady tone, with the streamers much longer 
and lasting  significantly longer. I was running three breakout points, and it 
seemed to  pick the points it grew a "main" streamer from at random. 
Nonetheless it  simultaneously grew "half-length" streamers out of the other breakout 
    There was also complete randomness to the direction  any of the streamers 
would go. It was definitely "wild" behavior! Most of the  time it completely 
ignored the aluminum ladder with the copper tubing strike  target set up at 
toroid level 50" away, only to strike to the ground at the  bottom of the 
ladder, or to go sideways in the air or to the much more distant  holly bush. This 
despite the fact that the ladder was in the grass on wet soil,  *and* had a # 
10 stranded copper wire tying it to the bottom of the secondary,  *and* I had 
put a 1' long piece of rebar in the ground between the coil and the  ladder and 
tied _it_ to the same RF ground point with another piece of # 10  copper! 
    Now, to the point of this post: After a few minutes  worth of operation, 
in the middle of a run, I saw part of the SISG assembly  flame up (more like 
"torch" up!), complete with little a fountain of burning  sparks (hot metal). 
The coil kept running fine while I kept it going for a  couple more seconds. 
The flames went out when I stopped. Since it was dark and I  was lazy, I started 
the coil again so I could see where the problem was. Sure  enough, the flames 
shot up immediately. This time we were able to see the  problem.
    One of the SIDACs bit the dust. 2nd board from the  "minus" side, which 
was the 5th section from the "minus" side, the middle SIDAC  in the string of 
three in that section. The case was split open along the plane  of the tab. In 
fact, I pulled the metal tab out like a loose tooth. The  heatsinks for the 
IGBTs were merely warm. I took a piece of aluminum tape and  jumpered over the 
SIDACs on either side of the bad one. When I started the coil  again, there was 
a little bit of flame for a few seconds in the same area, but  it quickly 
extinguished. I ran the coil for a few more minutes with no more  problems, and 
no discernable decrease in performance. 
    My *guess* is that the poor SIDAC suffered a direct  secondary strike. I 
really hope that was the case, otherwise the SIDACs might  not be as 
"indestructible" as we thought! But on the bright side: There's no  visible damage to 
the rest of the entire SISG assembly, and obviously it kept  right on working 
fine - even when part of it was on fire! After the "repair", I  put another 
piece of heavy sheet steel under the primary to protect the SISG  side.
    Now, WRT the discussion about grounding conductors  for our RF ground: 
When I resumed running the coil with the steel sheet  protecting the SISG, the 
two #10 copper wires that ran to the ladder and the  rebar in the ground were 
draped over the top edge of the steel sheet. However,  we noticed that in 
operation we were getting bright sparks from the two wires to  the top of the steel 
sheet! The wires are THHN, so they have 00V rated PVC  insulation with a  
protective nylon jacket. The bottom edge of the .060"  thick steel sheet was 
resting directly on the concrete driveway. I think  it's very interesting that 
either a voltage was induced in the steel sheet that  wasn't grounded by its 
bottom edge being in contact with the driveway, *or* the  impedance of the few 
feet of the two copper wires to paralleled to  grounding points was so high that 
a significant voltage existed on them  near the RF ground tie point next to 
the grounded sheet steel! Either way,  something wasn't grounded very well!
    Any thoughts? 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities

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