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Re: [TCML] Pig SISG: Destroying the Indestructible
The center IGBT may have run into a problem, but I think it's pretty
cool the rest of the circuit didn't care! The word "robust" comes to
mind regarding the SISG.
The metal sheet arcing. Obviously a high enough impedance existed to
develop a voltage capable of arcing through the pvc insulation to the
sheet. There are some significant peak currents at rf frequencies. I
expect the peak currents developed high enough peak voltages along the
length of wire, and the metal sheet contributed a little to the
potential difference as well. I would think if the sheet were connected
electrically to the ground point that you would not have seen this (but
then, it may have made an unwanted target). Maybe replace with an
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
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!
Center for the Advanced Study of Ballistic Improbabilities
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