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Re: [TCML] Pig SISG: Destroying the Indestructible



Hi Phil,

Interesting run!

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 acrylic sheet.

Take care,
Bart

FIFTYGUY@xxxxxxx wrote:
Folks-
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 points. 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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