[TCML] First ever Pig SISG.... inadvertently

[FIFTYGUY@xxxxxxx]

Folks-
 
    FWIW, last night I became the first person to run a  SISG coil from a 
pole pig. 
 
    Before anybody gets *too* excited, it was just my  medium 6" coil, and I 
really hadn't started the evening with plans to use the  Pig at all.
 
    Yesterday I finished mounting the SISG4 PCB's I got  from Mark Dunn. All 
six boards, since I was planning on running at high voltage.  I was originally 
planning on running the SISG with my 12" pig coil, but I  figured I could 
give it a try on my 6" coil first. After all, I've never run  anything but 
spark-gap coils before, and if I was gonna fry all that silicon, I  wanted to at 
least see it work!
 
    I had been using a SRSG with this 6" coil, so I had  a 15/60 NST and a 
Maxwell 37667 cap (.030 uF). I hastily made up a bridge  rectifier, with 40 
diodes in series in each leg (UF4007, 1A 1000V, 75nS  fast-recovery). Added a 200 
Ohm 100W power resistor in each DC leg to limit  the peak current through the 
diodes if something went wrong.
 
    Well, I was mighty disappointed when I couldn't get  anything to happen. 
Only thing that happened as I cranked up the variac was  noticeable arcing on 
the brushes near the top end. No noise or sparks from  anywhere on the coil. 
But at least no smoke or tripped breakers.
 
    A quick test showed that the rectifier was in fact  working, as a nice DC 
arc set fire to an alligator clip gapped on the rectifier  output. Hooked 
everything back up and still nothing. This is where having  test equipment ready 
to go would have been very helpful... and one of several  times during the 
evening that I almost threw in the towel.
 
    I thought about it for a coupla minutes, and then  figured I might as 
well use the "bigger hammer" method of troubleshooting. After  all, it was a 
clear, cold night outside on the driveway, and if something's  gonna fail, I want 
some flames to show me where the problem is! So I  disconnected the NST and 
rolled out the Pig.
 
    As I was hooking up the pig, I realized I may have  inadvertently solved 
the problem. I suddenly recalled the importance of having  the cap tuned to 
the power supply with a SISG coil. But my .03uF LTR cap was  only workable with 
the 15/60 because I had been running a SRSG. I paused to  contemplate 
decreasing the cap size to work with the NST and SISG, but then I  would have had to 
take the time to increase the primary turns, and would've lost  bang size. I 
figured hooking up the pig was easier, so that's what I did.
 
    Well, as soon as I lit off the pig I at least got  *some* kind of 
response. The neighboring kids showed up, and were very helpful  in pointing out 
where the sparks/flames were coming from. For some reason  everything wanted to 
fail on the side of the coil facing *away* from me, and  they were on the "dark" 
side of the coil not illuminated by the garage lights so  they could see 
better.
 
    First off, the primary arced hot to the ground  terminals a couple inches 
below it. For a few minutes I thought it was  inter-turn arcing on the 
primary (which *really* confused me!) until the kids  managed to catch it and point 
it out. I re-spaced the primary away from this  area and that took care of 
that problem.
 
    The *next* problem was interesting.  The inner  turn of the primary arced 
*hot* to the bottom (ground) wire of the secondary. I  spaced the secondary 
up a coupla inches, and tried intervening insulators, to no  avail. I thought 
the primary was arcing to the secondary, but on a little more  reflection I 
realized that it was the secondary arcing to the primary! In  my haste I had 
intentionally neglected to hook up a grounding wire from an RF  ground to the 
ground terminal on the coil. I had never run this coil that way  before. I poked a 
10" piece of rebar into the wet ground, ran a #10 wire to it  four feet away, 
and problem solved! I never realized how important having *any*  ground 
reference was, but it was a night and day difference with this setup. 
 
    I finally got some streamers off the thing - all of  24", and somewhat 
brush-like. That's when I had a sigh of relief that at least  the SISG was 
actually working. But it was pretty anemic. The control panel for  the pig has 
ancient, low-tech meters - a moving vane AC ammeter and a voltmeter.  It was 
interesting to see the voltmeter climb as I cranked up the ganged variac,  until 
suddenly it hit the breakdown point of the SISG and the current went to  the 
ballast limit (60 Amps). 
 
    The Pig, by the way, is a 10kVA 14/4kV unit I got  from Resonance 
Research. So putting 15,000 watts into a coil and only getting a  24" brush discharge 
was pretty pathetic. This wasn't the first time I had run  the pig into this 
coil, though. A while back I had done almost the same thing,  trying to figure 
out why I couldn't get the calculated output from this coil  with the SRSG. 
It seemed to me that it wanted more power. So I gave it "more  cowbell" with 
the pig. The Pig had also hit the current limit on this occasion,  but with only 
50" sparks. The power resistors in the Terry filter glowed bright  orange, 
and cracked their ceramic cores when they cooled down. So I knew I was  putting 
a lot of power *into* the system, I just couldn't figure out where it  was 
going!
 
    Last night I got a hunch and changed the Maxwell  cap with another one I 
had gotten from the same source. But got the same  results. Both caps still 
read the correct capacitance value, and the coil tuned  with the primary taps 
where the calculations and the Terry Tuner said it should  be. So I added a .010 
uF MMC string in parallel to the Maxwell.
 
    After some retuning, *that* made a big difference!  The tuning point is 
surprisingly sharp with the SISG. At least, much sharper  than it was with the 
SRSG. I think that's because the SISG just will not fire  until the voltage is 
high enough, whereas the SRSG would allow lower-bang  discharges to get the 
streamer going, which then would add enough  topload to bring it into tune. 
 
    So I managed 60" strikes to ground off the  thing before the night was 
over. I have no idea where the BPS was, but it  seemed perfectly happy running 
almost eh same intensity from two breakout points  simultaneously. My 8x36" 
home-made toroid is so smooth, if I had no breakout  point it wouldn't do 
anything at all! Just faint corona off the strike  rail! One piece of aluminum foil 
tape, taped *flat* to the edge of the  toroid, suddenly allowed full-length 
sparks!
 
    No primary strikes, but many hot ground  strikes. I did run it for about 
half an hour total. Still pulled 60 Amps  through the Pig primary. I did have 
another little flame when one of the diode  strings in the rectifier arced 
over, but after some physical re-location and no  repairs the thing worked 
exactly the same. 
 
    After all that, the IGBT heatsinks (all 24 of  them!) were universally 
"barely warm". At least they were all doing their part.  All the caps were cold. 
But if I run it again tonight, I'm going to make the  entire primary cap an 
MMC.
 
    My first solid-state coil! I was very eager to see  streamers without the 
distraction of a spark gap. I was amazed at how loud the  arcs off the 
topload were! 
 
 
    ****Many thanks to Mark Dunn and Terry Fritz, for  their advice and 
patience!****
 
 
 Here's my SISG tips:
 
1. "Maxclips" for mounting the IGBTs work fine. I mounted the heatsinks to  
the board, then put compound on the IGBTs and heatsinks, then dropped the IGBTs 
 into place on the board, then screwed on the clip, then went back and 
soldered  the IGBTs in. I figured this way, everything was in a happy clamped 
position  before it got soldered solid. 
 
2. The heatsinks are electrically hot on the SISG4 boards, so I took the  
time to round off the corners to reduce any corona. Probably  unnecessary...
 
3. I used a 47 Ohm (DigiKey P47W-2BK 2W 2% metal film)  "charging"  resistor. 
I used a trim pot for the "turn-off" resistor (DigiKey 3296Y-502LF,  5k, 
one-turn, top-adjust). I measured all of them before I installed them, and  from 
the factory, they are all set to the "middle", at 2.222 k +/- 1 Ohm! Turned  
down to "20%" is about 400 Ohms. I ran them all at the factory setting of 2.222  
k.
 
4. I put everything together with a 140W soldering gun. I never understood  
the "use a low-wattage iron with heat-sensitive components" theory. I have more 
 luck blasting it quickly, before the heat has time to spread up the  lead...
 
5. "Balancing" resistors are 1% Metal Film (DigiKey 1.00MXBK 1/4W). I  
figured these should be as precise as possible, after all I'm running 24  sections 
in series... 
 
6. When I first went to order, DigiKey was out of the .001 uF caps.And the  
.0012 caps! But the next night, they suddenly had a big stock of the *new*  
.001's (DigiKey PF2104, the -JB RoHS style). So it looks like that little  
supply-chain hiccup has been resolved. 
 
    I'll let everybody know what happens when I run the  "standard" SISG with 
the "real" 12" pig coil. By my reckoning, the SISG  shouldn't last long!
 
-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities



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