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Re: [TCML] Pig SISG: Destroying the Indestructible
In a message dated 3/22/08 8:09:12 P.M. Eastern Standard Time,
>I have purchased from Mark Dunn many of the parts for a 20 stage SISG
Keep up the good work, Mark!
>to run on a smaller TC
It seems to me the SISG seems to work well in a certain "middle-sized"
range of coils with the current cost-effective IGBTs available.
>but also later on my 18 inch coil that runs 2 PT's. So it will be a bit
I'd think more than a "bit"!
>Hence I am interested in the results of higher powers.
OK, I'll try to make a concerted "scientific" effort here.
>Are you sure that stage still works?
No idea yet, but as I have to make a repair to the board, I'll check it
as I go. Keep pestering me if I don't report back.
>The IGBT would have been over volted and destroyed if the voltage across
the SIDACs went higher >than 1200V.
I'd think so. On the other hand, the IGBT appears to be intact. Most things
in the path of the primary current, especially electronics, tend to fail
spectacularly. Plus, I think when the middle SIDAC in that section went, it was
effectively a short (much soot, flame, and carbonized remains). Shorted SIDACs
are no problem, since they *drop* the section firing voltage. An open SIDAC
might be a problem, since it means that section will never fire (although I
wonder if the 1 Meg balance resistor across an open SIDAC would screw things
up). If the other sections fire, the non-firing section might become
over-volted and fail?
Now that I took a closer look, not only did that one SIDAC fail, but the
24V 1500W TVS (D31 in Mark's schematic) failed as well. The case is split
axially from one lead to the other, with a bit of scorching evident as well. I
can only assume it failed as short as well: since the coil continued to run
just fine, with no other spectacular failures, the primary current was still
flowing through that stage. Unless the IGBT (or body diode) was shorted with
no externally visible damage, the only primary current path around the TVS
would have been through the 1N5819 diode (D32) and then through a number of
smaller components that should have been instantly blasted by that current.
Everything else in the entire SISG array except the SIDAC and the 24V
TVS looks just fine. FWIW, the connecting lead between SISG boards would be an
attractive point for arcs to hit under the primary, and that TVS has one lead
on the same trace as that jumper wire connects to. I'm really starting to
think a direct hit from the topload was to blame.
>I don't recall the details of your coil. Could you give me the basic
>and any info you have on spark length and power input?
Sure! And I'll bet Bart will enjoy doing his calculations on these specs
as well. ;)
Power source: 240VAC (measured) residential single phase (120VAC to
neutral/ground each leg)
Line filter: 100A single phase commercial unit
Ballast: handmade, open "E" core of laminated transformer steel,
quadrafiliar wound 12 AWG, limits line current on Pig to 60A max
Voltage control: four ganged 30A variacs, two per leg in parallel, no
balancing chokes, but calibrated to all run within 1V of each other. Allows max
output of 280V from 240V line.
Ammeter: old moving vane type, direct in line
Voltmeter: old, moving coil, calibrated
Transformer: 10kVA 14.4kV / 240V Wagner pole pig
Rectifier: full bridge, two legs still 40 seriesed UF4007 fast-recovery
1kV diodes, other two legs "bar" rectifiers unknown specs (20kV 1 Amp
fast-recovery?) but very heavy duty.
No DC filtering, charging chokes
Each DC leg has a 200 Ohm 100W wirewound power resistor in series
immediately after the bridge to protect the rectifiers against current surges.
These get merely warm on runs of a few minutes.
Primary cap: two Maxwells in parallel. One is a 31976, .075uF, 40kV;
other is the ubiquitous 37667, .030uF type. So total .105uF
Primary: shallow inverted cone, 7" ID, .375" OD copper tubing, 3/4"
spacing on center, currently tuned at 9-1/2 turns (so now 7 unused outer turns!)
Primary inner (bottom) turn 1-1/2" below bottom turn of secondary
Gap: SISG, six of Mark Dunn's SISG4 boards in series, nominal 21600V
breakover voltage. For (Rx5), I'm using 5kOhm trim pots, DigiKey #CT6EP502, left
at their factory center position settings (2.222kOhm +/-1 Ohm). For (Rx4)
I'm using the 47 Ohm 2W 2% metal film resistors, DigiKey #P47W-2BK. For what
it's worth, I'm using 1% metal films for (Rx1,2,3), DigiKey #1.00MXBK Using
AAVID Maxclips to mount the IGBTs to the heatsinks.
Secondary form: 6.25" OD thinwall PVC sewer pipe
Secondary winding: 25 AWG, .019" wire OD with insulation, 30.5" winding
length. Deduct 1-1/4" of winding length from the top half due to repairs.
Corona ring at top: 3" minor diameter, 12" major, centerline 2" above
top winding of secondary
Main toroid: 8" minor diameter, 36" major diameter, centerline 9" above
centerline of corona ring
Hope this helps!
Center for the Advanced Study of Ballistic Improbabilities
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