Hello,

Thanks for the replies and your ideas. I ended up doubling the snub capacitance by placing two more caps on the bus, as seen here: https://drive.google.com/file/d/1AZ76udfhaEa-8PIddRayiyQVBYZEMVU7/view?usp=sharing. You can also see the probe connections for the following scope shot:
https://drive.google.com/file/d/1I0mnVANob44GsXKjn4vJSQVXD2BWrxFR/view?usp=sharing

That seems to have taken care of the issue well enough. The transients seem high, but I don't think I need to worry because they're well under 1200v. Heating also isn't an issue; I ran the coil for a decent period of time (~8 minutes with short breaks in between songs) at full power, and the heat sink was only warm to the touch.

As a side note, when I was testing the bus at 350v (not pictured), the OCD tripped after just 2 cycles. Scary in my opinion.

I would also like to thank everyone on this forum for their detailed builds, especially Mads for his DRSSTC 1 write up. I really learned a lot following your build on my previous Coil. All this has kept me entertained over the past 2 years . It's safe to say that I wouldn't have been able to make this coil without this community.

Here are some more videos:

https://drive.google.com/file/d/1KyCgH2_GW8i_lfpE901m7jdYXsrRBR48/view?usp=sharing

https://youtu.be/jtbsVDrVPQY

https://drive.google.com/file/d/1wynRvVx7Rr0uifXccZ6RAO8KxCkHofqi/view?usp=sharing

https://drive.google.com/file/d/1uOeRaNgaVVidVXlBwK4LK9k_m7skJI0F/view?usp=sharing

Thanks again for the responses.

I looked for some that would not conduct during the maximum bridge output voltage plus some safety margin and took the largest one digikey had in stock. That is not to say that that is the way best way to go though... there might well be a cheaper alternative

I went the same route and found myself V250LA4P‎. Maximum clamping voltage is 650v (perfect for 650v IGBTs), and I don't plan to run my bridge at over 200v anytime soon(staying within typical varistor limits). Datasheet: https://www.littelfuse.com/~/media/electronics/datasheets/varistors/littelfuse_varistor_la_datasheet.pdf.pdf

The ground strike is hitting the second ring in from the outside.  Is that also a ground ring, or is it an outer primary turn? If a ground strike, is there a solid path from that ground ring to ground and/or counterpoise, not a wire that runs right by circuitry?

High primary current from a ground strike is a reasonable possibility too.  That is the first ground-strike in the video, and while playing a low note that likely has longer on-times.  Many factors affect how much current any given IGBT can handle, including how clean gate waveforms are and if phase lead is adjusted properly.  If H-Bridge parasitic inductance is high and phase is lagging, high current may cause a failure due to resulting switching spikes exceeding IGBT voltage rating.

The ring it hit was the ground ring. Not only was it the first ground strike in the video, it was the first ground strike ever for this coil (arcs are finally long enough  ). I did take some time to make good ground connections; I have a small terminal bar and a good RF ground. Regardless of whether or not the IGBTs died from overcurrent or overvoltage by virtue of overcurrent, I will add MOVs for extra protection and lower my OCD threshold.

One more thing that I think might help: The arc hit the sharp part of the hose clamp I used to connect the ground ring to ground, so I will work on making a smoother connection as well. ALSO I took a leaf out of Dave's book and tuned the coil by adding a string of caps to my MMC.

Hi All,

I've made a lot of progress on my DRSSTC, and I finally got sparks longer than 1 m!
As you can see, I let out the magic smoke. The most likely explanation is that my OCD was set too high and it did not trip during a ground strike (it may be hard to see, but the final arc hit the ground ring). I have 4x IXXN110N65. The datasheet (attached) says it has a peak current of 470A @ 1 ms, but I know this does not help much when dealing with 75kHz DRSSTCs. That being said, does anyone have any anecdotal evidence of the maximum current an SOT-227 IGBT can handle? Also, could TVS diodes be an alternative to Zeners on the gates?

Hello once again,

Success! Or at least somewhat. I moved the OCD and feedback CTs to before the cap bank as you suggested. I ran a test with 190VDC bus (I think part of my problem the other day was a too low bus voltage) and got sparks so small I could only hear them, not see them. Please note that the resonance is ballpark tuned (10% lower than secondary rule), I still need to do fine adjustments. After this somewhat successful test, I tried playing with my OCD circuit. I raised the threshold a little and saw negligible effects, and proceeded to disconnect the OCD CT entirely. Here are my results: So the pros: it works with decent sized arcs
Cons: There is something wrong with my OCD circuit
Any tips or tricks for OCD? I borrowed the OCD circuit chunk from https://www.stevehv.4hv.org/DRSSTC1/DRSSTC1OCDsch.JPG. My connection to the transformer is pretty long, there may be some issues with noise.

Hi all,
I am a high school senior in the US and I started making a DRSSTC back in January 2020, taking inspiration from Mads' DRSSTC I. I made the oneTesla kit a number of years ago and decided to come back to the wonderful land of Tesla coils. I have also made a spark gap tesla coil, which can be seen off to the right in the video below. Working on and off during the coronavirus lockdown, I had my first successful test last month (200VDC bus from variac): It blew right after I took this video, which I attribute to my poor bridge design: https://drive.google.com/file/d/1YcG2pWm8kf7eFIVq646JIEfXXsDc9pin/view?usp=sharing. Whole rat's nest of wires. I have since remade the bridge with IXXN110N65C IGBTs and busbar, found here: https://drive.google.com/file/d/1Ah39WuPqT2fYzhU6FvLrx1yl8nClExOo/view?usp=sharing. Since making the new bridge, I have not been able to get it running. New MMC is 3x7 CDE942C20P15 for 0.35uF @ 6KVDC, or around 900VAC derated @74kHz. Bus is 15,000uF 400V Kemets(2 x 7,500). Driver board is a variant of oneTesla's: https://drive.google.com/file/d/1pFHJJzeRBw8jN1Er48E7NNonjFVLksk1/view?usp=sharing. I use a 15v 1:1:1:1:1 GDT, something I plan to change in the future. I also added an OCD circuit after blowing the first set of IGBTs, which is not shown on the schematic. For an interrupter, I used oneTesla's SD interrupter. It's designed for much smaller coils, so I plan on modifying that in the future as well, but it works for now.

After firing up the new coil last night, no sparks flew. I have been stuck scratching my head. I was running with 130VDC from a variac. I tried flipping the feedback CT, but still no luck. I did notice, however, that the MMC gets charged, which is curious (After signaling the coil to fire and nothing happened, I shut everything down and measured a quickly declining voltage across the MMC). Any thoughts/comments/suggestions on what to try next? If you have any clarifying questions, please let me know and I'll try to get back ASAP.