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Re: Terry's DRSSTC - First light ;-))

Original poster: "Mike" <mike.marcum@xxxxxxxxxxxx>

Those arcs look hotter than hades (almost like a cw coil). I gotta build me one of these. Is there any way to build a "simple" version without alot of the protection circuits and just use huge igbt's (abeit expensive, seen some rated 6500v and 2000A, but cost $2200, ouch) and design the primary to where that limits the current? I got a xeltek 8020A 20mhz scope but no fancy toys with lots of bells and whistles. Is 100mhz absolutely necessary?

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, March 06, 2005 2:29 PM
Subject: Re: Terry's DRSSTC - First light ;-))

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 10:41 AM 3/6/2005, you wrote:
Hey Terry,

Congrats on some great sparks :-)

Thanks!! Worked great!!

I really like the sequence of still frames you posted of the failure,
the whole driver is nicely lit up :-).

It is amazing how bad it looks on video but how little damage is really done. the small IGBTs make real nice fuses ;O)) I will have to take the H-Bridge out to see if the traces survived this time. Since the card seems to look so good I may just patch it in any case.

It still seems to be somewhat of a mystery as to why primary strikes
are so catastrophic to a DRSSTC.  Some of my coils have survived many
primary strikes, other times just a hint of a streamer forming on the
primary will kill the thing.

I noticed on the real video that the last streamer hit to the primary was very bright. There was some serious current going through it. I suspect it was the buss discharging to ground, or the other rail, through the streamer. If so, it may have been a very high current shot that took out an IGBT and the opposite one went in turn.

It seems that some coils are more prone
to failure from a primary strike as well.  It might be possible that
the streamers short out primary turns making for a nice short circuit
load on the driver.  I cant see how the low primary impedance and all
those snubber caps and TVS couldnt keep voltage transients to a
minimum during such an event.

The current limit would not be able to stop a sudden dead short. It is too fast and turning off the IGBT would be hard. Once the current goes over about 1000 amps, it is all over for the IGBT. In the video, you can see what was probably the IGBT die ejecting to the left ;-)

Either way, you really have to protect
all that stuff from the streamers!

Strike rail is next...

I put metal caging around the base
of my coils, which seemed to work fine until i switched to primary
feedback (now the fence sucks up a lot of power for some reason).
Still a bit puzzled about that...  In any case, a well placed strike
ring as well as a breakout point that guides the sparks outward goes a
long way to keep those arcs out of there ;-).

I seem to also be able to run the electronics box well far away. I just run long 6 foot #12 wires to it right now.

So what primary current were you running?

250 amps peak.

What ON time and BPS?

10 - 500uS and 50 - 300 BPS.

you try letting it arc to grounded targets at all?  And finally, how
long were those streamers?

No ground strikes other than the last one :o) I think about 3 feet. It was a little windy but at the last it was doing real good. I will have to hook it to a coil that can reach 20kV on the primary next to get real long sparks ;-)

Anyway, keep fine tuning that system and im sure you will end up with
something very reliable.

Seems real good now. The primary strike thing might not be easy to actually protect against other than not doing that.





On Sat, 05 Mar 2005 23:44:21 -0700, Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>
> Hi All,
> I hauled the thing over to Jerry's house tonight to run it at full > power!!