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Solid State Disruptive Coil Spark Gaps

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Terry Fritz

Mon Jun 12 2006, 06:59AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

Tonight I used the SISG for the lightning ball experiment in the other forum here. I did not think much about using it first since there was no time and it had to go!!

It fired twice:

1. 1800V 60e-6F into probably ~15ohms (97.2J). The timing on the SISG was set for 400uS and it turned off with about 800 volts still on the cap. The SISG survived this.

2. 5400V 60e-6F into probably ~15 ohms (875J). All the IGBTs shorted to zero ohms.

So ether the IGBTs over heated for the raw energy or the short timing opened them and and heating once again got to them. The IGBTs showed not physical damage at all (SUPER -TO247 package).

I have not had anytime to think more on it but I wanted to write it all down before I forgot.

More -- After some calculation, the instant thermal heating on the die in the first case was well below its rating. Breaking open the circuit at 400uS was also fairly easy.

However, in the second case the die critically over heat in about 50uS. The higher current (into about 8.22 ohms) dissipated 10X the power and the IGBTs never had a chance. Then they get hit with another 28 joules to finish them off for sure.

So easily explained and no problem at all for Tesla coil stuff. But the 8.22 ohms is probably good for a conventional spark gap now in the case of the plasma ball experiment.

The 800V at 400uS gives great knowledge there

A very complex dynamic high current ohmmeter...

Cheers,

Terry

vasil

Mon Jun 12 2006, 01:22PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Just one question:
SISG topology can work with MOSFETs also instead IGBTs?
I have some small secondaries (higher frequnecies) around....

Marko

Mon Jun 12 2006, 04:26PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yep, but mosfets will blow themselves up when you put 100's of amps trough their high ohmic resistances (power waste is I^2*R). You need IGBT with constant voltage drop to handle that current.

I think you don't need some seriously fast GBT's while there is only one hard transistent per bang..

Terry Fritz

Mon Jun 12 2006, 04:30PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

SISG topology can work with MOSFETs also instead IGBTs?

Sure! Mosfets will not be able to take the super currents of IGBTs, but for smaller lower current stuff that is not a problem. The speed is not an issue since it is a simple "once per bang" switch. You will need an anti-parallel diode to allow high currents to pass in the opposite direction. I am not sure the FET body diode will like that in many cases. You may be able to use a pretty plain big diode for that since again speed is not an issue.

Cheers,

Terry

Terry Fritz

Sun Sept 17 2006, 05:25AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi All,

The SISG is the only coil I use now

)

I only run at 4800V "still" due to space limits. It could go to 6800V if I take out the jumpers >:-))

In preparation for high speed pictures follwing TDU's work, I have a number of "big" streamer pictures online now:

The schematic is here:

A scarry coil in that it is "silent" till it "goes off"!!! And it packs more than leathal power!!! But a neat and easy performer so far

) Scares me and Gerry

))

A warning not to go playing with the streamers!! There is very little between you and "electric chair" currents there!!!

But the SISG continues to run flawlessly, happily, and impressivly... The Cp cap is underated as I have it, but a proper new cap will be ordered when this one blows.

)

Cheers,

Terry

Mark Dinsmore

Thu Sept 21 2006, 04:56AM

Registered Member #484 Joined: Thu Jul 06 2006, 07:29PM
Location:
Posts: 4

Hi,

Has anyone ever thought about using a laser capacitor charging type power supply to run a SISG? They are essentially high power constant current power sources, capable of kilovolts of output and at least hundreds of watts of power. The constant current (which I think is adjustable) would give you a nice, stable, and adjustable break rate, and eliminate the dissipation from the charging resistors. It would be essentially a very high power relaxation oscillator. They would be bulletproof in this application, as they are designed to charge caps that are suddenly discharged when the laser fires. They show up on ebay from time to time, or there might be some way to convert a tig welder into a similar source.

Mark

Terry Fritz

Thu Sept 21 2006, 05:19AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

As far as I know, a real dedicated HV cap charging supply should work fine. I use a MOT, diodes, and resistors just because anyone can easily reproduce those.

Cheers,

Terry

Tesladownunder

Tue Oct 24 2006, 04:47PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

If this is to be regarded as a stable sytem usable for long running museum demos then primary strikes need to be considered. I am still concerned about a primary strike letting smoke out even though the IGBT's are on at that stage.
Have you actually tested primary strikes or do you rely on dragons to keep them at bay?

Peter

teravolt

Wed Oct 25 2006, 03:00AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

I like Reaching's modified SCR scematic to. why not get rid of the mosfet portion all together and replace the SCR with a hocky puck of about 1200v and use sidacs, or zeners, or a comparitor to trigger it. An inductive circuit would have to be in series to make enough inductive kikback to turn off the SCR. what does any body think. N.

J. Aaron Holmes

Wed Oct 25 2006, 04:55AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

teravolt wrote ...

I like Reaching's modified SCR scematic to. why not get rid of the mosfet portion all together and replace the SCR with a hocky puck of about 1200v and use sidacs, or zeners, or a comparitor to trigger it. An inductive circuit would have to be in series to make enough inductive kikback to turn off the SCR. what does any body think. N.

Those big "puks" look real neat at first, but then you look at the numbers. The turn-on times are awful. The turn-OFF times even worse. Add to that a very substantial voltage drop and they're pretty much unusable for this purpose. They'd eat a crazy amount of power during turn-on, I think (if your SIDACs didn't catch on fire first

).

Lately Dynex and a few other power semiconductor makers have started making what they call "pulse power" thyristors. Those may show more promise, but are undoubtedly very expensive. They are specifically meant to replace conventional thyratrons, and come in voltages all the way up to 12kV! I suspect the price tags are in the several-$k range, not unlike the bigger 6.5kV IGBT bricks.

Regards,
Aaron, N7OE

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