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Solid State Disruptive Coil Spark Gaps

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Steve Conner

Tue May 23 2006, 01:24PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

ON Semiconductor make SIDACs, you may be able to get samples. They really aren't quite the same as diacs: a diac is a small signal device for triggering, a SIDAC is much beefier.

If you don't have access to the big electronics distributors (maybe your mom won't let you use her credit card

) I heard that SCRs can be used as ghetto SIDACs. If you leave the gate terminal disconnected, they will self trigger when you exceed their rated voltage. You might need to connect the gate to the cathode through a largish resistor to get it working stably. Also, you'll still need the TVS to stop the IGBT gate from getting fried.

Now I think about it: if we used SCRs instead of SIDACs, wouldn't that allow us to make a triggered gap? It would just be a case of pinging a few of the SCR gates at the earthy end of the stack, and the rest would trigger by themselves.

SCR gates are easy to drive, so you could conceivably even trigger them all. I've seen a commercial gadget for this, that consists of a ferrite toroid transformer for each SCR, with a single heavily insulated cable passed through the middle of all the toroids. You dump current through the cable and it triggers all the SCRs at once. This would never work to drive IGBTs, but using the SCRs as helpers, it's a different story.

That would get rid of the objection that SCRs aren't supposed to be used in this self-triggering mode (although I once read a Philips datasheet where it said that it doesn't hurt them) and also get you a nice triggered gap with a very wide operating voltage range.

Terry is a genius

HV Enthusiast

Tue May 23 2006, 01:58PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

Now I think about it: if we used SCRs instead of SIDACs, wouldn't that allow us to make a triggered gap? It would just be a case of pinging a few of the SCR gates at the earthy end of the stack, and the rest would trigger by themselves.

Yep. I've built a high rep rate high voltage switch (35kV) using SCRs in just this fashion for a client two years ago. Works great. Although the SCRs do self-trigger, we used TVRs in parallel with all devices so they would break-down before the SCR. Our particular switch was used to discharge a 0.1uF @ 35kV at rates up to 100Hz.

wrote ...

SCR gates are easy to drive, so you could conceivably even trigger them all. I've seen a commercial gadget for this, that consists of a ferrite toroid transformer for each SCR, with a single heavily insulated cable passed through the middle of all the toroids.

We used individual drive transformers with separate primaries, but i've also seen them drive with a single primary.

wrote ...

This would never work to drive IGBTs, but using the SCRs as helpers . . .

Sure it would. I've already built a circuit like this. In fact, you don't even need transformers at all. One single gate signal turns on the entire stack
of IGBTs. I posted this circuit awhile back, but can repost again if interested. Works extremely well and I was planning on using it to create a MOT based SSTC, but at the time, the DRSSTC hit the scene, and I abandoned the work on that particular project. It would work well as a spark gap though ,but would require an external voltage to run and trigger, as opposed to Terry's which is just drop in and go.

Dan

Steve Conner

Tue May 23 2006, 03:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I'm talking about a switch that uses SCRs to trigger IGBTs, in the same way that Terry's switch uses SIDACs to trigger IGBTs. Are you saying that you built that already?

HV Enthusiast

Tue May 23 2006, 03:29PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Yes, i have. This was used in a crowbar switch. But you don't need the IGBTs. The SCRs alone can handle the current. With SIDACs, you need the IGBTs because SIDACS cannot handle the current.

I've actually done two designs, the first one being with IGBTs alone and using a single drive signal to turn on the the entire stack. The second design used SCRs alone in place of the IGBTs. This is more in regards to your comment about the "triggered" design, not the self-triggered design like Terrys.

As far as self-triggering, i don't see any reason why you could not replace the IGBT stack i was using with a single SIDAC with a voltage divider and a similar discrete firing circuit to initiate the IGBT stack.

Reaching

Tue May 23 2006, 05:35PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

mhh, i got something to work with an scr ,some zeners and a few resistors but i dont know if it would work in tc duty.
if i understand it correctly, the sidacs have a minimum on state current, they will stay on until the minimum hold current drops, exactly like a scr, i wonder that they really turn off. my test setup uses a 300volt zener ,a 100k resistor and a 600volt scr. if i turn up the voltage to say 302volt, the scr turns on and stays on, maybe thats the way to go for those who cant get sidacs. but i dont know if this setup will turn off as a spark gap replacement.

hehe, okay, Update

you dont even need sidacs, it works with scrs, i build a single module which should switch at 360volt hooked it up to my 400volt dc transformer , made a small primary,secondary,mmc really far out of tune, lol, but i got 5mm arcs with a nice "humm"

now i know, it works with scrs instead of sidacs with a small circuit change. now i can order some parts and build my scrsgtc (or sisg without sidacs,)

heres the modified shematic without sidacs but with an scr

Terry Fritz

Tue May 23 2006, 08:48PM

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

Sweet

This is probably smaller and cheaper than the SIDACs! It is fun to see the new ideas people come up with for this!! I bet someone is going to put a microprocessor on it soon

))

SISG could also stand for Silicon Spark Gap (SiSG) too. Then "SiSG" could sort of cover all the solid state spark gap replacements... But we'll just have to see where the future goes

)

I gotta get my old "spark" gaps on E-bay soon!!! Sell them before they are worthless... Might be too late

Cheers,

Terry

Marko

Tue May 23 2006, 09:06PM

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

I think you should add a resistor, some few hundred ohms in series with SCR to prevent blowage from masisve current going trough the gap, you want the IGBT to switch it (not the SCR

)?

For some more serious coils maybe.

PS.
I don't know if this is needed for Sidac's but I had SCR's burn up easily in similar conditions.
I think it's going to share too much current with IGBT and (maybe) blow up if overstressed.

Steve Conner

Wed May 24 2006, 11:34AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That's pretty cool Reaching! Try removing the three zeners and see if it still works with the SCR self-triggering.

Reaching

Wed May 24 2006, 12:14PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

mhh, i tried it yesterday too and i blew up an scr trying to self trigger it. but i dont know if it will work when you put say 6 boards in series. cause the scr needs a few mA to switch on in my case 8mA, so thats 3,6watts at 450volts so use heavy 2 watt zeners or so. i tried R1 as 100k and the scr switches on, but i dont know what happens if you put 6 boards in series. i dont have the right equipment to measure whats going on in this circuit. in a few days i can finish a nice test setup with 6 boards, a mot ,74nF 4kV mmc and a 7,5x20cm secondary with a nice sphere toroid, that should work fine for the beginning and for further tests.

colin heath

Thu May 25 2006, 06:17PM

Registered Member #123 Joined: Fri Feb 10 2006, 12:58PM
Location:
Posts: 162

Reaching said

if i understand it correctly, the sidacs have a minimum on state current, they will stay on until the minimum hold current drops, exactly like a scr, i wonder that they really turn off. my test setup uses a 300volt zener ,a 100k resistor and a 600volt scr. if i turn up the voltage to say 302volt, the scr turns on and stays on, maybe thats the way to go for those who cant get sidacs. but i dont know if this setup will turn off as a spark gap replacement.

SCR's or Thyristors as us Brits know them

Will turn on without a gate current if rate of rise of voltage or current exceeds the rating of the device (thats why they use snubbers). However they will not turn off until the voltage or current reverses. Great on an AC system but difficult on DC.

Cheers

Colin

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