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drsstc primary strike protection

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Ben Solon

Fri Mar 30 2012, 09:19PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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Posts: 600

hey, i noticed here

that steve uses the .1 caps to protect the power electronics from primary strikes. the caps however are limited by their voltage rating and may punch through if the stike is bad enough though. i just had a thought: would something like a gas discharge tube with a breakdown just over the maximum voltages seen in normal operation? this way it would both suppress overvoltage due to bad conditions on the bridge(just like the tvs diodes and snubbers) and the primary strikes that would kill the protective features(tvs diodes and snubbers).

dude_500

Fri Mar 30 2012, 11:08PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

Well, overvoltage shouldn't really burn out a TVS. That's the whole point of them. They breakdown and ensure that the voltage never exceeds their designed breakdown voltage. You can't put "too much voltage" across a TVS, because the device will prevent the voltage from going that high. All you can do is put too much power through them to physically burn them up. I don't think a typical ground arc could ever do that though.

That being said, I'm not exactly sure where you would put a TVS diode. Across the 0.1uF protection cap?

Ben Solon

Sat Mar 31 2012, 01:23AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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yes, that makes sense, but with a really powerful or long strike, you *may* possibly burn them out. the 1.5KE220CA which may people use can only dissipate 7W on an infinite heatsink. 1500W pulsed but still. with a tube, you can dump a lot more power into them. if you have a tvs rated for 1.5kW and breakdown of 600v, you can only pulse 2.5A. a tvs diode is really nothing more than a spark gap anyways.

But then why don't people do this? i have never seen a coil with a tvs protection for this purpose. strike rails are all fine and good, but unless you have the primary in a cage, there is still a small possibility that you can get a strike.

and where to put them? because the dc bus is at an ac neutral "ground", and the arcs are looking for rf ground, you could put a tvs or tube from the igbt emitter-collector connection(both for a full bridge) to rf ground. this way, the resonant ring up on the primary circuit has 1200v before it shorts the tank and primary. but it only has 600v before an arc grounds itself.

and argue with me

i want to learn!

dude_500

Sat Mar 31 2012, 06:01AM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

Keep in mind a 10kw coil being dumped into a TVS through an arc will dissipate very little actual power into the TVS because of the voltage divider between the TVS diode and the arc itself. The arc has many times the voltage of the TVS rating, so the arc will dissipate the vast majority of the energy. It can be thought of exactly like a resistor divider if you know the actual topload voltage.

Ben Solon

Sat Mar 31 2012, 03:09PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
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ok, now that i think about it the arc does eat up a lot of energy with heat/light/etc.

so to use this design where would you put the tvs diodes? same place i pointed out , or where steve was his caps?

Steve Conner

Sat Mar 31 2012, 06:22PM

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

I must admit that I don't see the point of using TVS in this way.

What would blow your IGBTs would be the differential mode overvoltage. This is clamped by the IGBTs' antiparallel diodes and the DC bus capacitors, so there is no point in putting TVS across the IGBTs, except as band-aids for an excessively high inductance bridge layout. And even then. you may find that modern IGBTs have a bigger avalanche rating than the TVS that are supposed to be protecting them.

The common-mode overvoltage appears between primary and secondary of your GDT, causing it to arc over. The 0.1uF cap shown in Steve's design seems to do a perfectly good job of keeping it down. It's what I've always used in my DRSSTCs. (one of them also has the ferrite clamp on the GDT wires, but I built another without it, and it didn't seem to make much difference)

Technically speaking that capacitor is a Class Y unit and shouldn't be more than 4700pF for safety reasons, but I ignore that. It's hardly the biggest electrical safety violation around Tesla coils.

If you used a gas discharge tube instead of a capacitor, things could get messy, because they are a short circuit once they arc over. From the negative rail of your DC bus, that would be a short between the mains and ground through the bridge rectifier. Things would go boom.

If you used a TVS, it probably doesn't have the right safety approvals for Class Y service, being made of silicon and not at all self-healing.

Steve Ward

Sat Apr 07 2012, 05:22AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Ive actually gone a step further and build full EMI filters on my DC bus for power electronic protection. There is a "Y-cap" from each side of the bus to ground, then a common mode choke where the DC power enters the tesla coil housing. At the same time, i do have very good strike rail and shielding placement around the electronics of the tesla coil, so i dont test this safety system too much.

Terry Fritz

Sat Apr 07 2012, 10:28PM

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

I buy about $1000 worth of TVSs a year so maybe I know something. Ok, Mine are like 24V 5000W...

But a little TVS is not going to take a giant power hit despite the fact that they are really tough. You would be better off with big MOVs in general that have a lot of thermal mass to absorb real power.

But the advantage of TVSs is that they are super fast!! Like pico second reaction times! In my world of super power LiPO batteries and Neodymium magnet motors, the folks really can hit some super (~5nS) spikes that would go right past a slow old MOV and cut up the FET die.

So my suggestion would be to use big MOVs to take the power of the hit, but also parallel bit higher voltage TVSs that can clamp very high speed spikes. The TVS takes the fast spike and then the MOVs take over for the power dissipation.

Just a thought there...

Terry

BTW - This long forgotten paper might be helpful too. That really did work.

Steve Conner

Sun Apr 08 2012, 07:26AM

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

Wow, Terry Fritz.

I don't see why the DRSSTC protection thing wouldn't work, but the odds of a dead short across the inverter output are pretty slim. Except in some of the more adventurous builds that use multiple inverters with the outputs combined, where failure of one inverter would short out the others. Steve Ward and a few others have used ballasting inductors to mitigate this.

Just for the record, is there an official site with all of your papers on it? We regularly get questions about the Terry Filter here, and it would be nice to be able to point people to the original experiments.

Terry Fritz

Mon Apr 09 2012, 11:44PM

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

Steve said,

I don't see why the DRSSTC protection thing wouldn't work, but the odds of a dead short across the inverter output are pretty slim

That is what was blowing my DRSSTC up when the streamers hit primary. The primary strike currents were turning the opposing FETs on when they were not supposed to be on and cross conducted very dramatically. I still have the videos too... But mine was a little odd.

Remember the sudden speeds at which a strike can dump dramatic currents...

Just for the record, is there an official site with all of your papers on it?

Even I just go here:

The Internet seldom forgets*.

drsstc.com Did catch some later stuff like the paper of interest.

I you want a copy of everything you cam PM me... I forget the size but I finally bought a DVD burner just to distribute that many years back. I have not logged into DRSSTC.COM in 5+ years... But the Cobalt SUN server still sits here on my desk so we still talk

I still have a NOS one too in storage, but the SUN things never die!!!!

I usually let others answer the Terry Filter things since the kids probably know more than Daddy there now

Terry

*I may have been among the first to actually loose some good Internet site stuff too

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