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Solid State Disruptive Coil Spark Gaps

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Steve Conner

Fri May 05 2006, 03:20PM

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

Ohh, now that is clever!

I wouldn't worry about it being incapable of quenching. If the coil is properly designed, the sparks should eat up the energy quick enough that it doesn't really matter. Just as long as it turns off quick enough that it doesn't latch on permanently and short the charging supply.

Can anyone think of a way to make it triggered? (ie simulating a rotary gap instead of a static) I'd be inclined to start by just clobbering the top of the stack with a positive HV pulse to try and turn a few of the SIDACs on.

I don't think Bert's method would work either for triggering or quenching. Turning on one stage only makes about 1000V of difference to the breakdown voltage, and turning a single stage off would cause uneven voltage distribution.

Bert

Fri May 05 2006, 04:26PM

Registered Member #118 Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72

Hi Steve,

I agree that triggering and quenching in this manner will become increasingly difficult as the number of stages is increased, but it should easily work for MOT powered coils. A more clever approach may be necessary for higher voltages.

I disagree that there will be little impact on performance with no quenching - poor quenching on disruptive coils shows a very marked performance impact. Once the bottom switch is "softly" turned off (say at the first or 2nd notch), it merely needs to stand off the induced primary voltage from transformer action from the resonating secondary. Assuming that similar (i.e., low) coupling coefficients are used as for other "disruptive" coils, a single 1200 - 1700 volt IGBT may do the trick (assuming it also has a free wheeling diode). The fact that there is now an uneven voltage distribution across the entire switch shouldn't matter as long as we can turn it off at the right time. Since it also becomes increasingly easier to quench during subsequent notches, perhaps one could aim for 2nd notch quenching instead of first.

For synchronous triggering, how about connecting a small HV pulse transformer secondary in the main current path and hitting it with a triggering pulse. You could do this on the bottom of the IGBT chain (using a negative-going spike) to keep the insulation needs of the transformer reasonable. You could even put the triggering transformer in the ground return of the charging path in order to keep it out of the high current TC primary path.

Bert

*mod edit fixed double post*

Hi Steve,

Hmmm.... after thinking about it a bit more, you may be right re: quenching not being as great a factor, since this is a more efficient "spark gap" it wouldn't dissipate as much energy as regular spark gaps, thereby allowing more energy to cycle back into the secondary on each transfer.

Looks like the real proof is in the testing....

Bert

Terry Fritz

Fri May 05 2006, 09:47PM

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

Hi,

I posted another paper at:

Mostly dynamic power and resistance measurements just to double check things. The server is real busy these days so you might have to try a little to get in.

I was contacted by a person that has a big async DC MOT coil (60 inch arcs) and he can make PC boards. He is an engineering manager type so he knows stuff and is willing to spend some money so the project is underway!!!

I am not planning on doing anything with quenching or triggering at the moment. Too much to do just to get the first one going!

Cheers,

Terry

*mod edit fixed double post*

Hi,

Yet another paper...

High current tests...

The IGBT I used sort of conks out at 380 amps and falls out of saturation even with "40V" on the gate! But it does not blow up and works well to 350 amps peak. Any IGBT could be used, so I may look for another...

Cheers,

Terry

Tesladownunder

Sat May 06 2006, 05:53AM

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

The reason I worry about the quenching is that with my SIDAC string in place of a spark gap, sparks were only 2 inches with 5kV input. However, add a series spark gap and sparks increased to 6 inches. This does not sound like a high current problem causing the SIDAC to become inefficient, but more like a quenching issue.
If one had to arrange the IGBT's to turn off then the simple system gets more complicated.

Great idea though and look forward to testing with a coil.

If it works I will try to find out when I said something like "just wait until you get SIDAC bricks and see what they do for TC's"

Peter

robert

Sat May 06 2006, 11:15AM

Registered Member #188 Joined: Thu Feb 16 2006, 05:18PM
Location:
Posts: 67

Just out of interest, it would perhaps work with a (fast) SCR too if a antiparalell diode would be used.
It just takes a little different gate circiut.
Now i dont really know if suitably fast SCRs are avialible at the necessary currents and if the price would be acceptable but chances are that it is.

Steve Conner

Sat May 06 2006, 05:21PM

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

Just to remind folks that 4hv has a rule against double posting. Please don't double post unless 48 hours or more have elapsed _and_ you have new information to add. Otherwise, please just edit your post using the edit button, which looks like this Edit

We do this because we don't want people posting just to bump their thread and get it marked new.

I combined Terry's and Bert's double posts into single posts.

robert: SCRs don't have high enough di/dt capability to be useful in a Tesla coil. If they did, we would use them in preference because they give more bang for the buck than IGBTs. The guys on TCML went through all that when we were planning the OLTC a couple of years back. I once calculated that once the frequency of your coil drops below 25kHz, inverter grade GTO SCRs start to give more value for money in both OLTCs and DRSSTCs. (GTOs switch faster than regular SCRs even when you circuit commutate them rather than using the GTO mode.)

Terry Fritz

Fri May 12 2006, 02:03AM

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

Hi,

I has been 48++ hours and new test results are in

)))

I still am a bit naughty for posting from another list... >:))

But the whole world of tesla coiling "changed" today(!!!!)... I have "been around" ""long enough" to "know" too ;))))))))))))))

But all should enjoy

)))

Sell those "old" spark gaps on E-bay while you still can :o))))

Cheers,

Terry

HV Enthusiast

Fri May 12 2006, 03:05AM

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

Looks good Terry and i have to admit, you got me.
Anyways, i just started drafting a prototype spark gap circuit to run with my 4kV/38mA coil. I just ordered some Sidacs and i already have some IGBTs I can use. I'll cut a board tomorrow and hopefully by early next week have this thing running and get some data from it. I'll probably being running about (6) IGBTs in series. Should be interesting!

Desmogod

Fri May 12 2006, 03:15AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

Curious considering my "Future of Tesla Coiling" thread was posted about 24 hours prior to this.
Do you have any pictures of operation, ideas on efficiency etc?

HV Enthusiast

Fri May 12 2006, 10:36AM

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

Desmogod wrote ...

Curious considering my "Future of Tesla Coiling" thread was posted about 24 hours prior to this.
Do you have any pictures of operation, ideas on efficiency etc?

Check the two papers Terry has already posted as well as his latest post. He has lots of data there.

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