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Tesla coils => Dual Resonant Solid State Tesla coils (DRSSTC) => Topic started by: profdc9 on May 08, 2018, 04:20:55 AM

Title: UD2.9 skip pulse assembled
Post by: profdc9 on May 08, 2018, 04:20:55 AM
Hello,

I am working on a new version of the Universal Driver called UD2.9, which includes the pulse skip ability.  By closing a jumper, pulse skip functionality is active, otherwise the standard overcurrent detection is active.  I also added an external disable feature for adding, for example, thermal shutoff.

I have assembled and bench tested it, so far so good. :)  Here's a picture of the assembled board:

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Hope to put this in the coil soon and not have blown IGBTs!

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: Mads Barnkob on May 08, 2018, 08:20:28 AM
Looks good!

A few questions that might lead to small optimizations:

- Could you move the electrolytic capacitors further away from the heat sinked voltage regulators?
- Could you space plastic parts further away from the large power resistors?

Looking forward to see it run a coil :)
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 08, 2018, 10:18:34 PM
I'll take a look into your suggestions.  Moving the capacitors further away from the regulators is tricky because if they are too far the regulation might not be stable.  I would have to add another smaller set of capacitors close to the regulators.  It's probably best to use 105C capacitors if this is an issue. 

I think I can probably move the burden resistors around though and not have to rearrange too much.  I tried to design it for QCW like use, so I am aware that heat could be a problem.

Hopefully I'll get a chance to test it soon.

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: Hydron on May 09, 2018, 08:39:07 AM
The notes about the capacitors and resistor are good ones, but I would not be worrying too much in this application. The capacitors will never be life limited by heat in a tesla coil controller (unless it's running in a permanent installation), and you can always leave longer legs on the resistor if it gets scorching hot and needs to be moved away a little.

I look forward to hearing the results of further testing - looks good so far (and sorry I wasn't able to be of more help scrutinising the design!)
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 09, 2018, 05:45:26 PM
No worries.  I am having a great time building a DRSSTC and having more fun doing electronics than I have in years. :)  It's a good intellectual challenge.

Dan
Title: Skip Pulse Scope Traces for UD2.9 driver
Post by: profdc9 on May 12, 2018, 04:13:28 PM
Sorry for the third post in a row, but I think I have some interesting results to share.

So this morning I put the UD2.9 skip pulse in.  I have not tested it yet with the secondary in, only the primary with some soup cans as a load.  I am being cautious, so I set the overcurrent low (200 A) and the bus voltage low (30-40 V that is doubled).

Here is a trace of the voltage and current on the bus with the skip pulse jumper removed, so it behaves like an ordinary driver that trips and ends the pulse at a single overcurrent event:



Here I replace the jumper, and it allows the pulse to continue until the interrupter is over:



So far so good.  The current is a bit rougher than I would like it but perhaps that is because I need to reduce the capacitance on the overcurrent detector comparator output, or just need to use it with a load.  Hopefully I can try the coil tonight when it gets dark out.

Dan


Title: Re: Skip Pulse Scope Traces for UD2.9 driver
Post by: Hydron on May 12, 2018, 04:42:12 PM
Looks great, waveforms exactly match what I'd expect (though had to squint a bit - the screenshot function on the scope is your friend!).

The current waveform isn't any rougher than what I'd expect from a DRSSTC primary tank with no load. You're right that it might look nicer once you have a streamer/ground strike as a load (system Q will be lower with a load) but in any case isn't something to worry about.
Title: Re: Skip Pulse Scope Traces for UD2.9 driver
Post by: profdc9 on May 12, 2018, 04:52:45 PM
For some reason I can't get my scope to recognize a USB thumb drive!  I don't know if I damaged it or what.

This circuit is the same one I sent to you except I changed a couple of component values.  I blew two of the IGBTs with the old driver demonstrating it for some guests.  It's getting hotter here (32-33 C) and I wonder if that's affecting how hard I can run the coil before the transistors fail.

Dan
Title: Re: Skip Pulse Scope Traces for UD2.9 driver
Post by: profdc9 on May 12, 2018, 05:28:46 PM
Oops, I learned that my scope only recognizes USB drives when power is turned on.  Here are some better screenshots.

This is without skip pulse, the jumper removed:

 [ Invalid Attachment ]

This is with skip pulse, with the jumper replaced:

 [ Invalid Attachment ]

Much better looking now.

Dan
Title: Re: Skip Pulse Scope Traces for UD2.9 driver
Post by: profdc9 on May 13, 2018, 03:30:05 AM
Ok, I have demonstrated the skip pulse mode on the coil.  It played through the whole song "Still Alive" by J. Coulton (from Portal) without blowing the IGBTs.  The overcurrent was set to 250 A.

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This used a Digispark programmed to be a OneTesla MIDI interrupter.

I think the main benefit of the skip pulse is that it allows the peak currents to be lower but still provide good arcs.  For example, the bus voltage can be higher, and if the rise of the current is too rapid, the pulse is very short.  The skip pulse mode lengthens the total time of the pulse.

As you can see, I do not have single streamers, but it did help me get some more power out of the coil I think.   I'll release the UD2.9 design soon with the update to the DRSSTC pack.

Dan
Title: Re: Skip Pulse Scope Traces for UD2.9 driver
Post by: oneKone on May 14, 2018, 05:45:45 AM
Nice work Dan,

It honestly amazes me how fast your creating these boards.
Title: Re: Skip Pulse Scope Traces for UD2.9 driver
Post by: Teravolt on May 14, 2018, 06:12:32 PM
nice work Dan what CAD are you using to create the gebers and do you plan to mass produce
Title: Re: UD2.9 skip pulse assembled
Post by: futurist on May 14, 2018, 08:19:32 PM
I merged topics to have everything at one place
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 15, 2018, 03:14:16 AM
I have three more videos of three songs played with pulse skip, which are very demanding because of the high frequency.

"Always With Me, Always With You" by Joe Satriani:

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Two marches by Sousa, "Liberty Bell March"

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and "Stars and Stripes Forever"

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The MIDI files were altered for two channel by me so I included them below.  I am going to upload the new boards shortly.  I do not plan to mass produce them, but you can send them to board houses if you wish.

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 20, 2018, 03:34:01 AM
Three more songs I sequenced and put on the pulse skip driver.  I had to do Tocatta in Fugue three times before the breaker did not blow, that's why the sparks are a little weaker.

Tocatta in Fugue by JS Bach

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Symphony No 5, 1st movement by L Van Beethoven

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and my personal favorite, "Spain" by Chick Corea

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My edited MIDIs are attached.  Thanks for watching!

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: Hydron on May 20, 2018, 10:58:04 PM
Haha, I had the same problem with Tocatta and breaker trips too. Impressive playing that one turned up to 11 though!
Title: Re: UD2.9 skip pulse assembled
Post by: Mads Barnkob on May 22, 2018, 10:01:49 PM
It is really some great work you have pulled off with the speed of getting the designs out and test them :)

Is it more the MIDI editing that gives the hick'ups than the pulse skipping? I think so as its only the first and last videos that show that behavior, all the classic pieces seems pretty smooth without stops.

I love 01:08 to 02:10 of Tocatta in Fogue, that is just a insane test for a DRSSTC!

How was your MMC, IGBT and primary/secondary coil temperatures after a 7 minute run like this?
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 23, 2018, 08:09:46 PM
I am using a full bridge of 8 IGBTs, two IGBTs per side, at 250 A maximum, so assuming the current is shared equally, that would be 125 A, which is slightly over the continuous current limit of the FGH80N80SMD at 25 C.   Of course, the transistors are not run continuously.  So the load is probably fairly low as compared to typical higher peak current DRSSTC.

I have an infrared camera that can measure temperature and I'll take a picture with it when I run it.  After I run it for 5 minutes or so, it is warm to the touch, but certainly not hot.  Probably 40-45 C or so.  The heatsink I use is an "L" channel I cut from aluminum, with a silicone heatsink pad between the IGBTs and the L channel.

The MIDI editing does produce some hiccups, you can listen to the original MIDI if you want to hear the hiccups.  I also did not do a perfect job of eliminating all of the third notes, because some of them can be hard to find, so some notes end before they should.  Examining the interrupter code, I am not sure what happens if the interrupts for TIMER0 and TIMER1 fire at the same time, if a pulse is lost or not or delayed, and that causes hiccups.

My fiber optic cable is a little flaky too, I need to get a better one.  Unfortunately I got the wrong kind and I am going to have to special order the right kind.  I know when the fiber optic cable is very loose, the connection is intermittent and the output gets a little more staticky.  But perhaps its the pulse skip too a little bit, perhaps I should run without it.


You should see the bewildered looks I get from my neighbors.  But they are generally favorable.

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 24, 2018, 03:15:42 AM
Here are some more songs I sequenced and played on the new driver.

"Little Wing" by Jimi Hendrix

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"Layla" by Derek and the Dominoes  (I tried my best)

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"Take Five" by Dave Brubeck and Paul Desmond

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"Cliffs of Dover" by Eric Johnson

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I attached a zip file with the midi's in it.

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: Uspring on May 24, 2018, 12:09:03 PM
That's a nice set of videos and thank you for your midis. I'll have to try them out some time.

I was was wondering about the pulse skip. I usually tune my coils to just not hit the OCD even for long bursts. The idea is to supply max voltage and current simultaneously to the primary. The period of max input power is quite short, though. Primary current rises, goes through a max and then drops considerably again. Tuning my primary lower would increase input current (and power) during the later stage but also causes my OCD to trigger and kill the burst before that.

With pulse skipping, the OCD triggering can be avoided, so you could have possibly max input power even during the later stages of the burst. Did you observe something like this? I don't know, whether having lots of power to the end of the burst increases arc length. My coils don't show this, but it well may have been caused by the small amount of power during the end of the burst.

Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 24, 2018, 04:22:00 PM
With the pulse skipping, the OCD is triggered all of the time.  Rather than set the OCD to trigger at a high current, say 500 A, I set it to about half that, or 250 A.  After the OCD is triggered, there is a flip flop that is cleared and shuts off the bridge for the next cycle.  If the OCD does not reoccur during that cycle, the bridge is turned back on at the next rising edge.  So really it should be called "cycle skip" because a low and a high transition must both be skipped to prevent flux imbalance from accumulating on the gate drive transformer.

I have seen similar spark lengths both with and without pulse skipping, but the power available to my coil is not that great, since I currently only have 15-20 A and 120 VAC to work with, before the lousy power factor is accounted for, so perhaps only 1 kW of actual delivered power.  The idea is to deliver power more continuously during the whole burst rather than only briefly at the end of the burst before the overcurrent trips.  There is probably some compromise pulse length that achieves a good arc length while minimally stressing the IGBTs.  Short pulse lengths can achieve good results but high peak currents shorten the life of the IGBTs.  Long pulse lengths can work too, but non-interrupted SSTCs definitely underperform interrupted DRSSTCs.  QCW systems have low duty cycle but very long ramp-up time (millisecond scale).  The idea behind pulse skipping is that it allows one to lower the current and increase the peak current time.  This strategy is probably good to a limited extent, but if the peak current is too low then the arc length is likely going to be reduced.  I have not yet figured out the compromise, but the UD2.9 controller is going to help do that.  I designed a new interrupter for the purpose of exploring these issues and I am waiting for the PCB to be manufactured.  The new interrupter has two selectable pulse width lengths as well as bps length, and also interrupts on the peak of the AC cycle to try to improve power factor and allow burst duty cycle control, and so I should be able to try longer pulse lengths (I am currenly limited to about 160 microseconds).  This interrupter is in the DRSSTC pack and you can look at the schematic if you wish.

Dan
Title: Re: UD2.9 skip pulse assembled
Post by: Uspring on May 25, 2018, 12:00:00 PM
Thank you for your answer.

A burst length of 160us might be too short to take notice the effect of the arc limiting primary current significantly. So likely you will notice pulse skipping up to the end of the burst.

Quote
Short pulse lengths can achieve good results but high peak currents shorten the life of the IGBTs.  Long pulse lengths can work too, but non-interrupted SSTCs definitely underperform interrupted DRSSTCs.  QCW systems have low duty cycle but very long ramp-up time (millisecond scale).

"Underperform" depends on how you define power input. In terms of line power requirements long bursts are costly. But if you relate arc length to power during the burst, QCWs are very effective. My DRSSTC can create a 2m arc form a 320V 700A input, while Loneoceans QCW1.5 can reach almost that size (1.75m) with a 300V and 150A source.

There is a large unexplored region of intermediate length bursts nicely accessible with pulse skip technology.
Title: Re: UD2.9 skip pulse assembled
Post by: Teravolt on May 25, 2018, 08:52:38 PM
hi profdc9, are you counting the cycles to know when to retrigger the pulse skipping or are you retriggering based on the current level? are you using a microprocessor to manage? 
Title: Re: UD2.9 skip pulse assembled
Post by: profdc9 on May 25, 2018, 09:43:13 PM
The schematic of the pulse skip is below.

It is a small modification of the UD2.7C interrupter.  Whereas on the UD2.7C interrupter an overcurrent event clears the overcurrent flip-flop and can only be set again by another interrupter rising edge, the UD2.9 is selectable to that each rising edge of the current, the flip-flop is set.  If the overcurrent does not reoccur, the enable flip-flop is set and the gate driver turns back on.  The jumper J16 closed allows the rising edge of the current to set the flip-flop.  If it is closed, the flip-flop can only be set again by the interrupter rising edge, which is the old behavior.

So no microprocessor is required, it is just a simple modification of the UD2.7C driver.

Dan
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