SGTC MK1 - An Accomplishment in Progress

[jturnerkc]

This is my first, "lets get serious" SGTC build.
This will also be my first post here at the HV forum!
Technically I made a small SGTC first, but it was simple enough, so we're going larger, but still budget-minded scale now.
I'm not jumping to the NSTs, MOTs, etc. just yet - but I'm very happy with what I've been able to get out of this build.
I certainly have a few kinks to work out, and I'm still a relative novice in the world of high voltage, so please feel free to join me on this adventure.   

Watch it GO at 120 FPS!

https://drive.google.com/open?id=1CkyjDH6enRkY398ecstW0iXVxJOWfRqc

*UPDATE*
See ongoing discussion for current status.

*ORIGINAL* SPECS below:

Power Supply:
    - 0-40vDC 10A Switching Power Supply (Line Filter added)
    - ZVS Driver with center-tapped Primary (5+5)
    - CRT Flyback Transformer/Ignition Coil (rated output unknown; estimated 15-20kV, 5-6mA)
Input Power:
    - 28-36vDC
Secondary Coil:
    - 2.4" Diameter
    - 16" Winding Height
    - 28 AWG
    - Ratio: ~6.7:1
Primary Coil:
    - .25" copper tubing (20 ft.)
    - Minor Diameter: 6.4"
    - Major Diameter: 10.46" (at tap)
    - Turns at Tap: ~4.7
MMC:
    - 20x 0.33uF, 1200vDC (series) = 16.5pF, 24kVdc
Spark Gap:
    - Two Electrodes, ~.5" diameter (small, drawer knobs), Spaced ~.2"
Topload:
    - 1.5"x10" ring with top and bottom plate, covered with Aluminum Tape
    - Effective Capacitance: ~9pF
    - Breakout added
Arc Length:
    - ~12-15" (great looking arcs!)

I'm considering encasing the flyback transformer in oil or other medium to help with heat dissipation at higher input voltages.

I certainly welcome, and would love, any feedback, constructive criticisms, or ideas anyone may have to make this build even better!

[davekni]

Using a ZVS oscillator into a flyback makes me think you have enough electronic experience to transition to IGBT drive (DRSSTC) whenever you decide to.

Do you have any spec's on the flyback transformer, or know what input voltage and frequency it was designed for?  There will be a limit to getting more power by cooling.  As the flyback ferrite core gets close to saturation, it's losses will go up faster than the output power (efficiency drops).  If you can get multiple identical flyback transformers, paralleling them is an option.

Is your MMC made from the Chinese induction-cooker capacitors?  The 1200V 0.33uF rating makes that seem likely.  I'm having good success so far with those.

[jturnerkc]

Using a ZVS oscillator into a flyback makes me think you have enough electronic experience to transition to IGBT drive (DRSSTC) whenever you decide to.

Do you have any spec's on the flyback transformer, or know what input voltage and frequency it was designed for?  There will be a limit to getting more power by cooling.  As the flyback ferrite core gets close to saturation, it's losses will go up faster than the output power (efficiency drops).  If you can get multiple identical flyback transformers, paralleling them is an option.

Is your MMC made from the Chinese induction-cooker capacitors?  The 1200V 0.33uF rating makes that seem likely.  I'm having good success so far with those.

Thank you for your interest and support, Dave!

The DRSSTC certainly seems like a daunting task compared to the relatively basic construction of the SGTC. I do think solid state is next on the list, but I think I'll need to study up a bit more on the solid state implementation before i make that jump.

Finding specs on the FBT was a bit more difficult than I initially imagined, considering it's an old, discontinued, chinese model, but I believe I've determined it was designed for a horizontal frequency of 15.625kHz, which would imply it was used in a TV monitor.

The flyback cooling may or may not be necessary. It was a consideration based on an observation made after a short run. I had a bit of hot glue just securing the flyback from freely moving around - a temporary measure that quickly proved useless once the flyback warmed up - and would prefer a 'cleaner' means of mounting and to use some decent terminals for tidier connection between the other components. I considered a small housing, simply because there doesn't seem to be a good mounting position for the flyback. I imagined filling the housing with oil and suspending the flyback could keep it cooler, if only by a bit.
If the heating gets any more significant it will be worth looking into further.

You're spot on with the MMC - it is comprised of the induction-cooker type capacitors and I'm also having good success with them. They are certainly a bargain considering what they can put up with.

[davekni]

15.625kHz is the analog PAL-standard TV horizontal frequency, used in Europe.  That's 64us, minus the 12us PAL blanking time, so 52us active time.  Presuming the 110-130V is the DC voltage of the flyback driver circuit, then the transformer input can handle 130V * 52us = 6.76mVs from peak negative to peak positive current.  (Half that time-voltage area if starting from 0 current.)  If you scope the transformer input in your ZVS circuit, hopefully the voltage-time area under one half cycle is less than 6.76mVs.

At 36Vdc in to your ZVS, peak voltage should be PI times that, or 113.1V.  For a sine wave, the area under a half cycle will be 2/PI times the peak voltage times the half-cycle time, so 72V * half_cycle_time, or 36V * period or 36V / frequency.  So, all you really need to measure is the frequency of your ZVS oscillator.  (I'm assuming your ZVS circuit is a standard Royer oscillator topology.)

Concerning transformer temperature, many power ferrite materials are designed for minimum loss at 100C.  I don't know if such materials are common in old flyback transformers.  I'd not be too concerned about it running "hot".

One final note on using TV flyback transformers with symmetric waveforms such as sine waves from a ZVS.  For a given output voltage, the internal diodes will see almost twice the reverse voltage.  In normal flyback use, the transformer output has high positive pulses and comparatively low negative pulses, reducing the diode's peak reverse-bias voltage requirements.  So, keep the output voltage (spark-gap breakdown voltage) not too far above half of the flyback transformer's output rating.

[jturnerkc]

15.625kHz is the analog PAL-standard TV horizontal frequency, used in Europe.  That's 64us, minus the 12us PAL blanking time, so 52us active time.  Presuming the 110-130V is the DC voltage of the flyback driver circuit, then the transformer input can handle 130V * 52us = 6.76mVs from peak negative to peak positive current.  (Half that time-voltage area if starting from 0 current.)  If you scope the transformer input in your ZVS circuit, hopefully the voltage-time area under one half cycle is less than 6.76mVs.

At 36Vdc in to your ZVS, peak voltage should be PI times that, or 113.1V.  For a sine wave, the area under a half cycle will be 2/PI times the peak voltage times the half-cycle time, so 72V * half_cycle_time, or 36V * period or 36V / frequency.  So, all you really need to measure is the frequency of your ZVS oscillator.  (I'm assuming your ZVS circuit is a standard Royer oscillator topology.)

Concerning transformer temperature, many power ferrite materials are designed for minimum loss at 100C.  I don't know if such materials are common in old flyback transformers.  I'd not be too concerned about it running "hot".

One final note on using TV flyback transformers with symmetric waveforms such as sine waves from a ZVS.  For a given output voltage, the internal diodes will see almost twice the reverse voltage.  In normal flyback use, the transformer output has high positive pulses and comparatively low negative pulses, reducing the diode's peak reverse-bias voltage requirements.  So, keep the output voltage (spark-gap breakdown voltage) not too far above half of the flyback transformer's output rating.

That is some great info and some specifics that will certainly help me out! I think I'll definitely look into grabbing another flyback to run in parallel, as you suggested earlier.
Sadly, during a run last night, i started smelling the undeniable scent of burning electronics. I noticed the high voltage pin on the flyback was actually arcing to the neighboring pin. This probably explains why it was getting so hot! I'm guessing it was doing that last time and I just didn't notice.  I never cut the other pins off, as I didn't think it would turn out to be an issue at the time. Everything is a learning experience, I suppose.
Of note: Feedback. Enough to receive a small shock from the power supply chassis. 
I was hoping that there was no internal damage to the FBT, but before I could run some further tests...my power supply seems to have stopped working. I'm getting zero DC voltage. I disconnected the supply and I can't see any burnt components or bloated caps, nothing. I'm pretty sure i was nowhere near 360W on the DC side when it was running. I'm going to look a bit closer tonight, remove the board, and see if i can figure out what's happened, but it seems my project is on hold until i can get the PS working again. I do have some batteries laying around that I could at least test the flyback with in the meantime. Hopefully the ZVS wasn't affected by whatever happened.

I'll update as I uncover more.

Thanks again, David!

[davekni]

Most TV flyback transformers have the primary high-voltage output (positive rectified output of ~20-30kV) on a read wire lead, not on any of the "pins".  One of the pins is the return (negative) side of the primary HV output.  I'd expect the red HV lead and the return pin to be wired to your spark-gap.

TV flyback transformers are a bit more complex that just the above.  They usually have some resistive dividers from intermediate taps to make adjustable 1-3kV outputs for other CRT electrodes (focus if I recall correctly).  The resistive dividers are intended to be returned to ground (to the primary HV output return pin).  I've had issues with those resistive-divider return pins arcing to other pins if left unconnected.  Usually doesn't cause internal damage, but does causing charring.

Drawing sparks to your DC supply is likely related to it frying.  If your Tesla coil secondary return (bottom end) isn't grounded, but instead has some electrical path back to the DC power, the return current will arc across components within the supply, likely causing failure.

Your system has three separate sections.  I'd recommend that each has a ground connection, to your power line safety ground.  The three are:
1) 24-36Vdc and circuitry powered by it (ZVS and the input side of the flyback transformer).  Negative DC lead is typically grounded.
2) Flyback output, MMC, spark gap, and Tesla primary coil.  Again, negative lead (flyback return pin) is typically grounded.
3) Tesla secondary coil.  This is the most important point to ground, as top-load capacitance is mostly to ground.

[jturnerkc]

Most TV flyback transformers have the primary high-voltage output (positive rectified output of ~20-30kV) on a read wire lead, not on any of the "pins".  One of the pins is the return (negative) side of the primary HV output.  I'd expect the red HV lead and the return pin to be wired to your spark-gap.

TV flyback transformers are a bit more complex that just the above.  They usually have some resistive dividers from intermediate taps to make adjustable 1-3kV outputs for other CRT electrodes (focus if I recall correctly).  The resistive dividers are intended to be returned to ground (to the primary HV output return pin).  I've had issues with those resistive-divider return pins arcing to other pins if left unconnected.  Usually doesn't cause internal damage, but does causing charring.

Drawing sparks to your DC supply is likely related to it frying.  If your Tesla coil secondary return (bottom end) isn't grounded, but instead has some electrical path back to the DC power, the return current will arc across components within the supply, likely causing failure.

Your system has three separate sections.  I'd recommend that each has a ground connection, to your power line safety ground.  The three are:
1) 24-36Vdc and circuitry powered by it (ZVS and the input side of the flyback transformer).  Negative DC lead is typically grounded.
2) Flyback output, MMC, spark gap, and Tesla primary coil.  Again, negative lead (flyback return pin) is typically grounded.
3) Tesla secondary coil.  This is the most important point to ground, as top-load capacitance is mostly to ground.

I misspoke and did intend to refer to the return "pin".
I do have the secondary coil of the TC grounded with a makeshift counterpoise at the moment, which I'm now speculating is not sufficient. I did not have the return pin of the Flyback grounded, it simply went straight into the spark gap. I can certainly understand why this circuit should be grounded. The grounding of this circuit is a source of confusion for me. Should it be isolated from the both of the other ground points?
When you refer to the negative DC lead being grounded, would the power supply not serve that purpose via the mains ground or are we talking about physically grounding the negative DC lead separate from the mains?
When you recommend each section have a "ground connection to [my] power line safety ground", are you suggesting that these can all share a common ground? I did not think this was the case.

After looking into the power supply further, I've determined that the transistors are definitely pooched, as well as the base resistors and associated diodes. I have a couple other random resistor failures. I've yet to test the IC, but everything around it was fine, with the exception of two resistors, so i do suspect it is something i should check as well. I may save the power supply repair for another time and just grab a cheap one for now, so I can get back to this project.

[davekni]

A counterpoise is helpful, especially for high frequencies caused by an arc.  I recommended connecting the counterpoise/secondary bottom to earth ground as well, via power wiring or water pipes.  The counterpoise alone may have several times the capacitance of the topload, but that can leave it with several percent of the top-load voltage, plenty enough to cause problems.

The flyback secondary return pin (negative side, since the red output wire is the positive side) could theoretically remain floating (along with the spark gap, MMC, and Tesla primary coil).  However, I recommend grounding it for two reasons.  Key one is the flyback's internal resistive divider and taps for focus etc. electrodes.  Other reason is due to stray capacitance from MMC/Tesla primary needing a return path.

Concerning the DC supply, it may or may not have a grounded output internally.  Adjustable lab supplies rarely have internal ground connection to the DC output.  This allows the user to ground the + or - or neither side (for say wiring two supplies in series).  Fixed supplies such as for laptop computers are more likely to be internally grounded, typically on the negative output lead.  Some aren't grounded, however, even when the power cord has a ground wire.  The power ground is used just for internal shielding between the supplies internal input and output side.

So, yes, I'm suggesting grounding all three sections to the power-line ground, which might as well be at the same point from a single power cord.  There's no short-circuit caused by this.  All three sections are electrically-isolated, coupled only magnetically.  First-to-second coupling is magnetic through the flyback.  Second to third is the Tesla primary-to-secondary magnetic coupling.  Grounding all three provides the single electrical connection to return any current due to stray capacitance.  A short-circuit would be created only if you ground two different nodes within one section, such as both the + and - outputs of the DC supply.

Good luck with getting it going again!

[jturnerkc]

A counterpoise is helpful, especially for high frequencies caused by an arc.  I recommended connecting the counterpoise/secondary bottom to earth ground as well, via power wiring or water pipes.  The counterpoise alone may have several times the capacitance of the topload, but that can leave it with several percent of the top-load voltage, plenty enough to cause problems.

The flyback secondary return pin (negative side, since the red output wire is the positive side) could theoretically remain floating (along with the spark gap, MMC, and Tesla primary coil).  However, I recommend grounding it for two reasons.  Key one is the flyback's internal resistive divider and taps for focus etc. electrodes.  Other reason is due to stray capacitance from MMC/Tesla primary needing a return path.

Concerning the DC supply, it may or may not have a grounded output internally.  Adjustable lab supplies rarely have internal ground connection to the DC output.  This allows the user to ground the + or - or neither side (for say wiring two supplies in series).  Fixed supplies such as for laptop computers are more likely to be internally grounded, typically on the negative output lead.  Some aren't grounded, however, even when the power cord has a ground wire.  The power ground is used just for internal shielding between the supplies internal input and output side.

So, yes, I'm suggesting grounding all three sections to the power-line ground, which might as well be at the same point from a single power cord.  There's no short-circuit caused by this.  All three sections are electrically-isolated, coupled only magnetically.  First-to-second coupling is magnetic through the flyback.  Second to third is the Tesla primary-to-secondary magnetic coupling.  Grounding all three provides the single electrical connection to return any current due to stray capacitance.  A short-circuit would be created only if you ground two different nodes within one section, such as both the + and - outputs of the DC supply.

Good luck with getting it going again!

I was under the impression that grounding back to mains was a last resort, if anything, with Tesla Coils. No doubt, you know much more than I do about the subject. I had read, from a number of TC information sources, that there's the risk of affecting other electronics in the home, etc. and that high frequency through the water pipes, etc. can be undesirable as well. My understanding was the the best ground was an actual, physical earth ground, separate from the mains ground, at least for the Tesla Secondary.
Drawing a simple schematic, it sounds like you're saying I can just ground everything back to the mains outlet that I'm using to power the supply. Am I misunderstanding something? I realize that the sections are only magnetically coupled, but if a common ground is used, would that not then create a case where they are no longer electrically isolated?

[davekni]

Personally, I've always grounded to power mains, besides using aluminum window-screen on the floor (or on grass when outside).  However, my first Tesla coil was only 6 years ago.  Others here have a much longer history.  Perhaps I'm just lucky to have success with line grounding.  (I've never used line-grounding alone.  Always had some aluminum screen/foil/sheets on the floor too.  Perhaps using the line alone would be problematic.)

Yes, grounding all three sections makes them no-longer electrically isolated.  But it doesn't create any short-circuits.  Only one node of each section is grounded.  What it does provide is a return-path for currents caused by stray capacitance out to the universe.  Otherwise the return currents can arc across the transformers that separate the sections, or through other control circuitry within the power supply.

You could try line-grounding only the first two sections, and keeping your counterpoise separate for Tesla secondary return.  The risk there is if you do get a strike to your Tesla primary, the current spike to the line ground will be higher than if your counterpoise were included in the line grounding to absorb some of the current spike.

[jturnerkc]

Personally, I've always grounded to power mains, besides using aluminum window-screen on the floor (or on grass when outside).  However, my first Tesla coil was only 6 years ago.  Others here have a much longer history.  Perhaps I'm just lucky to have success with line grounding.  (I've never used line-grounding alone.  Always had some aluminum screen/foil/sheets on the floor too.  Perhaps using the line alone would be problematic.)

Yes, grounding all three sections makes them no-longer electrically isolated.  But it doesn't create any short-circuits.  Only one node of each section is grounded.  What it does provide is a return-path for currents caused by stray capacitance out to the universe.  Otherwise the return currents can arc across the transformers that separate the sections, or through other control circuitry within the power supply.

You could try line-grounding only the first two sections, and keeping your counterpoise separate for Tesla secondary return.  The risk there is if you do get a strike to your Tesla primary, the current spike to the line ground will be higher than if your counterpoise were included in the line grounding to absorb some of the current spike.

I have a new power supply getting dropped off today, so I plan to test the ZVS and flyback out. Hopefully the ZVS is ok, and hopefully I'll be able to go for another light tonight.
Just so I understand correctly - in your setup, you were grounding all sections back to mains - would that imply that i can simply ground the DC negative and the flyback return, straight back to the AC mains ground terminal on the power supply? Would it be easier to ground everything to the counterpoise, and just ground the counterpoise to mains?
I'm not particularly worried about a strike to the primary, at least on this build, but I am still questioning whether or not to ground the counterpoise of the secondary separately, to a nearby pipe or something, just so it's not feeding directly back into the same outlet, and may eventually put a proper earth ground in somewhere.
I'll defer to your experience on this one. 

[davekni]

Either option should work.  Grounding the counterpoise separately to a pipe or whatever should be fine, as would tying in with the other connections to the power supply's ground terminal.

BTW, if the power supply has a separate ground terminal on the front, that's a good indication that the DC output is not grounded internally.  So, the DC output does need to be grounded, the negative output to ground by convention.  The fllyback output return (negative output pin) could be connected locally to the DC negative or with a separate wire back to the supply ground.  Locally is probably easier.  Then ground the counterpoise either to a pipe or back to the supply ground or both.

I usually have foil or screen against the floor or ground that extends from my coil back to the power supply, where it ties to the power line ground.  Is your counterpoise something like that?  I usually have more foil/screen extending past the coil and out to either side.  My scope input cables (coax) route against the foil, either just above or just below it.

Have fun!

[jturnerkc]

Either option should work.  Grounding the counterpoise separately to a pipe or whatever should be fine, as would tying in with the other connections to the power supply's ground terminal.

BTW, if the power supply has a separate ground terminal on the front, that's a good indication that the DC output is not grounded internally.  So, the DC output does need to be grounded, the negative output to ground by convention.  The flyback output return (negative output pin) could be connected locally to the DC negative or with a separate wire back to the supply ground.  Locally is probably easier.  Then ground the counterpoise either to a pipe or back to the supply ground or both.

I usually have foil or screen against the floor or ground that extends from my coil back to the power supply, where it ties to the power line ground.  Is your counterpoise something like that?  I usually have more foil/screen extending past the coil and out to either side.  My scope input cables (coax) route against the foil, either just above or just below it.

Have fun!

Wired up the new power supply last night. Started at 24v, but was not able to pull an arc off the HV return pin. I can hear the telltale hum of the flyback, but barely a spark when trying to pull an arc. I checked the ZVS and the FETs seem to meter fine, no shorts, and all resistors and diodes test fine.
After a closer inspection, I noticed a very slight bulge in the flyback housing. Sure enough, there's a few small cracks that formed as a result and I can only assume this flyback is done for. It must have been subjected to a beating long enough to experience an internal failure.
Luckily, I actually came across a pair of matching flybacks and should have those in a couple days.

For the counterpoise, i was using 4 12x12" sheets of 3/32" steel plates, attached together in a large square with aluminum tape (so it can be folded up), and placed on the garage floor underneath the secondary.

[profdc9]

You seem to have solved many problems I had when trying to do this.

One problem I had when trying to use a TV flyback transformer is that the peak voltage can be very high and damage the capacitors.  The open circuit voltage of some flyback transformers can be 30-40 kV!  The other problem is that I destroyed many transformers because when the arc is quenched, it causes a brief burst of high voltage, MHz-frequency RF in the transformer that over the course of a few minutes arced through and destroyed the internal insulation of the flyback transformer.   Then I tried to construct my own beefier  transformer on some huge U-shaped ferrite pieces, and those worked better, but even those were destroyed after a short run. 
I also had problems and could not use a static spark gap, because even with a strong fan blowing on the spark gap it was too hot.  So eventually I had to make a rotary spark gap out of an angle grinder, which is very difficult because it spins very fast and has to be balanced perfectly or will shake itself and everything else to pieces.

Eventually I went to dual MOTs and used a Terry filter which removes the RF spikes, and it was reliable.  This is one reason why DRSSTC can be so much easier, because there is no spark gap to cause voltage spikes, and especially good rotary spark gaps are hard to reconstruct.

Anyways that is a great build and very impressive you got it to work that well.

[jturnerkc]

You seem to have solved many problems I had when trying to do this.

One problem I had when trying to use a TV flyback transformer is that the peak voltage can be very high and damage the capacitors.  The open circuit voltage of some flyback transformers can be 30-40 kV!  The other problem is that I destroyed many transformers because when the arc is quenched, it causes a brief burst of high voltage, MHz-frequency RF in the transformer that over the course of a few minutes arced through and destroyed the internal insulation of the flyback transformer.   Then I tried to construct my own beefier  transformer on some huge U-shaped ferrite pieces, and those worked better, but even those were destroyed after a short run. 
I also had problems and could not use a static spark gap, because even with a strong fan blowing on the spark gap it was too hot.  So eventually I had to make a rotary spark gap out of an angle grinder, which is very difficult because it spins very fast and has to be balanced perfectly or will shake itself and everything else to pieces.

Eventually I went to dual MOTs and used a Terry filter which removes the RF spikes, and it was reliable.  This is one reason why DRSSTC can be so much easier, because there is no spark gap to cause voltage spikes, and especially good rotary spark gaps are hard to reconstruct.

Anyways that is a great build and very impressive you got it to work that well.

Thank you!
Did you happen to have your tank circuit grounded as David suggested above?
Could a low-pass filter be designed and added to snub the higher frequency slapping the flyback?
I struggled with this and transients during another unrelated project, but the same solution wouldn't work here.
I'm guessing the old vintage flybacks would hold up better since they don't have any other components like the "newer" flybacks. Might be worth a shot on top of the rest, but I am curious to see what kind of performance I'd get out of simply grounding the HV return pin on the flyback. Hmm...

[davekni]

I'm presuming you mean low-pass-filter (not high) between the flyback and spark gap.  Low-pass allows the "slow" MMC charging, but blocks the very-fast voltage drop of the spark-gap firing. 

profdc9 does bring up a good point that I'd missed.  The very-sudden voltage drop of the spark-gap firing may be what fried your flyback.  A high-voltage-capable inductor in series with the flyback output should help.  (That's probably the most critical part of any low-pass filter for this use.  A low-pass-filter with capacitors too will have resonances, which will ring to below ground after the spark.)

Inductors that can handle the sudden voltage of the spark-gap firing aren't trivial to find or build.  I learned that the hard way with my inductor-coupled Marx generator.  The charging inductors get a sudden 48kV spike when the Marx spark gaps fire.  My initial home-wound coils arc'ed across winding sections.  Changed to automobile spark coil secondaries, but they lasted only a few minutes.  In both cases, I think that uneven stray capacitance caused the voltage to distribute unevenly across the coil, causing insulation failure.  I finally ended up with strings of tiny commercial 1mH inductors in series, 133 inductors for one large inductor.  (Needed 24 total, so lots of tedious construction.)

A high-voltage-capable resistor in series with the flyback output would also help, but efficiency will suffer.

[jturnerkc]

I'm presuming you mean low-pass-filter (not high) between the flyback and spark gap.  Low-pass allows the "slow" MMC charging, but blocks the very-fast voltage drop of the spark-gap firing. 

profdc9 does bring up a good point that I'd missed.  The very-sudden voltage drop of the spark-gap firing may be what fried your flyback.  A high-voltage-capable inductor in series with the flyback output should help.  (That's probably the most critical part of any low-pass filter for this use.  A low-pass-filter with capacitors too will have resonances, which will ring to below ground after the spark.)

Inductors that can handle the sudden voltage of the spark-gap firing aren't trivial to find or build.  I learned that the hard way with my inductor-coupled Marx generator.  The charging inductors get a sudden 48kV spike when the Marx spark gaps fire.  My initial home-wound coils arc'ed across winding sections.  Changed to automobile spark coil secondaries, but they lasted only a few minutes.  In both cases, I think that uneven stray capacitance caused the voltage to distribute unevenly across the coil, causing insulation failure.  I finally ended up with strings of tiny commercial 1mH inductors in series, 133 inductors for one large inductor.  (Needed 24 total, so lots of tedious construction.)

A high-voltage-capable resistor in series with the flyback output would also help, but efficiency will suffer.

Correct. I meant low-pass, to attenuate the high frequency. Bit of dyslexia there, I guess.
I definitely noticed, even with the available higher voltage inductors I've found, that I'd probably be looking at a couple hundred bucks just to put something like that together (just as a rough guess).
What calculations did you use to determine the inductor specifications you required? I'm not sure I have the patience to wire up 3000 little baby inductors but perhaps I'll come across something in the future that would fit the bill.
I was about to ask about using a flyback secondary, but if the auto ignition coils failed, I'm betting a flyback wouldn't fair any better, and I certainly would prefer to not destroy another one just yet.

Would grounding the the return pin serve any purpose in reducing these spikes?
What about a simple grounded safety gap in parallel with the flyback and main spark gap, similar to those used with NSTs? I did come across a design where the individual was actually just using a simple horn gap, like a mini jacob's ladder as some sort of safety, but seems like it was more-so intended to protect from the effects of a primary strike, and not the voltage cause by the spark gap collapse. I'm not sure how that could be sufficient enough, but wouldn't it be great if the fix were that easy?
Perhaps I can look into hv resistors, for now, and sacrifice some efficiency to save my flybacks.

Needless to say, certain aspects of this project are pushing the limits of my electrical understanding, but that was technically the intent in the first place.
I really appreciate the information, detailed explanations, and patience!
There's really not any in depth information to be found on SGTC's using ZVS/Flyback combo anywhere, that I've found. Considering the preliminary results I was able to obtain with my current build, I'd love to hammer this out and be able to provide a proper, thorough, reference.

[davekni]

Grounding the flyback HV return pin might help slightly.  At least it should prevent any internal arcing from the flyback secondary back to primary.

A string of small inductors isn't all that hard or expensive.  I needed 3000+ because I needed 24 inductors each capable of 48kV.  You need only one inductor at perhaps 30kV or whatever your spark-gap firing voltage is.  100 inductors is probably fine.  Each inductor sees only 300V, which shouldn't be enough to break down even the thin magnet wire insulation.  200 inductors would give lots of margin.  Here's links to a couple Digikey pages for inductor parts that should work, roughly $20 and $30 for 100 parts:
https://www.digikey.com/product-detail/en/taiyo-yuden/LHL08TB153J/587-5891-1-ND/7675011
https://www.digikey.com/product-detail/en/bourns-inc/RLB1014-104KL/RLB1014-104KL-ND/2561370
Checking more distributors might turn up a lower price for these or other similar parts.  I just searched for the lowest cost and highest inductance that could handle 40mA.  Do you know what your flyback output current is?  I was just taking a guess that it wouldn't be above 40mA.

The key detail to making the inductor string is to have uniformly distributed stray capacitance.  That keeps the voltage evenly distributed.  I made a soldering fixture of a string of 6.35mm magnets.  Cut the inductor leads to ~6mm, then lined up a row on each side of the magnet string, staggered so the leads touched in a series configuration.  Then it was easy to run down the string bonding the touching lead pairs with solder.  I made 19-long inductor strings this way, 10 on one side and 9 on the other side.  Each string went into 1" heat-shrink tubing.  The resulting insulated strings were layered (stacked).  I'd share pictures, but my fixture is buried in my storage shed at the moment.  If you want to go this route, I'll get it out and add further description. 

[jturnerkc]

Grounding the flyback HV return pin might help slightly.  At least it should prevent any internal arcing from the flyback secondary back to primary.

A string of small inductors isn't all that hard or expensive.  I needed 3000+ because I needed 24 inductors each capable of 48kV.  You need only one inductor at perhaps 30kV or whatever your spark-gap firing voltage is.  100 inductors is probably fine.  Each inductor sees only 300V, which shouldn't be enough to break down even the thin magnet wire insulation.  200 inductors would give lots of margin.  Here's links to a couple Digikey pages for inductor parts that should work, roughly $20 and $30 for 100 parts:
https://www.digikey.com/product-detail/en/taiyo-yuden/LHL08TB153J/587-5891-1-ND/7675011
https://www.digikey.com/product-detail/en/bourns-inc/RLB1014-104KL/RLB1014-104KL-ND/2561370
Checking more distributors might turn up a lower price for these or other similar parts.  I just searched for the lowest cost and highest inductance that could handle 40mA.  Do you know what your flyback output current is?  I was just taking a guess that it wouldn't be above 40mA.

The key detail to making the inductor string is to have uniformly distributed stray capacitance.  That keeps the voltage evenly distributed.  I made a soldering fixture of a string of 6.35mm magnets.  Cut the inductor leads to ~6mm, then lined up a row on each side of the magnet string, staggered so the leads touched in a series configuration.  Then it was easy to run down the string bonding the touching lead pairs with solder.  I made 19-long inductor strings this way, 10 on one side and 9 on the other side.  Each string went into 1" heat-shrink tubing.  The resulting insulated strings were layered (stacked).  I'd share pictures, but my fixture is buried in my storage shed at the moment.  If you want to go this route, I'll get it out and add further description.

That does sound worth a try.
Does the inductors' own frequency matter here? Should DC resistance be lower to reduce energy storage and minimize any heat buildup, or higher to dissipate more power?
I feel like I already know the answer to this question, but is there any reason a perf board couldn't be used, lining inductors up in rows and soldering in series, instead of forming a long string? I can imagine a scenario where the proximity of all those magnetic fields would be undesirable, but would it particularly matter in this application?

Received a couple flybacks today, but eh... apparently wasn't paying close enough attention. They're a bit smaller than expected (about half the size of my original flyback! I had to laugh at myself a bit...). Is there a practical way to run the primaries in parallel and secondaries in series? I'll try to spec these out as much as I can, but I'm thinking these might be a bit too small.
I'd sure love to find some datasheets on these things... They are identified as MFB-10/332A or E144238. The equivalent is the HR 2287 T17, but I'm struggling to find any detailed info on either.

[davekni]

Placing two flyback transformer secondaries in series will almost-certainly fry one of them.  Flybacks are generally designed to have the HV negative (return) terminal somewhere near the same potential as the primary.  In series, one of the secondary negative pins will be many kV away from it's primary.  Internal insulation on the negative side isn't designed to handle that voltage.

Paralleling both the primaries and secondaries should work, and is probably closer to what you need.  The output voltage is likely enough from one flyback.  Using two in parallel will double the output current, so charge your MMC in half the time.

Yes, perf-board will work fine.  The string method is faster for the large quantities I needed.  Yes, the inductors do interact magnetically with adjacent ones.  Alternating the orientation (180 degree rotation) for radial-lead inductors as I used increases inductance (makes magnetic loops).  That's what I wanted.  It does lower saturation current, however.  Making a string of radial-lead inductors all in the same orientation lowers inductance, but increases saturation current.  I'd recommend making the entire string either the same or alternating, rather than mixing the two options.  If the inductors are spaced out a few mm, then direction won't matter much.  The inductors I used have a small white dot mark on top to show orientation.

For the inductors I suggested previously, resistance isn't enough to make much difference.  For example, the 150-ohm ones, 100 in series will have 15k ohms.  If charging at 30mA from the flyback(s), that's 450V drop, not too much compared to ~30kV.

To avoid excess voltage across any given inductor during the spark discharge, stray capacitance within the inductor string should be reasonably uniform.  I'd suggest a physical layout on your proto-board matching this schematic:


In other words, don't wire a zig-zag.  Make series-connected rows of inductors, wiring the right edge of each row to the left edge of the next row.  That way the electric field will be roughly-uniform from top-to-bottom.  Leave a few mm between rows, whether or not you decide to space out inductors within each row.  Mount the proto-board inductor string in a plastic case or otherwise spaced away from metal.

Good luck with your fun project!