Author Topic: SGTC MK1 - An Accomplishment in Progress  (Read 2170 times)

Offline Uspring

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #40 on: November 27, 2019, 02:03:02 PM »
Quote
I doubt frequency matters as long as it's in the 10+kHz range.  The ionized air path definitely decays significantly in 1-2ms, but I don't think it decays much in <100us.

I tend to agree with this. Plasma is caused by heat and that takes some time to cool off. But even during longer pauses hot air remains, which is a lot thinner than at room temperature. That reduces breakdown voltages along the hot path, which causes an easy reignition there. One can make nicely working Jacobs ladders even at line frequency.


Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #41 on: November 28, 2019, 12:44:10 AM »
My guess about insufficient voltage is that the MMC is charging to barely the spark-firing voltage.  When the voltage is marginal, a spark gap can fire or not depending on just how a corona streamer forms - what dust particles happen to be in the air, UV photons that happen to ionize some air, etc.  If the issue was limited current, the MMC would reach spark-gap firing voltage, just more slowly.

Another possibility occurred to me:  The flyback secondary winding has enough internal capacitance to resonate, and that is coupled to your primary ZVS resonant circuit.  That makes two resonant frequencies, one where the two winding voltages are in-phase and one where they are 180 degrees out-of-phase.  (The two resonant frequencies are discussed in some of the Tesla coil discussions, as the Tesla coil primary and secondary are two coupled resonant circuits.)  Perhaps the ZVS is occasionally locking into the higher-frequency out-of-phase mode.  I saw that occasionally a couple days ago in a ZVS-driven flyback experiment of my own - when the output was loaded more heavily.  The flyback is inefficient in that mode.

Concerning the nature of arcs, I'm just learning with my recent Jacob's ladder project.  My only previous experience was with spark-gap sudden discharges, not with continuous arcs.  Perhaps others here can assist.  If I had to guess, I'd say the fire-y arcs are higher current.  I doubt frequency matters as long as it's in the 10+kHz range.  The ionized air path definitely decays significantly in 1-2ms, but I don't think it decays much in <100us.

Many meter resistance ranges top-out at 20meg, so a 264meg resistor may have shown up as open.  An easy way to look for high resistances is with a DC voltage source and a volt meter.  Meters often have 10meg or 1meg input resistance on voltage ranges.  Apply a DC voltage to the HV output wire, then measure voltage on the pins.  If the DC voltage is negative and above ~30V, then the HV return pin can be found that way.  The internal HV diodes often have 20-30V forward drop, so at least that much voltage is required to see continuity from HV wire (positive output, which is the diode cathode) to the HV return pin.

If you want to head down the analytical path, I'd suggest measuring the flyback output turns.   There are a few ways to do so depending on what tools you have around.  Scope?  Probes good for a few hundred volts?  AC signal generator (some source of low voltage in the kHz range)?

That does make sense and I would agree that the issue would then appear to be voltage. I assumed 24V would be plenty to produce the energy needed. Without being able to measure the output, I couldn't be sure and didn't want to risk frying the little flyback.

It's very interesting that you mention two resonant frequencies. I'm not sure it's of note, but with a multimeter on either ZVS output, I was reading a relatively steady 70kHz. During a quick test the other night with a basic oscilloscope, I noticed the frequency would occasionally drop as low as 50kHz, but only for a very short time. Through my narrow perspective though, the waveforms appeared to be relatively as expected considering the crude Jacob's ladder I was using as a load, so I didn't think much of it. This may or may not be of any significance. We're getting into new territory for me now, and I'm learning more on every exchange. I need to widen my field of view when looking at these things and consider all variables.

I found it very curious that the two different flybacks had such drastically different arc formation. I might even speculate that my spark gap woes could be contributed to this. The arc formation of a typical Jacob’s ladder is exactly the kind of arcs this little flyback makes on its own and sustains them a considerable distance. It's quite impressive.

I experimented briefly with current limiting and was able to obtain a steady, controlled, arc across the spark gap (no capacitors connected). When I get back to it, I’ll try increasing voltage a bit while limiting available current and see what kind of results I get.

I did get ahold of another, larger, flyback today. Fortunately, I actually have the schematic for this one. (attached) I will do some comparisons and report the results.

Meter resistance range is also a good point. The one I'm using was actually a recent purchase and I can't recall the range off-hand. I'll have to double check. I hadn’t considered checking the voltage drop with an actual DC input, but that's a great idea.

I did put together a small signal generator that can output triangle, sine, and square wave up to 1MHz and can be finely adjusted. It can use any DC input. I usually just use a 9V battery.
I technically have a scope. It’s just a cheap, handheld, single-channel, bare-bones, oscilloscope that I purchased as a kit some time ago for very basic purposes. It gets the job done..mostly.
« Last Edit: November 28, 2019, 03:47:45 AM by jturnerkc »

Offline MRMILSTAR

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #42 on: November 28, 2019, 05:40:21 AM »
I have a Marx generator which is powered by an average-size flyback transformer driven by a ZVS driver. I use a 24 volt DC supply to power the ZVS driver. The flyback and ZVS driver have no problem with the 24 volts. Perhaps this will allay your concerns about the 24 volt power supply.
Steve White
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Retired electrical engineer

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #43 on: November 28, 2019, 06:20:37 AM »
Your small flyback transformers may be designed for a bit lower voltage and higher current.  As Steve suggests, it's probably OK to run the ZVS input voltage a bit higher.  The biggest issue may be the unconnected focus pin, if it decides to arc inside the potted windings.  Those small flybacks look new enough that they are likely from a color TV or monitor, which suggests they'd be good for at least 20kV output.

Measuring the turns ratio would help decide what input voltage is safe.  One way is to feed your signal generator sine wave to the flyback input through a resistor, and measure the flyback HV output with your meter.  Hopefully your signal generator can product enough power to get at least 100-200V on the HV output with only the meter load.  Set the signal generator for something in the 20-50kHz range, lower if you can get enough output voltage.  Measure the voltage across the flyback primary with the scope, to see what actual voltage is achieved from the signal generator and resistor.  Adjust the generator and/or resistor to get 100Vdc on the meter.  Record the flyback primary peak voltage from the scope (or P-P and divide by 2).  Adjust the resistor or generator for 200V, and record that input voltage.  The turns ratio is then (200V - 100V) / (peak_input_for_200V - peak_input_for_100V).  The output diode forward drop cancels in the calculation by using deltas (changes in voltages).  (This presumes a 1.0 coupling factor.  The real turns-ratio will be higher by 1/K.)

An alternative method:  Charge a capacitor (0.1 to 5uF) to 100V.  Then connect the capacitor to the flyback secondary, negative to the HV output wire and positive to the HV return pin.  Measure the flyback primary waveform with your scope.  Repeat with the capacitor charged to 200V.  For the primary peak voltage, use the initial fast edge voltage, not the following ring-down.  Calculation is the same as above, except that the actual turns-ratio will be lower by a factor of K.  (It's worth repeating each capacitor discharge multiple times, taking the scope reading from the trace with the cleanest waveform - initial fast step followed by ring-down without subsequent fast steps.  Mechanical touching of wires often makes mechanical bounce.  The good traces will avoid that noise, or at least have it well past the initial step.  Or, use a TRIAC, such as BTA8 or BTA16 or BTB16 or ... as the switch.)

K (coupling factor) is probably between 0.8 and 0.85 based on the couple flybacks I've measured.  If you want to measure K, perhaps the easiest is to run your ZVS at as low a voltage as it can handle.  Measure the frequency with the flyback secondary open (not arcing), then again with the secondary shorted.  If it's like what I see, the frequency change will be ~2.5:1.

Your new flyback has focus taken from an intermediate stage instead of the HV output.  My larger flybacks are that way too.  I'd still suggest grounding that pin (pin 7) to be safe.  Does the specification list anything about output voltage or current?  How about the design input Vcc voltage?

For comparing the flyback arc characteristics, measuring HV return current would be informative.  A series resistor, perhaps 100 ohms to ground.  Measure the voltage with your scope, or add a parallel capacitor and measure DC value with your meter.

Based on how well you are understanding all the information, it's hard to picture that you are just starting.  Impressive progress!
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #44 on: November 30, 2019, 09:41:17 PM »
I have a Marx generator which is powered by an average-size flyback transformer driven by a ZVS driver. I use a 24 volt DC supply to power the ZVS driver. The flyback and ZVS driver have no problem with the 24 volts. Perhaps this will allay your concerns about the 24 volt power supply.

Thanks MRMILSTAR. The capacitors are rated at 630 VAC or 1200 Volts DC. The voltage measured across the capacitors when the flyback is operating and producing a spark is approximately 50-60 Volts, but a safe drain to source rating for the FETs would be 200 Volts, so I'm not particularly concerned whether my ZVS can handle over 24V, I was questioning myself whether I should push the tiny flybacks any harder, especially with the amount of current being drawn (over 10A peaks when not being limited).
The larger flyback I recently got ahold of is only pulling ~5A at peak (when not limited), with the same 24V input.

Your small flyback transformers may be designed for a bit lower voltage and higher current. As Steve suggests, it's probably OK to run the ZVS input voltage a bit higher. The biggest issue may be the unconnected focus pin, if it decides to arc inside the potted windings.  Those small flybacks look new enough that they are likely from a color TV or monitor, which suggests they'd be good for at least 20kV output.

Your new flyback has focus taken from an intermediate stage instead of the HV output.  My larger flybacks are that way too.  I'd still suggest grounding that pin (pin 7) to be safe.  Does the specification list anything about output voltage or current?  How about the design input Vcc voltage?

For comparing the flyback arc characteristics, measuring HV return current would be informative.  A series resistor, perhaps 100 ohms to ground.  Measure the voltage with your scope, or add a parallel capacitor and measure DC value with your meter.

Based on how well you are understanding all the information, it's hard to picture that you are just starting.  Impressive progress!

Another interesting note about the larger flyback: My ZVS driver uses two 0.33 uF capacitors. Each capacitor is connected from a FET drain to ground. With a primary inductance of 22 uH resonating with 0.167 uF, the resulting frequency is approximately 80 kHz (if I'm doing the math right). Observed operational frequency is closer to 70kHz (which is pretty much spot on for what I was seeing with my small flybacks). However, with the larger flyback, this drops to an average of ~40kHz. Unfortunately, I can't compare this to the first large flyback because I didn't take frequency readings on it.

I ran it on the crude jacob's ladder I use just as a load. After about 10 minutes, maybe, temp reading on the secondary/housing was 60C. The primary and core were noticably cooler. ZVS fets were cool as well. This is quite different from the results when running the smaller flybacks, where EVERYTHING was hot.
Grounding pin 7 on the larger flyback, probably contributed to this difference, I can imagine.
Speaking of pin 7 - were you recommending to ground pin 7 and leave the HV return pin un-grounded, or tying the two pins together and then grounding?
From what I was able to gather, I believe this flyback is rated for 24.2kV, but I don't have an exact number on the ouput current. I came across some information in a repair manual that seems to contradict that though, and implies a 30.5kV "limited EHT rating". An uneducated guess would tell me I could safely assume around 150W?

It may or not be of note, but figured I'd mention that I'm using a bifilar winding for the primary this time (still 5+5). I did the same with my first flyback, but not the smaller ones. The difference may be negligible, but I thought I'd give it a try again, thinking it may help, to some extent, reduce the resonance imbalance cause by leakage inductance, and perhaps help stabilize frequency fluctuations I was seeing in the smaller flyback tests. Waveforms appear to be effectively the same, both when drawing and arc and not. Although the frequency is lower, I'm not seeing the fluctuation I observed with the small flyback, but all-in-all it appears more likely that the alternative winding method is not making any noticeable difference.

As far as measuring turns in the secondary, my signal generator definitely can't put out that kind of voltage (i put it together to be pocket size and was really just using it as a trigger), so I would have to use the capacitor method. I will definitely make a note of that info and perhaps try it in the future, but I think I'll set that aside for the time being. I do think you're right on with the .8-.85 coupling coefficient though.

I appreciate the support and encouragement. I'm not super new to electronics, but still consider myself a novice. This is, however, my first foray into high voltage. Before even attempting any of this, I spent months reading and simulating, etc. Even with all that, I think I'm picking up things here, with you, that I never found mentioned in my research. I'm a quick study, so I always appreciate a challenge, so I'm loving this. I can't thank you enough for your time and sharing your experience!
« Last Edit: December 01, 2019, 01:14:57 AM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #45 on: December 01, 2019, 02:56:39 AM »
Bifilar winding shouldn't matter much.  For the ZVS, the leakage inductance just adds to the inductance from positive DC input, so is a slight advantage.

I'd missed your original note about the two 0.33uF caps being from drain-to-ground.  Most ZVS circuits I've seen have a capacitor between the two drains, but not to ground.  Having capacitors to ground causes the FETs to carry the resonant current in addition to the DC input current, so increases their power dissipation.  Having capacitors to ground as you do may have advantages, though.  I was thinking it might help keep the oscillation to the lower resonant frequency of the two for the case of coupled resonant circuits.  Attempted simulation of that topology a few days ago, but LTSpice didn't converge well, and I didn't take time to play with the algorithm parameters (charge tolerance, ...).

That's likely why you're measuring 70kHz instead of 80kHz - the HV winding capacitance scales by turns^2 and adds to the primary capacitance in the lower-frequency mode.  (The flybacks I've been playing with drop even more due to secondary capacitance.)

BTW, due to my not reading carefully about caps to ground, I'd been presuming 0.66uF primary rather than 0.167uF.  So, that may have messed up some of my previous-post calculations.  However, with 0.33uF from each drain to ground, the effective resonant capacitance is actually 0.33uF, not 0.167uF.  That's because at any given point in time, one of the two 0.33uF caps is shorted by a FET.  Which cap toggles, but only one is active at a time.  The higher FET current is due to that cap shorting, carrying the resonant current from the other cap.  You can see this easily in simulation - measuring frequencies and FET currents in both topologies.

How did you determine 22uH?  Was that something I calculated (from bogus assumptions)?  The best way I know is to measure frequency with a much larger capacitor (ie 3-30uF) so the secondary capacitance is less significant.  (For best accuracy, measure with two different large capacitors and do the algebra to calculate inductance and parasitic capacitance - two measurements and two variables.  That's what I just did for a flyback last week - one I intentionally fried the internal diodes to get AC output.)  Ring-down is the best method with large capacitors.  Running the ZVS will result in low frequency and high current and therefore core saturation, which lowers inductance.

A larger flyback is likely to have a bit more inductance (for 10-turns), but probably not enough to explain 70kHz to 40kHz (about 3x inductance).  Inductance could actually be 3x higher, or perhaps the resonant mode is different.

The HV return pin definitely needs to remain grounded, along with pin 7.  Otherwise the HV return current passes through the high-value resistors of the pin 7 internal network.

150W seems reasonable, 5-6mA at 24-30kV.   With ZVS sine-wave input, the current can probably be a bit higher, and the voltage a bit lower.  (Lower voltage because sine-waves are symmetric, while flyback waveforms have much less reverse voltage.  Higher current because the diode forward-conduction will have higher duty cycle with sine-wave drive, so lower peak current.)

For measuring turns ratio, the signal generator doesn't need to make more than 1-2V peak.  The secondary turn count is likely at least 1000, so at least 100x your 10-turn primary.  However, it needs to get 1-2V peak into a fairly low impedance, which it may not be capable of doing.

For your new flyback for which you attached a schematic, pins 2 to 6 is shown as 90Vpp.  That would be a great place to measure with your scope.  Ground pin 2 and measure pin 6 or visa-versa.  With no HV load, measure pin 2-6 Vpp relative to ZVS input voltage.  That will determine what input DC voltage is safe to run.

Fun project!  Thank you for the detailed information.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #46 on: December 01, 2019, 07:32:44 PM »
Bifilar winding shouldn't matter much.  For the ZVS, the leakage inductance just adds to the inductance from positive DC input, so is a slight advantage.

I'd missed your original note about the two 0.33uF caps being from drain-to-ground.  Most ZVS circuits I've seen have a capacitor between the two drains, but not to ground.  Having capacitors to ground causes the FETs to carry the resonant current in addition to the DC input current, so increases their power dissipation.  Having capacitors to ground as you do may have advantages, though.  I was thinking it might help keep the oscillation to the lower resonant frequency of the two for the case of coupled resonant circuits.  Attempted simulation of that topology a few days ago, but LTSpice didn't converge well, and I didn't take time to play with the algorithm parameters (charge tolerance, ...).

That's likely why you're measuring 70kHz instead of 80kHz - the HV winding capacitance scales by turns^2 and adds to the primary capacitance in the lower-frequency mode.  (The flybacks I've been playing with drop even more due to secondary capacitance.)

BTW, due to my not reading carefully about caps to ground, I'd been presuming 0.66uF primary rather than 0.167uF.  So, that may have messed up some of my previous-post calculations.  However, with 0.33uF from each drain to ground, the effective resonant capacitance is actually 0.33uF, not 0.167uF.  That's because at any given point in time, one of the two 0.33uF caps is shorted by a FET.  Which cap toggles, but only one is active at a time.  The higher FET current is due to that cap shorting, carrying the resonant current from the other cap.  You can see this easily in simulation - measuring frequencies and FET currents in both topologies.

Thank you for pointing out the power dissipation! That wasn't something I even looked at when simulating. The difference seemed negligible, operation-wise, but I hadn't made considerations for power dissipation. I don't particularly recall the reason for doing that other than some proposed method I may have read and probably didn't understand. I can't exactly remember my thought process there.
Your note regarding resonant capacitance - I should have known better on that one. That's certainly quite clear to see in simulation.
I think i did originally misspeak regarding my ZVS configuration because I was not using the same configuration for my sim. That's my fault for the confusion there and now I understand that may have contributed to some of my other confusions.
Quote
How did you determine 22uH?  Was that something I calculated (from bogus assumptions)?  The best way I know is to measure frequency with a much larger capacitor (ie 3-30uF) so the secondary capacitance is less significant.  (For best accuracy, measure with two different large capacitors and do the algebra to calculate inductance and parasitic capacitance - two measurements and two variables.  That's what I just did for a flyback last week - one I intentionally fried the internal diodes to get AC output.)  Ring-down is the best method with large capacitors.  Running the ZVS will result in low frequency and high current and therefore core saturation, which lowers inductance.

The 22uH was based on some rough guesses entered into an inductance calculator and also seemed to match up closely to results another individual observed using effectively the same ZVS/flyback configuration. I made some other assumptions that were incorrect, as you pointed out, so this may or may not be near accurate. It made sense at the time as the results were close to my observed output when using the smaller flybacks.

Quote
A larger flyback is likely to have a bit more inductance (for 10-turns), but probably not enough to explain 70kHz to 40kHz (about 3x inductance).  Inductance could actually be 3x higher, or perhaps the resonant mode is different.

Right, I was not expecting that significant of a drop! I have that second large flyback, now, that I'll throw on there today and see what kind of frequency and other readings I get in comparison.

Quote
The HV return pin definitely needs to remain grounded, along with pin 7.  Otherwise the HV return current passes through the high-value resistors of the pin 7 internal network.

150W seems reasonable, 5-6mA at 24-30kV. With ZVS sine-wave input, the current can probably be a bit higher, and the voltage a bit lower.  (Lower voltage because sine-waves are symmetric, while flyback waveforms have much less reverse voltage.  Higher current because the diode forward-conduction will have higher duty cycle with sine-wave drive, so lower peak current.)

For your new flyback for which you attached a schematic, pins 2 to 6 is shown as 90Vpp.  That would be a great place to measure with your scope.  Ground pin 2 and measure pin 6 or visa-versa.  With no HV load, measure pin 2-6 Vpp relative to ZVS input voltage.  That will determine what input DC voltage is safe to run.

Excellent tip! Thank you for that! That will certainly help to know that value.
I should have time today to continue working, so will update with new results asap.
Thank you, again!
« Last Edit: December 02, 2019, 04:16:52 AM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #47 on: December 02, 2019, 04:09:37 AM »
When measuring pin 2 to 6 Vpp, there's no need to derate the 90 Vpp rating.  The diode reverse voltage is the output peak-to-peak voltage.  (The output peak voltage will be lower because of sine-wave drive, with as much reverse voltage as forward voltage.)

The inductance for a flyback I wound with 10 turns is ~60uH.  It's probably physically a bit larger than your small ones.

Can you provide any more details about your comment: "I found it interesting that the voltage across the caps was quite low when tied to ground"?  The peak voltage across each cap should be the same grounded or drain-to-drain.  Peak-to-peak will be half, because each cap sees a one-sided waveform, not going more than a diode-drop below ground.

Having some of the resonant capacitance to ground could have an advantage when using IGBT parts for a ZVS oscillator.  It will make the IGBT current negative just before switching off.  I haven't explored the effects yet, but it might be useful for IGBTs with slow turn-off.

Looking forward to your new update!  Hopefully it's a successful day of experimenting.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #48 on: December 02, 2019, 05:49:05 AM »
When measuring pin 2 to 6 Vpp, there's no need to derate the 90 Vpp rating.  The diode reverse voltage is the output peak-to-peak voltage.  (The output peak voltage will be lower because of sine-wave drive, with as much reverse voltage as forward voltage.)

The inductance for a flyback I wound with 10 turns is ~60uH.  It's probably physically a bit larger than your small ones.

Can you provide any more details about your comment: "I found it interesting that the voltage across the caps was quite low when tied to ground"?  The peak voltage across each cap should be the same grounded or drain-to-drain.  Peak-to-peak will be half, because each cap sees a one-sided waveform, not going more than a diode-drop below ground.

Having some of the resonant capacitance to ground could have an advantage when using IGBT parts for a ZVS oscillator.  It will make the IGBT current negative just before switching off.  I haven't explored the effects yet, but it might be useful for IGBTs with slow turn-off.

Looking forward to your new update!  Hopefully it's a successful day of experimenting.

I tried to go back and correct my voltage statement, but you got to it before I did.
I was only reading half of the waveform and out of ignorance was not figuring peak-to-peak for both caps and confused myself a bit. As you pointed out, one is always shorted, so I was only reading the Vpp of that particular cap.

I'm not well versed in LtSpice. It's a little too deep for me at this point, but I've managed to make good use of the Falstad simulator. I've attached the circuits for the two different ZVS topologies. I haven't entered my 'real-world' values as this was just a base model for operational comparison purposes. I also still don't have accurate values on my flyback primary/secondary. These can be a pain to simulate, I've noticed. A rough guess has yet to get me any further in mirroring my actual circuit, at least as closely as possible, especially when trying to including the rest of the TC circuit.
*Note: the 1u resistors were used just to prevent capacitor loop error in Falstad. I modeled it this way simply because I wanted to show 2 capacitors. The values I used for the transformer are: 83mH primary, Ratio is 1:2, and K is .99 (again, just a working model). For clarification on the screenshot - the top two waveforms are each of the capacitors, the third waveform down is one FET, and the 4th waveform is the same FET's power consumption. The left column is the typical ZVS topology, and the right column is how I currently have mine configured.)

I noticed the frequency increase as well as a slight increase in power consumption, as you mentioned. The FET waveform changes and the current drops slightly negative, as you pointed out, as well as the voltage. I'm curious about that brief positive spike just before the drop and subsequent ramp up. FET voltage remains relatively unchanged (~1V), but current increases quite a bit. The rms voltage of the capacitors effectively match the FETs, which makes sense (i think). Frequency and current seem to be the most noteworthy.
Looking back, I believe this topology is proposed to be used in higher frequency applications than is needed here...which I suppose makes sense, as no components need to be added/removed to achieve a higher frequency. Regardless of its intended application, it would appear, to my eyes anyways, that I should go the drain-to-drain route with the capacitors.

I think i did originally misspeak, and may have gone back and corrected my claim, regarding my ZVS configuration because I was not using the same configuration for my sim at the time. That's my fault for the mix-up there and now I understand that may have contributed to following confusions.

After flipping through some old notes, the drain-to-ground implementation seems likely to have spawned from notes I made on a conversation regarding FETs burning out in an H-Bridge motor driver application a while back.

I spent most of the time I had set aside today practicing my soldering skills putting together 120, 100mH, inductors, so I don't have much to update on just yet, other than that I am glad it was not more! I used some small boards I had laying around and put together a little two-tiered stack of 60 inductors each. I didn't have time to wire it up in the circuit, but it's ready to go.
« Last Edit: December 03, 2019, 02:09:04 AM by jturnerkc »

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #49 on: December 07, 2019, 09:48:21 PM »

The small positive current spike is likely because the FET turns on slightly before its drain voltage reaches 0, so relatively-suddenly discharges its drain cap the last few volts.  BTW, the relative drain current between the two topologies depends on the relative resonant current to load current.  If you change the 1k load in simulation, it changes the load current, but not the resonant current.  A low-impedance resonant circuit would make the extra FET loss of the drain-to-ground topology worse.

Yes, the drain-to-ground topology smooths out the drain voltage during the transition points.  That may have value with slower switch devices (IGBTs).  In my bit of simulation, drain-to-ground caps made it more difficult to get the ZVS to oscillate at the lower frequency parallel mode of the input and output windings.  I'm not sure why.  I'll be experimenting with that for a DIY plasma ball eventually, but December is busy for me.

Coupling of 0.99 is high for your simulation transformer, but won't matter much until you add an output capacitor to simulate the flyback secondary's internal stray capacitance (wire and diodes).  With a realistic output capacitance and coupling (80-85%), you can experiment with the two resonant frequencies.


The parameters I used for the transformer are not representative of my actual application. I entered a higher inductance and just left the coupling parameter at default. In simulation, it made it easier to observe the waveforms, considering I can only increase the division so much. I found that just increasing the inductance would reduce the frequency and make those observations easier. This seemed like a simple enough solution at the time. Although it was not completely representative, it simply helped me visualize what is generally happening at different locations within the circuit, during different conditions.

The ZVS circuit can definitely be fickle in simulation.

A few updates:

The larger secondary and new topload that I was going to be using will be set aside for the time being, as I use the previous secondary and topload (since I already had that tuned for the most part). The calculated resonant frequency of the larger secondary (backed up by observations when testing), would indicate that I either need to increase the # of turns in my primary or double the capacitance of the MMC to reduce the primary resonant frequency. Simply adding another 20 caps in parallel would lower the frequency enough to come within ~2% of resonance (using JavaTC) I'm going to go ahead and wire up another 20 capacitors and set them aside until I decide what to do. That would be cheaper and less time consuming than purchasing more copper and re-'winding' the primary.
Decisions. Decisions.

In the meantime, I reduced the inductor string to 6H. Even after taking resistance readings, voltage drop, etc. the day before - the next day went to check everything again before connecting, and nothing. I ended up finding a single inductor reading as an open circuit. Odd, but I just need to swap that one out apparently. Each level of my 'inductor bank' had 60 inductors, so I removed that level and left myself with 60, so 6H.



During some simulation, however, I found that it may be beneficial to actually use 2 separate strings. When an inductor string is placed on both the HV output and return. It was important to ground the HV return and then attach the inductor between the ground point and the spark gap. Another observation I made, when simulating this configuration was that placing the MMC in parallel and the spark gap in series (swapping the components around) appeared to positively affect the circuit. I judge this from a certainly novice standpoint, but I see less noise and a more uniform operation when simulating in this way. The drain to ground implementaion also seems to aid in the scheme of things.

Here is how I have it laid out in simulation. (I know the flyback representation is probably not ideal, but I wanted to try and simulate the effect of grounding the focus pin with the HV return. I'm using three diodes due to 3 diodes being represented on the flyback schematic.



Testing last night was not entirely successful. I believe the issue is actually with my spark gap and crude adjustment method. I'm going to be working on improving that today and hopefully give it another run tonight with more thorough results. At this point, I'm just trying to get back to my previous results, before losing the first flyback, with these added stability improvements.

One note: the DC circuitry does not appreciate being tied to the same ground as is being used for the HV return (and pin 7). The first bang in the spark gap would immediately reset my regulator, and/or disrupt DC output of the Power Supply. I'm not exactly sure which, but I would lean towards the latter being the reason the regulator reset and shut the ZVS off. Then again, my oscilloscope reset itself as well and it was only connected to one of the zvs outputs. Either way, I didn't like it, so, for now, I'll be keeping the rest of the TC circuit isolated from DC and mains. Something odd like this is kind of what I expected and the reason I hesitated to use a shared ground, especially back to the mains ground.
The TC secondary is grounded to only the counterpoise, so I grounded the HV return (& pin 7) to the counterpoise for the time being and that did remedy the power disruption I was experiencing. I am considering running a line from the counterpoise to a nearby water pipe just for an added measure. In addition, I'm definitely noticing a difference in behavior depending on what I do with pin 7, but I can't determine what the best measure is. Performance seems better when it's simply left disconnected or grounded to the counterpoise, instead of grounding both the HV return and pin 7. Tying the HV return and pin 7 together seems to adversely affect performance.

Speaking of the counterpoise, I'm now using a 3'x3' piece of aluminum with a perforated pattern. I'm curious if it will work well.

 
« Last Edit: December 08, 2019, 05:01:14 AM by jturnerkc »

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #50 on: December 09, 2019, 12:53:57 AM »
Great detail.  Hopefully I can make a coherent response with all the thoughts it triggers.

For the inductor sets, I'd recommend mounting them side by side or end to end, which ever way extends the rows or columns in the same sequence they are wired within each board.  In other words, input to the inductor string on one edge and output on the opposite edge.  With the inductor boards stacked, the capacitance between boards bypasses inductors for the highest-frequencies of the spark gap.  (This may be less important if driving the MMC directly instead of the spark gap - see farther below.)  My other possible concern is the etched copper on the breadboard used for inductor support.  Is the copper just at the fingers and plating through holes?  Or, does it have any larger traces or planes (intended for distributing power or ground in a typical circuit)?  Such traces could bypass the start-to-end of each inductor array with close-enough gaps to potentially arc over.  Even if not arcing, they would add capacitance that bypass inductors much as the board stack.  6H vs. 12H isn't an issue.  It's that with 6H, the voltage is split among only 60 inductors, so higher voltage per inductor, so higher risk of breaking down the inductor's internal magnet wire enamel.

For the larger lower-frequency secondary, the trade-off is of energy-per-spark vs. spark repeat frequency.  Twice the capacitance is twice the energy, which will take twice as long to recharge for given ZVS input power.  I don't have any particular recommendation.  Others here are more likely to have experience with which would produce a more impressive system.

For simulation, note that the flyback schematic had diodes between secondary winding sections, and the focus tap came after the first diode.  Focus is a DC tap, not AC as in your simulation.  Unrelated thought on focus:  since this flyback has focus coming from a tap rather than the full HV output, there's a better chance of keeping it from arcing when open.  It will be at only 1/3rd of your output voltage.  I'd still recommend tying it to the HV return pin, but perhaps not critical.  (I suspect your small flybacks take focus from the full HV output as do the smallest ones I have.)

For grounding, I'd suggest a ground plane under the entire "low" voltage side, the front half in your image.  Given the relative proximity to the higher-voltage spark-gap components, I'd recommend that the ground plane include a vertical part between the two halves.  Ie. a piece of aluminum bent into an L, with horizontal leg under all the LV circuitry and vertical leg separating the two halves, with some cutout or pass-through for the flyback primary leads.  I'd ground this sheet-metal piece to the line ground at the input to the power supply (at the point the line cord enters your system), and to the negative supply output terminal.  (For added protection against supply confusion, run the supply output wires (both + and -) for a few turns through a large ferrite bead or other common-mode choke.  Ground the - side after the choke.)

Concerning power-supply confusion due to sparks, that's my guess.  However, if a ZVS circuit stops oscillating for whatever reason (usually a low-Q resonant circuit), it's supply current ramps up "indefinitely", to the short-circuit capability of the FETs.  That would shut down the supply (of course).

The larger counterpoise should help.  Is it on a concrete slab or other surface built directly on the ground, or on a typical indoor raised floor?  The former will have better capacitance to ground.  If the latter, then I think some connection to a water pipe or line ground would be wise.  For the middle section (flyback secondary/MMC/spark-gap circuitry), I'd still recommend some form of grounding of the HV return pin.  Otherwise stray capacitance could cause a voltage spike on that pin when the spark-gap fires, eventually breaking down insulation within the flyback.  If the power supply still has issues after adding a ground plane under it, perhaps grounding the HV return down to the counterpoise would be less problematic.

Wiring the flyback HV output to the MMC instead of the spark gap isn't normal (to my knowledge), but could have advantages in your case.  It does make the inductor string critical.  After spark-gap firing, the MMC voltage resonates negative almost as far as it was charged positive.  Negative voltage on the flyback could be problematic depending on it's internal leakage inductance, as the diodes would all be forward-biased.  However, as you pointed out, the MMC doesn't have the sudden voltage step of the spark-gap.  That makes the slew rate more manageable, lowering the influence of stray capacitance.  So, it's a trade-off of gentle slew rate for higher peak-to-peak voltage across the inductor string.  Thus, if you feed the MMC, having all 120 inductors is more important, to share the higher peak-to-peak voltage, but the physical layout of the inductors isn't quite as critical.  (I'd still avoid the stack configuration, however.)

If I missed anything for which you wanted feedback, please let me know.

One more thought for grounding the flyback HV return:  Some DRSSTC designs use a bidirectional TVS (Transient Voltage Suppressor) between the Tesla primary and ground.  Sometimes a string of TVS devices rather than only one to get more voltage.  DRSSTC H-Bridges are often powered by a VBus that's directly tied to line voltage (diode bridge from line), so cannot ground that circuit.  A TVS string or capacitor conducts high frequency spikes to ground.  (For the DRSSTC case, spikes are usually Tesla secondary-to-primary arcs.  In your case, they are also normal spark-gap operation.)  For your design, a TVS would be better than a capacitor.  It allows for a moderate spike (500V or whatever TVS voltage you pick), but protects the flyback from larger spikes.  If direct grounding to the counterpoise or to the new ground plane still confuses your DC supply, grounding through a TVS might solve that issue while still limiting voltage to something the flyback transformer insulation can handle.
« Last Edit: December 09, 2019, 03:35:33 AM by davekni »
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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #51 on: December 09, 2019, 03:41:42 PM »
For the inductor sets, I'd recommend mounting them side by side or end to end, which ever way extends the rows or columns in the same sequence they are wired within each board.  In other words, input to the inductor string on one edge and output on the opposite edge.  With the inductor boards stacked, the capacitance between boards bypasses inductors for the highest-frequencies of the spark gap.  (This may be less important if driving the MMC directly instead of the spark gap - see farther below.)  My other possible concern is the etched copper on the breadboard used for inductor support.  Is the copper just at the fingers and plating through holes?  Or, does it have any larger traces or planes (intended for distributing power or ground in a typical circuit)?  Such traces could bypass the start-to-end of each inductor array with close-enough gaps to potentially arc over.  Even if not arcing, they would add capacitance that bypass inductors much as the board stack.  6H vs. 12H isn't an issue.  It's that with 6H, the voltage is split among only 60 inductors, so higher voltage per inductor, so higher risk of breaking down the inductor's internal magnet wire enamel.

That was in the back of my mind – how much stray capacitance that might add, and what the effect and magnetic field interaction might be when stacking the two. It was out of convenience more than anything, so can easily be remedied. I appreciate you clearing up those suspicions up for me. I did consider the voltage-per-inductor if I were to split the strings. I'm sure I'm pushing them a bit, but right now I'm only running for a few seconds at a time - not giving anything too much of a beating.

I'm glad you noticed the boards, as well - as that was another thought in the back of my mind. There are no traces or planes, just plated through-holes. I have never had any issues with arcing using these boards in 'high voltage' applications, but I know i should not be using these particular style boards. There's certain "upgrades" I'd like to make before going much further. I used some lexan and drilled holes to make a proper MMC array yesterday. I’ll be mounting all 40 so the capacitance can be adjusted. Will hopefully have that finished up within the next couple days. I'll probably leave the inductors on the boards they're on to avoid desoldering 120 inductors, probably ruining a few along the way. I'm not completely satisfied with my spark gap right now either. Might revert back to the ol' drawer knobs as I tweak the one I worked on so lovingly.

Quote
For simulation, note that the flyback schematic had diodes between secondary winding sections, and the focus tap came after the first diode.  Focus is a DC tap, not AC as in your simulation.  Unrelated thought on focus:  since this flyback has focus coming from a tap rather than the full HV output, there's a better chance of keeping it from arcing when open.  It will be at only 1/3rd of your output voltage.  I'd still recommend tying it to the HV return pin, but perhaps not critical.  (I suspect your small flybacks take focus from the full HV output as do the smallest ones I have.)

That's a good point and something I had done in some simulations, but left out in the example I provided. I've updated this in my current representation.

Quote
For grounding, I'd suggest a ground plane under the entire "low" voltage side, the front half in your image.  Given the relative proximity to the higher-voltage spark-gap components, I'd recommend that the ground plane include a vertical part between the two halves.  Ie. a piece of aluminum bent into an L, with horizontal leg under all the LV circuitry and vertical leg separating the two halves, with some cutout or pass-through for the flyback primary leads.  I'd ground this sheet-metal piece to the line ground at the input to the power supply (at the point the line cord enters your system), and to the negative supply output terminal.  (For added protection against supply confusion, run the supply output wires (both + and -) for a few turns through a large ferrite bead or other common-mode choke.  Ground the - side after the choke.)

I do have a few pieces of 1'x1' steel plates that could be suspended beneath the low voltage side. Is that what you're picturing? Are you suggesting the vertical plane for shielding purposes? Another plate could be attached vertically between the two halves. Then again, a 1x2 sheet of aluminum would be lighter and could just be bent. More decisions.

Quote
The larger counterpoise should help.  Is it on a concrete slab or other surface built directly on the ground, or on a typical indoor raised floor?  The former will have better capacitance to ground.  If the latter, then I think some connection to a water pipe or line ground would be wise.  For the middle section (flyback secondary/MMC/spark-gap circuitry), I'd still recommend some form of grounding of the HV return pin.  Otherwise stray capacitance could cause a voltage spike on that pin when the spark-gap fires, eventually breaking down insulation within the flyback.  If the power supply still has issues after adding a ground plane under it, perhaps grounding the HV return down to the counterpoise would be less problematic.

The counterpoise is placed directly on my concrete garage floor, so ground level.
I’ll make a short video of the different configurations I’ve tried, and the observable effects.
This whole stray capacitance factor is doing my head in.

I feel stupid even bringing this up (I'm sure it's simple enough), but  as you can see from the attached image on my last post, I've placed the entire setup on a stand, which is metal. How, significantly, might this now be affecting operation? I know that anything in the vacinity of the coil will affect the resonant frequency, but how greatly, if frequency is accounted for, can this change the behavior?
Too add to my feeling of ignorance, I'm left scratching my head today after observing what resembled lower-voltage sparks being created at one of the bolts holding the stand together . At the time, there was absolutely no live wiring in contact with the frame, and the stand is sitting on 2x4's on top of the counterpoise (the wood shelf and top are not even attached to the frame). I attached a wire from the bolt to my grounding bar, leading back to the mains ground...still sparks from the bolt. The spark appears when the spark gap fires. What is this voodoo?

Here’s a video where you can see the spark from the stand [go ahead and disregard the BPS here]:
https://drive.google.com/open?id=1HkMU6Wq_A7iXT1Z1vB4ykZQa03OFjEHc

Quote
Wiring the flyback HV output to the MMC instead of the spark gap isn't normal (to my knowledge), but could have advantages in your case.  It does make the inductor string critical.  After spark-gap firing, the MMC voltage resonates negative almost as far as it was charged positive.  Negative voltage on the flyback could be problematic depending on it's internal leakage inductance, as the diodes would all be forward-biased.  However, as you pointed out, the MMC doesn't have the sudden voltage step of the spark-gap.  That makes the slew rate more manageable, lowering the influence of stray capacitance.  So, it's a trade-off of gentle slew rate for higher peak-to-peak voltage across the inductor string.  Thus, if you feed the MMC, having all 120 inductors is more important, to share the higher peak-to-peak voltage, but the physical layout of the inductors isn't quite as critical.  (I'd still avoid the stack configuration, however.)

I have seen the circuit configured in both ways, but, you're right, almost always with the spark gap in parallel. The capacitor-in-parallel configuration seems more common in “spark gap transmitter" schematics than “tesla coil” schematics, but, even then, can be found interchanged.

An issue I was having yesterday – I haven’t conclusively determined the cause – was the varying BPS. I experimented with adjusting voltage/current as much as I was comfortable, but seemed to always have the same result.
Could this have anything to do with the single inductor string possibly affecting the flyback's charging of the MMC or affecting the oscillations in a way that the MMC is noit fully discharging, or possibly overcharging, etc. for some reason?
An inductor string on both the HV out and return, I would think, should help further prevent interference between/isolate the flyback output and the tank circuit as it oscillates. Just a suspicion.

These new variables are making it harder for me to troubleshoot this solo.

Here’s a video of the effect during a quick 18V test: 
https://drive.google.com/open?id=1HpKlL3n0VsK1rMohkfpC5O3ucFM3YPM9
and a better one with view of the spark gap:
https://drive.google.com/open?id=1Ivh2nUFPYYyEWtffFCxdjwl6uGNmo1NY

In the videos, the breaks are not exactly inconsistent, per say, but the speed varies. It will be constant at one speed, then all the sudden start firing much faster, then return back to where it was. It's easy to see the effect on arc formation.

It's not the best example... - I'd like to record the actual spark gap at 120fps, as well, to observe that more clearly.
I might even be able to use some neutral density filters on my lens to actually be able to see the arc in the gap, as well. Hmm...that would be interesting.

Quote
One more thought for grounding the flyback HV return:  Some DRSSTC designs use a bidirectional TVS (Transient Voltage Suppressor) between the Tesla primary and ground.  Sometimes a string of TVS devices rather than only one to get more voltage.  DRSSTC H-Bridges are often powered by a VBus that's directly tied to line voltage (diode bridge from line), so cannot ground that circuit.  A TVS string or capacitor conducts high frequency spikes to ground.  (For the DRSSTC case, spikes are usually Tesla secondary-to-primary arcs.  In your case, they are also normal spark-gap operation.)  For your design, a TVS would be better than a capacitor.  It allows for a moderate spike (500V or whatever TVS voltage you pick), but protects the flyback from larger spikes.  If direct grounding to the counterpoise or to the new ground plane still confuses your DC supply, grounding through a TVS might solve that issue while still limiting voltage to something the flyback transformer insulation can handle.

That’s an interesting idea – something I should probably have known from other projects where I had to deal with transients. Instead of the HV return pin directly to ground, you’re proposing to place TVS diodes in series along the way – am I getting that right? Would it not be better to use MOVs with a higher clamping voltage and tolerance for higher energy/temperatures? Would large enough spikes be expected that I could throw in a little  NE-2 neon in the mix as an indicator?
« Last Edit: December 10, 2019, 04:08:34 AM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #52 on: December 10, 2019, 06:04:28 AM »
Wow, a lot of great information.  I'll attempt to comment top-down.  (When I quote, it often ends up jumbled on the reply, so I'm skipping such.)

The inductor boards aren't an issue with only plated holes/pads.  No need to change.  I just saw the edge-connector pads and thought they might be like some old breadboard I have that include power and ground traces distributed across the hole array.  (Stacked boards are a capacitive-coupling issue, not magnetic, in this situation anyway.)

If making a 40-cap MMC with ability to run 20, the two strings need to be well separated.  I'd suggest two separate strings that can be bolted together.  With two strings in place, and one string open at one end, there's your full ~20kV primary voltage from the open string to the adjacent connected string.

If the 1'x1' steel is galvanized (zinc plated), it could work.  Plain steel has a VERY thin skin depth due to being ferromagnetic.  The separate sheets would still need good connections along their edges even if galvanized.  If you don't mind the cheap construction, aluminum foil taped to cardboard works fine.  (My DRSSTC has the control circuitry in a cardboard box lined with foil.)  The ideal would be a complete enclosure (Faraday cage) around the LV circuitry, but two sides is probably sufficient and leaves it much more open for debug.

Counterpoise on concrete floor is great!

I noticed the stand, but initially guessed that just the four posts/legs were metal.  Now I'm thinking that there are square metal frames for the top and shelf, with wood squares for the surface.  Is that correct?  It's the only I can explain the interesting sparks in your first video.  They appear to be sparks of burning steel, not electrical sparks.  That could make sense if the top (square) ring is steel bolted together with marginal electrical connections.  It's inductively coupled to the Tesla primary, inducing enough current in that loop to spark where the steel touches (high-current welding-style sparks, not high-voltage sparks).  The steel legs may be OK (why I didn't comment on your last post), but any closed metal loop near or above the primary is quite problematic.  Could easily explain your inconstancy.  Possibly you could break (insulate) the joints, but it would be better not to have any steel close to the Tesla primary either.  It will concentrate the magnetic field, then cause a lot of hysteresis and eddy-current losses within the steel, lowering overall efficiency.

Inconsistency is likely the steel frame as mentioned above.  Another possibility is that the spark gap doesn't remain conductive long enough, so the Tesla primary L/C doesn't ring down all the way, leaving either a negative or positive charge on the MMC after firing.  Much less likely, however.

For flyback HV return grounding, yes, I'm suggesting a string (or just 1 if it's voltage is high enough - whatever keeps your power supply sane) from HV return to ground, perhaps down to the counterpoise for ground presuming the counterpoise does have some wire to a line or pipe ground.  MOVs have higher capacitance, so would pass more of any high-frequency spike, so won't protect the supply as well.  (I'm hoping the ground plane is sufficient to keep the supply sane, so no TVS diodes are needed.)  You could add a neon bulb - it may arc rather than a normal neon glow for the short pulse if the voltage is high enough, which may fry the bulb.

BTW, if you end up with a stand with any metal parts, my general rule is no electrically-floating metal.  All metal is either part of an electrical circuit or tied to ground.  This is more for ESD issues in equipment (printers in my case), but I'd recommend for almost anything.  If nothing else, it makes behavior consistent, rather than allowing occasional arc-over between metal pieces.
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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #53 on: December 10, 2019, 10:27:54 AM »
In regard to counterpoise and grounding, I wrote a long article on the subject and it goes just as well for SGTC as for DRSSTCs.

Grounding rods are almost always a must, the capacitance ratio between your counterpoise/artificial ground plane/metal sheet on the ground has to be around topload 1 : counterpoise 10, in order to have a solid distribution.

Read it all here: http://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/grounding-circuit-protection-and-emi/
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #54 on: December 11, 2019, 01:14:32 AM »
After some further research, I'm considering the following determinations here. Bare with me and let me know what you think.

Metal stand aside, for a moment while we consider the spark gap behavior as well as another observation I hesitated to share until I understood further what might be happening - that being, a few instances when a cap in the MMC would actually discharge, I assume arcing to another capacitor, which has never happened before in any of my testing. The leads of the capacitors are spaced a good deal apart, so this was very surprising when it happened. I was able to recreate (more so observe) the condition multiple times, albeit not on 'command', exactly.
Anyway, back to constructive analysis:
While using inductors as a sort of ballast in a charging circuit that incorporates a spark gap, the inductive kick effect, that the inductor(s) are there to mitigate, can increase the level to which the tank capacitor charges. This would be due to the inductors ability to store energy which would otherwise be dissipated in other forms, primarily heat, I assume. The stored energy in the inductor is then released some time later in the charging cycle.
With that in mind, it may also be worth considering that although the breakdown voltage is largely proportional to the electrode spacing, it reduces with increasing temperature. If the electrodes and the air in the spark gap are allowed to heat up excessively the breakdown voltage of the spark gap decreases considerably. In this instance the spark gap fires at a higher rate, but with the tank capacitor at a much reduced voltage. With the time of year and the ambient conditions changing quite a bit from when I first started this project, it may also be a factor to consider. To add to that - obviously, when any system that uses a gap-based "switch" powers up, the gap is cold, and the first breakdown occurs at a higher voltage than when running. If I increase the distance between electrodes, a very high initial voltage needs to occur before the spark gap will start firing. However, once running, the firing voltage is lower and less power is being transferred. As a consequence, the flyback, inductors, and MMC must be rated to withstand the high power-up transient, but the overall performance would depend on the lower breakdown voltage when the system is running.
Another takeaway - with my current, 20 cap series, MMC setup rated at only 24kV, I may be dancing on thin ice. It would seem, regardless, I should probably add more caps to the series, considering the rated output of the current flyback.
To an extent, the above effects may be partially responsible (as well as the proximity of the steel stand) for the observed inconsistency as the spark gap heats up. The increased firing rate of the spark gap would cause further heating and the static gap to become truly overloaded. As can be heard in the video, a characteristic increase in pitch is noticeable in the sound from the spark gap, and it is accompanied by almost total loss of spark output. However, in my case, there was still fluctuation in speed, which might indicate more than one, possibly even more than a few factors are contributing to my inconsistent results. Granted, with a static spark gap, is it difficult to take repeatable measurements as the behavior is different from one supply cycle to the next.

You're right about the stand. There are square, metal, frames for the shelf and top, and now that you point it out, that actually makes complete sense. Being inductively coupled, and a closed "loop" (for lack of a better description), that spark is forming at the "weakest" connection in the "loop" (ie. janky bolt that's not tightened down all the way), which is why that's the only bolt I'm seeing that exhibited. Still very interesting to see that when the stand is grounded. So many things to consider here!

I'm going to remove everything from the metal stand and repeat previous tests. I will be sure to update with new information!
« Last Edit: December 11, 2019, 01:45:49 AM by jturnerkc »

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #55 on: December 11, 2019, 06:08:48 AM »
Sparks between MMC capacitors sounds like your most serious problem.  Unless leads are extremely close, it would require MUCH more than 1200V to spark.  Do you have a bleed resistor across each cap, all the same value?  That's a requirement for any series-connected MMC.  Otherwise non-uniform corona will slowly redistribute voltage throughout the array until you have a serious over-voltage on one or more caps.  If a lack of bleed resistors isn't the issue, then a close-up image of the MMC array and wiring would help to see what else could possibly be the issue.  I'd also suggest measuring each of the 20 caps to make sure that whatever the issue may be hasn't damaged them.  (Damage usually shows as capacitance drop initially - a few percent, followed by rising leakage current if abuse continues.)

For your concern about 24kV max, I wouldn't worry much.  I've destructively tested a couple of these caps in Tesla use.  It required +-2150 volts for a couple hours to induce failure.  At +-2000V they lasted for a couple days of continuous firing before I gave up waiting and upped to +-2150V.  These same two caps had already been ran continuously at +-1700V for a week.  (When I say "continuous", I'm referring to ongoing 1% duty cycle, 500us of 80kHz repeated every 50ms.)

For the circular (square) current loop of your stand top, grounding doesn't matter.  It's a local loop.

Do you have the fan blowing on your spark gap energized?  If so, I doubt you'll see much voltage change.  What is the gap distance?  My 6kW Marx generator had huge voltage shifts before adding air flow, but was plenty consistent with quite moderate air flow.

One more thought on consistency:  It's possible that the flyback is generating enough current to sometimes sustain the spark within the gap, so delaying any subsequent MMC charging.  That would be especially likely without air flow.  That was a big problem with my Marx generator even with air flow.  The 6kW input had plenty of current to maintain spark gap arcs.  I had to add a circuit to pause the supply for ~1ms after each firing to allow time for the arcs to blow out.

I doubt the inductor string matters much.  It's current just before firing isn't going to be as high as the flyback will generate just after firing.  The only extra current added to the inductor string due to firing is from the energy of the stray capacitance of the flyback output and its wiring, perhaps 1 or 2mJ.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #56 on: December 13, 2019, 05:05:55 PM »
Secondary LC removed from metal stand and placed back on previous base, which is just sitting on a cheap wood computer desk, exactly where it was before. Basically zero metal in the desk besides the two drawer rails beneath. The counterpoise was moved out from under the metal stand and placed directly under the desk, centered under the secondary LC.
I moved the shelf with the primary LC and ZVS, power supply, etc. to sit on top of the metal stand for convenience and placed a couple feet away.
So as it sits:
     - The Negative terminal of the ZVS driver is tied to mains ground, as is the metal stand.
     - The HV return and focus pin (7) are joined and ‘grounded’ to the counterpoise at the spark gap. (still not sure about this pin 7 business)
     - The counterpoise, itself, is not attached to earth or mains ground.
     - The bottom of the secondary is attached to the counterpoise.
     - The secondary LC is approximately 4’ above the counterpoise.
     - 6H inductor between the Flyback output and spark gap.
     - 16.5nF MMC

Before a quick “let’s see what happens” sort of test, I measured all 20 capacitors. Each measured between 330-335nf. 
I removed the variable of the new spark gap, and used my old starter gap (ie, two drawer-knobs (nickel [I think]; unplated) on screws through an electrical box.) There is no airflow on this gap, but is more ‘finely’ adjustable
During the first test I used an input of 18V and limited current to 6A. The spark gap fired consistently, with the occasional hesitation.
I didn’t take any other measurements during testing, partly because the MMC experienced the “something” again, after about 30-40 seconds of the first run-time. It doesn’t happen immediately, or in any type of pattern. It seems slightly prone to occur sooner if I let it run a moment, then stop the input for a few seconds and reapply voltage again. Everything is really speculation for me at this point
Note: before testing I  added more solder to all MMC connections. There was no possibility of any faulty connections. The individual capacitors are wired in an odd zig-zag pattern (simply just to fit as many on the small boards as I could), but no leads come within ~½” from each other. As previously mentioned, however, the plated through-holes do give me cause for concern.

Here’s a tour (this is after re-soldering and the “events”) (also: if video looks like garbage quality, try downloading)
https://drive.google.com/open?id=1KLX0-wxhPm45ne9nwg5_x22xdWKtkPja

All resistors are 1M with the exception of a few 1.1M. I ran out of 1M apparently. I have plenty more on the way for the new MMC though.

I went as far as to remove the regulator and essentially go back to the original configuration with the exception of the inductor string and new flyback, of course. I supplied a 24V input (adjusted with a trimmer on the supply. The supply is rated for 10A. The spark gap continued firing consistently, but the circuit still provoked a reaction in the MMC.
I made a video recording at 120fps and still could not catch more than the flash, so I have no idea where it’s actually coming from within the MMC. I thought an arc should have been visible, but this is just a loud pop with no arc or spark to be seen. The flash is really only visible in the video, and even then the origin can’t be determined. It happens very quickly.
What kind of voltages would have to be present here for a flash-over to form? Is it possible the plated through-holes, if enough voltage is present, are providing a path for arcing to occur?

Here’s the best video I could get: (same - if video looks like garbage quality, try downloading)
https://drive.google.com/open?id=1K65AYEjPaQVol-eoSUEmOMvaWGghPmCN

UPDATE***:
I think I've spotted it...maybe. Watch the far left end of the strip of hot glue on the left side. It flips up a bit during each occurrence (it's not stuck to the board anymore). It looks like it might be happening between the boards, beneath that piece of hot glue. Just speculation at this point.

I really thought I’d be back to or close to my previous results by now, with improvements – this is quite frustrating, especially considering this is the same MMC I’ve been using since the beginning of this project and have had zero issues until now. Setbacks, setbacks. All in the name of learning.

I intended to complete my new MMC yesterday, however, some of the capacitors I was planning on using required de-soldering and ended up being more trouble than it’s worth. I’ll probably just buy some more. They’re cheap enough.

I think that’s it for now. I should have some more time to put in tonight for debugging and will report any further findings.

Thanks again!
« Last Edit: December 13, 2019, 11:30:14 PM by jturnerkc »

Offline MRMILSTAR

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #57 on: December 13, 2019, 08:56:27 PM »
Its just my quick observation but all those plated-through holes look like a disaster waiting to happen. You have hundreds of potential flash-over points on that board. I would never consider such a construction method for high voltage circuitry.
« Last Edit: December 13, 2019, 09:35:23 PM by MRMILSTAR »
Steve White
Cedar Rapids, Iowa
Retired electrical engineer

Offline John123

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #58 on: December 13, 2019, 09:08:28 PM »
Could the ozone levels around the coil, hv spiking and UV from sparks actually encourage that prototyping board to flash over at a much lower voltage?

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #59 on: December 13, 2019, 10:50:03 PM »
Its just my quick observation but all those plated-through holes look like a disaster waiting to happen. You have hundreds of potential flash-over points on that board. I would never consider such a construction method for high voltage circuitry.

You're right on and that has been in the back of my mind for a while now, and it's my current thought and concern, exactly. It's the only thing that makes any sense to me right now, and, I would imagine, is likely the culprit here.
Perhaps my previous flyback was a bit more wimpy than this guy, so I'm only just now seeing the error of my ways. Against my better judgement, I used these because they were what I had around. You're right though, i shouldn't be using these boards for this application.
I should have the new MMC together this weekend. The caps will be mounted on a sheet of lexan, so should have no chance of flash over or anything.

Could the ozone levels around the coil, hv spiking and UV from sparks actually encourage that prototyping board to flash over at a much lower voltage?

That's an interesting question John. I don't think the TC was running long enough to create high enough levels of ozone to affect it like that. We're talking 10-20 seconds of run time.
« Last Edit: December 13, 2019, 11:10:55 PM by jturnerkc »

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #59 on: December 13, 2019, 10:50:03 PM »

 


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