Author Topic: SGTC MK1 - An Accomplishment in Progress  (Read 1586 times)

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #20 on: November 18, 2019, 12:07:38 AM »
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!

Right. I definitely understand running two flybacks in series would normally be an issue. Trying to run the two primaries in parallel, and then the secondaries in series would almost certainly result in a failure.
I was considering a possible circuit that I thought might work. Let me know what you think. Essentially there's a bare (extracted flyback) core transformer driven by the ZVS, that then oscillates the two flybacks, instead of running parallel primaries.

These flybacks I just picked up are angry little guys. Series might be way overkill - so I'll stick with running a single flyback for now. I will need to keep an eye on temperature though, considering the size of these FBTs.
I think I'll try out your inductor suggestion and see how that goes as well. Thanks for the clarification on that!
« Last Edit: November 18, 2019, 12:19:44 AM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #21 on: November 18, 2019, 01:59:24 AM »
Your proposed series connection could work, but it requires the upper secondary winding of your custom transformer to handle 15~30kV relative to the lower secondary and primaries, which are essentially at ground potential (compared to 30kV).  15~30kV is whatever voltage a single flyback output can generate.  If you can construct a custom transformer with such HV insulation, I'd suggest making your own flyback large enough to drive your project directly with one stage.  If you do try your series design, I'd add a connection from the negative (return) secondary pin of the upper flyback to it's primary.  This provides a path for any leakage current from you custom input transformer, preventing it from damaging the upper flyback.

Starting with a single flyback sounds like a good idea.  If you get enough voltage to trigger your spark gap, then series isn't needed.  Add a second in parallel if you want to double the firing frequency.  BTW, do you know roughly what voltage your spark gap is set for?

Small fans are quite cheap these days.  I'd suggest placing one blowing directly on the flyback(s).  Of course, there's still higher temperature inside than on the surface, but a fan will significantly improve the power capability.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #22 on: November 18, 2019, 03:30:02 AM »
Your proposed series connection could work, but it requires the upper secondary winding of your custom transformer to handle 15~30kV relative to the lower secondary and primaries, which are essentially at ground potential (compared to 30kV).  15~30kV is whatever voltage a single flyback output can generate.  If you can construct a custom transformer with such HV insulation, I'd suggest making your own flyback large enough to drive your project directly with one stage.  If you do try your series design, I'd add a connection from the negative (return) secondary pin of the upper flyback to it's primary.  This provides a path for any leakage current from you custom input transformer, preventing it from damaging the upper flyback.

Starting with a single flyback sounds like a good idea.  If you get enough voltage to trigger your spark gap, then series isn't needed.  Add a second in parallel if you want to double the firing frequency.  BTW, do you know roughly what voltage your spark gap is set for?

Small fans are quite cheap these days.  I'd suggest placing one blowing directly on the flyback(s).  Of course, there's still higher temperature inside than on the surface, but a fan will significantly improve the power capability.

With the smaller flyback, since i have not yet established its actual voltage output, I have the spark gap set to fire at ~9kV (rudimentary measurement).
I did a short test run and achieved a respectable output. I'm running on a bit lower power (about 25V to the ZVS), until I'm satisfied it can handle a bit more. I hesitate to push this baby flyback too hard on its own before adding some additional protection.
On another note, I've finally finished up the new topload. I've also had an additional secondary I wrapped a while back and have been waiting to run (different dimensions, but near the same resonance) so will be swapping those out.

I'll have some new specs up shortly.
I suppose this one will have to officially be...
« Last Edit: November 18, 2019, 04:23:49 AM by jturnerkc »

Offline Mads Barnkob

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #23 on: November 18, 2019, 12:21:53 PM »
Your proposed series connection could work, but it requires the upper secondary winding of your custom transformer to handle 15~30kV relative to the lower secondary and primaries, which are essentially at ground potential (compared to 30kV).  15~30kV is whatever voltage a single flyback output can generate.  If you can construct a custom transformer with such HV insulation, I'd suggest making your own flyback large enough to drive your project directly with one stage.  If you do try your series design, I'd add a connection from the negative (return) secondary pin of the upper flyback to it's primary.  This provides a path for any leakage current from you custom input transformer, preventing it from damaging the upper flyback.

Reversing the polarity on one of the primaries and turn one flyback transformer around, mid-point ground them and you will not surpass the voltage rating of the single transformer. But if it makes it necessary to redo the whole grounding scheme.
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Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #24 on: November 19, 2019, 12:49:52 AM »
Your proposed series connection could work, but it requires the upper secondary winding of your custom transformer to handle 15~30kV relative to the lower secondary and primaries, which are essentially at ground potential (compared to 30kV).  15~30kV is whatever voltage a single flyback output can generate.  If you can construct a custom transformer with such HV insulation, I'd suggest making your own flyback large enough to drive your project directly with one stage.  If you do try your series design, I'd add a connection from the negative (return) secondary pin of the upper flyback to it's primary.  This provides a path for any leakage current from you custom input transformer, preventing it from damaging the upper flyback.

Reversing the polarity on one of the primaries and turn one flyback transformer around, mid-point ground them and you will not surpass the voltage rating of the single transformer. But if it makes it necessary to redo the whole grounding scheme.

Ok, that's genius and clearly works, at least in simulation. David does mention a point I had not considered in my original sim, however...the internal diodes. With a couple AC flybacks it looks like you'd get double the voltage across the spark gap, while each flyback is only putting out half of the voltage the spark gap sees! I'll definitely keep this in mind for the future.

I've swapped out the secondary of the SGTC and added my newly constructed topload. I should have some updates and specs on Mk. II in a few days!

Thank you both for all your insight and suggestions so far!!
« Last Edit: November 19, 2019, 11:21:13 PM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #25 on: November 19, 2019, 05:47:34 AM »
Are you using standard TV flyback transformers as I've been assuming?  Those almost always have internal diodes, so output DC, positive on the HV lead and negative on the return pin.  If you were using bare AC flyback transformers, then you would have HV diode(s) between the secondaries and spark gap.

The normal DC flyback transforms wouldn't work back-to-back in series.  Both ends would be positive, so no voltage between them.  Your series schematic doesn't show diodes either internal or external.  Is that what you are simulating - no diodes?  If the simulation values (inductances, capacitances, etc.) are reasonable, you won't be getting much voltage on the MMC, since it can charge for only one half-cycle of the flyback frequency.
David Knierim

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #26 on: November 19, 2019, 06:25:49 AM »
Are you using standard TV flyback transformers as I've been assuming?  Those almost always have internal diodes, so output DC, positive on the HV lead and negative on the return pin.  If you were using bare AC flyback transformers, then you would have HV diode(s) between the secondaries and spark gap.

The normal DC flyback transforms wouldn't work back-to-back in series.  Both ends would be positive, so no voltage between them.  Your series schematic doesn't show diodes either internal or external.  Is that what you are simulating - no diodes?  If the simulation values (inductances, capacitances, etc.) are reasonable, you won't be getting much voltage on the MMC, since it can charge for only one half-cycle of the flyback frequency.

That's a good point, and a good observation. I did not use diodes in the sim.
Maybe Mads can chime in.
Although I don't have the exact specs on these small flybacks i acquired, i think it's safe to assume there are diodes employed. I've attached some shots. Like I said, they are pretty small. I've not found much data on these in my searches. I certainly don't plan on pushing them much or in series. I don't think these little babies could handle much of anything like that. I'm only using one for the time being.
I may consider winding my own. It actually sounds like a fun challenge. A relatively easy build that I'm sure would be pretty satisfying after completion. 
Right now, I'm working on finishing up the new TC secondary and grounding scheme, as well as a fancy new stand.
« Last Edit: November 19, 2019, 05:56:12 PM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #27 on: November 20, 2019, 05:30:25 AM »
Thank you for the images.  I'd be quite surprised if they don't have internal diodes.  I can't see how your SGTC would work at all if they didn't include diodes.

Should have been obvious to me, but I hadn't realized that you were winding your own primary.  Flyback transformers don't typically have center-tapped primary windings, and you mentioned center-tapped primary in your initial post.  Custom primary winding has a likely-key advantage besides center-tap:  The coupling factor will be lower than with the built-in primary winding.  As I discuss some in my ZVS Jacob's ladder post:
    https://highvoltageforum.net/index.php?topic=831.0
Having a coupling factor below 0.86 allows the ZVS oscillator to run over the full range of output loading from short to open.  That's close to what you have as the MMC charges from 0V to the spark-gap trigger voltage:  shorted load to some lower load current (higher voltage).  It would be interesting to measure the coupling coefficient of your flyback.  However, that's not necessary unless you have trouble with the oscillations dropping out, which would cause run-away input current to the ZVS oscillator.

What frequency is the ZVS running with your new small flyback transformer?  What is the cross-sectional area of the flyback's ferrite core?  Knowing those two values would allow estimating the ZVS input voltage permitted before saturating the core.  I'll use 50kHz and 50mm^2 area for an example, and a presumed saturation flux density of 0.4T.  Period = 20us, divided by 2PI = 3.183us/radian.  50mm^2 * 0.4T = 20uVs/turn, for 200uVs for  your 10-turn primary.  200uVs / 3.183us = 62.83V peak.  ZVS peak voltage is ideally (no losses) PI * DC_input_voltage.  So, input voltage would be 62.83V / PI = 20Vdc.  If the core could handle 0.5T (the highest I've seen for any fferrite material), then Vdc could be up to 25V.  Of course, this is just for my example guess for frequency and core cross-sectional area.

If you want to optimize power from the small flyback, it can help to separately measure temperatures of the primary winding, ferrite core, and secondary winding.  That indicates where to improve.  Hot primary might be improved with litz-wire.  With your single-layer primary, litz is of less advantage than for multi-layer windings.  So, I'd guess that won't be the hottest.  If the core is hottest, it could be due to flux saturation and/or high frequency losses.  A hot secondary is likely due to the internal diodes, either just total current or switching losses if the frequency is high.

Looking forward to hearing how your fun project progresses!
David Knierim

Offline Mads Barnkob

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #28 on: November 20, 2019, 07:16:24 PM »
They would have to be put in series, positive|negative - positive|negative for doubling the output voltage, whether or not you will destroy one from imbalance or not... its not exactly practical to add balancing resistors :)

Here is one that did that, on a single ZVS driver
/>
From my own experience, you have to push them real hard before they start to have corona on the outside enclosure and flash-over is not far after that, at the end of this video (input voltage was around 140VDC)

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Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #29 on: November 21, 2019, 01:11:49 AM »

They would have to be put in series, positive|negative - positive|negative for doubling the output voltage, whether or not you will destroy one from imbalance or not... its not exactly practical to add balancing resistors :)

Here is one that did that, on a single ZVS driver
[video]

From my own experience, you have to push them real hard before they start to have corona on the outside enclosure and flash-over is not far after that, at the end of this video (input voltage was around 140VDC)
[video]

Right. That first video was effectively what my sim was set up to recreate. As David pointed out, however, I did not take internal diodes into account.

I'll skip the series idea for now. Even though these are small flybacks, they seem to be packing quite a punch. However I’m quite confident one or both would be destroyed running in series.


Thank you for the images.  I'd be quite surprised if they don't have internal diodes.  I can't see how your SGTC would work at all if they didn't include diodes.

Flyback transformers don't typically have center-tapped primary windings, and you mentioned center-tapped primary in your initial post.  Custom primary winding has a likely-key advantage besides center-tap:  The coupling factor will be lower than with the built-in primary winding.

Having a coupling factor below 0.86 allows the ZVS oscillator to run over the full range of output loading from short to open.  That's close to what you have as the MMC charges from 0V to the spark-gap trigger voltage:  shorted load to some lower load current (higher voltage).  It would be interesting to measure the coupling coefficient of your flyback.  However, that's not necessary unless you have trouble with the oscillations dropping out, which would cause run-away input current to the ZVS oscillator.

What frequency is the ZVS running with your new small flyback transformer?  What is the cross-sectional area of the flyback's ferrite core?  Knowing those two values would allow estimating the ZVS input voltage permitted before saturating the core.  I'll use 50kHz and 50mm^2 area for an example, and a presumed saturation flux density of 0.4T.  Period = 20us, divided by 2PI = 3.183us/radian.  50mm^2 * 0.4T = 20uVs/turn, for 200uVs for  your 10-turn primary.  200uVs / 3.183us = 62.83V peak.  ZVS peak voltage is ideally (no losses) PI * DC_input_voltage.  So, input voltage would be 62.83V / PI = 20Vdc.  If the core could handle 0.5T (the highest I've seen for any ferrite material), then Vdc could be up to 25V.  Of course, this is just for my example guess for frequency and core cross-sectional area.

If you want to optimize power from the small flyback, it can help to separately measure temperatures of the primary winding, ferrite core, and secondary winding.  That indicates where to improve.  Hot primary might be improved with litz-wire.  With your single-layer primary, litz is of less advantage than for multi-layer windings.  So, I'd guess that won't be the hottest.  If the core is hottest, it could be due to flux saturation and/or high frequency losses.  A hot secondary is likely due to the internal diodes, either just total current or switching losses if the frequency is high.

I will take some temperature readings at different run times during the next light.
Is there a relatively simple method to measure the DC bias?

Could bias be removed or reduced using a tertiary winding?

Could modifying the ZVS frequency/duty cycle of the primary be used to minimize bias and risk of saturation?
Just wondering this makes me want to modify the ZVS for variable frequency, but to make that possible on a resonant circuit i'd have to find the perfect variable inductors and/or capacitors.

I ran a little Jacob's ladder to grab some frequency readings. Seems my ZVS is operating around 70kHz.
After a few test runs, I can't tell what's hotter - the primary, the core, or the secondary. They're all quite warm to the touch after about 1 minute, give or take. My ZVS mosfets and inductor were getting quite warm as well. I was running this setup with both the DC negative and HV return pin connected to mains ground. I'm not sure if it's related, but after grounding both, my amp draw seemed to  decrease.
I will try to get some of the other measurements you suggested, asap. I'm curious to get to the bottom of all this.
« Last Edit: November 22, 2019, 12:23:44 AM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #30 on: November 21, 2019, 05:44:40 AM »
I hadn't intended to bring up DC bias, but that is a good point.  The half-wave rectification of the internal flyback diodes will create some DC magnetic field in the core.  Core saturation will show up as distortion to the sine-wave shape of the ZVS output (flyback primary) leads.  Saturation will show up as steeper edges and flatter top compared to a true sine wave.  If one ZVS output has steeper edges than the other, that's the result of the DC field component caused by the output diodes.  Compensation could be done by moving the "center" tap a bit off of center.  Then the DC from the ZVS supply input inductor will flow through more turns one direction than the other, so add/subtract DC bias.  DC bias compensation may not be necessary depending on how close you happen to be to saturation.  I have a ZVS simulation with full-wave rectification on the output.  I'll change it to half-wave to get some feeling for the bias issue.  (Saturation isn't simulated, so it won't be perfect.)

Frequency is determined by the flyback primary inductance and your ZVS resonant capacitor value (and the secondary winding capacitance adds a bit too).  Changing frequency involves just changing ZVS capacitor value.  What value do you have now for your 70kHz result?  Did you get a chance to measure core cross-section (ferrite thickness and width inside your primary winding) - at least a guess?

Do you have any information on the average DC output current?  For example, if you know the spark repeat frequency and the spark-gap voltage (9kV last you mentioned), then the average current can be found by 16.5nF * 9kV * spark_frequency, where 16.5nF is your MMC capacitance.

You may not be far off from optimum already.  If the flyback is staying below 100C, it may be fine as is.  (I'd still suggest adding the output inductor string to protect from nano-second fall times.)  In other words, analysis and optimization is fun, but you may prefer to go with what is working and not drag out the details.  This is a fun project, but don't let me take it too far down the analytical path if that's not your interest.
David Knierim

Offline jturnerkc

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #31 on: November 22, 2019, 01:38:57 AM »
I hadn't intended to bring up DC bias, but that is a good point.  The half-wave rectification of the internal flyback diodes will create some DC magnetic field in the core.  Core saturation will show up as distortion to the sine-wave shape of the ZVS output (flyback primary) leads.  Saturation will show up as steeper edges and flatter top compared to a true sine wave.  If one ZVS output has steeper edges than the other, that's the result of the DC field component caused by the output diodes.  Compensation could be done by moving the "center" tap a bit off of center.  Then the DC from the ZVS supply input inductor will flow through more turns one direction than the other, so add/subtract DC bias.  DC bias compensation may not be necessary depending on how close you happen to be to saturation.  I have a ZVS simulation with full-wave rectification on the output.  I'll change it to half-wave to get some feeling for the bias issue.  (Saturation isn't simulated, so it won't be perfect.)

Frequency is determined by the flyback primary inductance and your ZVS resonant capacitor value (and the secondary winding capacitance adds a bit too).  Changing frequency involves just changing ZVS capacitor value.  What value do you have now for your 70kHz result?  Did you get a chance to measure core cross-section (ferrite thickness and width inside your primary winding) - at least a guess?

Do you have any information on the average DC output current?  For example, if you know the spark repeat frequency and the spark-gap voltage (9kV last you mentioned), then the average current can be found by 16.5nF * 9kV * spark_frequency, where 16.5nF is your MMC capacitance.

You may not be far off from optimum already.  If the flyback is staying below 100C, it may be fine as is.  (I'd still suggest adding the output inductor string to protect from nano-second fall times.)  In other words, analysis and optimization is fun, but you may prefer to go with what is working and not drag out the details.  This is a fun project, but don't let me take it too far down the analytical path if that's not your interest.

Can you share the schematic you're using for your sim? I'd like to replicate, in mine, what you're looking at.

For the ZVS, I actually just used the same capacitors that I used in my MMC. There are two .33uF 1200vdc; one between each FET drain and ground.
The small flyback cores measure ~9.7mm in diameter, ~49.4mm in height and ~35mm wide. Cute little thing, isn't it?

I do not yet know, for sure, what the output current is on the secondary. I can estimate the spark gap voltage at ~9kV, maybe 10, but I can’t confirm that with any certainty. I'd like to put a high voltage probe together and at least see what the voltage output is. At least I'd have some info to work with... Same with spark frequency. I can guess and throw some numbers into a calculator, but they may or may not be close to accurate. If I do get any more flybacks, I’m definitely going to make sure I can find the datasheet first!

I ran the same makeshift jacob's ladder with the flybacks in parallel tonight and it was running around 90kHz.
I did not take temperature readings with them in parallel, but with a single flyback running (zvs ~70kHz) I took readings after at least 1 minute of run time and got these readings:
Secondary/Housing: ~44°C
Primary: ~33°C
Core: ~46°C
I should have done a few different runs at different time intervals, but ran out of time tonight.

I certainly expect those temperatures to change quite a bit when actually running the tesla coil. I should get that done over the weekend.

I really do appreciate you taking the time to walk me through these things, especially those I had not considered, and help me improve this circuit. I couldn't ask for better insight and I do enjoy the analytical aspect and trying to optimize performance. What was working also resulted in destruction of my original flyback and I would prefer a more reliable circuit that I can run more than 1 minute with relative certainty that I’m not going to fry another one, or something else, during a demonstration. I'm even more determined now that I've destroyed a flyback and rendered a power supply useless (for the time being).
The complexity and number of components involved in building an SSTC or DRSSTC don't seem to be very practical for me at this time, so I am determined to optimize this current setup (or something like it). I've seen some assembled SSTC and DRSSTC drivers on eBay. I'd certainly be curious to try solid state as it seems much more stable, but hesitate to buy anything pre-assembled from overseas. It would be fun to "play with", but I'd, of course, prefer to build my own.
« Last Edit: December 01, 2019, 12:35:33 AM by jturnerkc »

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #32 on: November 22, 2019, 06:21:22 AM »
Those temperatures seem quite reasonable.  The only issue would be if the secondary power dissipation was concentrated mostly in the diodes, which could make their internal die temperature much hotter than secondary surface temperature.  That's why I was asking about secondary current.  A wild guess for that flyback is ~5mA average (DC) secondary rated current.  You could probably push it a bit higher, especially since the sine-wave drive has the diodes conducting with a higher duty cycle than a normal flyback waveform.

Here's the ZVS I'm using to simulate something like your setup.  The voltages and currents are different, as this is derived from my Jacob's ladder ZVS.  The input ramps from 0 to 160V (most of what it sees from rectified 120V line power).  The secondary is 1:1 to the primary.  That's just for convenience in comparing input and output waveforms on the same simulation plot.  It's easy to scale the output manually.  It also uses a separate high-coupling-factor center-tapped coil to feed DC to the oscillator.  That's left over from some of my old ZVS induction circuits - works roughly the same as center-tapping the working coil (L3 in this case).  You can modify it to be like your circuit.


I wound a flyback from my stash (11 x 13.5mm rectangular core area) with 10T and made some measurements.  Coupling factor was 81%, and the secondary has somewhere around 1600 turns.  Your flyback has 74mm^2 core area, so can handle a bit more volt*seconds than my example calculation using 50mm^2.  I think you've built a very nice close-to-optimum system for these flybacks.

A resistor in series with the negative ground-return side of the secondary would allow measuring current.  1000 ohms would have 5V drop at 5mA.  The waveform will be higher pulses with zero between, which you could measure with a scope.  If using a meter to measure the voltage-drop, I'd suggest adding a 0.1~1uF capacitor across the resistor so the high-frequency pulses don't confuse the meter.  If using a scope, you can also see the spark frequency, as the current will drop as the MMC charges, then suddenly rise again after each spark.

In simulation I played with feeding the ZVS from only one side (removed L1 and L2, made L4 200uH and wired it to either VP or VN).  Moving the DC feed all the way to one side was about right to compensate for the DC component of output current.  It compensated well at the medium-to-low current conditions (higher output voltage).  At low output voltage and high output current, even that wasn't fully compensating for output current.  I'd initially guessed the "center"-tap would need to be moved only 1 or 2 turns from center.  Turns out that all the way to one end is good.  Feeding to VP should be compensating, adding current to the end of L3 that matches the positive end of L5 where current is leaving.  I'm 99% sure that's what worked in simulation, but was too distracted today to write it down.  Feeding to the wrong side doubles the total DC current bias to the core.  To check for yourself, wire it each way and plot the sum of L3 and L5 current to see the net DC component.  (Another reason it's easier to simulate with 1:1 turns ratio.)

Yes, if/when you get to DRSSTC, I'd build your own.  Much more fun and more educational too.  You're off to a great start!
« Last Edit: November 22, 2019, 06:55:12 AM by davekni »
David Knierim

Offline davekni

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #33 on: November 23, 2019, 05:01:46 AM »
Thinking a bit more about filtering between flyback output and spark-gap:  A simple R/C filter is probably sufficient, avoiding the cost/work of soldering 100 inductors.  In this case, the R/C filter is removing high-frequencies generated by the spark gap, so would have the R on the spark-gap side:


Something like the above filter will waste a few % of input power, but should protect the flyback from fast spark-gap falling edges as well or better than the inductor string.  The resistor is constructed from 10 in series to avoid the cost/trouble of finding a single 20-30kV capable resistor.  Even with 10 resistors, each will see 2-3kV.  Resistors rated for a few kV aren't too hard to find, but any 1-2W rated resistor will likely be fine.  The voltage is there for a very short time.  I used 2W 10k resistors, six per string, for my small Marx generator (9kV per string).  That's only 1.5kV/resistor, but I did some testing to much higher voltage on those 2W parts.

Values for the above low-pass filter aren't critical.  Take some care in finding a capacitor, however.  Many cheep ceramic 20-30kV capacitors have bad capacitance vs. voltage curves, dropping down by 80-90% at rated voltage (only 10-20% of capacitance remaining).

One other caution, which I think you're already doing.  Make sure there's some load (MMC or arc gap) on the secondary when the ZVS starts up.  ZVS oscillators often produce a startup-burst that is ~2x the normal run voltage.  Energy builds up in the input inductor until oscillation starts.  Than that energy transfers to output oscillation until used up.
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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #34 on: November 23, 2019, 04:15:31 PM »
Thinking a bit more about filtering between flyback output and spark-gap:  A simple R/C filter is probably sufficient, avoiding the cost/work of soldering 100 inductors.  In this case, the R/C filter is removing high-frequencies generated by the spark gap, so would have the R on the spark-gap side:


Something like the above filter will waste a few % of input power, but should protect the flyback from fast spark-gap falling edges as well or better than the inductor string.  The resistor is constructed from 10 in series to avoid the cost/trouble of finding a single 20-30kV capable resistor.  Even with 10 resistors, each will see 2-3kV.  Resistors rated for a few kV aren't too hard to find, but any 1-2W rated resistor will likely be fine.  The voltage is there for a very short time.  I used 2W 10k resistors, six per string, for my small Marx generator (9kV per string).  That's only 1.5kV/resistor, but I did some testing to much higher voltage on those 2W parts.

Values for the above low-pass filter aren't critical.  Take some care in finding a capacitor, however.  Many cheep ceramic 20-30kV capacitors have bad capacitance vs. voltage curves, dropping down by 80-90% at rated voltage (only 10-20% of capacitance remaining).

One other caution, which I think you're already doing.  Make sure there's some load (MMC or arc gap) on the secondary when the ZVS starts up.  ZVS oscillators often produce a startup-burst that is ~2x the normal run voltage.  Energy builds up in the input inductor until oscillation starts.  Than that energy transfers to output oscillation until used up.

Looks like the well-known "Terry filter" used by many spark gap coilers for their NST-based tesla coils.
Steve White
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Retired electrical engineer

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #35 on: November 23, 2019, 06:34:56 PM »
Oh!  I'd seen the phrase "Terry filter" on this forum, but had no idea what it was.  Thank you for the insight.
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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #36 on: November 25, 2019, 09:20:41 PM »
Thinking a bit more about filtering between flyback output and spark-gap:  A simple R/C filter is probably sufficient, avoiding the cost/work of soldering 100 inductors.  In this case, the R/C filter is removing high-frequencies generated by the spark gap, so would have the R on the spark-gap side:

Something like the above filter will waste a few % of input power, but should protect the flyback from fast spark-gap falling edges as well or better than the inductor string.  The resistor is constructed from 10 in series to avoid the cost/trouble of finding a single 20-30kV capable resistor.  Even with 10 resistors, each will see 2-3kV.  Resistors rated for a few kV aren't too hard to find, but any 1-2W rated resistor will likely be fine.  The voltage is there for a very short time.  I used 2W 10k resistors, six per string, for my small Marx generator (9kV per string).  That's only 1.5kV/resistor, but I did some testing to much higher voltage on those 2W parts.

Values for the above low-pass filter aren't critical.  Take some care in finding a capacitor, however.  Many cheep ceramic 20-30kV capacitors have bad capacitance vs. voltage curves, dropping down by 80-90% at rated voltage (only 10-20% of capacitance remaining).

One other caution, which I think you're already doing.  Make sure there's some load (MMC or arc gap) on the secondary when the ZVS starts up.  ZVS oscillators often produce a startup-burst that is ~2x the normal run voltage.  Energy builds up in the input inductor until oscillation starts.  Than that energy transfers to output oscillation until used up.

I think I'll go ahead and try out the inductor string first since I recently acquired 160 inductors, which I’ll probably try to get wired up tonight. I'll keep this in mind, though.

Meanwhile, during some testing with the small flybacks in parallel, once again I spotted the return pin of one of them arching to a neighboring pin (even after I had already cut all other pins, added extra insulation around the return pin, and sealed everything up with hot glue. I think I'll use epoxy or something next time.) Needless to say I don't particularly feel like digging glue out at the moment. The other flyback does not have the same issue.  Unfortunately, one doesn't seem powerful enough to charge my current capacitor configuration (or rather, my capacitor bank is too large for the flyback) and is not firing consistently.
I’m convinced these little turds aren't going to produce the results I'm looking for. They’re great for a Jacob’s ladder though.

I’ve already sourced two larger ones locally, but will take me a couple days before I can extract them. We’ll try this again with a properly sized transformer like I started with. I miss my thick, angry, streamers!

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #37 on: November 26, 2019, 05:54:16 AM »
If it's not firing consistently, then it sounds like insufficient voltage rather than insufficient current.  The latter would cause it to be slow (low repeat frequency), but not inconsistency.  At risk of frying the little flybacks, a bit higher ZVS input voltage would help.

The pin arcing may be caused by a feedback resistor within the flyback.  Several of my flyback transformers include a high-value resistor from the positive HV output lead back to a pin adjacent the negative HV return pin.  For example, the one I've been experimenting with recently has 264meg ohms from output lead to pin 7.  Pin 6 is the HV return.  I don't know if the 264meg resistor was for feedback to regulate voltage, or part of a voltage-divider to generate focus voltage (~2kV).  Either way, if your flyback has such an internal resistor, it's better to tie the associated pin to HV return.  (I have 20meg from pin 7 to pin 6 to monitor HV voltage.)

If that HV feedback pin is left floating, it may arc to the HV return or not.  The problem is that it might arc inside the flyback rather than at the pin.  Internal arcing will damage insulation and perhaps start a more extensive internal failure.  That's why I'd suggest tying it to the HV return, directly or through a resistor.
David Knierim

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #38 on: November 26, 2019, 09:33:32 PM »
If it's not firing consistently, then it sounds like insufficient voltage rather than insufficient current.  The latter would cause it to be slow (low repeat frequency), but not inconsistency.  At risk of frying the little flybacks, a bit higher ZVS input voltage would help.

The pin arcing may be caused by a feedback resistor within the flyback.  Several of my flyback transformers include a high-value resistor from the positive HV output lead back to a pin adjacent the negative HV return pin.  For example, the one I've been experimenting with recently has 264meg ohms from output lead to pin 7.  Pin 6 is the HV return.  I don't know if the 264meg resistor was for feedback to regulate voltage, or part of a voltage-divider to generate focus voltage (~2kV).  Either way, if your flyback has such an internal resistor, it's better to tie the associated pin to HV return.  (I have 20meg from pin 7 to pin 6 to monitor HV voltage.)

If that HV feedback pin is left floating, it may arc to the HV return or not.  The problem is that it might arc inside the flyback rather than at the pin.  Internal arcing will damage insulation and perhaps start a more extensive internal failure.  That's why I'd suggest tying it to the HV return, directly or through a resistor.

That's an interesting observation. I checked all the pins when I received the units and didn't have any resistance readings that stuck out like that.

When using the larger flyback, I was able to get consistent charge and firing at only 12V, and eventually kicked it up to about 30V which resulted in those really nice streamers pictured in my original post. I ran this small one at 24V, but didn't want to push it much further at the time.

When first receiving the flybacks and simply pulling an arc off the HV return, the small one seems more interested in making long, fire-y, rising, arcs instead of the shorter, more concentrated, arcs output from the larger flyback. It's an interesting contrast. Would the difference in arc behavior be attributed to voltage or current? Frequency?
Perhaps I can record a short video to illustrate more clearly what I'm speaking about and post all the specs I can come up with.
« Last Edit: November 26, 2019, 09:46:43 PM by jturnerkc »

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #39 on: November 27, 2019, 05:58:09 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)?
David Knierim

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Re: SGTC MK1 - An Accomplishment in Progress
« Reply #39 on: November 27, 2019, 05:58:09 AM »

 


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