Show Posts

This section allows you to view all posts made by this member. Note that you can only see posts made in areas you currently have access to.


Messages - davekni

Pages: [1] 2 3 ... 7
1
Yes.  I've built 5 or 6 ZVS circuits and ran lots of simulations.  When power is applied, current in the power-feed inductor(s) from the supply ramps up fairly quickly.  Initially there's no oscillation, so both FET drains (or IGBT collectors) are at 1-gate-voltage above ground.  Eventually the positive feedback gets oscillation going.  By that time the power-feed inductor current is much higher than in normal operation, perhaps even saturating and/or shutting down the DC input supply.  If the supply doesn't shut down, the high current causes a separate issue.  All that stored energy causes the oscillation voltage to spike way above normal.  (The oscillation amplitude rings for several cycles.)  To handle this voltage, either the FETs need to be rated for much higher voltage than needed during normal use, or TVS diodes are needed to clamp the voltage.  I've done both.

Before this design I played with lots of ideas to kick-start the oscillation, from just unbalanced part values to extra circuitry to hold off one gate signal until power is up, or hold of both gates with staggered release.  At best, these got the oscillation voltage overshoot (and ring) down to 2x rather than 3-5x.  At this point, the only solution I've found is what's in this design:  Let the oscillation start slowly at low current, feed by bleed resistor, R2 in this circuit.  Once oscillation is going, applying full voltage isn't so problematic.  It takes much less time for the oscillation to ramp from the low amplitude than it does to start with no oscillation at all.  I still have TVS diodes, as I'm pushing the voltage, 600V rated IGBTs for a normal-operating peak of 533V (PI * sqrt(2) * 120V, for 120VAC line power input).

I'm about to build a small low-voltage ZVS with current-limited buck-regulator input, using the ZVS input inductor for double-duty as the buck-regulator inductor.  Of course, the buck-regulator itself isn't zero-voltage or zero-current switching, so somewhat defeats the point of ZVS.

2
If you wind one turn around a core, and send 1 amp through it, it creates one ampere-turn, which induces a specific magnetic flux, depending on the core geometry/gap and material properties.  If you wind a second turn, with one ampere, then there is a total of two ampere-turns, which creates twice the magnetic flux.  That's two turns with one ampere.  If you go back to a single turn and send two amperes through it, that's another way to get the same two ampere-turns, so the same flux.

Perhaps what you are thinking of is the volt-second rating of a winding.  That does go up linearly with turn count.  A typical flyback core may have a volt-second rating of 50uVs/turn (50 microvolt-seconds per turn).   Say it also has a rating of 1uH/turn^2 (one microhenry per turn squared).  A one-turn winding will have 1uH.  Applying 50V to it for 1us will ramp current to 50A, which would be saturation (50uVs).  A two-turn winding will have 4uH.  Applying 100V to it for 1us will ramp current to 25A, again saturation (100uVs / 2 turns = 50uVs/turn).

Hope this helps.  If not, please feel free to ask again.

3
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« 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.

4
If I'm interpreting the scope image correctly, it does look like phasing is quite off, likely enough to be problematic.

I'm loosing track of which resistor is R1 and which is R2.  Do you have a second resistor between clamp diodes and 4069 input?  Or is there just a single resistor (with series capacitor) from CT output to the clamp diodes?  Is it that single resistor you are calling R1?  (As I mentioned previously, the second clamp-diode to 4069 input resistor isn't critical.)

I'd mistakenly thought your scope had only one channel.  With two, the external trigger isn't needed. Keep one probe on the 4069 output and trigger on that channel.  Move the other probe from one point to the next, taking images at each place.  (Adjust vertical as needed.)  For these measurements, I'd suggest going back to the normal 1k for the CT output resistor.  Start with the antenna for the second channel (as in your last image, but repeat just to make sure the setup is stable, no stray POT leads for the resistor now).  Then move to the CT output, then after the 0.1uF blocking capacitor, then to after the 1k resistor (scope at the clamp diode junction), then to UCC27425 pin 2 or 4, then to UCC27425 outputs (pins pins 5 and 7, each separately, to make sure there's no issues with the chip), then to the node between the blocking capacitor and gate-drive transformer primary.   If you are using your 120V supply (and not line power directly), make sure the negative supply output is grounded and move to the lower two H-Bridge gates (one at a time), then to the H-Bridge outputs (should be 0 to 120V mostly-square waves), and finally to the node between that output blocking cap and the Tesla primary coil.  That's a lot of images, but it will almost certainly explain the entire phase-shift situation, where the delays are unexpectedly large (or blocking caps too small or whatever).  Again, your call on whether it's worth making all those measurements.

5
"Does core saturation current stay fixed no matter what DC input voltage, frequency and number of turns I use?"

The input voltage and frequency affect the actual current, but not the saturation limit.  Turns does affect saturation current, inversely proportional.  In other words, the saturation level is ampere-turns.

"What would be a typical ballpark saturation current for 10 windings externally wound on a gapped flyback transformer core?"

One flyback transformer I have is ~38T on the primary, with saturation around 1A.  So, a 10 turn coil would have ~3.8A saturation current.

The circuit you posted looks fine - controlled by current limit.  My circuits are mostly made of discrete transistors, so not something anyone would want to copy.

6
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« 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.

7
If it's a TV flyback (not from a computer monitor), the two common standards have very similar horizontal timing (NTSC in the US and PAL in Europe).  I recall PAL at the moment: 15625Hz horizontal, 64us total, of which 52us is active picture and 12us is blanking (retrace) time.  A high-quality TV showing the entire picture would have slightly less than 12us retrace time, say 10us out of 64 total.  Cheaper TVs often used the entire time and cropped the image edges a bit in the process.

Are you defining duty cycle as the drive transistor on-time?  Presuming so, then normal TV use would have it on for 52-54 out of 64us, or 81-84% duty cycle.  For non-TV use, the optimum depends on many factors, so there's no one correct answer.  It depends on frequency, DC input voltage, transistor voltage capability, and the nature of the desired arc.  Low voltage at high current needs wires closer to start an arc, but can sometimes stretch to longer after started and be thicker.

Sine waves from a ZVS circuit can work well if you are after high current at a lower voltage (ie. 50-60% of rated flyback output voltage). ZVS may be better driving a a user-added primary winding on the exposed ferrite core leg (lower transformer coupling factor), with a center-tap on the primary so the ZVS has only a single inductor for power feed.

For driving in flyback mode, my preferred topology is one with a current-sense resistor in the source/emitter of the drive transistor, so each drive period lasts until the current reaches the flyback's rated input current.  If spec's aren't available, a cap discharge ring-down test can determine the core saturation current.  The spec current is less than that, perhaps 60-80% of saturation.  If the core gets over 100C, then it's being driven too hard.  60-80C would be safer to account for hotter inside the windings.  This topology auto-corrects duty cycle for maximum current.

8
Since these already are inductors, they'll have an air gap.  If you want to adjust inductance, then the gap needs adjusting as Steve said (or the winding count changed).  So, you'll need to separate the two "halves" of the core, but likely not need to de-laminate as you would if starting with a transformer.  My guess would be that the existing gap is created by E section(s) having a shorter center leg than outer legs.  That's common in ferrite cores, but I'm not sure about silicon-iron laminations.  If that's the case, then you can add space to lower inductance, but not decrease gap to increase inductance.  Of course, if the existing inductance is close enough, that's the easiest.

Another use of an inductor is between a 3-phase bridge rectifier and the bulk-cap.  Improves power factor (and lowers the DC output voltage a bit, with higher current capability, more than the voltage reduction, so a bit more power) under load.  Just be careful of having too much inductance and the resulting bulk-cap voltage spike when the load is abruptly removed (ie. end of burst of a DRSSTC).

9
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« 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.

10
Voltage Multipliers / Re: CW multiplier resistor string suggestions
« on: December 08, 2019, 09:57:51 PM »
That's the value of searching on Digikey first, searching by parameter, then looking at oemstrade for better prices.  OEMs trade works reasonably well with partial part-number strings IF there are reasonably few matches.  Adding a single extra digit to your search makes it reasonable, "VR68000001".

BTW, the VR68 series has the value encoded in the four digits after the long string of 0's, three digits followed by a power-of-ten digit.  So, the VR68000001503JAC00 part is 150E3 or 150k.

11
Voltage Multipliers / Re: CW multiplier resistor string suggestions
« on: December 08, 2019, 06:50:11 AM »
Steve,

The VR68 series goes down to 100K.  Arrow happens to have 150k in stock, VR68000001503JAC00.  Digikey has all the way down to 100k, but at a bit higher price.  Or, to you already have a set of 470k parts?

12
Electronic circuits / Re: Cost reduction in consumer electrics
« on: December 06, 2019, 03:48:34 AM »
Still safer than early days of residential electrification.  When my great grandparents first got electricity, bath water heating went like this:  Fill tub with cold water.  Take heater from hook on wall and place in tub.  Heater is bare resistance wire wound around a slab of wood, with a cord going to an exposed (no box) knife-switch on the ceiling above the tub.  Close the exposed knife switch and wait for water to warm.  Open the switch and hang the heater board back on the wall hook.  Bathe.

Not that I'm suggesting relaxing standards.  It's just fun to think about where we've come from.

13
Voltage Multipliers / Re: HV resistor in oil
« on: December 05, 2019, 05:21:07 AM »
Steve,

You could use well less resistance if you want to make sharper sparks.  The 30mA diodes are likely rated well higher for peak current, probably over 100mA.  Any sudden discharge event will draw current for only a short pulse.  Even that short pulse sends only half of the discharge current through diodes.  Half will go through each capacitor chain.  The one chain connected to ground and your resistor string will not contribute to diode current during a discharge event.

For limiting average diode current, the capability of the ZVS/flyback is likely sufficient.  To reach 30mA average into even a shorted load, the flyback needs to output +-60mA, with each diode conducting for half-cycles.  That would be +-6A flyback input current if the turns-ratio is 100:1 (not including the resonant current).  With a 2meg output resistor string, the current would share some among the first several diodes, so could handle more than +-60mA.  It would require simulation for a more precise analysis, but I suspect 2meg total would be enough.

Of course, 15meg would be more conservative if that's your bent.  It would also lower the dV/dt of the caps during discharges - don't know what your caps can handle.

For mounting, how about making the single long string without covering, then wind it around a smaller internal piece of PVC pipe, perhaps an extension of something used to mount the caps and diodes?  Or make a zigzag up one side of the internal PVC.  Either way, there are many options for keeping the string in place.  Drill holes in the pipe and have excess lead stubs from the resistors stick into the holes.  Or, make small saw-cuts along one side of the pipe and wrap string or fishing line around the pipe and resistors,  with the string in the saw cuts.  The string could wrap up and back down and tie together.  Or, use epoxy on the resistor leads or on string instead of saw cuts.  Or, cut small scraps of PVC (say semicircles of pipe) and use PVC glue to attach them between turns of resistors...

14
Voltage Multipliers / Re: HV resistor in oil
« on: December 04, 2019, 04:31:16 AM »
Steve,

Thank you for the EBay link!  At the picture looks to be in very nice shape.

Is there a list of AC flyback part numbers?  I haven't found anything showing that F0239 is AC-output, nor any way to search EBay (or other places) for AC flyback transformers in general.

This question isn't critical.  At the moment I have only one HV AC transformer plan (DIY plasma globe), which I have by frying diodes in an old DC flyback.

15
Beginners / Re: Full bridge showing half the voltage
« on: December 04, 2019, 04:26:38 AM »
If your new schematic is accurate, the bridge rectifier you are using to measure voltage is connected to the gate-drive on the right side rather than to the right-side H-Bridge output.

16
Voltage Multipliers / Re: HV resistor in oil
« on: December 03, 2019, 04:34:46 AM »
Steve,

For more specific electronic parts such as VR68 resistors, I purchase from electronic distributors.  There's a wonderful web site:
    http://www.oemstrade.com
Enter a part number, or the initial string, and it searches most distributors, displaying price@quantity (and quantity in stock). with links to the distributor's page for that specific part.  It's wonderful for finding the best price.  (For work, it's also helpful to see if a part is widely available.)

Arrow (arrow.com) often has one of the best prices, and has free shipping for >=$50 orders.  For a couple years they had free overnight shipping on all orders of any size.

DigiKey often has the highest price, but not always, occasionally the lowest.  They have the widest range of parts in stock and the best search tools (IMHO).  So, I often start at DigiKey, search for the parameters I want (ie. >=10kV 10meg resistor), look for the lowest price options on DigiKey, then enter those part numbers into Oemstrade.  (Sometimes I need to order a larger variety of parts or more unusual ones, so end up ordering from DigiKey or Newark or Mouser.)

Also, if exact value isn't critical, look on Oemstrade for several similar parts.  Sometimes one of the discount distributors such as chip1stop or Verical will have excess stock and dramatically-lower prices on specific parts.

I looked through the videos from backwoodsBrophil that you linked, and commented there with a question about source of his AC-output flyback transformers.  He's using old Sanyo FO241 transformers from Ebay.  I don't see any source for those at the moment.  Do you have AC-out HV transformer(s) for this project?  If so, where did you find them?  Or, are you frying the diodes in a DC-output flyback?

Saw your comment on backwoodsBrophil's video about more diodes than necessary.  My experience suggests this is necessary.  Series diode strings have a problem at the turn-off point.  Some diodes in a series string will have shorter Trr, so turn off first.  The first diode turning off now sees the entire reverse voltage.  It goes into avalanche breakdown, and must handle the remaining reverse recovery charge from the other diodes w/o frying.  My larger Marx generator is fed from a +-11kV home-made transformer and two-state multiplier to get 44kV.  Each diode needs to handle 22kV reverse and ~0.5A forward current.  Made strings of 1.5A 1kV 75ns Trr diodes.  (Also tried some 120ns diodes separately.)  Had frying issues until using 40+ diodes per string.  The diodes died slowly, increasing leakage current and lowering breakdown voltage from the avalanche current.  (They were avalanche-energy-rated diodes.  Initial try with diodes not rated for avalanche failed almost instantly.)  For my initial 30-diode strings, I kept testing each diode and replacing the failing ones.  When I didn't test often enough, the failures cascaded throughout the strings.  With enough diodes, the few fastest ones fry and there's still enough good diodes left.

BTW, my low-current CW multiplier uses pairs of 20kV 5mA diodes, even though the input voltage is ~20kVpp.

You may get as much or more voltage than backwoodsBrophil even with fewer stages.  If I heard his video correctly, he has 500pF caps.  With 20 stages, he's getting quite a bit of loss from stray capacitance.

17
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« 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.

18
Voltage Multipliers / Re: HV resistor in oil
« on: December 02, 2019, 03:42:16 AM »
I use Vishay's VR68 series, such as VR68000004703JAC00 for 470K, or VR68000001005JAC00 for 10meg.  They are rated for 1W, 10kV, and much smaller than many HV parts.  Being interested in low current for human touch (standing hair on end etc.), I used a string of 32 10-meg parts on the output of my CW multiplier.  I use the 68meg ones for bleed/balancing resistors on my larger Marx generator.

19
Phasing isn't too critical for initial operation unless it's way off.  It requires +-60 degrees to get down to 50% output (cosine function).  The more sensitive use for phasing is to improve IGBT efficiency by having them switch off just before current reaches 0.  If your IGBTs aren't getting too hot, precise phasing can wait.  Yes, R2 could be used for phase adjustment.

Does your scope have a separate trigger input?  If not, measuring phase with a single channel will be difficult.  If it does have a trigger input, connect that to a fixed place, ideally the enable input.  Trigger on the lead edge of enable.  Then use the single channel to measure points around the loop:  CT output, 4069 input, 4069 output, gate drive transformer inputs (each side separately), and H-Bridge outputs (if your probe can handle 200V).  Set the scope for say 5us/division to see the startup phasing.  This exercise should show phasing and hopefully give clues on the surprisingly-rapid HV output start-up.  Scope your antenna pick-up as well with the same trigger and horizontal scale.  Images of all those should be enough information to figure out the puzzles.

Of course, your call on whether it's worth all that measurement and analysis.  I have fun figuring out puzzles, so tend to ask for lots of information.

I'm still hoping someone else can comment on the fast rise and slow fall of your antenna waveform.

20
Spark gap Tesla coils / Re: SGTC MK1 - An Accomplishment in Progress
« 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.

Pages: [1] 2 3 ... 7

* Recent Topics and Posts

post Re: How do you keep in contact with forum people...?
[General chatting]
Mads Barnkob
Today at 09:31:58 AM
post Re: My first DRSSTC on bricks
[Dual Resonant Solid State Tesla coils]
johnf
Today at 07:56:39 AM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
AndreiRS
Today at 03:59:49 AM
post How do you keep in contact with forum people...?
[General chatting]
AndreiRS
Today at 03:23:31 AM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
klugesmith
Today at 12:16:53 AM
post Re: My first DRSSTC on bricks
[Dual Resonant Solid State Tesla coils]
Laci
December 12, 2019, 08:35:24 PM
post Re: How To Make a Stencil For Spray Paint With Cheap Hand Tools
[General chatting]
johnf
December 12, 2019, 07:53:35 PM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
John123
December 12, 2019, 02:52:58 PM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
John123
December 12, 2019, 02:42:19 PM
post Re: Full bridge much "weaker" than half bridge?
[Solid state Tesla coils]
nick
December 12, 2019, 11:17:36 AM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
davekni
December 12, 2019, 04:51:08 AM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
davekni
December 12, 2019, 04:26:22 AM
post Re: How To Make a Stencil For Spray Paint With Cheap Hand Tools
[General chatting]
AndreiRS
December 12, 2019, 01:21:35 AM
post Re: Using same winding on separate rails?
[Transformer (iron core)]
Twospoons
December 11, 2019, 10:49:04 PM
post Re: A dynamical arc model v2
[Dual Resonant Solid State Tesla coils]
Uspring
December 11, 2019, 05:28:46 PM
post Re: Full bridge much "weaker" than half bridge?
[Solid state Tesla coils]
Fumeaux
December 11, 2019, 03:11:42 PM
post Re: Royer oscillator (ZVS) driven Jacob's ladder, E80 core transformer
[Transformer (ferrite core)]
John123
December 11, 2019, 02:22:10 PM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
John123
December 11, 2019, 02:09:19 PM
post Full bridge much "weaker" than half bridge?
[Solid state Tesla coils]
nick
December 11, 2019, 01:10:57 PM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
kamelryttarn
December 11, 2019, 08:48:36 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
December 11, 2019, 06:08:48 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
December 11, 2019, 05:38:17 AM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
davekni
December 11, 2019, 05:01:31 AM
post Re: Using same winding on separate rails?
[Transformer (iron core)]
John123
December 11, 2019, 01:52:43 AM
post Re: Using same winding on separate rails?
[Transformer (iron core)]
klugesmith
December 11, 2019, 01:41:04 AM
post Re: Worlds Weirdest Microwave Oven, From A Weapons Factory - The Husqvarna Cupol
[Transformer (iron core)]
klugesmith
December 11, 2019, 01:37:13 AM
post Re: Using same winding on separate rails?
[Transformer (iron core)]
John123
December 11, 2019, 01:19:58 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
December 11, 2019, 01:14:32 AM
post Re: Using same winding on separate rails?
[Transformer (iron core)]
klugesmith
December 11, 2019, 01:04:08 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
December 10, 2019, 11:24:50 PM
post Using same winding on separate rails?
[Transformer (iron core)]
John123
December 10, 2019, 10:52:50 PM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
John123
December 10, 2019, 03:35:40 PM
post Re: Why did 4HV die??
[General chatting]
John123
December 10, 2019, 03:21:07 PM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
Mads Barnkob
December 10, 2019, 03:04:28 PM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
John123
December 10, 2019, 02:57:45 PM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
kamelryttarn
December 10, 2019, 11:23:48 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
Mads Barnkob
December 10, 2019, 10:27:54 AM
post Re: Worlds Weirdest Microwave Oven, From A Weapons Factory - The Husqvarna Cupol
[Transformer (iron core)]
Mads Barnkob
December 10, 2019, 10:20:17 AM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
Mads Barnkob
December 10, 2019, 10:16:32 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
December 10, 2019, 06:04:28 AM
post Re: Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
davekni
December 10, 2019, 05:28:54 AM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
davekni
December 10, 2019, 04:12:29 AM
post Duty cycle when driving a CRT TV flyback transformer
[Transformer (ferrite core)]
John123
December 09, 2019, 10:52:59 PM
post Replacement guide for Windows Media Center
[Computers, Microcontrollers, Programmable Logic, Interfaces and Displays]
MRMILSTAR
December 09, 2019, 08:29:16 PM
post Re: Sense coil fabrication?
[Induction launchers, coil guns and rails guns]
Uspring
December 09, 2019, 04:23:33 PM
post Re: Possible use for large inductor (laminated core)
[Transformer (iron core)]
MRMILSTAR
December 09, 2019, 03:59:02 PM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
December 09, 2019, 03:41:42 PM
post Possible use for large inductor (laminated core)
[Transformer (iron core)]
kamelryttarn
December 09, 2019, 09:43:11 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
December 09, 2019, 12:53:57 AM
post Re: CW multiplier resistor string suggestions
[Voltage Multipliers]
davekni
December 08, 2019, 09:57:51 PM