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Messages - davekni

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1
Looks like a successful project already.

Quote
Question - It would be nice if we could interrupt this coil with another MOSFET connected to a PWM driver. I know Arc Angel (of youtube fame) was doing this, but I am unable to get in contact with him  Does anyone have info on doing this? It was suggested that we use a TC4420 MOSFET driver driven by a PWM generator circuit that might work. Does anyone have opinions on this
The closest thread I'm aware of on this forum is about interrupting a ZVS oscillator feeding an HV transformer.  This should be similar.
https://highvoltageforum.net/index.php?topic=1391.msg10563#msg10563

You'll need a FET to switch input current as you mentioned.  Will also need a catch diode after the FET to handle ZVS input inductor current after FET is switched off.  The FET and diode need to be close to the power supply or preferably have a filter capacitor on the input (power supply side) in close proximity.  Minimize wiring inductance of these three parts (filter capacitor, FET, and diode) to minimize FET voltage spike at switch-off.

It will also help to add a power resistor bypassing the FET, so that the OFF state is not fully off.  ZVS oscillator circuits tend to have startup transient issues (high initial current spike from supply, then high voltage spike on ZVS switching FETs).  There are many posts on the forum about ZVS issues (and successes).  A resistor keeping the ZVS weakly oscillating makes startup more gentle.

Good luck!

2
Quote
It will require adequate impedance in the transformer primaries or the drive will fault as it will be "expecting" to see 10 horsepower motor windings or the equivalent.
This is likely your most difficult challenge.  My guess is that you need inductors between VFD and transformers.  HV transformers are not likely to handle VFD switching-frequency voltage swings, and the VFD is likely to have trouble with HV transformers secondary winding capacitance if not isolated by primary inductors.

Quote
I have a box of 30 12KV 1A NTE rectifiers (block mount on heatsink)
At <1kHz, series connection may work.  At 30kHz, the biggest issue I found with diode series strings is variation in reverse recovery charge.  The first diode to recover gets the entire string's reverse voltage, conducting recovery current of other diodes in avalanche breakdown.  Sorting diodes by recovery charge helped, as did using diodes with avalanche energy handling specifications.

BTW, if transformer secondaries are wired as 3-phase, only 6 diodes are required, not 12 as shown in your diagram.

Quote
3 strong step-up transformers
If you can find a 3-phase transformer, that will be lighter and more efficient than 3 separate 1-phase transformers.  Not sure where you will find either, nor the HV inductor.  Good luck with that.  (I used resonant charging on my SRSGTC, using intentional inductance of MOTs and a bit more added inductance in primary since MOT inductance was not quite enough.  Charging tuned to 120Hz to match my 60Hz line frequency.  Synchronous, so no diodes.)


3
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 15, 2022, 11:59:37 PM »
Quote
The only thing left for me to try is to remove the interrupter switch (which I actually don't need) and jumper its connector on the board
Good idea to jumper the interrupter switch.  However, given the interruption frequency ramp in your video, I doubt the switch was the problem.

Before risking more IGBTs, it might be worth replacing the 555 socket and examining under the socket before soldering in a new one.

Intermittent problems are hard.  Without finding and fixing the source of the issue, another failure seems likely.  Might be worth adding a fast-acting fuse to AC line input.  Since IGBT failure takes at least a second after interrupter fails to continuous-enable, perhaps a fuse would open first.  (In most cases, fuses aren't useful for protecting IGBTs and FETS.  They fry faster than the fuse.  However, in your case of more mild overload, a fuse might help.  Presumably fuses are cheaper than IBGTs.)

4
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 15, 2022, 09:43:07 PM »
Quote
The 300 ohm reading was made on the board without 555 and without 0.33uF capacitor because it was the first component that I removed to test its capacity and resistance, and from the measurements it turns out to be good
As a general consideration when chasing intermittent problems, measuring a component to be good is no guarantee.  It might measure good at one time, then fail at another time.

However, since you measured 300 ohms on the board with 0.33uF capacitor removed and 555 removed, the only possibility left is a failure of the socket or board itself.  Some contamination or metal (ie. tin from solder) whisker is intermittently shorting that node to ground.  Are there any spots that were overheated during soldering?  Carbon from charred organic material seems most likely to create 300 ohms.  Metal whiskers are generally lower resistance and ionic conductivity (ie. salty finger prints) are higher resistance.

5
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 15, 2022, 07:49:51 PM »
Quote
The reading of 300 ohms was made with the chip out of the socket. But after re-soldering the socket the reading went from 300 ohms to a few tens of megohms.
Was the reading of the chip or of the board?  I'm going to guess that the measurement was of the board.

Quote
After the next failure I have not made other measurements but I don't understand what could cause such a problem on the board
300 ohms seems unlikely for a circuit board contamination failure.  Most likely is an intermittent failure of the 0.33uF capacitor.  Appears to be ceramic in your photo.  Cracks can form when bending leads close to the body, causing metalization of one layer to contact metalization of an adjacent layer connected to the opposite lead.

6
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 15, 2022, 06:47:53 PM »
Quote
So the problem is not with the 555 chip. I have made other tests with a multimeter and the only thing that does not fit me is that between pin 2 (and therefore also on 6) and GND I see a resistance on about 300 ohm.
I agree, 300 ohms is likely a problem, and would cause 555 output to stay high.  (Unless your meter measures resistance with unusually high voltage, enough to forward-bias diodes.  To check for this, measure at a higher resistance range such as 20k full-scale.  If it is still 300 ohms, this is likely the cause of your failures.)  I'd guess the failure is internal to 555 chip.  Other less-likely possibility that comes to mind is some resistive short on the circuit board.

Either way, it appears to be intermittent.  Intermittent failures are the hardest to find.  At the time of any given measurement such as repeating the above 300 ohm test, the failure may be present or may not be present.  No easy way around this difficulty.  If you can get the 300 ohm measurement to repeat, measure the chip out-of-socket and measure the socket pins, to determine if the failure is of the chip or board.

7
Quote
The coil was, the whole time, fed through the 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters ( https://highvoltageforum.net/index.php?topic=2015.0 ) which did not even get warm to the touch.
Variac wipers are often hot spots, causing the winding to fail at the wiper setpoint.  Not sure if checking overall Variac temperature is a good way to judge margin for short-term overload.

Impressive video and demonstration!  I'm wondering if you could achieve even longer arcs (at least a little bit) with further detuning and/or a shorter breakout point.  It doesn't seem to be struggling to break out initially as would be the case if detuned was close to the limit.  Of course, perhaps it is at the limit.  Hard to tell by viewing a video only.  In my case, an interruption sequence of ramped pulse widths also helped.  The ramp from short to long helped an arc form with slightly more detuning than otherwise worked.  Final enable pulse at the end of the ramp jumped to 2x longer, trying to get a tiny bit more length from an arc that had already reduced secondary frequency below primary.

8
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 15, 2022, 03:40:42 AM »
Quote
However this is the link of the video of the operation, maybe it can be useful to explain me better
Yes, I'd agree with your interpretation.  Looks like interruption is failing to continuously-enabled.  Are you adjusting any potentiometers (50k or 2M) just before operation changes to continuous?  It sounds like the interruption frequency is increasing for about 1 second prior to failing to continuous-enable.  If you were not adjusting frequency (2M pot), then the failure is likely some issue with the 555 circuit.  If you were adjusting frequency, then the failure may be coincidental to the frequency change, so may be unrelated.

If the 50k (pulse width) potentiometer were to fail open (the most common failure), that would result in continuous enable.  Presuming all solder joints are good, adding a jumper from the unused potentiometer pin to the wiper (center) pin avoids the common failure of open wiper.  Then an open wiper failure becomes maximum 50k ohms rather than infinite.  Not too likely to be your specific failure unless you happened to be adjusting both pots simultaneously.

Quote
But I have a question: can I test without replacing dead IGBTs?
Yes, but do remove dead IGBTs first.  If the IGBTs failed with gate-source shorted besides source-drain, then leaving them in circuit would overload driver chip.






9
Thank you!  Yes, a typo that I carried through the post, though I did calculate with r rather than 1/r.  I've edited (corrected) my original post so that no one will get confused looking at this thread in the future.

10
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 14, 2022, 09:25:23 PM »
Do you have access to an oscilloscope?  If so, it shouldn't take too long to figure out the issue.  Probe UCC27425 enable and output pins.  Probe without power to half-bridge, only +5V and +12V supplies on.  Verify that you can adjust enable pulse width and frequency, and that both UCC27425 outputs are low when enable is low.  If that looks good, repeat measurements during operation, set to low duty cycle to minimize risk to IGBTs.

Quote
as soon as you turn on the coil, the burts appears "correctly" and after a few seconds it changes without me changing anything
One possibility is a bad connection between socket and IC.  Sockets can be helpful, but also problematic.  Intermittent connection is one possibility for this behavior.

11
Solid State Tesla Coils (SSTC) / Re: Problem with SSTC
« on: May 14, 2022, 08:02:22 PM »
How do you have the "Interrupter ON/OFF" switch set?  It doesn't show up in the schematic.  If I recall correctly from another similar post, it is in series with the 5k resistor from 555 to UCC27425 enable.  If open, then the coil runs continuously, ignoring 555 output.

Since parts came from Mouser, the other issue is unlikely.  There are counterfeit UCC being sold with non-functioning enable inputs.  Hopefully those counterfeit chips have not made their way into normal electronics distribution channels such as Mouser.

Good luck with your problem solving!

12
Quote
Sure! It starts around 4-5 seconds into it.
Agree with Mads.  I don't see or hear anything indicating a problem.  Might be a transition where the base of the previous arc is hot and ionized enough to make the next enable pulse start immediately with significant arc length.  If bridge voltage and current are still properly phased (ideally bridge output voltage switching slightly before current zero-crossing), it should be fine.

13
Scoping current waveforms before and after onset of "flanging sound" might also help.  Even better would be both current and H-Bridge output (or UD2.7 output to GDT if you don't have isolated or differential probes).

14
Solid State Tesla Coils (SSTC) / Re: Hi frequency ringing with GDT
« on: May 12, 2022, 06:39:55 AM »
Quote
Where the probe is connected (isolated by isolation transformer) :
Some or even most of the ringing may be signal in the FET source lead inductance.  Connect scope ground lead as close to FET body as possible, and route ground lead adjacent probe to minimize loop area (minimize mutual inductance with power wiring).

If I'm seeing images correctly, FET source connection is soldered close to FET body.  That's great.  Best if GDT connection to source doesn't share much if any distance along the wire conducting FET source current.  In other words, solder GDT source wire close to FET, or on FET source lead separately from power (high current) source connection.  Tighter twisting of wires from GDT to FETs and to driver would help as well.  (IMHO, that is more important than moving the resistor to the FET gate lead.)

Reducing half-bridge power ringing will also help.  In case it's helpful, here's my example of low-parasitic-inductance half-bridge construction:
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795

Quote
By the way, I there any way to predict what would be the perfect primary for my setup.
There are a couple discussions on this forum about optimum DRSSTC primary impedance.  Haven't thought much about SSTC.  Mads (Kaiser Power Electronics) web pages may have guides.

Quote
And for the damped wave un my unloaded circuit, It put the fet in activity for quite a long time at the end of each interupter cycle. How can I get rid of it ? I would like to audio modulate it but with that wave, I'm going to have problems !
I think you are talking about the lower-amplitude ring-down after half-bridge drive ends.  That is not likely a problem.  Have you zoomed-in to see the ring frequency?  May be at high frequency, resonance of primary inductance and FET capacitance.

15
Quote
Clear thanks ! I still have the sphere piece of steel I cut from the bottom... So I think I could use it to test... Not sure it is light enough, but it has the good curvature ! I will try to approximate the voltage to check. With 50cm sparks, should be above 1 million, right ?
How thick is the steel?  May be fine. Somewhat smaller stainless steel balls I bought have thin walls.
As USpring pointed out, voltage is unlikely to be much above 750kV due to excessive corona discharge from top sphere.  Higher local field strength at smaller ground ball can start arcs of 50cm below 1MV.

Quote
And then I will try to measure the voltage. I keep you informed.
Great!  I'm looking forward to seeing your results.

16
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Too much work and winding to risk  smacking it with 60 or 400Hz!
I quite agree.  Given ferrite core and 20kHz design frequency, it is certain to function poorly at 60Hz or 400Hz.  Output voltage capability before saturation will be roughly proportional to frequency.  At 400Hz, max output voltage will be about 100V rather than 5kV.  (I'd misunderstood initially.  Thought you were asking about running a 60Hz transformer at higher frequencies.)

17
Not clear where 27kHz comes from.  I'd guess upper useful transformer frequency is limited first by secondary winding capacitance and leakage inductance.

For a given SGTC design (given MMC value and spark gap voltage), transformer RMS current increases with frequency, as it must charge the MMC more often.  That's one limit for SGTC use.

Do you have a ballast (series inductance) for your transformers?  I'm guessing they don't have intentional internal series inductance as NSTs do.  At high enough frequency, transformer's leakage inductance might be enough for ballast.  However, that may be too high a frequency for transformer power rating.

Quote
I would assume an SGTC would really benefit from 400Hz or so with lots more breaks and a more annoying buzz, but that going up to the KHz would just create a constant arc unless there was a mechanical break device.
Synchronous rotary spark gap (SRSGTC) would be a great option.  I'd guess that 400Hz could be achieved with a static spark gap and fast air flow through the gap.  If you added arc-extinguishing features as found in circuit breakers along with fast air flow, I'd guess 1kHz is possible.  I don't have any specific experience to justify this guess.  Closest is my 120Hz Marx generator using static gaps and rather gentle air flow.  (Limited to <50Hz without any air blowing at spark gaps.)

18
Quote
Good idea ! How do you link the disk weight to output voltage ?
Electric field E on top of the sphere (presuming semi-infinite distance to other objects) is V/r, where V is the sphere voltage and r is the sphere radius.  Energy density (J/m^3) of that field is 1/2 * E^2 * e0, where e0 is air (vacuum) permittivity, 8.854pF/m.  Force is energy per unit distance, so J/m^3 is the same as N/m^2.  What matters for voltage measurement is force (weight) per unit area.  As long as the disk is thin and disk diameter somewhat smaller than top-load radius, weight/area is the only needed parameter.  In other words, if you can lift a 1mm thick aluminum disk, it doesn't matter if it is 10mm or 100mm diameter.  Either way, 1mm thick aluminum (2.7g/cc = 2700kg/m^3) is 2.7kg/m^2 for 1mm thickness.

With a bit of algebra, V = r * sqrt(2 / e0) * sqrt(force / area) = r * sqrt(2g / e0) * sqrt(mass / area) where g is gravitational acceleration on earth's surface, 9.8 N/kg.
For MKS units, that becomes:
     V = (1.49MV * r_in_meters) * sqrt(kg / m^2).
For your 0.25m radius top load:
     V = 372kV * sqrt(kg / m^2).
Thus, if you can lift 1mm thick aluminum (2.7kg/m^2), voltage is 372kV * sqrt(2.7) = 611kV.

Measurement weights can be constructed various ways.  Coins are easy if you can find ones with appropriate areal density.  Sharp fields around rims will have some effect.  I've used aluminum foil disks formed to top-load curvature, with various weighting sandwiched between pairs - flattened bits of clay or other putty, coins and double-sticky tape, etc.  Weigh and divide by area to get areal density.

I believe the above is all accurate.  If I've made a mistake, presumably USpring will add a correction or clarification :)

19
Quote
And much more recently... I just finished a Van De Graaf generator. Passive one. PA / PTFE pulleys. Rubber Theraband belt. 50cm topload.
Impressive machine!  Would be interesting to measure output voltage.  One measurement option is to see how heavy a thin conductive disk can be lifted off the top by electrostatic repulsion.  Requires care to accurately center disk on top.  Disk slides off once electrostatic repulsion reduces normal force to the point where friction no longer keeps the disk in place.  Thus actual electrostatic force required to dislodge disk is not quite as much as disk's gravitational force unless centering is perfect.

20
Dual Resonant Solid State Tesla coils (DRSSTC) / Re: QCW questions
« on: May 08, 2022, 06:43:31 PM »
Quote
David, I tried  :)
Looks like that is working well.

Quote
so I just set the 'current' pot to minimum, and checked if there is output voltage.yes there is.
Current limit accuracy (including temperature drift and change with output voltage) is going to be poor down at 2mA.  True for almost any supply capable of 1amp or more.  That power supply circuit powers U1 and other circuitry after current sense resistor R7 (0.47 ohms).  Also, 2mA is only 1mV across R7, well under sensing opamp U3's worst-case input offset voltage specification (+-6mV).

To test leakage current, I'd suggest feeding collector through 10k resistor.  Measure voltage before and after resistor to see how much current the IGBT is drawing.  If more than 40uA, that is a clear violation of a worst-case specified limit, so proof of counterfeit (or damaged) parts.  (Spec. is 40uA at 650V at 25degreesC.)

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May 10, 2022, 07:52:45 PM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Nicolas
May 10, 2022, 10:04:27 AM
post Re: Transistor choice
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
May 10, 2022, 09:31:39 AM
post Re: Litz Wire?
[General Chat]
Mads Barnkob
May 10, 2022, 09:29:19 AM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General Chat]
Mads Barnkob
May 10, 2022, 09:23:38 AM
post Re: Transistor choice
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AstRii
May 10, 2022, 07:24:33 AM
post Transistor choice
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
May 10, 2022, 03:33:10 AM
post Litz Wire?
[General Chat]
abstruse1
May 10, 2022, 03:16:29 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
DashApple
May 09, 2022, 07:41:51 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
johnf
May 09, 2022, 05:39:51 AM
post Re: Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
davekni
May 09, 2022, 12:44:13 AM
post Re: First tesla coil building / Questions before ignition
[Spark Gap Tesla Coils (SGTC)]
Nicolas
May 09, 2022, 12:24:37 AM
post Home made Wimshurst machine and Van De Graaf generator
[Static Electricity]
Nicolas
May 09, 2022, 12:21:40 AM
post Ground, Grounding, Grounded?
[General Chat]
abstruse1
May 09, 2022, 12:13:01 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 08, 2022, 06:43:31 PM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 08, 2022, 07:43:13 AM
post Re: 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters
[Transformer (Iron Core)]
Mads Barnkob
May 07, 2022, 07:59:36 PM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General Chat]
Robert3z
May 07, 2022, 06:41:41 PM
post Re: Capacitor Esoterica
[General Chat]
abstruse1
May 07, 2022, 05:57:22 PM
post Re: 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters
[Transformer (Iron Core)]
Hydron
May 07, 2022, 03:43:57 PM
post 3 Phase 400 VAC 6 A Variac Stack With Voltage/Current Meters
[Transformer (Iron Core)]
Mads Barnkob
May 07, 2022, 03:21:16 PM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 07, 2022, 06:52:29 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 07, 2022, 04:57:41 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 06, 2022, 08:54:52 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 06, 2022, 02:54:44 AM
post Re: DRSSTC No Breakout On Topload
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Dhaygoh
May 05, 2022, 05:26:27 PM
post Re: DRSSTC No Breakout On Topload
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Dhaygoh
May 05, 2022, 05:14:32 PM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 05, 2022, 06:29:00 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 05, 2022, 05:56:01 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Rafft
May 05, 2022, 05:11:45 AM
post Re: QCW questions
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 05, 2022, 04:37:42 AM
post Re: DRSSTC No Breakout On Topload
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
May 05, 2022, 04:19:47 AM
post Re: Capacitor Esoterica
[General Chat]
johnf
May 04, 2022, 11:22:09 PM
post Re: DRSSTC driver in the chip shortage
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Hydron
May 04, 2022, 11:04:19 PM

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