Author Topic: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)  (Read 1298 times)

Offline davekni

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13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« on: June 27, 2021, 01:22:32 AM »
For comparison, I made a 13.56MHz (more common ISM freuency) HFSSTC.  As with my initial 6.78MHz version, gate drive is from a crystal oscillator, not from drain feedback.  The arc plasma behaves much more like a flame at 13.56MHz than at 6.78MHz.

Circuit is similar.  Ended up with 26pF of external drain-source capacitance.  Not planned initially, but routing FET source (ground) as a plane adjacent the heat-sink (FET drain) added this capacitance.  As Steve pointed out, added capacitance widens drain pulses, lowering peak voltage.



Running inside at 450W (90V 5A) from bench supplies:



Simulation schematic:  (Full SiC FET part number is NVH4L160N120SC1)



Ran into one new issue initially.  Since these are actually dual-resonant (DRSSTC), the upper pole ended up at the second harmonic (27.12MHz).  Here's a scope capture at low power (no arc) showing the harmonic.  Black is SiC FET gate (at 5V/div.  Probe readout pin is missing), green is FET drain, cyan (light blue) is one side of GDT input (for trigger), and red is antenna (scope probe) near secondary:



Reducing the breakout size fixed the harmonic issue by raising the upper pole frequency above 2x lower pole.  Perhaps second-harmonic isn't really an issue.  I never ran it that way at high power.  Arcs are larger than the initial secondary structure, but are damped (resistive) enough to not show significant second-harmonic.

After fixing, here's scope traces running at 450W:



Same scope traces except without gate, running ~900W in my garage:



Running normally with carbon rod breakout, up to 1.1kW DC input power:
/>
There is room to push power higher, as drain peaks are aroun 800V.  For now it is limited by the voltage of rectified 120VAC line.

With glass tube over stainless for sodium yellow:
/>
Strontium chloride on breakout for red:
/>
Boron (ammonium borate) for green:
/>
Finally, just for fun, steel spring breakout (sparkler):
/>
« Last Edit: June 27, 2021, 01:27:36 AM by davekni »
David Knierim

Offline Weston

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #1 on: June 29, 2021, 04:11:16 AM »
Looks cool! And even better, the 13.56MHz ISM band is international without conditional approval like the 6.78MHz band  :D

It looks like you are using the AOT42S60 as a varactor of sorts? That seems like a cool technique. What capacitance / tuning range do you get out of it?

How well does your arc model match the measured results? I have been wanting to make another HF coil and try to synthesize a matching network that leads to better self starting behavior but have been hindered a lack of an arc model.

Offline davekni

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #2 on: June 29, 2021, 04:47:44 AM »
Quote
How well does your arc model match the measured results?
Waveforms are reasonably close.  However, I can find other sets of simulation values that produce similar results.  If I had access to higher-frequency impedance analyzers or signal generators, characterizing winding parasitics and interconnect parasitics would eliminate some of the value guesses, allowing more certainty on the arc model.

Quote
It looks like you are using the AOT42S60 as a varactor of sorts? That seems like a cool technique. What capacitance / tuning range do you get out of it?
Yes, "of sorts" is the key phrase.  Drain capacitance is highly non-linear.  At 0Vdc, drain voltage is about 30Vpp, so almost a short.  At 300Vdc, drain voltage is about 250Vpp.  (Unfortunately I can't find my notes on that voltage.  May have been 230Vpp or 270Vpp.)  At that voltage the two FETs have 200pF total, so 1.2nF when combined with the 1nF fixed capacitance in parallel.  With lower voltage, say 50Vdc, the waveform is quite distorted - little swing on the negative half-cycle and more on the positive half-cycle.  This non-linearity doesn't matter much, only average capacitance.  For one, it is in series with the main 75pF vacuum capacitor with ~4kVpp.  Second, this primarily-second-harmonic distortion doesn't affect overall arc generation.  Resulting range is ~5% capacitance (~71pF to ~75pF), for ~2.5% frequency range.  Since AOT42S60 is good for 600V, there is room to increase range by reducing the 1nF parallel capacitance (implemented as 5 parallel 200pF 2kV 1206 ceramic capacitors).

BTW, the spring-breakout (sparkler) video shows self-starting, as the spring wire is small diameter.  The carbon rod can self-start if sharpened.  However, it quickly erodes to a rounded end that doesn't self-start.  I'm wondering if SiC would keep a point any better.
« Last Edit: June 29, 2021, 04:54:31 AM by davekni »
David Knierim

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #3 on: July 01, 2021, 07:36:17 AM »
FYI, gate capacitance is apparently quite trans-linear, the downsides being high resistance (for power devices anyway) and low voltage range (-Vgs(max) < Vgs < Vgs(th)).

Not sure if RF devices exhibit the same response; it may be specific to VDMOS and not LDMOS.  That is, I would expect LDMOS may have more Cin(Vgs) dependency than VDMOS (which has essentially only Cin(Vds) dependency).

Tim

Offline Uspring

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #4 on: July 05, 2021, 05:51:21 PM »
Another interesting post by davekni beyond the borders of known TC technology.  :)
From your arc load circuit, I get a peak voltage of about 5 kV for 900W. Explains, why you don't get a self starting breakout, since also the secondary fres is quite high initially. There is probably not much of a resonance effect then.
I wonder if the thick flame like arcs are due to the CW nature of your coil or to the high frequency. Possibly the difference in appearance of the 6 and 13 Mhz arcs results from larger repulsive space charges at the lower frequency.

Offline davekni

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #5 on: July 06, 2021, 01:23:41 AM »
Quote
From your arc load circuit, I get a peak voltage of about 5 kV for 900W. Explains, why you don't get a self starting breakout, since also the secondary fres is quite high initially. There is probably not much of a resonance effect then.
I wonder if the thick flame like arcs are due to the CW nature of your coil or to the high frequency. Possibly the difference in appearance of the 6 and 13 Mhz arcs results from larger repulsive space charges at the lower frequency.
Yes, voltage is low.  The 6.78MHz version was a bit higher, but still low compared to normal coils.

CW does make shorter thicker arcs.  Both of these HFSSTCs run from DC with relatively little ripple.  My 100kHz SSTC isn't quite CW, as it runs from full-wave-rectified 60Hz without bulk capacitors.  It's arcs are still fairly short and thick, but not at all flame-like.  Look more like short thick versions of my 100kHz QCW coil arcs.  Concerning 6.78 vs 13.56MHz, it may be less space-charge at 13.56MHz since voltage is lower, and/or more randomness in the spatial location of the space-charge repulsion.  Don't have any good ideas for experiments.  Any attempt to probe with anything conductive causes a heavy arc to the conductive object (as in starting the arc).
David Knierim

Offline davekni

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #6 on: October 03, 2021, 07:05:50 PM »
I'd really like to have self-starting capability for this HFSSTC.  So far, all the HFSSTCs I've seen including this one require arc initiation by holding some conductive object near the breakout point.  Anyone have self-starting HFSSTC examples to share?

The issue is that a sharp breakout point is necessary to start an arc automatically.  I can sharpen the graphite rod, but the flame wears it back to rounded.  Sharp tungsten glows bright white at the tip, blinding out the flame.  Other ideas for a robust sharp point are welcome!

Lacking a real solution, I made a little "cheat". :)  I've drilled a small hole in the graphite rod end.  Before each start, I place a new fine copper wire strand in the hole, sticking out the top.  It quickly melts away after starting the arc flame:

/>
BTW, it will occasionally self-start with the rounded tip, especially just after a run when the tip is still hot.  Fried an SiC FET pushing the voltage higher trying for self-start.

I'm wondering if industrial-quality diamonds have enough impurities to be sufficiently conductive at room temperature.  If so, anyone know how to mount one?  Perhaps would need significant contact area with copper to conduct heat away.

Would silicon carbide hold a point any better than graphite?
David Knierim

Offline davekni

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #7 on: October 27, 2021, 12:45:41 AM »
Two more failed tries at self-starting:

Bought an industrial diamond, as long and thin as I could find without paying too much.  Its electrical conductivity at room temperature was much too low.  Held it in an alligator-clip up to a running arc.  Arc jumped around the diamond.  Never any hint of arc to the diamond.

Then bought an SiC wireless heating element.  Cut a somewhat-pointed tip using a diamond wheel.  Wasn't quite sharp enough for self-starting.  Tested it by normal manual starting.  At 1kW, the tip was slowly melting and burning to a more rounded shape.  Point eroded perhaps more slowly than graphite, but definitely not surviving sharp enough for self-starting.

Setting this project aside unless I hear of another reasonable option to try.
David Knierim

Offline alan sailer

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #8 on: October 27, 2021, 01:45:42 AM »
I offer this hesitantly since your power is so much higher, but here goes...

When I was running my much lower power set-up (maybe 120 watts) a friend wanted me to try various electrodes.
One of them was iridium, both as a wire and using an iridium spark-plug electrode. Both worked and self-started. I
didn't do any life testing as I liked the color of the tungsten rod. It self-starts when the rod is pointy fresh but that
soon goes away.

If you had a bucket of money I suspect that a large diameter iridium rod with a pointed tip might work. My theory is
that the large rod would conduct the heat away from the tip fast enough to keep it stable. Like any theory it could
easily be proven wrong by experiment. But I'm too cheap to do that experiment.

Cheers.

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #9 on: October 28, 2021, 02:41:50 PM »
Your secondary resonance frequency is probably way above the operating frequency at breakout, since you probably tuned for the max output when you have an arc with its capacitive loading. Possibly some retuning might help to get more voltage at breakout time.

Another idea is to have a breakout point at the side of the electrode, so that the arc travels up to the tip and does not heat up the the breakout point after ignition. Dunno if this works. You probably tried that.

A third idea is to heat up the electrode locally on top before you power up the coil. That will create a small pocket of hot and thin air with a lower breakdown voltage. If you heat the spot up into the region of thermionic emission, that might help, too. It would be somewhat like the ignition of a hot cathode fluorescent tube. As an experiment leading up to this, you could try heating the top electrode locally with a little torch before you power up.
You should be careful of getting the torch and yourself out of the way before applying power  ;)
Better is, of course, some means to heat up the top electrically.

Offline Duane B

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #10 on: October 28, 2021, 10:31:52 PM »
Going along with some ideas by Uspring, you could wind the secondary bifilar, then you would be able to send up a DC (or AC) voltage to the top of the coil to make a small ignition spark or heat something up.

In our linear accelerators there is a pulse transformer that develops 125 kV at around 85 amps. The secondary of this pulse transformer is bifilar wound so it can send up 240 vac to power a klystron filament transformer to produce 7.7 volts at 30 amps (elevated at the 125 kV voltage)

Another idea may be to make an AC ground at the base of the secondary with a capacitor, then send a DC voltage up the secondary to start corona.
« Last Edit: October 28, 2021, 10:58:48 PM by Duane B »
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Offline davekni

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #11 on: October 29, 2021, 05:25:52 AM »
Thank you all for suggestions!

Quote
If you had a bucket of money I suspect that a large diameter iridium rod with a pointed tip might work. My theory is
that the large rod would conduct the heat away from the tip fast enough to keep it stable. Like any theory it could
easily be proven wrong by experiment. But I'm too cheap to do that experiment.
I do spend money on experiments, but not enough for this one.  Would be much more than the $65 I spent on a diamond.  I also wonder if iridium would cool well enough to not glow blinding-white.  Tungsten has slightly higher thermal conductivity than iridium and stays sharp longer than other tips I've tried.  However, it glows so bright that the arc flame is difficult to view.

Quote
Your secondary resonance frequency is probably way above the operating frequency at breakout, since you probably tuned for the max output when you have an arc with its capacitive loading. Possibly some retuning might help to get more voltage at breakout time.
I've tried tuning some.  Can't get very much voltage increase without losing class-E operation.  A higher-impedance design would likely produce self-starting, but then not allow 1kW with class-E.  Load impedance drops (semi-constant arc voltage) as power increases, as you know.

Quote
Another idea is to have a breakout point at the side of the electrode, so that the arc travels up to the tip and does not heat up the the breakout point after ignition. Dunno if this works. You probably tried that.
Yes, I've tried that accidentally with unintentional sharp points.  Arc generally locks to that sharp point until the side point burns/melts away.

Quote
A third idea is to heat up the electrode locally on top before you power up the coil. That will create a small pocket of hot and thin air with a lower breakdown voltage.
Yes, hot does work.  Arc does restart when the carbon rod is still hot from immediately-previous operation.  Don't think I'll bother with the complexity of bifilar winding to bring heating power to the tip, though.

Quote
Going along with some ideas by Uspring, you could wind the secondary bifilar, then you would be able to send up a DC (or AC) voltage to the top of the coil to make a small ignition spark or heat something up.
Spark might be even harder, as bifilar winding would require HV insulation.  If I ever run out of new project ideas  :) perhaps I would build bifilar heating.

Quote
Another idea may be to make an AC ground at the base of the secondary with a capacitor, then send a DC voltage up the secondary to start corona.
Without a sharp point, voltage to make DC corona would be very high and therefore difficult.

Somewhat similar to the bifilar ideas, I have considered winding with capillary tube.  Then perhaps the arc would self-start with a small flow of argon or helium exiting the tip.

More suggestions are welcome!  It is fun to discuss ideas even when too expensive or difficult for actual testing.
« Last Edit: October 29, 2021, 05:30:00 AM by davekni »
David Knierim

Offline Steve Ward

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #12 on: November 25, 2021, 06:58:56 AM »
The simple way of auto-starting the discharge is to insert a ~1" piece of ~26-30awg bare copper wire into a small hole drilled into the top of the carbon electrode.  The sharp tip will initiate even a tiny discharge and then the copper will simply melt down to a small ball that resides in the hole for the duration of the experiment while the carbon electrode takes over.  Of course, you have to reload the wire each time.

Thanks for sharing, very nice to see it works so well with fixed driving frequency.  I notice the freq is pretty constant despite arc load on my self-oscillating types. I find the controlled gate drive appealing, and lack of high speed PLL also appealing. 


Offline davekni

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #13 on: November 25, 2021, 07:28:52 PM »
Quote
The simple way of auto-starting the discharge is to insert a ~1" piece of ~26-30awg bare copper wire into a small hole drilled into the top of the carbon electrode.  The sharp tip will initiate even a tiny discharge and then the copper will simply melt down to a small ball that resides in the hole for the duration of the experiment while the carbon electrode takes over.  Of course, you have to reload the wire each time.
Thank you for confirming my "cheat" solution.  That is exactly what I used in Reply#6 above.

Quote
Thanks for sharing, very nice to see it works so well with fixed driving frequency.  I notice the freq is pretty constant despite arc load on my self-oscillating types. I find the controlled gate drive appealing, and lack of high speed PLL also appealing.
I think the arc ends up being so low-Q that it adds more loss than detuning.
David Knierim

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Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
« Reply #13 on: November 25, 2021, 07:28:52 PM »

 


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[Spark Gap Tesla Coils (SGTC)]
304er
November 27, 2021, 05:27:27 AM
post Re: Kind of old single piece of glass plate capacitor
[Spark Gap Tesla Coils (SGTC)]
304er
November 27, 2021, 03:19:55 AM
post Re: Kind of old single piece of glass plate capacitor
[Spark Gap Tesla Coils (SGTC)]
304er
November 27, 2021, 02:31:46 AM
post Re: Kind of old single piece of glass plate capacitor
[Spark Gap Tesla Coils (SGTC)]
304er
November 27, 2021, 02:22:20 AM
post Driverless SSTC
[Solid State Tesla Coils (SSTC)]
bismallah
November 26, 2021, 11:59:27 PM
post Infrastructure and Architecture
[General Chat]
Da_Stier
November 26, 2021, 11:27:47 PM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
klugesmith
November 26, 2021, 11:17:55 PM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
MRMILSTAR
November 26, 2021, 08:24:39 PM
post Re: Some Starting Questions About My New DRSSTC ???
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
November 26, 2021, 07:48:07 PM
post Help on my "first" SSTC (LabCoatz SSTC 2.0)- Popping transistors
[Solid State Tesla Coils (SSTC)]
TiagoBS
November 26, 2021, 07:44:25 PM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
davekni
November 26, 2021, 06:58:16 PM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
plasma
November 26, 2021, 05:40:37 PM
post Re: "Exploding" water
[Capacitor Banks]
MRMILSTAR
November 26, 2021, 04:39:07 PM
post Re: Kind of old single piece of glass plate capacitor
[Spark Gap Tesla Coils (SGTC)]
MRMILSTAR
November 26, 2021, 04:31:20 PM
post Re: Next Gen DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Intra
November 26, 2021, 03:52:08 PM
post Re: "Exploding" water
[Capacitor Banks]
johnf
November 26, 2021, 09:17:07 AM
post Re: Kind of old single piece of glass plate capacitor
[Spark Gap Tesla Coils (SGTC)]
paulj
November 26, 2021, 08:45:34 AM
post Re: Some Starting Questions About My New DRSSTC ???
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ItsChloeUwU
November 26, 2021, 06:31:56 AM
post Kind of old single piece of glass plate capacitor
[Spark Gap Tesla Coils (SGTC)]
304er
November 26, 2021, 05:59:58 AM
post Re: FPS1000HD unboxing and first impression
[High Speed Filming]
Kamui
November 26, 2021, 05:12:14 AM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
304er
November 26, 2021, 05:07:10 AM
post Please Help Me Find Datasheed Of IGBT
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ItsChloeUwU
November 26, 2021, 04:08:22 AM
post Re: Contest of motor slowness, or lowness of voltage?
[Science, Research and News In Other Fields Than Electronics]
davekni
November 25, 2021, 07:35:57 PM
post Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
[Solid State Tesla Coils (SSTC)]
davekni
November 25, 2021, 07:28:52 PM
post Re: Practical limitations in size for Marx or CW
[Voltage Multipliers]
davekni
November 25, 2021, 07:22:32 PM
post Re: "Exploding" water
[Capacitor Banks]
304er
November 25, 2021, 07:01:03 PM
post Practical limitations in size for Marx or CW
[Voltage Multipliers]
abstruse1
November 25, 2021, 05:58:34 PM
post Gas Discharge Tubes for Marx Generator?
[Voltage Multipliers]
abstruse1
November 25, 2021, 05:55:53 PM
post Contest of motor slowness, or lowness of voltage?
[Science, Research and News In Other Fields Than Electronics]
klugesmith
November 25, 2021, 05:50:35 PM
post "Exploding" water
[Capacitor Banks]
MRMILSTAR
November 25, 2021, 04:48:10 PM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
paulj
November 25, 2021, 10:52:44 AM
post Re: Next Gen DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Netzpfuscher
November 25, 2021, 07:09:29 AM
post Re: 13.56MHz ISM frequency HFSSTC (or perhaps HFDRSSTC)
[Solid State Tesla Coils (SSTC)]
Steve Ward
November 25, 2021, 06:58:56 AM
post Re: My completed 14-stage Cockroft-Walton voltage multiplier
[Voltage Multipliers]
davekni
November 25, 2021, 05:51:03 AM
post Re: Some Starting Questions About My New DRSSTC ???
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
November 25, 2021, 05:13:17 AM
post Re: My completed 14-stage Cockroft-Walton voltage multiplier
[Voltage Multipliers]
MRMILSTAR
November 25, 2021, 04:50:11 AM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
davekni
November 25, 2021, 02:25:10 AM
post Re: My completed 14-stage Cockroft-Walton voltage multiplier
[Voltage Multipliers]
304er
November 25, 2021, 12:42:59 AM
post Re: 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
klugesmith
November 24, 2021, 11:03:52 PM
post Re: My completed 14-stage Cockroft-Walton voltage multiplier
[Voltage Multipliers]
MRMILSTAR
November 24, 2021, 10:00:47 PM
post Re: Next Gen DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Intra
November 24, 2021, 02:32:28 PM
post 60hz synchronous motor on 50hz?
[Spark Gap Tesla Coils (SGTC)]
paulj
November 24, 2021, 08:28:50 AM

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