Author Topic: Small DRSSTC Questions  (Read 971 times)

Offline Mike

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Small DRSSTC Questions
« on: May 20, 2022, 12:43:36 PM »
Hi all,

I recently built my first DRSSTC to moderate success, I'm able to get just over 0.5m streamers with a 32us pulse width. What I'm finding confusing however is that when I increase the pulse width beyond about 40us the streamer length suddenly reduces to about 0.2m and stays constant as I increase the pulse width all the way to the current maximum of 64us. I'm using a UD2.7 driver and JavaTC output has been checked against measurements of secondary inductance & resistance and primary & secondary resonant frequencies, all appear in good agreement (full text output from JavaTC is below).

I've so far tried increasing the amount of film capacitance right on the bridge, and running in burst mode both working on the assumption that there might be too much impedance between my bulk cap and the bridge, but to no avail. Scoping the coil is a little tricky as I only have a 2 channel scope,  and don't currently have any ferrites to make a second CT, though as I'm not currently hitting over current protection, I could probably repurpose that...

Not sure if it's relevant at all, but when I lift the coil further from the ground the output steamers are about the same size but are more likely to hiss and crackle, vs when the coil sits on the ground it is a much purer tone coming from the coil. I assume this is due to the reduced secondary capacitance pulling the secondary out of tune.

If anyone has any ideas for things I could try or has heard of a similar issue before I'd really appreciate your feedback. Thanks!

J A V A T C version 13.6 - CONSOLIDATED OUTPUT
20/05/2022, 10:32:43 pm

Units = Centimeters
Ambient Temp = 20ºC

----------------------------------------------------
Surrounding Inputs:
----------------------------------------------------
200 = Ground Plane Radius
200 = Wall Radius
200 = Ceiling Height

----------------------------------------------------
Secondary Coil Inputs:
----------------------------------------------------
Current Profile = G.PROFILE_LOADED
4 = Radius 1
4 = Radius 2
15 = Height 1
50 = Height 2
1500 = Turns
0.02 = Wire Diameter

----------------------------------------------------
Primary Coil Inputs:
----------------------------------------------------
Round Primary Conductor
5.5 = Radius 1
18 = Radius 2
16 = Height 1
16 = Height 2
6.5 = Turns
0.6 = Wire Diameter
0 = Ribbon Width
0 = Ribbon Thickness
0.067 = Primary Cap (uF)
0 = Total Lead Length
0 = Lead Diameter

----------------------------------------------------
Secondary Coil Outputs:
----------------------------------------------------
211.12 [kHz] = Secondary Resonant Frequency
90 [deg °] = Angle of Secondary
35 [cm] = Length of Winding
42.86 [cm] = Turns Per Unit
0.03333 [mm] = Space Between Turns (edge to edge)
376.99 [m] = Length of Wire
4.38 [:1] = H/D Aspect Ratio
205.2 [Ohms] = DC Resistance
47547 [Ohms] = Reactance at Resonance
0.105 [ kg] = Weight of Wire
35.844 [mH] = Les-Effective Series Inductance
37.814 [mH] = Lee-Equivalent Energy Inductance
37.041 [mH] = Ldc-Low Frequency Inductance
15.855 [pF] = Ces-Effective Shunt Capacitance
15.029 [pF] = Cee-Equivalent Energy Capacitance
25.403 [pF] = Cdc-Low Frequency Capacitance
0.165 [mm] = Skin Depth
12.848 [pF] = Topload Effective Capacitance
273.0364 [Ohms] = Effective AC Resistance
174 [Q] = Quality Factor

----------------------------------------------------
Primary Coil Outputs:
----------------------------------------------------
196.11 [kHz] = Primary Resonant Frequency
7.11 [% high] = Percent Detuned
0 [deg °] = Angle of Primary
479.88 [cm] = Length of Wire
2.93 [mOhms] = DC Resistance
1.323 [cm] = Average spacing between turns (edge to edge)
1.19 [ cm] = Proximity between coils
0 [cm] = Recommended minimum proximity between coils
9.83 [µH] = Ldc-Low Frequency Inductance
0.05781 [µF] = Cap size needed with Primary L (reference)
0 [µH] = Lead Length Inductance
91.947 [µH] = Lm-Mutual Inductance
0.152 [k] = Coupling Coefficient
0.126 [k] = Recommended Coupling Coefficient
6.58 [half cycles] = Number of half cycles for energy transfer at K
16.53 [µs] = Time for total energy transfer

----------------------------------------------------
Top Load Inputs:
----------------------------------------------------
Toroid #1: minor=8, major=30, height=52, topload
« Last Edit: August 11, 2022, 05:39:31 AM by Mike »

Offline AstRii

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #1 on: May 20, 2022, 05:05:40 PM »
Check your secondary if it does not flash over on the inner side. Maybe after you increase ontime, the streamers break through the inner side of the secondary, that would explain lower output while increasing input power.
Marek Novotny
Czech Republic, Czech Technical University in Prague
www.uhvlab.org

Offline davekni

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #2 on: May 20, 2022, 08:23:46 PM »
Quote
Scoping the coil is a little tricky as I only have a 2 channel scope,  and don't currently have any ferrites to make a second CT, though as I'm not currently hitting over current protection, I could probably repurpose that...
Scoping would be useful and not that hard.  No need for a second CT.  Scope across R1 of UD2.7 (51 ohm feedback current sense burden resistor).  Presuming phase-lead is enabled (SV1 pins 2 and 3 are connected), SV1 pin 1 is a good place to scope, as it is connected to R1.  Initially, I'd use current (R1 voltage) for one channel and one output pin of UD2.7 (GDT input) for the other channel.

As a second run, move the second probe to one side of H-bridge output.  Unless you have an isolation transformer feeding your coil, use a small capacitor (1-10nF) of sufficient voltage rating between scope probe ground clip and VBus.  Or, leave ground clip disconnected for this second probe.  Even though there will be line-frequency added to H-bridge output, the much higher frequency bridge switching events will be visible.

BTW, my first guess is that you need a little more detuning (reduced primary frequency).  However, that is just a wild guess without scope traces.  What enable repeat frequency are you using for these tests?  Also, what is your bus voltage and over-current setpoint?

Good luck!
« Last Edit: May 20, 2022, 08:27:14 PM by davekni »
David Knierim

Offline Mike

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #3 on: May 20, 2022, 11:23:19 PM »
Check your secondary if it does not flash over on the inner side. Maybe after you increase ontime, the streamers break through the inner side of the secondary, that would explain lower output while increasing input power.

This was a good theory, but I can confirm there is no internal flash over. The coil former is sealed and I monitored joint between the PVC and HDPE top cap and there's no arcing present anywhere near it .

Scoping would be useful and not that hard.  No need for a second CT.  Scope across R1 of UD2.7 (51 ohm feedback current sense burden resistor).  Presuming phase-lead is enabled (SV1 pins 2 and 3 are connected), SV1 pin 1 is a good place to scope, as it is connected to R1.  Initially, I'd use current (R1 voltage) for one channel and one output pin of UD2.7 (GDT input) for the other channel.

You're right, for some reason I had it stuck in my head that the driver was mains referenced, but of course it's isolated. I'll try and get those measurements in the next day or two, see what they show, thanks. I hadn't encountered the cap on earth lead trick before, clever.

Currently running a repeat rate of between 500 and 2000Hz. Lower gives significantly smaller streamers. Bus voltage is 325VDC and over current set point is set to 200A.

*Fixed auto correct errors, typing on mobile still sucks.
« Last Edit: May 20, 2022, 11:49:21 PM by miket6000 »

Offline davekni

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #4 on: May 21, 2022, 12:31:45 AM »
Quote
Currently running a repeat rate of between 500 and 2000Hz. Lower gives significantly smaller streamers.
I'd guess that you want lower rates and more detuning.  High repeat rates (high BPS) can lead to continuous-arc behavior (ionized air path remains from one pulse to the next), which typically makes shorter thicker arcs.  Especially at 2kHz.

Do you have a break-out point on top?  How long?  A breakout point allows more detuning while still starting an arc, which allows longer arcs before secondary frequency drops too far below primary frequency.
David Knierim

Offline Mike

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #5 on: May 21, 2022, 11:12:05 PM »
Ok, I managed to get into the garage and get the scope attached. It became painfully obvious that the current limit was preventing larger streamers. The point where the streamers shrunk is the same point where the current limit kicked in. I still don't know why the current limiter was resulting in smaller streamers than limiting the energy with the pulse width, however winding the current limit up resulted in larger streamers.

I haven't had a chance to play with the repetition rate yet but will look into that as well.

Scope traces are below. The current waveform is approximately 0.11V/A (462:1 CT and 51R burden resistor). The ground clip for the HV trace was left floating.

As to the other questions, yes there is a breakout point, it extends ~10cm beyond the edge of the top load.

« Last Edit: May 21, 2022, 11:14:53 PM by miket6000 »

Offline davekni

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #6 on: May 22, 2022, 12:09:21 AM »
Quote
Ok, I managed to get into the garage and get the scope attached. It became painfully obvious that the current limit was preventing larger streamers. The point where the streamers shrunk is the same point where the current limit kicked in.
Glad to hear of your success!

Quote
I still don't know why the current limiter was resulting in smaller streamers than limiting the energy with the pulse width, however winding the current limit up resulted in larger streamers.
I'm not thinking of any explanation either.  Perhaps someone else can offer possibilities.

Quote
Scope traces are below. The current waveform is approximately 0.11V/A (462:1 CT and 51R burden resistor). The ground clip for the HV trace was left floating.
Forgot to mention previously:  If no suitable Y-caps are available, connecting scope probe ground clips to safety ground of line power inside coil base box is better than leaving ground floating.  That provides some capacitance to line power and through rectifier diodes to Vbus.

Hopefully this is just an artifact of floating scope ground clip:  The bridge output trace shows some partial steps during primary current decay after enable ends.  If those are real, it would indicate IGBT gates are not held off sufficiently, which could lead to IGBT failure.  What circuit are you using between GDT outputs and gates?  Is it the standard resistor paralleled by diode?  Are the diodes fast-recovery or schottky?
David Knierim

Offline Mike

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #7 on: May 22, 2022, 01:30:18 AM »
Quote
What circuit are you using between GDT outputs and gates?  Is it the standard resistor paralleled by diode?  Are the diodes fast-recovery or schottky?
Yep, 10R resistors and a schottky for turn off.

After increasing the current limit to 300A and detuing the primary by a full turn (I was surprised it was this much) I'm now getting >0.6m streamers and ground strikes. I'll keep playing with further detuning to try and find the optimal operating point but I feel like I'm well on the way. I also modified the interrupter so I can drop the repetition rate down to 200Hz. I still find longer arcs up around 1kHz, beyond that they get thicker but not really longer.

Thank you very much for your help davekni, I feel I'm now well on the way and can just play with parameters. I'll set the scope up again at some stage to get a better view on whether those partial turn on's are real. If so I'll probably need a bridge redesign to improve the layout as I'm not sure there's much else that I could do to hold the gates off better.

Photo below shows >1m strike!
« Last Edit: May 22, 2022, 02:03:01 AM by miket6000 »

Offline davekni

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #8 on: May 22, 2022, 04:57:46 AM »
Quote
Photo below shows >1m strike!
Looks great!  Have fun playing with your new coil!
David Knierim

Offline Mike

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Re: DRSSTC streamers shrink with increasing pulse width
« Reply #9 on: May 25, 2022, 12:16:00 AM »
Looks great!  Have fun playing with your new coil!
I have been, now with video!

Offline Mike

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Re: Small DRSSTC Questions
« Reply #10 on: August 11, 2022, 06:12:48 AM »
Hi all, I have another question based on an observation playing with my coil.

At lower input voltages I see a current waveform exactly as expected, the current ramps up initially then drops down after a streamer forms and reduces the Q of the system (first image). At higher voltages however there's a "double bounce" in current where the current starts to ramp down (presumably because a streamer has started) but then ramps back up again (second image). The issue I have with this is that the second ramp up is hitting my maximum current limit (500A) and terminating the pulse. Mostly I'm just curious if anyone has any theories as to why the current would be larger in the second ramp up that it was in the first and what the cause could be.


Low Voltage (100VDC)


High Voltage (325VDC)

My best theory so far is that the streamer loading initially reduced the primary current until it brings the secondary into tune with the primary causing an increase in the energy being transferred and a corresponding increase in primary current. I'll attempt to adjust the tuning of the primary to see if this is the case in which case I might be able to balance the two effects and avoid hitting the current limit.

As a small aside I still notice the effect where setting the pulse width to the point where it disables at about the same time as the current limit is hit I see slightly larger streamers than if I increase the pulse width beyond that point and let the current limiter terminate the pulse 100% of the time. It's also clear that the primary impedance of my coil is a little on the low side for my bridge, just a small decrease in capacitance would make all the above a non-issue, but again, I'll need to wait for new caps. I think I'll do this anyway as the current 2p3s of 940C20P1K-F is already at its limit (the 500A limit already puts 6.6kV across a 6kV DC rated cap, though only briefly).

Cheers,
Mike.


 


Offline davekni

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Re: Small DRSSTC Questions
« Reply #11 on: August 11, 2022, 11:51:42 PM »
Quote
My best theory so far is that the streamer loading initially reduced the primary current until it brings the secondary into tune with the primary causing an increase in the energy being transferred and a corresponding increase in primary current. I'll attempt to adjust the tuning of the primary to see if this is the case in which case I might be able to balance the two effects and avoid hitting the current limit.
I see the same primary waveform shape.  At least two different phenomenon can cause similar waveforms.  First one is classic dual-resonant system behavior.  If the arc doesn't drain sufficient secondary energy, remaining secondary energy transfers back to primary.  Higher energy shows up as higher current and voltage.
Second phenomenon is close to your theory above.  However, the second primary current increase is after arc loading lowers secondary frequency to the point where energy transfer is hampered.  Primary current increases without a way to dissipate.  I'd guess this is what you are experiencing.

I see both in my DRSSTC.  More detuning will help (lower primary frequency) with the second cause.  However, that raises amplitude of first current peak before arc starts.  You may want to detune until the first peak barely doesn't trip OCD.  Of course, as you mentioned, a slightly higher primary impedance will likely help.

Quote
As a small aside I still notice the effect where setting the pulse width to the point where it disables at about the same time as the current limit is hit I see slightly larger streamers than if I increase the pulse width beyond that point and let the current limiter terminate the pulse 100% of the time.
Any chance D3 is missing or reversed in your UD2.7 driver?  (Presuming that's the driver you are using.)  That's a diode in the OCD circuit from comparitor to FF.  That could make OCD events keep driver disabled for longer than just to the next interrupt enable cycle.
David Knierim

Offline Mike

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Re: Small DRSSTC Questions
« Reply #12 on: August 12, 2022, 01:27:14 AM »
Thanks Dave,

I lowered the primary frequency further and once I could achieve breakout by increasing the pulse repetition rate to ~1kHz (which I assume allows the breakout point to heat sufficiently and ionises the air near the breakout point to enable initial breakout) I hit much better performance with streamers growing more than 20% beyond my previous max. Unfortunately this was too much for the MMC which promptly failed short circuit. Even more unfortunately I wasn't monitoring the current at the time because it requires removing the UD2.7 housing which I was suspicious could be contributing to some of the weirdness I was seeing (it wasn't).

Quote
Any chance D3 is missing or reversed in your UD2.7 driver?  (Presuming that's the driver you are using.)  That's a diode in the OCD circuit from comparitor to FF.  That could make OCD events keep driver disabled for longer than just to the next interrupt enable cycle.

D3 is correctly installed, so that's one culprit removed.

I'm now stuck waiting for new caps to arrive. I've elected to use the same 2p3s configuration of 100n caps, but upgraded from 940C20's to 942C20's. Hopefully the increased peak current rating helps. I might also scale the peak current back down to 450A to mitigate the over voltage issue until I get some new 4s3p boards made up.


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Re: Small DRSSTC Questions
« Reply #12 on: August 12, 2022, 01:27:14 AM »

 


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[Computers, Microcontrollers, Programmable Logic, Interfaces and Displays]
davekni
September 25, 2022, 07:33:57 PM
post Re: Ud failure
[Dual Resonant Solid State Tesla coils (DRSSTC)]
FonziDaytona
September 25, 2022, 07:06:33 PM
post What am I missing?!?!?
[Spark Gap Tesla Coils (SGTC)]
Conrad73
September 25, 2022, 06:35:43 PM
post Radiometer Copenhagen OSG 42b Oscilloscope Repair, From The 1950'ish
[Laboratories, Equipment and Tools]
Mads Barnkob
September 25, 2022, 03:18:22 PM
post Re: Recommended development boards for QCWDRSSTC
[Computers, Microcontrollers, Programmable Logic, Interfaces and Displays]
Rafft
September 25, 2022, 02:53:30 PM
post Recommended development boards for QCWDRSSTC
[Computers, Microcontrollers, Programmable Logic, Interfaces and Displays]
dru
September 25, 2022, 09:05:37 AM
post Re: Ud failure
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
September 25, 2022, 08:29:09 AM
post Re: Ud failure
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 25, 2022, 05:27:41 AM
post Re: Gauss meter for strong magnetic fields
[Laboratories, Equipment and Tools]
klugesmith
September 25, 2022, 02:12:21 AM
post Ud failure
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
September 25, 2022, 01:50:45 AM
post Re: controlling the negative gate bias voltage of the Gate drive transformer
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 24, 2022, 09:36:58 PM
post Re: Gate drive transformer with negative gate bias voltage
[Dual Resonant Solid State Tesla coils (DRSSTC)]
prabhatkumar
September 24, 2022, 06:56:35 AM
post Re: Gate drive transformer with negative gate bias voltage
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 24, 2022, 06:09:03 AM
post Re: Gauss meter for strong magnetic fields
[Laboratories, Equipment and Tools]
davekni
September 24, 2022, 01:32:08 AM
post Re: Some questions about my first drsstc.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
mthome4
September 23, 2022, 10:55:21 PM
post Re: Gauss meter for strong magnetic fields
[Laboratories, Equipment and Tools]
AstRii
September 23, 2022, 10:39:13 PM
post controlling the negative gate bias voltage of the Gate drive transformer
[Dual Resonant Solid State Tesla coils (DRSSTC)]
prabhatkumar
September 23, 2022, 05:48:17 PM
post Gauss meter for strong magnetic fields
[Laboratories, Equipment and Tools]
Peregrine
September 23, 2022, 02:01:10 PM
post Re: UD2.7 dual Purpose Use
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 23, 2022, 03:04:31 AM
post Re: UD2.7 dual Purpose Use
[Dual Resonant Solid State Tesla coils (DRSSTC)]
FonziDaytona
September 22, 2022, 04:15:36 PM
post Re: SSTC - PCB design
[Beginners]
Przemekk00
September 22, 2022, 12:39:51 PM
post Re: Possible Leads on Single Photon Camera
[General Chat]
alan sailer
September 22, 2022, 12:05:55 AM
post Re: Possible Leads on Single Photon Camera
[General Chat]
Mads Barnkob
September 21, 2022, 10:32:29 PM
post Possible Leads on Single Photon Camera
[General Chat]
alan sailer
September 21, 2022, 03:57:38 PM
post Re: Driverless Single Mosfet Relay interrupted SSTC? - [All kinds of problems]
[Solid State Tesla Coils (SSTC)]
davekni
September 21, 2022, 05:50:15 AM
post Re: Musical interrupter problems
[Solid State Tesla Coils (SSTC)]
RoamingD
September 20, 2022, 10:29:41 PM
post Re: Musical interrupter problems
[Solid State Tesla Coils (SSTC)]
Mads Barnkob
September 20, 2022, 08:24:30 PM
post Re: Driverless Single Mosfet Relay interrupted SSTC? - [All kinds of problems]
[Solid State Tesla Coils (SSTC)]
TiagoBS
September 20, 2022, 06:23:54 PM
post Re: Some questions about my first drsstc.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 20, 2022, 05:20:07 AM
post Re: TO247 mounting
[Beginners]
davekni
September 20, 2022, 05:15:35 AM
post Re: MidiStick V2.0: Next gen tesla coil interrupter
[Computers, Microcontrollers, Programmable Logic, Interfaces and Displays]
TMaxElectronics
September 19, 2022, 11:43:47 PM
post Re: TO247 mounting
[Beginners]
Weston
September 19, 2022, 09:58:56 PM
post Re: Some questions about my first drsstc.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
mthome4
September 19, 2022, 09:05:35 PM
post Re: Musical interrupter problems
[Solid State Tesla Coils (SSTC)]
RoamingD
September 19, 2022, 12:58:35 PM
post Re: UD2.7 dual Purpose Use
[Dual Resonant Solid State Tesla coils (DRSSTC)]
FonziDaytona
September 19, 2022, 12:13:46 AM
post Re: MMC capacitor crash
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 19, 2022, 12:09:22 AM
post Re: UD2.7 dual Purpose Use
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 18, 2022, 11:38:51 PM
post Re: UD2.7 dual Purpose Use
[Dual Resonant Solid State Tesla coils (DRSSTC)]
FonziDaytona
September 18, 2022, 10:41:54 PM
post Re: Musical interrupter problems
[Solid State Tesla Coils (SSTC)]
davekni
September 18, 2022, 10:31:32 PM
post Re: Musical interrupter problems
[Solid State Tesla Coils (SSTC)]
RoamingD
September 18, 2022, 10:09:33 PM
post Re: UD2.7 dual Purpose Use
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
September 18, 2022, 09:58:34 PM

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