Author Topic: Soft switching / feedback transformer problem  (Read 911 times)

Offline David

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Soft switching / feedback transformer problem
« on: December 24, 2020, 04:43:31 PM »
Hi everyone,

I am building my first Tesla coil. The project once started based on the schematic of the oneTesla TS coil driver + my own primary and secondary coils. http://onetesla.com//media/wysiwyg/downloads/tsschem.png
After problems with blown IGBTs because of hard switching, I decided to do an upgrade using the UD2.7 driver (I hoped to be able to adjust the soft switching with the trim inductor).
Link: https://www.loneoceans.com/labs/ud27/

The upgrade included:
- New DIY current sens transformers with a ratio of 1:30x33 (I used these cores: https://www.mouser.ch/ProductDetail/623-5977006401/)
- Larger IGBT. SKM75GB12T4 (I got a few used ones for free, Yay!)
So basically everything before the GDT + the IGBTs are replaced.

After the initial measurements with the new setup, I noticed, that the feedback signal is too weak for the driver to start working in sync.
So I removed the jumper "SV1" that adds a 51 Ohm resistor in parallel to the feedback input signal. Now the signal looks about what I expected, except that the phase was shifted: https://ibb.co/2Y11n5F

This was easy to fix by changing the polarity of the feedback signal:
https://ibb.co/3Rdqhzv

So far so good, but the IGBT is not switching at the zero current moments. I would have to use the adjustable inductor at the feedback input to adjust the timing. But - as described above - the feedback is not working when the jumper is placed.
because of the 51 Ohm resistor that seems to be too much load for the signal.

Does anyone have an idea what the problem is? I replaced the feedback transformer by a CST306-3A http://catalog.triadmagnetics.com/Asset/CST306-3A.pdf. This one has 3x the output voltage of my DIY transformers, but the problem is the same as soon as the jumper SV1 is placed.

Cheers
David



 

Online davekni

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Re: Soft switching / feedback transformer problem
« Reply #1 on: December 24, 2020, 10:16:42 PM »
Concerning initial IGBT frying issues with hard-switching, that is likely caused by the resulting voltage spikes exceeding IGBT rating.  With 1200V IGBTs, hopefully hard switching won't cause failures.  The voltage spikes result from parasitic inductance in H-Bridge construction.  Overlapping parallel copper planes are best for interconnect, especially for Vbus from IGBTs to caps.  To explain the general idea, here's an example for TO247 IGBTs:
https://highvoltageforum.net/index.php?topic=1324.msg9795#msg9795

Concerning feedback, I suggest replacing the jumper and then figuring out why there isn't enough signal.  Especially with your latest 200:1 CT, there should be sufficient signal to start oscillation.  One likely issue is that the initial bridge output voltage is already at the same VBus rail that the initial pulse drives it to.  Adding a bleed resistor across the opposite IGBT will fix that issue.

My favorite way to fix startup issues is to make the driver self-oscillating near the coil's resonant frequency.  That can be done to UD2.7 by adding a ~50k resistor from positive comparitor output (IC8 pin 7) to its inverting input (IC8 pin 3), removing R7, and adjusting the value of C33 to get a reasonable frequency.  This works only when jumper SV1 is inserted.  Adding the 50k resistor may be easier soldering by using the open pad for R7 as a connection to IC8-7 and and one end of D1, D2, or R2 for a connection to IC8-3.

Have you tried JavaTC on your coil design?  It's a great tool for calculating coil parameters given coil geometry.

For more detailed suggestions, pictures of your coil and circuitry would help, and values for your MMC (capacitor in series with primary coil).

Good luck!
David Knierim

Offline David

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Re: Soft switching / feedback transformer problem
« Reply #2 on: December 24, 2020, 11:20:23 PM »
Hi,

I don't see any ringing of the primary current in the moment of switching when the jumper is not placed. So I attached the secondary coil and turned up the input voltage of the bridge. I get an arc of about 5mm. ???

I figured out that the primary current gets to a max at about 30% of the grid voltage (I am using a variac). If the primary current is too low, is this caused by my "MMC" whose capacity is too low? Or could it be the IGBT that is too slow? My bridge (half-bridge) is actually still like the oneTesla TS (see link in the first post). The MMC is just one 0.068uF 3000V capacitor. Maybe, the feedback problem resolves if I find the reason why the primary current is low.

Here are the pictures of the setup.

PS: Yes, I used JavaTC to calculate the dimensions of the coils. They should resonate at about 230kHz.

Cheers
David

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Re: Soft switching / feedback transformer problem
« Reply #3 on: December 25, 2020, 12:22:08 AM »
Just noticed something about that original circuit:  There are no gate series resistors (nor diodes across resistors to speed turn-off).  With IGBTs, turn-off times are almost always longer than turn-on times.  The gate resistor//diode sets slow turn-on as well as damping ringing due to GDT leakage inductance.  Your IGBTs may be frying due to cross-conduction, when both are on simultaneously for a moment during switching.

When using IGBT bricks, the GDT wires should all go to the small terminals intended for gate drive.  Use the local small emitter terminals.  These are pins 5 and 7.  That avoids voltage spikes in gate drive due to emitter power wiring inductance, which is why those terminals are provided.  Change that wiring before running full-power or you are very likely to fry IGBT gates.

How far is 0.068uF from what JavaTC recommends for your coil?  If far off, that could be the feedback issue.  I think it is more likely an issue with initial state of the half-bridge output, fixable with a resistor across one of the two IGBTs.

Good to see twisted GDT and CT leads.  IGBT power connections would be tricky to implement as copper planes since you are patching onto an ECB designed for TO247 parts.  May still be fine given 1200V IGBTs (once GDT connections are changed to use local small emitter pins).
David Knierim

Offline David

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Re: Soft switching / feedback transformer problem
« Reply #4 on: December 27, 2020, 10:00:37 AM »
I added two 6.8 Ohm gate resistors and fast diodes. The gate signal still looks ok after this modification.

Also the wiring was changed. I now use the small terminals to connect the IGBT module to the GDT.

The main problem still is that I can increase the bridge voltage without the primary current goint up. I am not sure if the IGBT module is fast enough. The datasheet says "up to 20 kHz" (that will be for hard switching). If the IGBTs would not switch fast enough, I can imagine that this would be the result, as it never has the time to get to Vce(sat).

0.068uF is what I used in JavaTC. I didn't find out how to get recommendations on the primary capacity from JawaTC. See JavaTC data below.

So I startet a little experiment:
1. I incresed the primary coil which lead to a fres of 175 kHz. Primary current is still limited with this setup.
2. I switched back to FGA65 IGBTs, as I know that they are really fast. With these, the primary current went much higher! ...And they immediatelly fried because the gate signal lookes very strange. 

Do you think it would be better to buy some faster IGBTs, that are well known to work at 230-250 kHz?
I have seen that the IXGN60N60 is used very often. But it is no longer produced.
Just checked my drawers and I only have 4x 88N65M5 Mosfets here.



J A V A T C version 13.5 - CONSOLIDATED OUTPUT
27.12.2020, 09:57:51

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
3.75 = Radius 1
3.75 = Radius 2
2 = Height 1
23 = Height 2
1200 = Turns
35 = Wire Awg

----------------------------------------------------
Primary Coil Inputs:
----------------------------------------------------
Round Primary Conductor
4.25 = Radius 1
8.5 = Radius 2
2 = Height 1
5 = Height 2
6.8 = Turns
13 = Wire Awg
0 = Ribbon Width
0 = Ribbon Thickness
0.068 = Primary Cap (uF)
0 = Total Lead Length
0 = Lead Diameter

----------------------------------------------------
Secondary Coil Outputs:
----------------------------------------------------
253.29 [kHz] = Secondary Resonant Frequency
90 [deg °] = Angle of Secondary
21 [cm] = Length of Winding
57.14 [cm] = Turns Per Unit
0.03239 [mm] = Space Between Turns (edge to edge)
282.74 [m] = Length of Wire
2.8 [:1] = H/D Aspect Ratio
302.6882 [Ohms] = DC Resistance
49774 [Ohms] = Reactance at Resonance
0.04 [ kg] = Weight of Wire
31.276 [mH] = Les-Effective Series Inductance
33.423 [mH] = Lee-Equivalent Energy Inductance
32.934 [mH] = Ldc-Low Frequency Inductance
12.624 [pF] = Ces-Effective Shunt Capacitance
11.813 [pF] = Cee-Equivalent Energy Capacitance
22.809 [pF] = Cdc-Low Frequency Capacitance
0.1545 [mm] = Skin Depth
9.722 [pF] = Topload Effective Capacitance
406.0711 [Ohms] = Effective AC Resistance
123 [Q] = Quality Factor

----------------------------------------------------
Primary Coil Outputs:
----------------------------------------------------
235.12 [kHz] = Primary Resonant Frequency
7.17 [% high] = Percent Detuned
35 [deg °] = Angle of Primary
272.38 [cm] = Length of Wire
17.9 [mOhms] = DC Resistance
0.582 [cm] = Average spacing between turns (edge to edge)
0.401 [ cm] = Proximity between coils
0 [cm] = Recommended minimum proximity between coils
6.738 [µH] = Ldc-Low Frequency Inductance
0.05859 [µF] = Cap size needed with Primary L (reference)
0 [µH] = Lead Length Inductance
127.961 [µH] = Lm-Mutual Inductance
0.272 [k] = Coupling Coefficient
0.126 [k] = Recommended Coupling Coefficient
3.68 [half cycles] = Number of half cycles for energy transfer at K
7.45 [µs] = Time for total energy transfer

----------------------------------------------------
Top Load Inputs:
----------------------------------------------------
Toroid #1: minor=8, major=21, height=28, topload
Disc #1: inside=0, outside=15, height=0, grounded

Offline Mads Barnkob

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Re: Soft switching / feedback transformer problem
« Reply #5 on: December 27, 2020, 11:16:41 AM »
Your IGBTs can not limit the current from being inadequate to carry the current. IGBTs will conduct whatever they are allowed to according to gate driving voltage. The older and slower IGBT bricks also have higher gate charge, so maybe your driving circuit is not able to drive the slow IGBT properly?

When changing to other IGBTs, did you adjust gate resistors? Maybe you drove them into oblivion with extreme ringing.

Is your voltage on the DC bus sagging?
Is the spark output increased with higher DC bus voltage?

If you are thinking about IXGN60N60 miniblock isotop-227, there is better alternatives, they were expensive even when they could be bought. And we only bought those because the much better 40N40 got unavailable back then :)
 
https://kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics
https://www.youtube.com/KaizerPowerElectronicsDk60/join - Please consider supporting the forum, websites and youtube channel!

Offline David

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Re: Soft switching / feedback transformer problem
« Reply #6 on: December 27, 2020, 12:23:35 PM »
Hi Mads,

I just realised that I could use the two GDT outputs of the UD2.7 in parallel. However, this did not resolve the problem.

I did not use the gate resistors with the FGA65 IGBTs. They were installed exactly like in the oneTesla TS.

Another multimeter was added to display the voltage over both 1000uF caps. The voltage is not sagging.
No, After about 70-80V, the primary current is not getting bigger and neither the sparks are.  :'(

How about the GDT? It is the Pulse P0584NL -> https://www.mouser.com/datasheet/2/336/-369999.pdf
But according the formula 2A in the datasheet, it should not be in saturation:

ET must be smaller than 95.
In my case: ET = 10*24V*50% / 235kHz = 51

Also flux density (Formula 2B) is 2040 Gauss, which is below the max of 2100 Gauss.



Online davekni

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Re: Soft switching / feedback transformer problem
« Reply #7 on: December 27, 2020, 08:27:09 PM »
Your JavaTC results look good.  Your 68nF capacitor shows 235kHz primary, 7% below 253kHz secondary, an appropriate amount of detuning.

This latest scope trace shows 177kHz oscillation, rather than the expected ~215kHz lower pole.  If you have a signal generator available, I recommend measuring your secondary frequency.  Perhaps primary and secondary frequencies are too far off.

Even without an isolation transformer, it is possible to get some useful information scoping the half-bridge output.  Of course, do not connect the scope ground to the line-side circuitry, but rather to the line safety ground.  Trigger on the half-bridge output with trigger threshold set just above zero.  That way the scope will capture a burst when VBus- is close to ground potential (when hot side of line voltage is at a positive peak).  Comparing half-bridge output to GDT input and to CT output will help.

Are you still running with SV1 jumper removed?  If so, how are you scoping current?
David Knierim

Offline David

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Re: Soft switching / feedback transformer problem
« Reply #8 on: December 27, 2020, 10:00:21 PM »
Thanks David,

The latest scope trace shows 177 kHz because I experimentally enlarged the primary coil. Without that extra part, I am at 239kHz.
At the moment I am testing only with an empty metal can in the middle of the primary coil.

SV1 is still removed. I measure the feedback signal with the normal probe at the UD2.7 feedback connector (GND of the probe connected to GND of the UD2.7).

I measured Vce of the low side IGBT while oscillation. To be sure, I always do this with a 1000V differential probe. Yellow is the primary current.
I can't measure GDT input at the same time, as I only have 2 channels. Would you prefer to see Vce and GDT input only?
This signal looks really wierd and I don't really know what I see here.


The high frequency oscillation increases a bit when I increase the bus voltage. Here it is about 10% higher than before:


Online davekni

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Re: Soft switching / feedback transformer problem
« Reply #9 on: December 28, 2020, 12:06:54 AM »
Looks like the H-Bridge is not functioning, which explains the low current.  Perhaps the GDT phase (polarity) isn't opposite between the two IGBTs, or the IGBT power connections are mis-wired.  I suggest scoping both IGBT gates simultaneously with no Vbus power applied.  If those are both clean +-24V square waves of opposite polarity, then power VBus from a low-voltage bench supply, say 5V or 24V or whatever you have handy.  Again scope the two gate waveforms.  There may be a bit of additional ringing due to low VBus.  Also scope low-side gate and half-bridge output (low-side collector) together, then low-side gate and VBus together.
David Knierim

Offline David

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Re: Soft switching / feedback transformer problem
« Reply #10 on: December 28, 2020, 05:01:42 PM »
You were right, I acidentally connected both GDT outputs the same way to the gates. That must have happend after switching back to the IGBT module.

After fixing this, the gate signals look good when VBus is 0V


As soon as I adjust the variac to 20% VBus, they start to ring quite a bit.


Vce follows the ringing of the gate while it is turned off.
When scoping the gate signal of the low side IGBT and the Vce of the low side IGBT, it looks like this:


After disconnecting the variac and supplying 24VDC (Same power supply as for the UD2.7) directly to the 2x 1000uF bus caps, it looks a bit better, but Vce still completely drops in the middle.


Yellow = VBus (24V), Green = Vce low side



Online davekni

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Re: Soft switching / feedback transformer problem
« Reply #11 on: December 28, 2020, 10:09:51 PM »
My best guess is that the IGBT brick is fried.  Either that or mis-wired.  I can't think of any other possibilities that would fit your scope traces.  Depending on how much power was applied, the reversed GDT output winding may have fried the brick.  Or, perhaps your free bricks aren't all good to start with.  It would be worth running a simple static test on the bricks before using them.  Verify that each IGBT does not conduct current at 0Vge, conducts DC current at 12+Vge, and conducts current in reverse (internal anti-parallel diode).

Good luck!
David Knierim

Offline David

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Re: Soft switching / feedback transformer problem
« Reply #12 on: December 29, 2020, 12:38:46 AM »
Yes, we are getting there! :D I changed the IGBT module to another one and Vce looks fine now.
It does no longer drop during the IGBT is turned off.

I have about 3cm arcs @ 120V Vbus.
This is the maximum before the OCD is activated. I will have to increase the threshold to allow more primary current. The heatsink is not getting warm at all!
From your experience, how many Amperes do you think that the SKM75gb12t4 can handle in this application?

Also the feedback transformer is now able to start the oscillation - even with placed SV1 jumper! But the phase is a bit off. With the other CT, it is spot on.

Tomorrow, I will start to adjust the primary coil, feedback phase lead and the OCD limit.
I hope I will get a bit more than just 3cm. 15-30cm would be nice.

By the way, I see that most DRSSTC have something like a guard ring on top of the primary coil, which is connected the earth ground.
Should I add this? Or is this only for larger coils?





 

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Re: Soft switching / feedback transformer problem
« Reply #13 on: December 29, 2020, 02:45:24 AM »
Others may have more specific experience with SKM75gb12t4 or similar parts.  My rule-of-thumb is that IGBTs fry at about 4x nominal current or 2x rated peak current, which would be around 450A for this part.  So I'd suggest OCD around 350A.  If you have more parts to replace a fried one, then push it to 450A.  It might survive.  (Some data sheets have graphs that are better for extrapolation.  This one has graphs to only 150A, so it's hard to extrapolate.)

 Again, others may have better experience with guard rings or not.  I have rings on my large DRSSTC.  Depends on how robust your drive circuitry is to a strike from the top.  If the current takes a safe path to ground that doesn't include any of the low-voltage circuitry, it may be OK without a guard ring.  (Or if you don't get enough performance to reach from top-load to bottom.  I hope you have better luck than that.)
David Knierim

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Re: Soft switching / feedback transformer problem
« Reply #14 on: December 29, 2020, 10:16:31 PM »
After setting the OCD to 300A, I get nice, long arcs:


Now it is all about bringing the tesla coil to a reliable state. To do this, I added a small fan and a built in power supply (I don't want to carry a lab power supply with me when I want to use it.)
Also I am thinking about improvements to the circuit to prothect the IGBT. Some 1000V TVS disodes over both Collector-Emmiter won't hurt. Maybe also some 30V TVS for the gates.
A guard ring was installed today.

The problem that still remains is, that I cannot use the DIY CT, because it shifts the phase. Adjusting the inductor does not change a lot. I tried the Coilcraft 7M3-153 (11 to 19uH) and the 7M3-223 (17-28uH).

Anyway... Thanks Dave and Mads for your help! You really know what you are talking about.

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Re: Soft switching / feedback transformer problem
« Reply #15 on: December 30, 2020, 12:30:03 AM »
Do you have a picture and turn counts for your DIY CT?  The 200:1 ratio of the commercial part is on the low side of typical DRSSTC designs.  The 77-material core you listed should be fine.  By 28uH, you should get 500ns of phase lead relative to no inductor.

Gate TVS diodes are a good idea.  Vce TVS diodes may help, but can also hurt.  Typical primary currents are higher than 500V+ TVS diodes can handle.  There are cases on this forum where the TVS diodes fried shorted, which then fried IGBTs.  In other words, 1200V spikes would be OK (barely) for the IGBTs.  However, those same spikes might fry TVS diodes, which then fries IGBTs.

Great to see success!
David Knierim

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Re: Soft switching / feedback transformer problem
« Reply #16 on: December 30, 2020, 02:55:49 PM »
I tried to take a photo without disassembling the whole thing. I hope you can see how it is made.

There are two cores the red one has 30 turns and the blue one has 33 turns. One red turn is guided through the other core and soldered together.
The two transformers (OCD and Feedback) are identical.
The IGBT output wire goes up through the first transformer and down the second transformer and then to the MMC and coil.

My commercial 1:200 transformer is placed betweenMMC and coil. But Mr. Kirchhoff once told me, that this does not matter.

At full power, I am measuring 150Vpp at the UD2.7 feedback input, using the commercial transformer. (SV1 placed at 51Ohm position)
I = Vpp/(2 * sqrt(2) * 51Ohm) = 1.04 A

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Re: Soft switching / feedback transformer problem
« Reply #17 on: December 30, 2020, 06:45:01 PM »
You are correct that current is the same between MMC and coil as it is on H-Bridge outputs.  However, the voltage is much higher there, usually several kV.  That is likely more than the insulation rating of your CT.  You are running the risk of insulation failure and an arc from primary to CT secondary.

Very nicely constructed CT!  Don't see any issue there.  The difference must be in the 990:1 ratio compared to 200:1.  At 990:1 you should get 30Vpp instead of 150Vpp.  Phase lead should function fine at 30Vpp.  That issue is still a puzzle.  You can leave 200:1 (but moved to a lower voltage location) as long as you don't plan to increase primary current any further.  150Vpp is as much as I'd run into a UD2.7.  Or, switch to your 990:1 and scope on either side of the UD2.7 phase-lead inductor to see what's happening to that.
David Knierim

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Re: Soft switching / feedback transformer problem
« Reply #17 on: December 30, 2020, 06:45:01 PM »

 


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[Solid State Tesla Coils (SSTC)]
prabhatkumar
February 27, 2021, 04:48:49 AM
post Re: Gate drive push pull(FDS8958) heating up
[Solid State Tesla Coils (SSTC)]
davekni
February 26, 2021, 10:55:04 PM
post Re: Very old flyback
[Transformer (Ferrite Core)]
Alberto
February 26, 2021, 10:54:13 PM
post Re: Very old flyback
[Transformer (Ferrite Core)]
davekni
February 26, 2021, 10:10:11 PM
post Gate drive push pull(FDS8958) heating up
[Solid State Tesla Coils (SSTC)]
prabhatkumar
February 26, 2021, 09:56:58 PM
post Re: Search for opinions concerning the use or test of a yunbao / Leopard drsstc skp
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Maju
February 26, 2021, 09:07:13 PM
post Re: Very old flyback
[Transformer (Ferrite Core)]
MRMILSTAR
February 26, 2021, 08:30:42 PM
post Re: DRSSTC Performs only with 1cm Arcs - Troubleshooting
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
February 26, 2021, 08:01:54 PM
post Re: Very old flyback
[Transformer (Ferrite Core)]
davekni
February 26, 2021, 06:57:00 PM
post Search for opinions concerning the use or test of a yunbao / Ebay drsstc skp
[Dual Resonant Solid State Tesla coils (DRSSTC)]
JCF
February 26, 2021, 04:15:39 PM
post Re: GDT resources
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Jesperb123
February 26, 2021, 09:26:09 AM
post Re: DRSSTC Performs only with 1cm Arcs - Troubleshooting
[Dual Resonant Solid State Tesla coils (DRSSTC)]
ChrisBlis
February 26, 2021, 09:07:40 AM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
February 26, 2021, 07:20:13 AM
post Re: Mains Fullwave CWDRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Phoenix
February 26, 2021, 06:36:58 AM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
February 26, 2021, 12:18:08 AM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
February 25, 2021, 11:26:16 PM
post Re: Very old flyback
[Transformer (Ferrite Core)]
Alberto
February 25, 2021, 11:14:19 PM
post Re: Mains Fullwave CWDRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AeraCura_
February 25, 2021, 09:54:08 PM
post Re: 8 ft spark french tesla coil
[Spark Gap Tesla Coils (SGTC)]
paulj
February 25, 2021, 09:25:11 PM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
February 25, 2021, 09:09:45 PM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
February 25, 2021, 08:37:26 PM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
February 25, 2021, 08:29:10 PM
post Re: Very old flyback
[Transformer (Ferrite Core)]
davekni
February 25, 2021, 07:54:08 PM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
February 25, 2021, 06:49:00 PM
post Re: Large coil experimenting with long on-times.
[Dual Resonant Solid State Tesla coils (DRSSTC)]
fh89
February 25, 2021, 06:01:13 PM
post Re: Try to make a full Bridge with IGBT and GDT
[Dual Resonant Solid State Tesla coils (DRSSTC)]
JCF
February 25, 2021, 04:49:19 PM

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