Author Topic: I am having the worst time getting an inductor based Marx Gen to work  (Read 2471 times)

Offline bobfrancis1980

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I bought some chinese marx generators on ebay.

They are resistor based and work fine unmodified but are slow. I added a SF6 based high voltage switch to control the pulse output and that does work. The best it can do is about 1 pulse per second. My eventual goal was to have a bipolar marx gen with a 10 stage going into a 20 stage for positive voltage and a 10 stage going into a 20 stage for negative voltage. Similar to the million volt marx gen I found online.

Obviously using resistors will dramatically slow down the charging of the capacitors. That is why I am trying to use the marx gen I have but replace the resistors with inductors.

Unfortunately with a 3 stage inductor based marx gen prototype the current seems to go through the high voltage switch and through the first spark gap when the sf6 switch is in discharge mode but not the rest of the spark gaps and not through the output wire back to ground. The inductors are hand wound around ferrite bobbins using 10kV high voltage wire and average between 200 - 300 uH each.

For the life of me I don't know what I am doing wrong.

My ultimate goal was to get a pulse rate of about 10hz but when the 10 stage marx gen averages about 1 times a second output, I clearly can't use resistors if I want to reach this output frequency. Adding on the rest of the marx gen stages will be even slower.

I don't have a ballast resistor, could this be the cause?

Any suggestions?
« Last Edit: October 06, 2022, 11:14:07 PM by bobfrancis1980 »

Offline davekni

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #1 on: October 07, 2022, 05:58:38 AM »
Quote
Unfortunately with a 3 stage inductor based marx gen prototype the current seems to go through the high voltage switch and through the first spark gap when the sf6 switch is in discharge mode but not the rest of the spark gaps and not through the output wire back to ground. The inductors are hand wound around ferrite bobbins using 10kV high voltage wire and average between 200 - 300 uH each.

For the life of me I don't know what I am doing wrong.
Don't know capacitor value(s) for your Marx generator, but I suggest calculating the resonant frequency with 200-300uH.  Or make a simulation in LTSpice or other such analog simulator.  My inductor-coupled Marx generator uses ~170mH inductors.  Some explanation in my comment answers on Youtube:
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David Knierim

Offline bobfrancis1980

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #2 on: October 07, 2022, 05:32:43 PM »
The capacitors are 2000pF.

The inductors have resonant frequencies between 205kHz to 250kHz so that should be fine I am not going to exceed probably 10Hz pulse output when its all built. I don't have any air blowing through all the spark gaps so that will severely limit the output pulse rate, hopefully no less than 10Hz.

Study of an Ultra-Compact, Repetitive Marx Generator for High-Power Microwave Applications
uses 17.8uH inductors, mine are larger than that so it should work.

Could my problem stem from lack of a ballast resistor even though I have a high voltage switch?

I have noticed a number of marx generators include one in their schematic and it is used for stopping electricity going across the first spark gap after the trigger pulse.

Offline davekni

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #3 on: October 07, 2022, 08:55:29 PM »
Quote
The capacitors are 2000pF.

The inductors have resonant frequencies between 205kHz to 250kHz so that should be fine I am not going to exceed probably 10Hz pulse output when its all built. I don't have any air blowing through all the spark gaps so that will severely limit the output pulse rate, hopefully no less than 10Hz.
10Hz should be fine without air, presuming electrodes don't overheat.  I needed air to get above 45-50Hz.

Quote
Study of an Ultra-Compact, Repetitive Marx Generator for High-Power Microwave Applications uses 17.8uH inductors, mine are larger than that so it should work.
Thank you for reminding me of this paper.  It does make amazingly fast-rise-time pulses.  Coaxial construction with spark gaps down center makes for very low parasitic inductance in discharge path.  Pressurized air for higher voltages also helps shorten rise times (keeping dimensions small).  Also note that they are firing into a 50-ohm load.  Having an initial low-impedance load helps upper spark gaps fire quickly (voltage increases more rapidly across spark gap).
I suspect this more typical EBay Marx generator can't fire fast enough (<1us) before voltage drops too far.  (BTW, after spark gaps fire, resonant frequency is with two inductors paralleled across each capacitor.)  I don't have any specific information on how fast such a Marx generator can fire, so don't know for sure if that is an issue or not.  (Also, requires final output spark to fire fast too.  I do have some experience with that - short pulses jump shorter distances.)

Quote
Could my problem stem from lack of a ballast resistor even though I have a high voltage switch?

I have noticed a number of marx generators include one in their schematic and it is used for stopping electricity going across the first spark gap after the trigger pulse.
I'm not familiar with any need for a ballast resistor.  Can you point to a schematic with one?  I have high-value bleed resistors across capacitors for safety discharge (and for voltage balancing since each of my Marx capacitors is a series string).  Edit:  Is the ballast resistor in series with the input DC supply?  If so, I think that is to protect the input supply from sudden voltage drop to ~0V.  I do have an inductor between supply output and Marx ladder to prevent frying diodes in my supply.

Does your SF6 switch replace the first spark gap, or couple a trigger pulse to it?  If replacing the first spark gap, two possible issues come to mind.  Do you know the rise time of the switch?  I have no idea if an arc starting between closing contacts inside SF6 ionizes as rapidly as an air arc would.  Second is the lack of UV light coupling to other spark gaps.  I've read multiple places (including the article you linked) that UV light ionization of air within remaining spark gaps is critical to getting fast Marx spark gap triggering.  I presume the SF6 switch is enclosed and not providing UV light to remaining spark gaps.

Finally, do you know what the Marx generator caps are made of?  The EBay image looks a bit like the polystyrene/foil caps that I tried initially.  They had limited life (in terms of pulse count).  Might be OK depending on how long your 10Hz runs will be.  Failure mode appeared to be occasional very-tiny trapped air bubbles along foil edges.  Air formed corona (sometimes visible in a very-dark room during discharges).  Corona degraded polystyrene until it started carbonizing and cascading to a failure.

Good luck with your project!
« Last Edit: October 07, 2022, 09:28:10 PM by davekni »
David Knierim

Offline bobfrancis1980

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #4 on: October 19, 2022, 09:22:56 PM »
Well, I added the 50ohm resistor to the output of the marx gen but it still did not work.

Only the first stage of the marx would fire and it seemed like a steady stream of electricity rather than pulses from the capacitor. There was some smoke and it appeared that I had a short from the flyback transformer to one of the inductors burning some electrical tape.

I took the marx gen apart and tested the inductors and capacitors and they seem to be ok, just the electrical tape that got burned.

As far as a ballast resistor goes, I have only seen them on resistor based marx generators, that was where I got the idea from but I think you are right, they are not needed in a inductor based marx gen with a trigger pulse.

The K61C841 SF6 switch does replace the first stage's spark gap. The operate time and release time are both 15ms so more than quick enough to have a 10hz pulse fire rate according to the datasheet.

I ordered a 20kV thyratron, maybe that will fare better...


I am kind of lost, don't know what the next step is to try and get it working. I figured replacing the resistors with inductors would be a simple operation seeing as how the resistor based marx gen worked fine controlled by the SF6 switch but clearly I am missing something.

I would appreciate any suggestions. Thanks.

Offline davekni

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #5 on: October 20, 2022, 02:23:45 AM »
Quote
The K61C841 SF6 switch does replace the first stage's spark gap. The operate time and release time are both 15ms so more than quick enough to have a 10hz pulse fire rate according to the datasheet.
Sounds like the missing UV light from SF6 switch (instead of spark gap) is the likely issue.

If your goal is to hit precise 10Hz fire rate, two options come to mind.  One is what I used: Charging supply turns on at a 10Hz rate and remains on until spark gaps self-trigger.  Turns off on spark (sudden drop in output voltage from supply) and remains off until next 10Hz start pulse.

Other more conventional option is to use a triggered spark gap.  Set input voltage to not quite self-trigger gaps.  Triggering is by either injecting a short fast voltage pulse added to the bottom spark gap, or by including a small trigger electrode in a hole in the grounded main electrode of bottom spark gap.

With your relatively-low charging ladder inductance, spark gaps must fire rapidly before significant charge is lost through inductors.  Besides UV light, two other thoughts on fast triggering:  Minimize parasitic inductance in the erected state (inductance through capacitors and spark gaps).  The paper you referenced has very low parasitic inductance with their coaxial design.  Looking at their rise time (time constant a bit under 10ns into 50 ohms), parasitic inductance must total under 500nH.  May be even lower depending on how much other factors such as spark gap rise time and measurement bandwidth affect resulting traces.  Other consideration is having voltage just barely below spark-gap self-trigger voltage.  Gaps that are on the verge of self-triggering will react much faster to the voltage increase caused by first gap firing.  (Note that the paper referenced uses self-triggering for rapid-fire pulses.  A trigger pulse is used for single fires only.  No information on how close voltage is to self-triggering when a trigger pulse is used.)
« Last Edit: October 20, 2022, 03:44:09 AM by davekni »
David Knierim

Offline bobfrancis1980

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #6 on: October 20, 2022, 09:48:18 PM »
The first spark gap after the SF6 switch does fire but it stays on until the arduino -> optocoupler switches the SF6 switch off instead of just one pulse discharging that stage's capacitor. That first spark gap though does not initiate the second spark gap to fire.

When I used the marx gen with its original resistors rather than inductors the sf6 switch worked. When the sf6 switch was turned on one pulse occurred each time the sf6 switch was turned on, there wasn't a continuous current flow or anything like that that I am seeing with the inductor based marx.

I wonder if my inductors don't have enough resistance when the sf6 switch activates so they short circuit to ground instead of discharging through the spark gaps, idk.

Or perhaps its hard to see but maybe I am getting a short circuit through the ferrite bobbins? Supposedly they are not a conductor but I touched one of them after one longer test attempt and the ferrite bobbin was hot to the touch. I don't think the hv wire I am using had burned through the dielectric coating. As I recall it was the only bobbin that was hot.

Offline davekni

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #7 on: October 20, 2022, 11:51:25 PM »
Quote
The first spark gap after the SF6 switch does fire but it stays on until the arduino -> optocoupler switches the SF6 switch off instead of just one pulse discharging that stage's capacitor. That first spark gap though does not initiate the second spark gap to fire.
I think a schematic would help here.  The schematic I'm inferring couldn't have any spark gap continuing to fire, as the SF6 switch that replaces the first spark gap is shorting the incoming DC supply, preventing any power from continuing to other stages.  Or, is the SF6 connecting the incoming DC supply to the Marx generator?  If so, that might fit observed behavior better.

Quote
I wonder if my inductors don't have enough resistance when the sf6 switch activates so they short circuit to ground instead of discharging through the spark gaps, idk.
Inductors may be too small unless spark gaps trigger very rapidly.  Most of the previous discussion is about how to get other spark gaps to trigger fast enough before inductors discharge all the capacitors.  However, since I'm not understanding what your schematic is, can't say anything with confidence.

Quote
Or perhaps its hard to see but maybe I am getting a short circuit through the ferrite bobbins? Supposedly they are not a conductor but I touched one of them after one longer test attempt and the ferrite bobbin was hot to the touch. I don't think the hv wire I am using had burned through the dielectric coating. As I recall it was the only bobbin that was hot.
Ferrites are semiconductors, with conductivity varying steeply with temperature.  Typical MnZn ferrites are on the order of 10ohm-cm, so quite conductive compared to high-voltage use.  NiZn ferrites are much less conductive at room temperature, but become conductive as they get hot.

Core-only heating suggests AC core losses (hysteresis and eddy-currents).  Hard to imagine how you would get enough AC to heat cores at 10Hz fire rate.  Perhaps current is arcing through cores.
« Last Edit: October 21, 2022, 06:48:06 PM by davekni »
David Knierim

Offline bobfrancis1980

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #8 on: October 21, 2022, 07:52:59 PM »
I found and ordered some air core inductor bobbins from digikey close to the same size as the ferrite ones I have. It'll be a few days before they get here and I am able to wind the inductors.

They use a plastic instead of ferrite so I should be able to eliminate the chances they will conduct electricity. I'll get back to you in about a week and let you know how it goes.

Offline bobfrancis1980

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #9 on: November 03, 2022, 05:49:10 PM »
Well, I wound the inductors on the air core bobbins. They average around 40uH per inductor. Even though less inductance was used in the marx gen PDF example I linked to earlier I think the inductance isn't high enough.

I saw in your video that you used 1mH inductors with 133 of them soldered in series for 133mH per stage. How did you come to the conclusion that you needed such a high inductance, trial and error?

I ordered some polyimide 26AWG wire as it is considerably thinner but offers a little smaller dielectric resistance and larger size bobbins on ebay but those won't arrive for about a month.

Cheers.

Offline bobfrancis1980

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #10 on: November 03, 2022, 07:56:59 PM »
Well, I had some polyimide 26AWG left and an order of slightly larger bobbins arrived. The first bobbin I wound was just a little under 2mH so here's hoping when I wind the rest and test a 3 stage marx gen that they will work out.

Offline davekni

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #11 on: November 04, 2022, 04:57:54 AM »
Quote
I saw in your video that you used 1mH inductors with 133 of them soldered in series for 133mH per stage. How did you come to the conclusion that you needed such a high inductance, trial and error?
There is no magic number for required inductance.  More inductance allows erected voltage to last longer.  The longer the top voltage lasts, the longer spark can be generated for a given voltage.  (The referenced axial Marx generator feed into a 50-ohm load, so pulses were inherently short with no need for time to start an output spark.  Only the internal gap sparks needed to be fast.  That was helped by their very-compact coaxial layout with direct UV path between gaps and low parasitic inductance.)

I'd guess your key issue is the missing first spark gap (replaced with SF6-filled relay).  Without UV from that first spark gap, other gaps will be slow to trigger.
David Knierim

Offline highvoltageguypk

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #12 on: December 20, 2022, 05:13:21 AM »
Hi,
I have gone through the above discussions (just skimming)  and have few questions (never mind of asking it after 2 and half months):
Q1: Open load voltage?
Q2: Pulse rise time and FWHM?
Q3: Load? (50 Ohm I guess as you mentioned)
Q4: Rep rate: 10Hz or more?

After these parameters we can simulate desired Marx generator in Ltspice (or any other circuit simulator). From this simulation and some analytical calculations we can have the component values.
Another option to achieve fast rise time and high voltage is using Tesla transformer with high pressure or oil based switch but it depends upon your voltage requirements. Thanks.

highvoltageguypk :-)

Offline huntergroundmind

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #13 on: December 23, 2022, 10:27:25 PM »
Are the capacitors in your marx are foil -wound type like this https://cutt.ly/10VPbpy ? I suspect that hey can have high ESL and ESR (It has two long pieces of very thin foil wound, and connected in one point). Your inductors need to have inductance far larger than ESL of capacitors. Also as davekni said, uv coupling is important for synchronization of spark gaps. I can also propose you to try out modification with adding third electrode to spark gaps, and connecting them with for example 100kOhm resistors.

(schematic taken from: Foundations of Pulsed Power Technology ( Lehr Jane, Ron Pralhad))  Also if You want short pulses (similar as in paper you sent), the much easier approach is pulse forming line charged with Tesla resonant transformer (the best with high coupling), and high pressure spark gap. You then have only two spark gaps: primary low voltage one, and secondary one to discharge energy from PFL, and you do not have any problems with synchronization, and reliability :D
« Last Edit: December 23, 2022, 10:57:16 PM by huntergroundmind »

Offline highvoltageguypk

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #14 on: December 24, 2022, 04:10:24 AM »
I want to share my thoughts regarding the fast rise time pulsed generators.
Tesla based HV generators:
1. USAFRL designed in 90' JOLT system up to 1MV but later on they also shifted to Marx generators 1MV amplitude and achieved 2MV across the load.
2. Russians designed open magnetic core Tesla transformers ranging from 150kv to 1MV with input charging limited to 1kV
The only problem that I can see in Tesla based generator is the high pressure switch and pulse forming lines. This can cause the variation in the output pulse (+/-15%) and safety must be assured while dealing with high pressure switches.
Marx based HV generators:
1. Marx generator is reliable and compact. My point of view is that if some company like APLEC, ITHPP and many more are preferring Marx generator as a source for pulsed HV  then its mean Marx generators are reliable and generates stable output pulse as compared with Tesla transformers.
2. Yes there some complexity in Marx generators regarding the erection of spark gaps but it can be overcome with experience and time.
3. Multiple Marx generators can be triggered simultaneously as compared with Tesla Transformers.
highvoltageguypk :-)

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Re: I am having the worst time getting an inductor based Marx Gen to work
« Reply #14 on: December 24, 2022, 04:10:24 AM »

 


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February 08, 2024, 07:04:07 PM
post ESAB Bantam TBH 140 Welder Restoration
[Laboratories, Equipment and Tools]
Mads Barnkob
February 08, 2024, 06:47:35 PM
post Re: Anyone used iCCFL.com?
[Transformer (Ferrite Core)]
FPS
February 08, 2024, 12:27:47 AM
post Re: PLL "IHFSSTC" concept and schematic
[Solid State Tesla Coils (SSTC)]
Anders Mikkelsen
February 07, 2024, 09:49:11 PM
post Re: Anyone know of a good variable driver?
[Beginners]
NyaaX_X
February 07, 2024, 04:48:02 PM
post Re: Anyone know of a good variable driver?
[Beginners]
FPS
February 07, 2024, 08:17:37 AM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
Terry
February 06, 2024, 10:51:35 PM
post Re: OCXO Reference devolopment - or a story about scaleability
[Radio Frequency]
Mads Barnkob
February 06, 2024, 07:29:29 PM
post Re: Anyone used iCCFL.com?
[Transformer (Ferrite Core)]
MRMILSTAR
February 05, 2024, 05:18:33 AM
post Re: Anyone know of a good variable driver?
[Beginners]
Mads Barnkob
February 04, 2024, 09:16:25 PM
post Re: PLL "IHFSSTC" concept and schematic
[Solid State Tesla Coils (SSTC)]
davekni
February 04, 2024, 09:13:36 PM
post Anyone used iCCFL.com?
[Transformer (Ferrite Core)]
FPS
February 04, 2024, 09:00:30 PM
post Re: Anyone know of a good variable driver?
[Beginners]
FPS
February 04, 2024, 08:56:14 PM
post Re: OCXO Reference devolopment - or a story about scaleability
[Radio Frequency]
Da_Stier
February 04, 2024, 08:18:01 PM
post Re: Anyone know of a good variable driver?
[Beginners]
Mads Barnkob
February 04, 2024, 08:04:36 PM
post OCXO Reference devolopment - or a story about scaleability
[Radio Frequency]
Da_Stier
February 04, 2024, 07:56:55 PM
post Re: PLL "IHFSSTC" concept and schematic
[Solid State Tesla Coils (SSTC)]
Lucasww
February 04, 2024, 08:45:53 AM
post Anyone know of a good variable driver?
[Beginners]
FPS
February 04, 2024, 12:56:52 AM
post Re: looking for a pll sstc schematic with gate driver ic.
[Solid State Tesla Coils (SSTC)]
yourboi
February 03, 2024, 07:01:12 PM
post Re: SSTC bus inductance
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
February 03, 2024, 08:04:29 AM
post Re: SSTC bus inductance
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
February 03, 2024, 06:01:12 AM
post Re: is there a standard pinout of optical audio output connectors?
[Electronic Circuits]
Rafft
February 02, 2024, 05:04:44 PM
post SSTC bus inductance
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Benjamin Lockhart
February 02, 2024, 08:34:56 AM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
klugesmith
February 01, 2024, 07:03:41 PM
post Does anyone have a 3d print form for a spiral primary on 4" pvc?
[Solid State Tesla Coils (SSTC)]
yourboi
February 01, 2024, 06:23:45 PM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
Mads Barnkob
February 01, 2024, 07:53:53 AM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
klugesmith
January 31, 2024, 11:43:32 PM
post Re: Welcome new members, come say hello and tell a little about yourself :)
[General Chat]
Ranni81
January 31, 2024, 08:03:40 PM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
Hysteresis
January 31, 2024, 03:34:48 PM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
MRMILSTAR
January 31, 2024, 05:08:10 AM
post Re: How much power?
[Spark Gap Tesla Coils (SGTC)]
Mads Barnkob
January 30, 2024, 10:17:45 PM
post How much power?
[Spark Gap Tesla Coils (SGTC)]
Terry
January 30, 2024, 08:07:39 PM
post Ultrasonic Plastic Welding experiments with TA-40CS transducer
[Transformer (Ferrite Core)]
davekni
January 28, 2024, 11:37:30 PM
post Re: is there a standard pinout of optical audio output connectors?
[Electronic Circuits]
yourboi
January 28, 2024, 03:07:12 AM
post Re: First time DRSSTC Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Saattvik24
January 27, 2024, 02:27:29 PM
post Re: Lathe Restauration
[General Chat]
Mads Barnkob
January 27, 2024, 01:26:34 PM
post Re: Determine output voltage of DRSSTC
[General Chat]
Mads Barnkob
January 27, 2024, 01:15:51 PM
post Re: is there a standard pinout of optical audio output connectors?
[Electronic Circuits]
Mads Barnkob
January 27, 2024, 01:13:29 PM
post Determine output voltage of DRSSTC
[General Chat]
Pavol
January 27, 2024, 11:35:35 AM
post is there a standard pinout of optical audio output connectors?
[Electronic Circuits]
yourboi
January 27, 2024, 01:47:13 AM
post Re: First time DRSSTC Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
AstRii
January 26, 2024, 11:26:11 PM
post Re: First time DRSSTC Build
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Saattvik24
January 26, 2024, 08:24:30 PM
post Re: Next Gen DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Netzpfuscher
January 26, 2024, 05:19:18 PM
post Re: Anyone had success using analogue/digital HV panel meters?
[Beginners]
MRMILSTAR
January 26, 2024, 04:28:38 PM
post Re: Anyone had success using analogue/digital HV panel meters?
[Beginners]
Luca c.
January 26, 2024, 03:34:10 PM
post Re: Anyone had success using analogue/digital HV panel meters?
[Beginners]
FPS
January 26, 2024, 11:49:48 AM
post Re: Anyone had success using analogue/digital HV panel meters?
[Beginners]
TizianoBll
January 26, 2024, 11:04:37 AM
post Re: Next Gen DRSSTC
[Dual Resonant Solid State Tesla coils (DRSSTC)]
flyingperson23
January 26, 2024, 06:26:58 AM

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