Author Topic: Current Sense transformer trouble (solved)  (Read 1997 times)

Offline TMaxElectronics

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Current Sense transformer trouble (solved)
« on: June 06, 2020, 05:46:56 PM »
So I just finished rebuilding my first DRSSTC with my own design (for which i got some good advice here: https://highvoltageforum.net/index.php?topic=1058.0) and have got into some issues with the phase lead, or rather lack there of, of the drive signal.
I have an RC filter network in the driver to compensate for any lag of the circuitry, which works perfectly when driven from my signal generator (without any power on the bridge of course) and the gate drive is aligned +-100ns with the input waveform.

But when I test the circuit with the current sense transformer i wound for it, there is an added lag of 58°. I wound it myself with 2 x 55 turns cascaded onto these cores: https://www.digikey.de/product-detail/en/ferroxcube/T58-41-18-3C90/1779-1661-ND/8021375

Now my question is if I have made a mistake with the transformer or if this is an unavoidable issue and i need to introduce more phase lead into my controller. I would like to not rip up my circuit :P
I have so far also failed to understand where exactly the lag is coming from (as in which part of the equivalent circuit) and trying to think about which components i have to put where to counteract the lead is messing with my brain a bit, because i keep thinking as if i had a constant voltage source instead of constant current one.

EDIT: i forgot to note: i measured the actual current phase using a resistor in the resonant loop with the primary voltage reduced significantly so the peak current was about 40A. The CT output phase angle is in relation to that.

I have already done some simulation with LTSpice and would be able to fix the lead by using an inductor in series with the bias resistor, but in testing that didn't do anything, though this might be because i started being pissed and not taking a good measurement....
« Last Edit: June 27, 2020, 11:52:20 AM by TMaxElectronics »

Offline klugesmith

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Re: Current Sense transformer trouble
« Reply #1 on: June 06, 2020, 07:59:11 PM »
Your reference phase is the voltage on a current-sensing resistor.
How non-inductive is the resistor?
What would be the phase shift if resistor had, say, 10 nH of parasitic inductance?

If you are not using a differential input to the scope, what is the connection between scope channel ground and one side of the current sensing resistor, and other parts of the node called "ground" in simulation?

Suppose you connect the scope-probe ground clip to the probe point, and put that loop close to your circuit with 40 ampere RF current.  What's the apparent voltage, and at what phase with respect to the current?

This might be a place to try probing with a Rogowski coil, whose voltage output is naturally shifted 90 degrees from the current under test.  Then you can reverse the Rogowski coil, expecting that the voltage from magnetic coupling will reverse and any voltage from capacitive coupling will not reverse.

« Last Edit: June 06, 2020, 08:03:59 PM by klugesmith »

Online davekni

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Re: Current Sense transformer trouble
« Reply #2 on: June 06, 2020, 08:11:13 PM »
58 degrees seems quite high.  The current-sense transformers shouldn't be making that much difference, especially at low power where core saturation isn't an issue.

Are you using the same phase-lead circuit shared in your previous thread, with 100-ohm input resistor?  One difference relative to signal-generator testing is that the signal generator output impedance is likely 50 ohms, not a current source.  50 ohms in parallel with the 100 ohm input resistor makes 33 ohms net.  Your circuit will add a bit more phase lead with that lower input impedance.  For a test, try paralleling 50 ohms across the input along with your CT output.

You mentioned that your phase-lead circuit matched delays to the IGBT gate signals.  The IGBTs themselves add more delay, which should be compensated with additional phase-lead.  The goal is to have IGBT outputs switch slightly before current zero-crossing.

Can you post a picture of your current transformer and of scope traces showing phase of signals relative to your current-sampling resistor?  Signals to probe (along with current-sensing resistor) are current-transformer output, gate drive, and IGBT (H-Bridge) outputs.

How are you scoping the current-sampling resistor?  Do you have a differential scope probe?  If I recall correctly, this is a full H-Bridge design, so the primary circuit has no grounded location to place a current-sampling resistor.
David Knierim

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #3 on: June 06, 2020, 11:48:59 PM »
I'll do the maths for the inductances tomorrow, but i don't think that is the issue here.
I think it probably has something to do with my probing.

Initially i had the scope ground on the one side of the resistor, and was probing the other (i was running it from an audio transformer so the supply was floating), but then switched to this janky setup, because i also wanted to probe the gate drive waveforms at the same time.

I am not sure, but at 85 kHz the impedance of the transformer should be very high compared to the 10mOhm resistor, so phase shift should be negligible, and when running with an unloaded output from my signal gen I didn't measure any phase shift between input and output voltage, and a not quite 1:1 winding ratio (though i only care about the phase here).

I have since tuned some component values in the drive circuit, increasing the theoretical phase lead to ~38° at 85 kHz, but making it not follow the ideal as well as it did last time anymore. And the transistors now seem to switch at the zero crossing point (at least the zero crossing according to the sense resistor), but there is still quite a bit of ringing (i will also post the waveforms tomorrow) so i don't think it is actually the zero crossing point.

My brain has given up for the day so apologize if I made some stupid assumptions, there is no processing power left :P

Online davekni

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Re: Current Sense transformer trouble
« Reply #4 on: June 07, 2020, 12:48:13 AM »
As klugesmith pointed out, calculate what 10nH of inductance of your 0.01 ohm resistor will do to phase.  I suspect resistor inductance is at least that much, more likely 20-30nH.

If you have an extra of your CT cores, make one more single-stage CT of say 40:1 and use a low-inductance 10 ohm burden resistor.  20-30nH will have much less effect on 10 ohms than it does on 0.01 ohms.
David Knierim

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #5 on: June 09, 2020, 05:08:40 PM »
OK, so i just did the calculations, 10nH with that resistor gives me 28° shift :/ So that seems to be the source of my trouble, especially if it has more inductance....

EDIT: i just measured the phase shift the circuit generates at the moment (the config that switched at the zero crossing of the voltage across the current sensing resistor) and its ~-30° coincidence? I don't think so

However another issue "popped up" (sorry for the pun :P )


I was getting ready to make some more measurements but the IGBTs had different ideas.
So what i think is most likely to have killed them this time is that I overcompensated the phase, because of my wrong measurement with the resistor (>30° prob) and when I turned the voltage up a little higher the Hard switching killed them (again :( )
Interestingly only one side of the bridge died, and only one of those made a bang the other one was silent (still a complete internal short).
Or might this be something else wrong with the setup again? I made the bridge exactly how I showed in my other post (https://imgur.com/pvsLYZt)
Or maybe my house is built on an ancient burial ground of electronic components :P

Quote
make one more single-stage CT of say 40:1 and use a low-inductance 10 ohm burden resistor.
I'll try this now. I was hoping to use the resistor to get a reliable phase reference to test my transformers though, so can i do this properly without it? (measuring the error in a part with reference from another identical part seems a bit flawed to me)
Or should i just accept the lag in the CT and calibrate the transistors to switch to whatever it outputs? I somehow doubt using bridge output voltages would give me a reliable measurment of the switching behaviour, is this correct?

BTW this is how the CT looks, i haven't made a case for it yet: https://imgur.com/X7zXaTm
« Last Edit: June 09, 2020, 07:32:00 PM by TmaxElectronics »

Online davekni

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Re: Current Sense transformer trouble
« Reply #6 on: June 10, 2020, 12:37:45 AM »
Sorry about your hard failure.  What duty cycle were you running at the time?  Tests at low repetition rates and short enable pulse widths are unlikely to fail due to hard-switching with a good low-inductance layout as you now have.  (High VBus inductance can make large voltage rings at hard-switching points which over-voltage IGBTs.  With you layout that shouldn't be an issue.  The other problem with hard switching is power dissipation.  That would cause damage only with longer enable pulses.)  So, I'm wondering if there's any possible malfunction of the gate-drive circuitry.  Any unintended feedback from wiring inductance etc. that could have created a high-frequency gate signal or a partial-voltage gate signal or a signal with overlapping gate drive (negative dead time) could cause frying.

The reason for a second measurement-only current transformer is that your phase-lead circuit isn't a pure resistive load for your feedback CT.  Measuring voltage on the existing current transformer isn't an accurate measure of current due to the complex load impedance.
David Knierim

Offline ritaismyconscience

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Re: Current Sense transformer trouble
« Reply #7 on: June 10, 2020, 11:12:20 PM »
You seem to be using a wirewound resistor.
Ceramic resistors use a thin nichrome wire wrapped around a ceramic cylinder. This adds a significant amount of resistance to the resistor. Try using other types of resistors and that should help fix your problem.

Offline klugesmith

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Re: Current Sense transformer trouble
« Reply #8 on: June 11, 2020, 03:40:21 AM »
When you get down to 0.01 ohms, sensing 40 A, it's not easy to get good HF performance.

I think ordinary current shunts (10 A 75 mV in picture) will have some nH of inductance even if your sense connection is a twisted pair or coax connected up close.


Time for a Current Viewing Resistor with coaxial construction, and cost that depends on pulse energy and continuous power dissipation.
Pictured below is a series A from T and M, rated for 4 W, 16 J, 0.01 ohms, 400 MHz, 1 ns risetime.
http://www.tandmresearch.com/index.php?page=products
https://www.powertekuk.com/coaxial-shunt-series-sdn
« Last Edit: June 11, 2020, 03:47:48 AM by klugesmith »

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #9 on: June 11, 2020, 01:43:01 PM »
I think the best solution is the one davekni suggested. I really only care about the phase of the current and if i used a similar inductance resistor, but with 10Ohms behind a CT, the phase shift should be < 5°.
Could i perhaps even use the original CT i bought for the coil? (https://www.digikey.de/product-detail/en/talema-group-llc/AC1200/1295-1063-ND/3881374) Then I wouldn't need to wind a new one.
Making a non inductive resistor (or probably more likely just buying one) might be something to do if I ever want an accurate current calibration of the CT.

Thanks for your help. I'll give a quick update once I put the coil back together and measured the phase shift again :D

Offline Hydron

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Re: Current Sense transformer trouble
« Reply #10 on: June 11, 2020, 03:53:38 PM »
Do _not_ use the original CT you linked - this is meant for 50/60Hz applications and is useless for the 10s of kHz you need. As klugesmith pointed out a resistive shunt is also problematic.

The most proven, reliable and cheapest way to do DRSSTC current sensing is to wind your own _cascaded_ CT. This involves winding two CTs of say 20-32 turns each, with the output the first CT passing through the centre of the other, with the output taken from this second CT. The ratios can be multiplied to get the final effective CT ratio; the number of turns required for an effective ~1000:1 CT will only be 63 (32*31 = 992), a 500:1 would be 45 (20*25).

The low turn count means you will get very good high frequency bandwidth with minimal phase shift, and they should be fairly insensitive to ferrite material used - as long as it has a high enough inductance (i.e. avoid low inductance toroids designed for DC chokes) and isn't some special lossy ferrite or something then it's likely to work; I have used salvaged EMI suppression toroids with no problems.

If you wind such a 1000:1 CT you can simply load the output with a 10 ohm *non-inductive* resistor to give a 10mV/A current monitor with a nice high bandwidth (into the MHz). A bog standard 1/2W metal film resistor is probably good enough, but you should give up on the wirewound white ceramic ones I see in the pics. I have gone into depth on this topic here: https://highvoltageforum.net/index.php?topic=914.0

Edit: note that the cascaded CT approach should also be used for your driver's current sense inputs as well - if you're using a 50/60Hz one for this then it would explain all manner of problems!
« Last Edit: June 11, 2020, 03:59:52 PM by Hydron »

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #11 on: June 11, 2020, 09:49:12 PM »
Quote
Do _not_ use the original CT you linked
Ok i won't then ;)
I already made a cascaded current sense transformer for the feedback, and was looking for a phase reference to measure its phase shift to.
I have got some more toroids though so I will just wind another one.

Quote
A bog standard 1/2W metal film resistor is probably good enough
Should have some of those around too so I'll use those, thanks!

Offline Hydron

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Re: Current Sense transformer trouble
« Reply #12 on: June 12, 2020, 12:31:09 PM »
Ah good, was worried that your driver feedback was using a low frequency CT (though that would have solved the mystery of the blown IGBTs).

A well made cascaded CT will have a minimal phase shift in the TC frequency range, so you can basically assume that it's providing a correct reference measurement (other than picking up spikes from switching noise, which should be obvious). I have a number of fancier commercial high bandwidth CTs, but they are not necessary for this sort of stuff, though they did help validate the DIY ones.

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #13 on: June 12, 2020, 01:41:45 PM »
Quote
Ah good, was worried that your driver feedback was using a low frequency CT
eehhhhm i might have done in my first version (from my older post)  ::) :-[

I just measured things again and it now the readings make more sense: phase shift of the CT used in my feedback loop is as close to zero as I can measure, and i have now switched out some components to make the IGBTs switch on&off at -250ns and +250ns respectively (including their internal delays according to the datasheet). The difference is from the 500ns dead time.
I have also added a can over the driver ICs (as i had initially planned) and the top layer of the circuit board is flood filled with a shield ground:

So how do you see the chances of the IGBTs now? am i ready for a first test with the actual secondary?

This is the measurement setup i had for the second CT, i only had the small metal film resistors so i made a large 11,2 ohm resistor from 5 in parallel (should decrease the inductance too right?):


Is there any way to reduce the size of these image previews? They look waaaaay too large in the post

Online davekni

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Re: Current Sense transformer trouble
« Reply #14 on: June 13, 2020, 01:53:36 AM »
That measurement CT looks like only 28:1 or so into 11.5 ohms.  Useful at low current, but resistors will smoke if used during operation.  Even 28A RMS (40A peak) primary current will be 1A into 11.5 ohms for 11.5 watts.  That might be OK at a few percent duty cycle.  By 100A, low duty cycle won't be enough to avoid smoke.  Most DRSSTC current transformers are around 1000:1, usually made from two stages.  (Mine is only 500:1, made of a 40:1 CT fed to two 25:2 CTs.)  You could get to a single 100:1 stage with say 10 paralleled 10-ohm resistors for 1 ohm burden resistance.  500A peak becomes 5A peak, 25 watts peak, 12.5 watts RMS, so 1W average at 8% duty cycle.

500ns is likely more dead-time than you need.  It's best to have the IGBT turn-on at current zero-crossing, not after.  250ns dead time would produce that, and should be plenty to avoid cross-conduction.  (I use 200ns dead-time for my DRSSTC using isolated gate driver chips.)

I wouldn't worry about the "preview" images.  It's convenient to have them large, avoiding any need to expand them for most cases.
David Knierim

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #15 on: June 14, 2020, 09:01:07 PM »
Quote
500ns is likely more dead-time than you need.
I calculated 300ns with a safety factor of 1.4 but the drivers i have have a minimum of 500ns :/

So should I rather have the switch off be at -500ns and the switch on at as near as possible to zero?
I am not quite familiar with the mechanism that leads to the losses when switching in an IGBT.
At uni we didn't talk much about IGBTs at all.

Quote
Even 28A RMS (40A peak) primary current will be 1A into 11.5 ohms for 11.5 watts
For calibration I was only running it at 40A or so for maybe 5 seconds at a time. I almost expected the resistors to burn out by the end of it, but since I didn't need them after calibration I just ignored it.
I was too lazy to wind a larger one / another one :P

Online davekni

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Re: Current Sense transformer trouble
« Reply #16 on: June 14, 2020, 11:08:00 PM »
Yes, it's probably better to have turn-off at -500ns than to have turn-on at +250ns, given that you are stuck with 500ns dead time.  I forget what IGBTs you are using.  If the diodes have very low recovery time and charge, then phase lead is less critical.

This isn't a critical issue, but here's a crude simulation of a half-bridge showing what happens with a long dead-time straddling current zero-crossing.  Each output voltage transition becomes a tripple-transition, at turn-off, current zero-crossing, and turn-on.


With resulting bridge output voltage and coil current:


I've seen this behavior in real circuits, not just simulation.  After current zero-crossing, the turned-off IGBT's diode is conducting.  (This is true of lagging phase in general, even with a short dead-time all after zero-crossing.)  When the opposite IGBT turns-on, it is aggressively forcing the diode off, pulling a huge current spike if the diode is slow.  That current spike wastes power and can cause a resulting voltage spike in IGBT lead inductance.  At least your nice low-inductance interconnect doesn't add much additional inductance beyond the package's.
David Knierim

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #17 on: June 15, 2020, 01:20:38 AM »
I'm using the IXXN110N65C4H1.

So if I understand correctly, the issue with switching past zero is, that the the diodes conducting the freewheeling current (is that the correct term?) are not those in the IGBTs to be switched on but the opposite side one, so the IGBTs have to conduct a current similar to a shoot through one, flowing through the diode in reverse until it can recover? The diode in my transistor has 100ns Trr. And I am now pulling the gate low at -430ns, causing a peak current through the diode of ~80A (after 130ns off delay time) and pull high the other gate almost exactly at the zero crossing point, so after the Td(on) of 50ns the current would be at only 13A (both assume Ipeak of 500A) Those seem at least somewhat of reasonable to me.

I since run a test at 100Hz 100us for ten minutes without failure :D
BUT... when I finally put on the secondary and actually made some arcs, though only about 10cm  since I did chicken out and ran only at 120V primary voltage with 70us on time and just the most rudimentary of tuning, one of the gate drivers gave up the ghost.
At least it ran fine for 30sec  :-\ But hey. Better a 2€ gate driver than a 20€ transistor ;)

I guess I'll have to scope the thing while its running wit ha secondary after all...
I have currently not connected the shield to anything but itself, could that be a problem? It is a closed faraday cage around the gate drivers and the cable.
Otherwise i will check if the driver maybe decides to send out some extra pulses with the secondary in place (from interference. It's only in an aluminium box, maybe the two parts aren't connecting electrically). I'm thinking about just going back to a GTD if i'm honest, I have two fairly strong signals 180° out of phase going to the driver board after all.

Online davekni

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Re: Current Sense transformer trouble
« Reply #18 on: June 15, 2020, 05:40:44 AM »
The driver chip frying may have been due to partial-damage previously when 2 IGBTs fried.  Often when FETs or IGBTs fry they take out the driver circuitry too, feeding bus voltage to the gates as they short.  Still, I would recommend tying your shield to some node, VBus- being the most obvious choice.

I think you understand correctly.  The ~80A diode current will be in the same package as the IGBT that's about to turn on.  As long as it stays that way, diode recovery is no issue.  It has an entire half-cycle to recover, with at most the IGBT's forward drop (~2V) of reverse voltage.  Zero-current is a fine time to turn on the IGBT.  One way to tell if phase lead is good is to scope the bridge output to see that you don't get triple transitions.
David Knierim

Offline TMaxElectronics

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Re: Current Sense transformer trouble
« Reply #19 on: June 15, 2020, 06:38:09 PM »
I just switched the Gate driver IC, and now the thing works again. It was actually the one on the IGBTs that didn't die in my last poof, but maybe a bus voltage spike damaged it a bit.
After a bit of tuning i can get some sparks, even with 100us on time and at 50V on the bridge.

However, i found two more issues. First is the double switching davekni was talking about:

Though only minor, i will still add a tiny bit more of phase lead.

The other is, as I (and i think somebody else aswell) suspected, that the driver gets confused once the secondary is there:

Channel two is clamped across the CT for the feedback, channel four is the one i wound for testing and three is the output from my controller to the gate drivers.
As you can see, there is some weird additional switching happening. It is after the interrupter has switched of, so maybe there is a problem there. I think i have read somewhere that the fiberoptic receivers are a bit unhappy once some interference comes in, maybe thats a problem... I think i will try to scrape of some of the coating from the housing to make it conductive, and act as shielding, or add some copper tape.

EDIT: the last switching pulse also wasn't always at the zero crossing point, for the same reason the first one isn't. The first one isn't that problematic i guess, because the current is low, but if it happens just after the current peak it is probably quite bad.
« Last Edit: June 15, 2020, 06:43:54 PM by TmaxElectronics »

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Re: Current Sense transformer trouble
« Reply #19 on: June 15, 2020, 06:38:09 PM »

 


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post Re: Question about arduino polyphonic MIDI interrupter
[Dual Resonant Solid State Tesla coils (DRSSTC)]
TMaxElectronics
December 01, 2020, 04:22:10 PM
post Re: Question about arduino polyphonic MIDI interrupter
[Dual Resonant Solid State Tesla coils (DRSSTC)]
bozidar
December 01, 2020, 03:02:51 PM
post Re: Half-brigde with pot on mosfet gates question
[Solid State Tesla Coils (SSTC)]
TMaxElectronics
December 01, 2020, 12:08:56 PM
post transistor markings question
[Beginners]
NOOBPASTE51
December 01, 2020, 12:02:19 PM
post Re: Question about arduino polyphonic MIDI interrupter
[Dual Resonant Solid State Tesla coils (DRSSTC)]
TMaxElectronics
December 01, 2020, 11:55:01 AM
post Re: HF Litz wire as a primary coil
[Dual Resonant Solid State Tesla coils (DRSSTC)]
TMaxElectronics
December 01, 2020, 11:50:19 AM
post Re: Analog HFBR amplifier
[Electronic Circuits]
TMaxElectronics
December 01, 2020, 11:21:56 AM
post Half-brigde with pot on mosfet gates question
[Solid State Tesla Coils (SSTC)]
costas_p
December 01, 2020, 11:17:16 AM
post Re: Question about arduino polyphonic MIDI interrupter
[Dual Resonant Solid State Tesla coils (DRSSTC)]
bozidar
December 01, 2020, 11:11:50 AM
post Re: mini tesla (hopefully not a fail)
[Beginners]
bogdan
December 01, 2020, 09:29:24 AM
post Re: HF Litz wire as a primary coil
[Dual Resonant Solid State Tesla coils (DRSSTC)]
johnf
December 01, 2020, 07:03:56 AM
post Re: How to design FPGA based on EDA technology?
[Electronic Circuits]
johnf
December 01, 2020, 06:49:26 AM
post Re: SSTC or DRSSTC with GaN Transistors
[Solid State Tesla Coils (SSTC)]
Steve Ward
December 01, 2020, 04:59:28 AM
post How to design FPGA based on EDA technology?
[Electronic Circuits]
Genterman
December 01, 2020, 04:02:06 AM
post Re: DRSSTC trouble
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
December 01, 2020, 03:46:10 AM
post Re: DRSSTC trouble
[Dual Resonant Solid State Tesla coils (DRSSTC)]
buchtawill
December 01, 2020, 02:00:54 AM
post Re: HF Litz wire as a primary coil
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Steve Ward
December 01, 2020, 01:47:10 AM
post Re: DRSSTC trouble
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
December 01, 2020, 12:02:23 AM
post Re: Site Info desperately needed: http://www.stevehv.4hv.org/
[General Chat]
Mads Barnkob
November 30, 2020, 10:28:56 PM
post Re: I started making a drsstc
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Miki_407
November 30, 2020, 08:58:41 PM
post DRSSTC trouble
[Dual Resonant Solid State Tesla coils (DRSSTC)]
buchtawill
November 30, 2020, 08:53:00 PM
post Re: I started making a drsstc
[Dual Resonant Solid State Tesla coils (DRSSTC)]
davekni
November 30, 2020, 08:49:30 PM
post Re: Simple H-Bridge construction with low parasitic inductances (for SSTC or ...)
[Beginners]
davekni
November 30, 2020, 08:45:46 PM
post Re: mini tesla (hopefully not a fail)
[Beginners]
davekni
November 30, 2020, 06:56:13 PM
post Re: I started making a drsstc
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Miki_407
November 30, 2020, 01:16:04 PM
post Re: I started making a drsstc
[Dual Resonant Solid State Tesla coils (DRSSTC)]
Mads Barnkob
November 30, 2020, 12:56:05 PM

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