Author Topic: Problems with my first DRSSTC  (Read 1742 times)

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #20 on: October 11, 2019, 06:19:09 AM »
Thank you for all those pictures!  It's much easier to analyze issues when there is actual information rather than guessing how to fill "between the lines".  (One question, though:  What bus voltage was used for the scope images?)

I'm presuming the third image is of the current feedback, 450:1 on 51-ohm load.  It has a particular anomaly where the current reverses 3V to -3V very rapidly (~+26A to -26A).  If that condition is repeatable, it would be a good place to focus.

Looking at the wiring pictures, I see one important issue:  The IGBTs have double emitter terminals.  The intended use is as a Kelvin connection - one terminal is used for high current (C-E current), and the other only for gate drive.  That avoids inductive and resistive voltage drop from the high current affecting the gate signal.  The pictures show the collector current from the low-side IGBTs passing through the high-side IGBTs, in one emitter terminal and out the other.  That definitely needs fixing.  The gates may not be damaged yet, but that is likely contributing to your operational issues, and will likely fry the gates eventually.  For each of the four IGBTs, one emitter terminal should be used for the gate-drive signal (and nothing else), and the other for all power connections.  Most bricks have small emitter terminals for gate-drive return.  This part happens to have two large interchangeably connections, but it's still necessary to pick one for gate-drive return use only.

Some less critical suggestions:
The gate-drive transformer will have lower leakage-inductance if the four paralleled primary wires of the CAT5 cable are individually twisted and ran back to the driver board, then patched to a single pair of wires at the driver.  That reduces the inductance of the relatively-long wiring from the driver-to-transformer by 4x.  That's an improvement tweak - not the cause of your man issues.
Also, I'd periodically measure the capacitance and leakage current of each quarter of your MMC.  As I'd mentioned earlier, my initial DRSSTC MMC used WIMA FKP1 capacitors, and they failed quite quickly.
Finally, the local VBus bypass capacitors (snubbers I see them called, although I use that term differently, for R+C series networks added across switching voltages) have fairly high wiring inductance for the fast IGBTs you are using.  Multiple parallel (smaller if necessary) caps will lower inductance, or copper plane/foil connections for the Vbus + and - rails.

Good luck!  Thank you for the bountiful scope traces and pictures.
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #21 on: October 11, 2019, 11:34:59 AM »
Vbus is 24VDC.
Yes , the third picture is of the current feedback (450:1 on 51 ohm load) and yes , anomaly where the current reverses is repeatable. I think that's what happens when the gate signal have small duty cycle ( second picture).
I didn't know that for double emitter terminals. I'll fix it as soon as I can.
I'm using four paralleled primary wires but i patched them to a single pair of wires right next to the GDT.I shielded all wires with aluminium foil and put everything in shrinking tube.
In my local shop I can only find these caps for MMC so I hope that they ( 68nF 8kV) can handle  4.5 kV peak at 150kHz. I checked all the capacitors separately , with RLC meter , and they are all good.

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #22 on: October 12, 2019, 04:41:19 AM »
I'm guessing there's another issue besides just the emitter wiring.  At 24V and ~26A, the emitter connections seem unlikely to cause the anomalous behavior (fast current reversal event and resulting short gate pulse).  The other things I'd suggested are in the minor-improvement category, not causes of the issue either.  Still suggest fixing the emitter wiring before any higher-power testing.

Is the current-limit circuit fully wired on your board?  If there were to be an open (no load) on the current-limit half of the current transformer, that core is likely to saturate, causing odd signals on the other half of the current sense transformer.  Not sure that could explain exactly what you are seeing, but worth a check.

Would it be possible to capture that anomalous event at 24Vbus and ~26A with the following four scope signals:
    Current sense voltage (as you had before)
    Both H-Bridge outputs
    High side of primary caps (connection to the primary coil).
If you have only two scope channels available, make three captures.  The current sense signal would be common to all three captures, with the other probe monitoring each of the three other signals sequentially.  At ~26A, the primary cap high-side should be ~400V, so within capability of normal scope probes.

This is interesting debugging remotely, where I can't just move a scope probe from one point to the next to see where an issue lies.  If my probing requests become too tedious, please feel free to say so.  If you don't mind continuing to capture signals, I'm confident it will be possible to find and fix the issue.  (If someone else manages to recognize the likely cause from the symptoms, that will be great.  If not, then walking through signals with your scope will get there.)
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #23 on: October 17, 2019, 05:53:09 PM »
I found out there was a problem in my oscilloscope (settings) that caused the signal not to display properly  ;D
I have a USB oscilloscope ISDS205B (20MHz, 48Ms) that can measure up to +-60V so I can`t measure voltage at  high side of primary caps  :-\
I fixed the emitter wiring and now everything is almost fine.When I turned on the coil for the first 2-3 minutes everything worked again as before, some impulses were skipped, but after that everything slowly returned to normal.
Here is some scope images after that.
First two pictures- yellow trace is feedback input from CT and blue trace is primary current (1V=1Amp)
Pictures 202,203,204 -yellow trace is Vce from one transistor and blue trace is primary current
Pictures 205,206,207 -yellow trace is Vce from other transistor and blue trace is primary current

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #24 on: October 18, 2019, 06:30:06 AM »
Great!  Nice to see scope traces that look as expected.

The occasional startup issues at 24Vbus are reasonable for the UD2.7.  The 1.3A current of the first half-cycle fits calculations (15.7 ohms primary L and C at 149kHz on about 21V bridge output).  1.3A through 625:1 CT into 51 ohms gives 0.106V, barely enough to trigger the UD2.7.  This issue should completely disappear at higher Vbus.  (I'd previously suggested some UD2.7 patches if you want to get more reliable operation at low Vbus, but no need to change anything with Vbus above 24V.)

How are you scoping primary current on the blue channel?  (Cyan channel in my color-science world:)  Do you have a current probe, or a resistor in series?  I'm asking because the CT output appears to have 20-25 degrees of phase lead relative to that current.  Do you have L1 in the circuit now?  That would explain the phase lead of CT signal.  (The parasitic inductance of the CT would account for only 1 degree or so based on your N30 core and 25 turns for the second stage.)

Given that you have primary current scoped, primary voltage isn't necessary.  If you want to scope such in the future, inexpensive 100x scope probes are available, such as this P4060:
https://www.ebay.com/itm/P4060-1-100-High-Voltage-2KV-2000V-60MHz-Oscilloscope-Scope-Probe-100X/372463710732?hash=item56b891320c:g:eLIAAOSw8UZaLlMi

The occasional short positive spikes on the H-Bridge outputs might be due to not quite enough phase lead.  If the IGBTs switch after zero-current, the reverse diodes of a pair of parts are conducting.  The other pair then switches on, rapidly removing the diodes stored charge, then overshooting as the diodes snap-off.

The longer spikes when the drive pulse ends may be on the edge of problematic at 310Vbus.  This is caused by an L/C circuit ring, where C is the smaller Vbus capacitors at the H-Bridge and L is the wiring inductance back to the larger bulk Vbus capacitors.  That ring will grow in proportion to primary current at the turn-off point.  Over-current shutdown, by definition, occurs at high primary current, causing high voltage spike on Vbus at the IGBTs.  (This issue caused a rework in my Vbus wiring, as I'm using 600V IGBTs at 450Vbus.)  At your 310 Vbus, extrapolating the spikes, they should be a little below 600V, but without a lot of margin.

To reduce that Vbus spike at the end of high-current pulses, either increase the capacitance local to the IGBTs or reduce wiring inductance back to the bulk Vbus capacitors.  Using a few wire-ties to force the Vbus wires together will help.  Multiple pairs of Vbus wires, each pair twisted or tied to remain adjacent, will help even more.  That's what I did, as I had no room for more local-to-IGBT caps.

Have fun - nice progress!
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #25 on: October 22, 2019, 07:44:55 PM »
I`m scoping primary current with another separate CT (41 turns connected to 41Ohm resistor).
I've already adjusted phase lead before with proper oscilloscope as best as I can, so now I`m afraid to adjust it again (unfortunately I had limited access to that oscilloscope). First picture that I posted is taken by that oscilloscope.
After some experimenting at low voltages I decide to power bridge at full voltage (310VDC) and the coil didn't work at all (only some small sparks at 7KHz and 9us on time).
Then I replaced switching power supply which powers the driver (24VDC 10A ),because that's the only thing I didn't check, with classic iron core transformer with greatz , some capacitors and 24V regulator and finally the coil works (still skipping some pulses at low frequencies below 100Hz but it does not matter)  :D :D :D
I guess there was some interference between driver and switching power supply  :o
The coil produce around 30cm sparks at 200Apk , 1.5KHz and 30us on time. Given that the secondary is only 23cm tall I think this is a good result.
I'll post a video soon.
Thanks for all your help, especially davekni for the great tips and explanations.

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #26 on: October 23, 2019, 04:33:46 AM »
Yes, I can imagine a switching supply being troubled by the nearby electromagnetic fields.  The cheap DMMs get completely confused by stray fields.  On the other hand, I have good luck with old Dell laptop 19V supplies, which are switching.

Sparks might be longer with longer pulse width and lower repeat frequency (to keep average power constant).  Other's likely have better intuition on that.

Looking forward to the video!  Glad I could assist - thank you for the mention.
David Knierim

Offline Mads Barnkob

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Re: Problems with my first DRSSTC
« Reply #27 on: October 28, 2019, 08:41:19 AM »
The longer spikes when the drive pulse ends may be on the edge of problematic at 310Vbus.  This is caused by an L/C circuit ring, where C is the smaller Vbus capacitors at the H-Bridge and L is the wiring inductance back to the larger bulk Vbus capacitors.  That ring will grow in proportion to primary current at the turn-off point.  Over-current shutdown, by definition, occurs at high primary current, causing high voltage spike on Vbus at the IGBTs.  (This issue caused a rework in my Vbus wiring, as I'm using 600V IGBTs at 450Vbus.)  At your 310 Vbus, extrapolating the spikes, they should be a little below 600V, but without a lot of margin.

To reduce that Vbus spike at the end of high-current pulses, either increase the capacitance local to the IGBTs or reduce wiring inductance back to the bulk Vbus capacitors.  Using a few wire-ties to force the Vbus wires together will help.  Multiple pairs of Vbus wires, each pair twisted or tied to remain adjacent, will help even more.  That's what I did, as I had no room for more local-to-IGBT caps.

Low voltage testing is notoriously known to be misleading when it comes to switching spikes for IGBTs in a resonant circuit.

Quote from: From a Powerex application note
Coes – Output Capacitance
This is the output capacitance measured between the
collector and emitter terminals with the gate shorted to
the emitter for AC voltages. Coes is made up of the
collector to emitter capacitance (CCE) in parallel with
the gate to collector capacitance (CGC), or

Coes = Cce + Cgc

For soft switching applications, Coes is important
because it can affect the resonance of the circuit.



Kizmo explained this: As you can see the IGBT open state capacitance is largely depending on the voltage that is applied over Collector-emitter! And imagine what happens at your bridge when for example high side transistor turns on and low side transistor turns off. The emitter of the low side brick is tied to negative bus rail, and collector is quickly whacked to the full bus voltage which means the parasitic capacitance inside of the brick will be charged to your dc bus voltage at very high voltage rise time. And that charge current must travel through all parasitic inductances that your bridge has.

As you increase your bus voltage, your parasitic capacitance will decrease and amplitude of these transients related to the bus voltage will go down. My big coil does that too, at 100V bus the transients are almost 200% above bus voltage. But at 650V they are only maybe 20% over.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #28 on: November 01, 2019, 04:39:28 AM »
Mads,

Great point about IGBT capacitance vs. voltage.  That applies to the fast spikes that show up occasionally at the switching transitions.  (I had a similar issue on my resonant PFC at full voltage, but it was caused by diode recovery charge rather than capacitance, when switching after current zero-crossing.)

The lower-frequency ring at the end of the drive pulse, the spike that lasts a couple microseconds, isn't due to IGBT capacitance.  It's rather the local Vbus capacitance at the bridge, resonating with the wiring inductance from the bridge back to the bulk capacitor.  That ring voltage will be a relatively linear function of current at the end of the drive pulse.  (There is a small relatively-constant part due the the IGBT's diode forward voltage drop.  Most is due to the wiring inductance and local Vbus capacitor.)  This is the problem that caused me to redo wiring from bulk capacitors to H-Bridge on my 40-IGBT DRSSTC drive, to lower inductance.  This spike drops by that resonant impedance, so proportional to sqrt(wiring inductance) for given local H-Bridge VBus capacitance (360uF in my case - 12 30uF PP caps local on the H-Bridge).
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #29 on: November 09, 2019, 08:14:08 PM »
Unfortunately, the switching power supply was not a problem, because when I turned on the coil again at 310VDC two IGBTs exploded  :(
Luckily i had 2 spare IGBTs, and now my problem is the same as before. Sometimes the coil works properly and sometimes it does not. When I touch the feedback input with a screwdriver everything works as it should (Vbus 24VDC). I noticed when I disconnect 24VDC from bridge and touch the feedback input (with a screwdriver) I can hear audible buzz on GDT. And when I remove a screwdriver there is no more buzz on GDT. Then I connected only GDT (without OCD and feedback CT) and the same thing with a screwdriver happens. In attached images you can see gate-emitter voltages with (11.jpg) and without (10.jpg) a screwdriver.


This is written on the loneoceans site:
www.loneoceans.com/labs/ud27
Quote
Note that if you have no input signal in your FB jacks, the driver will not see any feedback, causing the drive to lock up. Here's a scope shot of a GDT output with a feedback input signal (left). But if you have a GDT attached with no signal generator input on the feedback, this will make a signal on the output of the GDT look like the above (right - where the driver doesn't know what to do), and will cause quite a loud audible buzz from the GDT with this associated output per pulse. This sound should diminish greatly with a feedback input signal.


So without feedback , the signal should look like last attached image.  Right?
Maybe my driver board is bad?

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #30 on: November 11, 2019, 02:01:12 AM »
If I'm understanding the scope trace context correctly, then yes there does appear to be some issue with the driver board.
For these scope images, are you sending roughly 15us wide enable pulses every 266us to the optical input?  I'm guessing that from the scope plots.

The gate drive appears to be low amplitude, just under +-7V for the largest pulses.  Also, it appears that the gate drive is not being shorted between enable pulses as it should be.  Between enable pulses, the NFET halves of U1, U2, U3, and U4 should all be turned-on (high output on pin 5 of IC9 and IC10).  When the two IGBTs failed, it's quite possible that they sent a spike back through the CT large enough to fry U1-U4.  Or, it could be the reverse, where something failed on the driver first, causing low gate voltages, which triggered the IGBTs to fry.
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #31 on: November 11, 2019, 10:19:52 AM »
Here is some scope images with and without a screwdriver and circuit of my driver (Vge - blue trace). 1kHz 200us on time - input signal. I connected only GDT ,without OCD and feedback CT .
A similar thing with a screwdriver happened before IGBTs exploded. I wrote this in one of my past posts.
« Last Edit: November 11, 2019, 10:26:40 AM by bozidar »

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #32 on: November 12, 2019, 06:20:41 AM »
Thank you for sharing the schematic.  Looks like there's a signal inversion issue relative to the original UD2.7 schematic.  IC3 needs to be an inverting driver, UCC27423.  With the non-inverting driver shown in your schematic, the gate drive is floating between enable pulses, causing probably all of your issues.
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #33 on: November 12, 2019, 06:44:36 PM »
When buying parts for the driver I couldn't find the UCC27423 chip, so I thought I could use the UCC27424 chip.

How the gate drive can float between enable pulses when enable pins are connected to 9V?

After some experimenting with LTSpice I added a small circuit  (Picture1.png) to improve impulses at the beginning of the input pulse(from interrupter). In attached images you can see gate-emitter voltage (blue trace) and output pulse from AND gate (yellow trace) (second image) , input (blue trace) and output signal (yellow trace) of added circuit (1.png and 2.png) (Vbus 0V). After that I connected 24VDV on the bridge and the coil works  :D
The only problem is that sometimes some pulses are skipping at some frequencies, much like the one in the picture from the earlier post (https://highvoltageforum.net/index.php?action=dlattach;topic=722.0;attach=4990;image). I don't think it should be able to damage IGBTs at full Vbus. What do you think?
Another question, it may be stupid but I have to be sure. Should I connect the ground to the driver board when it is mounted in a metal housing?
I connected the other end of the secondary to the main ground as well as the IGBT coolers, metal case for the driver board and strike rail.

Offline dexter

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Re: Problems with my first DRSSTC
« Reply #34 on: November 12, 2019, 11:08:44 PM »
When buying parts for the driver I couldn't find the UCC27423 chip, so I thought I could use the UCC27424 chip.

i made the same mistake on my first DRSSTC and couldn't get why my coil didn't start

the gates of the p channel mosfets are capacitively coupled so they need a high to low transition in order for one of the p's to conduct

if you use a non inverting driver they'll get a low to high transition and won't turn on

with an external signal the coil will work but you still loose the 1st cycle

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #35 on: November 13, 2019, 12:35:34 AM »
Can I just invert the input signals (2 and 4 pins of ucc27424) with 74hc14?

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #36 on: November 13, 2019, 06:36:57 AM »
Besides what dexter pointed out, there's another issue with non-inverting gate drive.  At the end of each pulse, the driver outputs are both low, leaving NFETs Q1 and Q2 off.  PFETs Q3 and Q4 will be initially on, but will turn off after 50-100us when C15 and C16 charge.  That leaves the gate-drive transformer input floating (no FETs on).  The residual gate transformer flux will then cause a gate drive transient, which can be seen in your previous scope plots.

Inverting inputs (pins 2 and 4) with HC14's should work.  It does add some extra delay, so you'll need more phase-lead (higher L1 inductance) to keep the gate signal leading the zero-current point.  (That's less than ideal, more delay and more compensation for the delay, but workable.)

Recommend not running at 310V until the IC3 inversion issue is fixed.  Without fixing that, the occasional half-voltage gate signals will likely fry IGBTs.

In general, I'd say yes, ground the driver board to its box.  That's the way my coil runs.  It's best to avoid ground currents running through the board, so I'd suggest connecting to the board ground near the power input connector.  The other connections to the driver board are all optically or transformer coupled, so shouldn't inject ground currents.

Your added circuit looks workable for kicking the oscillator at startup with low Vbus.  Have you looked at a simpler alternative:  Switch the right side of R18 (100k) from pin 7 to pin 8 of IC10.  That should make it self-oscillating at a frequency determined by C20.  (My DRSSTC has a discrete-transistor comparator implementation, but uses that topology to be self-oscillating.)  The CT feedback will override the self-oscillation frequency, so it still locks in fine.
David Knierim

Offline bozidar

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Re: Problems with my first DRSSTC
« Reply #37 on: November 13, 2019, 09:08:45 AM »
Ok then I will buy the UCC27423 chip, it`s better than with 74HC14 and UCC27424.
I have GBU808 bridge rectifier (800V 8A). Is that enough for a 230AC or should I buy another that can handle more current?
I had a FB2510 (1000V 25A) but it burned out when the IGBTs exploded.

Offline davekni

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Re: Problems with my first DRSSTC
« Reply #38 on: November 14, 2019, 05:35:19 AM »
The input bridge current depends on how hard you are going to run your coil - frequency and width of the enable pulses.  If you are measuring line current, just keep it under 8A or buy higher current diode bridge.  BTW, I recently bought some cheap bridges from China on EBay, supposedly 50A at 1000V, but they were worthless, perhaps good for 5A.  Years ago I'd gotten some of these that worked OK, at least at 15A.

Once the driver is swapped to non-inverting, it will require reversing either the CT feedback or GDT terminals to get the feedback phase back to correct.
David Knierim

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Re: Problems with my first DRSSTC
« Reply #38 on: November 14, 2019, 05:35:19 AM »

 


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November 20, 2019, 07:16:24 PM
post Re: my first DRSSTC is not working properly
[Dual Resonant Solid State Tesla coils]
zanikk
November 20, 2019, 06:07:23 PM
post Re: my first DRSSTC is not working properly
[Dual Resonant Solid State Tesla coils]
Mads Barnkob
November 20, 2019, 10:25:09 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
November 20, 2019, 05:30:25 AM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
klugesmith
November 19, 2019, 11:15:31 PM
post Re: coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Mads Barnkob
November 19, 2019, 11:28:10 AM
post Re: RF Amplifier for Cubesat plasma thruster
[Electronic circuits]
Weston
November 19, 2019, 08:56:49 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 19, 2019, 06:25:49 AM
post Re: my first DRSSTC is not working properly
[Dual Resonant Solid State Tesla coils]
davekni
November 19, 2019, 06:17:03 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 19, 2019, 06:03:37 AM
post Disc launcher attachment
[Induction launchers, coil guns and rails guns]
MRMILSTAR
November 19, 2019, 05:51:39 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
davekni
November 19, 2019, 05:47:34 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 19, 2019, 12:49:52 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
babass
November 18, 2019, 09:19:26 PM
post my first DRSSTC is not working properly
[Dual Resonant Solid State Tesla coils]
zanikk
November 18, 2019, 06:56:18 PM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
Uspring
November 18, 2019, 02:38:14 PM
post Re: coil Tesla in the space
[Dual Resonant Solid State Tesla coils]
Uspring
November 18, 2019, 01:46:06 PM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
Mads Barnkob
November 18, 2019, 12:21:53 PM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
shrad
November 18, 2019, 09:01:41 AM
post Re: RF Amplifier for Cubesat plasma thruster
[Electronic circuits]
shrad
November 18, 2019, 08:57:05 AM
post Re: IR wavelength in photoelectric smoke detectors?
[Light, lasers and optics]
klugesmith
November 18, 2019, 04:48:39 AM
post Re: SGTC MK1 - An Accomplishment in Progress
[Spark gap Tesla coils]
jturnerkc
November 18, 2019, 03:30:02 AM
post Re: SSTC low voltage at gate transformer and heating drivers
[Solid state Tesla coils]
davekni
November 18, 2019, 02:53:29 AM

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