Author Topic: SSTC GDT troubleshooting  (Read 282 times)

Offline Nylla

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SSTC GDT troubleshooting
« on: May 13, 2020, 08:46:28 AM »
Hello everyone, first time posting here, pretty cool forum you got going on!

Anyway, I’ll try to keep it short.

I’m somewhat experienced with electronics, and this isn’t the first time I find myself messing around with Teslacoils, although my previous projects were always pretty small.
That being said, with the whole covid thing going on, I was able to take back on am older project which for some reason I never managed to get to work properly.

My design is based off Mads’ SSTCII, which I’m sure you’re all pretty familiar with.
I have rebuilt the driver part from scratch and I’m currently working on the bridge part.

With the driver built on perfboard, I started messing around and probing at some of the outputs, never really able to get any decent waveform, they’re always really messy and don’t really look anything like GDT waveform, overall being very inconsistent, which made me wonder, am I probing it wrong?
How am I supposed to probe the GDT? does it need to be connected to the bridge?
Or do I have to probe at the ICs output? (Doubt it)
Also, another weird behavior of the driver circuit, is that when fed a 1khz square wave from the 74HC14, the resulting output is somehow in frequencies in the order of megahertz, how’s this possible?
And lastly, moving the GDT around somehow changes the (still very messy) waveform.

I’m using an N30 core, with magnetic permeability of 6300mH.
I’ll be glad to provide with more details and pictures whether necessary, in the meanwhile, thanks a lot in advance to anyone who wants to help!
« Last Edit: May 13, 2020, 08:44:49 PM by Nylla »

Offline oneKone

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Re: SSTC GDT help
« Reply #1 on: May 13, 2020, 12:29:04 PM »
Welcome to the forum.

If you could please post some pics that would be helpful.

In regards of where to probe I normally test before the gdt (just to make sure it's getting a nice square wave) and also at the gate of the MOSFETs/igbts. Some people use a capacitor as a load on the secondary side of the gdt, I just find it easier to connect whatever you're switching.

Hopefully some of this helps.

Offline Nylla

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Re: SSTC GDT help
« Reply #2 on: May 13, 2020, 05:11:05 PM »
Hey there!
Before I go on and send pictures I just wanna make sure I’m doing it right.

To probe at the GDT primary (I assume that’s what you mean by before the GDT), do you suggest clamping the probe ground to one of the GDT primary leads, and the input to the other, or should I probe one of the leads and connect the probe ground to the driver common ground?
Or, should I leave it floating?

Thank you!

EDIT:

I went on and made some new tests, I'll attach seme pictures.

I'm currently feeding a 500hz square wave into the gate drive ICs pin 2, while pin 3 are tied to 12VDC.
These are the results:

-1st picture is the two signals probed directly at pin 6-7 of the ICs, with floating probe ground.
-2nd picture is the signal probed at one of the ICs, except after the 0.22uF capacitor that's in series with the primary, again with floating ground, it is interesting to note how the frequency seemingly increases by quite a bit? Going from a 500hz signal to several kilohertz...
-3rd and fourth pictures are the signals probed at one of the GDT's secondary windings, respectively with and without floating ground.

Again, thanks to anyone who would like to take time to help me troubleshoot this!
« Last Edit: May 13, 2020, 08:43:21 PM by Nylla »

Offline Weston

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Re: SSTC GDT troubleshooting
« Reply #3 on: May 13, 2020, 09:56:42 PM »
You should be connecting the scope probe ground clip to the circuit ground. If you want to measure the voltage across the GDT you can probe out each side of it with a separate probe and use the math mode on the scope.

A signal with fast rise time (like what is generated from the gate drive output) can induce ringing in the scope probe itself. Ideally you would use the ground spring clip to have the shortest possible connection, but the ground clip is also ok. Without a ground clip your measurements are not going to be accurate.

Also, transformers (such as GDTs) have a maximum volt*second product that can be applied before they saturate and the output voltage drops to zero. GDTs for tesla coil use are typically designed to operate at 100KHz+ and will saturate when tested with a significantly lower frequency, like 500Hz. Thats why you are getting small pulses instead of a square wave.

Try testing with a higher frequency and with better ground connections to the oscilloscope. The first should give you something resembling a square wave, the second should reduce the ringing you see.

Feel free to upload pictures of your test / probe setup. This page explains the issue with a long ground lead a bit: https://teledynelecroy.com/doc/passive-probe-ground-lead-effects




Offline Nylla

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Re: SSTC GDT troubleshooting
« Reply #4 on: May 13, 2020, 10:41:49 PM »
This just shows how much I still have to learn.

I followed all your advice, and the result speaks for itself(picture attached), thank you SO MUCH.
Probably needs some bigger damping resistors, but besides that, it's looking pretty great!!

thank you thank you thank you!

Offline Fumeaux

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Re: SSTC GDT troubleshooting
« Reply #5 on: May 14, 2020, 08:08:24 AM »
There is this website which is a great source for gdt troubleshooting. There are the most typical problems listed and how to fix them.

http://www.richieburnett.co.uk/temp/gdt/gdt2.html

I think when testing the gdt's performance, you should use a frequency close to the driving frequency of the tesla coil (I guess 200kHz-300kHz). Additionally you should connect the secondary of the gdt to the load i.e. mosfets, resistors, diodes or whatever you plan to use, to get as close to realworld testing as possible. (you dont need to have the bridge finished or even powered on for the first tests)

You should post some pictures of the driver board and setup. If there are any errors, someone might be able to spot them before they become a problem.

Offline Mads Barnkob

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Re: SSTC GDT troubleshooting
« Reply #6 on: May 14, 2020, 11:34:23 AM »
This just shows how much I still have to learn.

I followed all your advice, and the result speaks for itself(picture attached), thank you SO MUCH.
Probably needs some bigger damping resistors, but besides that, it's looking pretty great!!

thank you thank you thank you!

Hi Nylla and welcome to HVF!

I assume this shot was without a gate of a MOSFET connected and no gate resistors?

Experiment with gate resistor value between 3R3 to 10R and see which gives the best "slight 5% overshoot that does not ring" and other than that a good square wave.
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline Nylla

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Re: SSTC GDT troubleshooting
« Reply #7 on: May 16, 2020, 01:33:21 AM »
Hey everyone!
Thanks a lot for your help, really appreciate it.

I was finally able to sit down and complete the bridge part of the circuit as well, and I was able to do some testing with the GDT connected to it.
Find some screenshots attached, the filenames are pretty self explanatory, but I'll try to go into more detail, all of them are with the probes ground on the driver/bridge negative.
Damping resistor was 10ohm on each mosfet, but I'll experiment with different values as soon as I'm able to go buy more resistors.
Also, on a side note, I noticed that it doesn't make any difference whether the ENBL pins of the ICs are connected to 12V or left floating, though it is my understanding that when the input is floating, the ICs shouldn't have any output, or am I understanding this wrong?


1-Is the GDT output while connected to the bridge, probed after the resistor, Math function is added, I'm unsure as to why channel 2 is so flat compared to channel 1.
2-Is the same thing, except with a resistive load on the bridge.
3-Same thing again, except I lowered the bridge's supply voltage, I think it's interesting to note that the ringing seems to increase exponentially as the supply voltage to the bridge is increased.
4-and 5 are the waveforms probed at the bridge's output, instead, which again seems to show a considerable amount of ringing as the voltage is increased.

Any ideas? Thanks!
« Last Edit: May 16, 2020, 03:46:30 AM by Nylla »

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Re: SSTC GDT troubleshooting
« Reply #7 on: May 16, 2020, 01:33:21 AM »

 


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