Kaizer SSTC II

[Unleashed]

Well, first of all, hi everyone, and hi Mads! Been a fan of your work for a while, hence why i'm trying to make a teslacoil based on your SSTC II project.
So far I've wound the coil, assembled the driver circuit on breadboard, and assembled the full bridge.
I haven't done too much testing, but I'm trying to get the GDT waveform right.

The tests I've done so far included varying the damping resistors and GDT inductance, by going from 15 turns to 20, did improve it a bit, from this


to this:


The core I'm using is an N30 core, with around 4800 mH of magnetic permeability.
The waveform also seems to change a little with a slight resistive load (around 2A) on the bridge, from the one above to this (which correct me if I'm wrong, is unusable).



This is the waveform i get when the GDT is not connected to the bridge


I think the problem might be a low GDT inductance.
Any suggestions?

[Mads Barnkob]

Welcome to the forum and thank you for the kind words, you should also take a look a Steve Wards mini SSTC, which this is based on: http://stevehv.4hv.org/SSTC5.htm

To be able to help you better, could you upload some more pictures of all your components and what has been put together.

Something is definitely not right when you have a duty cycle higher than 50% with a GDT involved, you must have the GDT connected to the UCC driver ICs and the GDT connected to the MOSFET gates to make a measurement of testing the GDT driving the load its going to drive.

Also give some more details/pictures on how you hooked up these measurements.

[Unleashed]

Hey Mads, thank you for the reply.
You're perfectly right, I completely forgot about adjusting the duty cycle.
I'm using a 555 oscillator to simulate the antenna, I'll definitely lower it to 50% and give it another try.

As for my setup, here's a picture.

I was powering it at around 15V from my power supply, with a 12ohm 5W
 resistor.
The measurements have been carried out with all the GDT terminals connected to the mosfets, and the oscilloscope was hooked between gate and source of one of the mosfets.

[Unleashed]

Ok, I adjusted the duty cycle, this is what I get
No load
Resistive load

[Mads Barnkob]

That looks much better, how are the waveforms with MOSFETs on as load? If you get the same little overshoot as in your shot with resistive load, you could use a larger gate resistor to dampen it a bit.

Remember that driving a MOSFET properly is not just dumping as much peak current into its gate as fast as possible, that will make it ring and distort. Switch mode is all about switching as fast as slow as possible

[Unleashed]

Hey Mads, thank you so much for your help.
The first picture is just the GDT connected to the mosfets, without a load on them, the second one is the same thing, except I have a resistor where the primary should be, instead of having it open.
Also, correct me if I'm wrong, shouldn't the bottom edges be the same as the top ones?

[Mads Barnkob]

The N30 material should be good enough, but what is the physical size, ? mm^2 cross section?

15 turns would normally do it, but you had some real bad drooping going on there, with 20 turns you are on the way, but still like 5V drooping, you need to reduce it to maximum a couple of volts to ignore it. But many more turns on the GDT and you will start to loose peak current drive and will be phasing slow sloping turn-on waveforms instead.

Maybe you were right in your initial comment on suspecting the GDT core size/material

[Unleashed]

Hey Mads, a little update.
My coil is acting completely weird, I'll try to sum it up as best as I can.
I wired it up and I wasn't getting anytning, no power drawn, nothing.
Moving the antenna around however did change something.
For some reason, the only way I can get the coil to do something (and it's just a small corona) is by having the antenna extremely close to the breakout point, so close that it'll just arc into it, which is weird.
But the weirdest behavior was that once I turned off the variac for tests, as soon as I would touch anything (I even got shocked), the coil would fire a small spark.
Even touching the main smoothing capacitor with the multimeter terminals did that.
Thoughts ?

[Mads Barnkob]

Hey Mads, a little update.
My coil is acting completely weird, I'll try to sum it up as best as I can.
I wired it up and I wasn't getting anytning, no power drawn, nothing.
Moving the antenna around however did change something.
For some reason, the only way I can get the coil to do something (and it's just a small corona) is by having the antenna extremely close to the breakout point, so close that it'll just arc into it, which is weird.
But the weirdest behavior was that once I turned off the variac for tests, as soon as I would touch anything (I even got shocked), the coil would fire a small spark.
Even touching the main smoothing capacitor with the multimeter terminals did that.
Thoughts ?

It sounds like the primary phasing is wrong, try to reverse the primary connections. Could you please upload a picture of the whole setup so we can see it all?