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SSTC running roughly - why?

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Hartstart

Sun Jan 10 2010, 01:12PM

Registered Member #2616 Joined: Sun Jan 10 2010, 01:00PM
Location:
Posts: 12

Hi!

I made my first SSTC some time ago. Its just basic Steve wards minisstc with antenna feedback and IRFP460 H-bridge. I power it from 0-220VAC variac. Problem is that when i add topload (small or large) coil wont run very happily. Sparks are short, current draw and mosfet heating are massive and instead of normal 50Hz hum i can hear hi pitch whining and popping indicating that something is wrong. I cant figure out what went wrong, without topload coil seems to be working much better.

Specs:

MiniSSTC driver
11cm by 35cm secondary, resonates @ 310kHz
Sharp breakout point
12 primary turns
0-220VAC fullwave rectified DC filtered with 500ÂµF

Friend of mine suggested that this could have something to do with antenna and e-field shape altered by topload. Would current transformer feedback work any better or should i try oldschool style external oscillator?

doctor electrons

Sun Jan 10 2010, 03:19PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

I would guess that your topload is throwing off the coils resonant frequency. I suspect all you need to
do is some retuning. I can only guess because i do not know how you did your tuning. I had the same situation
with my drsstc. Arc length would not get larger than an inch. The fix was in the tuning. I initially tried a
Sharp aluminum breakout point but it changed the capacitance of the toroid quite a bit and caused
the current to rise very high before breakout. I saw a video once of a guy tuning his secondary with a 4 foot wire
hanging from the toroid to simulate the detuning from a streamer. He showed the effect on an oscilloscope and
it was actually substantial.

Hartstart

Sun Jan 10 2010, 03:27PM

Registered Member #2616 Joined: Sun Jan 10 2010, 01:00PM
Location:
Posts: 12

MiniSSTC is self tuning circuit. Im using antenna feedback.

Just checked gate waveforms with oscilloscope and they are fine.

uzzors2k

Sun Jan 10 2010, 05:57PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Do you have enough turns on your GDT for the new, lower resonant frequency? Try CT feedback, or changing the antenna length. Tuning the number of primary turns is also something you should try.

Steve Conner

Sun Jan 10 2010, 06:15PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If the coil makes whining sounds, it's possibly an interference issue. Putting on a topload increases the electric field around the coil, and the increased E-field can start getting into places in the drive circuitry where it shouldn't. Which is to say, anywhere except the antenna. if you have an interrupter, it could be getting into that.

You might like to try shielding your driver board or moving it a bit further away from the coil.

Dr. Dark Current

Sun Jan 10 2010, 06:42PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I don't know how much current does the 74HC14 draw, but I think not much. I had this idea that some of these problems could be caused by the antenna current raising the supply voltage for the chip through the diodes. A 5.1 volt zener should fix this.

Of course it could be interference from the coil, it's best to put the driver circuit into a grounded metal box anyway.
This leads me to a question - is your gate driver grounded? It should be, otherwise the antenna has not much to "work against".

If nothing helps, try to replace your controller ICs and check all transistors if none is "half-dead".

Hartstart

Sun Jan 10 2010, 08:57PM

Registered Member #2616 Joined: Sun Jan 10 2010, 01:00PM
Location:
Posts: 12

Thanks for help!

Currently i have driver board just sitting on table and wired just like minisstc schematic tells (all grounds on control board are 'common negative' without real ground from mains). Secondary base is grounded to mains ground.

Would it work if i mount all electronics on metal plate and cover them with steel wire cage both tied to mains ground? Should i also connect driver board common negative to same ground?

Dr. Dark Current

Sun Jan 10 2010, 08:59PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think that would work, just don't short out anything. :)

I always ground the driver, never had problems with that, dunno why the schematic says the opposite. The ungrounded driver may even pick up a few kV from the coil and flash over your GDTs insulation etc.

doctor electrons

Sun Jan 10 2010, 09:32PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Hartstart wrote ...

MiniSSTC is self tuning circuit. Im using antenna feedback.

Just checked gate waveforms with oscilloscope and they are fine.

My drsstc is also self tuning, ct feedback. But it didnt really care!
Might be the same symptom, wrong diagnosis. Never hurts to check!
I ended up removing 1/4 turn from the primary with a 1/4 inch fuse holder as a tap. Solved
the problem in about 10 mins. Runs great, no strange noises and streamer length is about 36" with the current limiting
circuit cutting it back at about 400 amps.

raff

Mon Jan 11 2010, 12:18AM

Registered Member #2315 Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161

I THINK, this problem lies in the construction of GDT...

>>are your gdt leads twisted together?

>>if you have an LC meter, go for 15T and at least get 300uH inductance..

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