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Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
Hi everybody
I am making TS 555 controled sstc without feedback... and mosfet's some times go off, and blow up .. And i don't want spend money for mosfets , what to do?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
You might want to try running with feedback instead of a fixed frequency. The resonance frequency of the coil changes slightly from streamers or when pulling arcs. Running the coil a little off resonance creates a lot of reactive current and you lose the benefits of running a resonant load. I would simply replace the 382 kHz source with a self resonant feedback scheme. I can recommend PLL, but you could also use a a buffer chip of some sort with an antenna or current transformer.
Registered Member #1143
Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
can you schow sheme, but GDT should been driving using PNP and NPN transistors, because i couldn't get UCC drivers :( i think about some think like that TL494 is good for feedback imput? beacause my sstc olso can bee driving TL494 :)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
When you're driving a GDT with PNP and NPN transistors like that, you need a diode between collector and emitter of each transistor to take the magnetizing current.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Steve Conner wrote ...
When you're driving a GDT with PNP and NPN transistors like that, you need a diode between collector and emitter of each transistor to take the magnetizing current.
Then I wonder how I made it to work well without diodes? Do the diodes conduct at all when duty cycle is full and exact?
Registered Member #514
Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295
Linas wrote ...
Does someone have SSTC , which works without problem?
SSTCs and "no problems" don't mix
I dunno, Steve Ward's designs are pretty solid. I use a feedback driver similiar to his desings on my Mark II (see "add your tesla coil here"-thread), and haven't had too much trouble. I get 30cm discharges and haven't blown it up in three months now (and it has weekly run times >30 minutes).
Another option is PLL. It shouldn't be much harder to set up than a regular feedback driver and gives good results.
Either way you do it, feedback is essential if you want your mosfets to live. Going off tune causes hard switching, which in turn makes the mosfets go BOOM. Also, for best reliability, make sure you have a good safety margin on everything. Use beefier FETs, so that a sudden surge in power won't fry them.
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Mosfet's protection
, what to do? 
Do the diodes conduct at all when duty cycle is full and exact?
