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Rather small and rather problematic SSTC

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Inducktion

Fri Nov 25 2011, 12:59AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I made a really small tesla coil using some 1/2 inch or so diameter piping I had lying around.

It's being problematic, because I believe the frequency required to get it to run is extremely high. I attached some pictures...

I tried running it off of 30 vDC, 40 vDC, and almost 80 vDC. Nothing resulted in breakout, or anything that I can tell. The gate drivers get really hot, and the signal on the mosfet's gates are sine waves...but they don't get hot at all!

I'm using

as the schematic, without the 555 timer attached. I'm also using a schmitt trigger nand gates wired as two inverters, instead of the 74LS14. ALSO, I'm using just one 1 uF MKP cap as the DC blocking cap, instead of the two .68 uF caps.

Is it my coupling? My GDT? Should I perhaps try a totally different method of getting this thing to work right, since the required frequency will probably end up being in the 4-8 mhz range?

Marko

Fri Nov 25 2011, 01:11AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

bleh, it looks like your phasing is wrong.

Also, this circuit is kind of crappy enough not to start alone in CW without the interrupter, some people do it by touching the antenna or such... You'd probably want to use an interrupter with it anyway.

Marko

Polonium210

Fri Nov 25 2011, 01:15AM

Registered Member #3450 Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61

If your using that schematic without the 555, what are you using for an interrupter circuit? Are you trying to run CW? Also your gate drive chips might prefer a larger, lower frequency secondary.

Inducktion

Fri Nov 25 2011, 01:24AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Dr. Pork wrote ...

bleh, it looks like your phasing is wrong.

Also, this circuit is kind of crappy enough not to start alone in CW without the interrupter, some people do it by touching the antenna or such... You'd probably want to use an interrupter with it anyway.

Marko

What would a better circuit be for this small of a SSTC anyway?

Marko

Fri Nov 25 2011, 02:26AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

So have you tried swapping your primary phasing? You should get some breakout that way (hopefully, unless your LS gates are just too crappy) but considering your poorly heatsinked mosfets and high losses it's likely that you won't get much of CW before they blow.

Right topology for this coil... if it resonates in Mhz range, probably class E... (copying richie burnett's work is actually a very good start)... or a properly built driver with phase lead compensation and variable deadtime which you could tune to class DE. (finn's predikter and Conner's PLL are probably the only drivers like such in existence, but both would need heavy modification to run at a frequency this high).

Ofcourse, you could just heatsink your mosfets better or/and use many in parallel to see what you can get out of it before it blows... btw, if you want to be mean you could use "direct UCC" driver like Idid in the past with some success... (look at my "big bad"the CW coil thread in projects, it uses this circuit). Instead of TL494, you can discharge 1nF cap over a pushbutton into one UCC input to start oscillation.

Marko

Inducktion

Fri Nov 25 2011, 02:32AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

So, I measured my inductance to be ONLY 1.4 mH. (just using my DMM, so it might be lying)

Going to have a ridiculously high frequency....yay...

Also, I DID get the coil to work, sort of. Lit up some neon bulbs and flourescent bulbs, but, no breakout at all.

Maybe I should use uzzor's PLL class E ? It seems easy enough to build.

Marko

Fri Nov 25 2011, 02:50AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Third time, have you swapped your primary phasing? You can get the bulbs to light with wrong phasing too (they will tend to be brightest around the middle or so.

Also, remove the whatever monstrosity you have for topload and leave a thin wire protruding from the top of your coil. Observe if you can get a small breakout from it... if not, swap phasing. At over 80V DC I think you should see at least something...

Regarding class E coils, build the richie burnett's one, because pretty much all other I know suck (more like class OMG than class E)

Marko

Tetris

Fri Nov 25 2011, 02:51AM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Thats one very tiny coil.

Inducktion

Fri Nov 25 2011, 03:47AM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Okay, so I managed to measure the resonant frequency!

It's around 2.56 mhz, at 2.76 pF. (I built a function generator out of an XR2206. Chip is a beast)

That isn't as near as bad as I thought it was going to be, thankfully. Driving it will still be hard, but not as hard as a 4 mhz coil.

Also killed my schmitt trigger driver. It was kind of poop anyway...

Dr. Dark Current

Fri Nov 25 2011, 10:51AM

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

I would go for a class E drive at this frequency, or maybe a half-bridge of FETs with very low gate charge (try IRF730, or IRF630 for 120v mains).

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