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Registered Member #1548
Joined: Mon Jun 16 2008, 09:22PM
Location: Ohio
Posts: 61
Hello all.
I have created a SSTC based heavily off Richie Burnett's SSTC...well, it's pretty much the same thing. But for a first coil, why not try to copy another persons design, right?
But anyway, here are my specs:
Circuitry:
Same drive circuitry
Same full-bridge design but with 20N50T MOSFETs
140VAC -> Fullwave rectified
Primary:
5 inch OD
8 turns of solid 14 AWG wire
Cardboard form
Secondary:
4.25 inch OD PVC pipe
Approx. 27 inches of 24 AWG winding
Around 25 coats of polyurethane varnish
Topload:
Toroid with 3 inch minor and 11 inch major diameter
Breakout point straight up from center
For my ground I used a copper pipe pounded about 5 feet into the earth, attached to it is a 50 foot 14 AWG grounding wire that runs to my lab area.
This is a picture of my coil pretty much done. You can see the old 10 AWG primary in the background.
Here is a close up shot of my bridge.
In this picture, there are about 5-6 inch streamers from an insanely small toroid.
Here is a picture of streamers after a new, smaller primary, as well as a properly sized toroid. Arc length in this pic is about 7 or 8 inches
Since the last pictures were taken, I decided I would decrease primary turns in attempt to increase arc length. I hit about 11 to 12 inch streamers, and this is when things started to go wrong...
First, the resistors that the bridge V+ goes through to sweep the freq. down began to smoke and burn when it was connected. Fortunately nothing else was harmed, and I just disconnected this. Arc length maintained in the same ball park, but performance may have been deteriorated somewhat. Any likely causes of this? Maybe just shoddy resistors?
Then when I hit 7 turns, I had the coil running at max 140 Vin, and after about a minute or minute and a half, the 7A fuse blew. I replaced the fuse and slowly turned the variac up. The fuse blew again at 14 Vin. I go in with my multimeter and find that two FETs on the same side have their drain and source at a dead short. They are not removed from the circuit, but it really should not matter(on the other side, the FETs drain-source resistance is about 525 ohms). There is no physical damage to any parts, so that is probably a plus. Could this be from the FETs pumping to many amps and not sufficient heatsinking? I have no thermal grease on the FETs, just screwed to the heatsink...very poor thermal conductivity. Perhaps someone more experienced on this matter could point me in the right direction? Would it be a good idea to just add a turn to my primary, replace the FETs, and go at it again? Or could there be an underlying problem? Any thoughts would be greatly appreciated, and feel free to ask questions.
Again, this is my first coil, I know its not the greatest...but it's something!
Registered Member #2018
Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74
Nice coil and nice arcs!
The problems of the dead bridge may be caused by a bad gate signal quality. If the MOSFETs don't turn on and off fast enough, they will die. This is due to switching losses. I would check the signals on the gates of the power FETs with a scope. Turn-on time should be 200ns or better from my experience.
EDIT: Another thing: Your secondary may not have a good aspect ratio (it is very tall if you compare with its diameter. Normally the ratio of height to diameter is not more than about 3:1.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Nice coil!
Ah yes, you also want to make sure you are not getting too MUCH gate drive. If the mosfets are not made for the freq you are using, the gates require more current to switch so fast, and they could get damaged/melted. I am sure this is not the problem, but i felt i should throw it out there.
That reminds me i need to check my gate drive signal, as i have a new true RMS volt meter. Sweeeeet.
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Solid state tesla coil based off Richie Burnette's
