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Mini SSTC Bugs

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LAN_403

semiconductor

Sun Feb 22 2009, 04:18AM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

I am attempting my first SSTC without much electronics experience. After some searching I found a schematic on Steve Ward's site. It looked fairly simple but it has proven otherwise. I started with a breadboard but it made all my components heat up! So I transfered everything to a prototype board.

I know the E core is not a good core to use as a GDT, I have since changed that to a 5000u farrite toroid.

Here is what my setup looks like.

It is just in the lash-up stage. Here is what it will look like after I put all the guts in a box and mount the coil on top.

Here is a pic of my half-bridge and driver circuit.

This is how I will lay it out in the box, It's all just sitting loose to see how it fits.

I can't seem to figure out what I have done wrong. I keep blowing FET's. I don't have a scope, just a good multi meter. Is there any way of testing for dead FET's with a meter?

Any suggestions or notes of improvement to this circuit would be appreciated!

Arcstarter

Sun Feb 22 2009, 04:51AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Suggestion with the GDT is take those three (actually 6) wires and then wrap them together as well. I think :P. :/

Frosty90

Sun Feb 22 2009, 08:59AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Is there any way of testing for dead FET's with a meter?

generally you can set your meter to the ohms scale and measure the resistance between the gate/drain/source. often (for me anyway), they seem to fail with about 1 to 10 ohms between gate/source and dead short between drain/source

Mads Barnkob

Sun Feb 22 2009, 12:00PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Use a breakout point, this TC will fry your fets if you try to let it breakout on its own from a spun toroid.

Check your gate driver resistors, they can burn over, tried that a couple of times with my mini sstc.

semiconductor

Sun Feb 22 2009, 05:27PM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

What kind of caps should I use in my driver circuit? Right now I'm using tantalum's for all accept the antenna(.1 monolythic ceramic) and the timming cap (2uF lytic). Both my UCC's and the 555 seem to heat up and the .1uF Tant on the GDT seems to blow a puff of smoke!!! ????

Steve Conner

Sun Feb 22 2009, 06:46PM

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

2uF sounds way too big for a timing capacitor in something that should oscillate at Tesla coil frequencies. What schematic are you using, and are you sure you read it right?

Also the DC block cap on the GDT should be non-polarized.

Arcstarter

Sun Feb 22 2009, 09:58PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

He is using 2uf for his 555 interrupter, i believe.

And that is why your driver died, is the tant on the GDT! I didn't notice it was a tant. You should use film non polarized capacitor on the GDT. Just post pictures of different looking caps, we could point out the one that you should use. Those red capacitors that are covered in plastic that are found in televisions are film. You said you where going to look for a TV this weekend, so its perfect :P.

You can find the value of the capacitors there. Disregard the letter that comes after the number, as that is just its tolerance.

Tants hate any reverse charging, and they are not good for gdt driving anyway. Hopefully this was the fault.

P.S. Did you make the new driver board yet?

semiconductor

Mon Feb 23 2009, 12:44AM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

Steve M, I am using Steve Wards mini SSTC schematic. It was kinda vague on values of components( for someone with not much electronics experience. I am however an electrician and can follow a schematic, so this shouldn't be a problem.

Arcstarter

Mon Feb 23 2009, 02:33AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

One question. You did use the BOTTOM schematic on that page, right? It had a few minor updates, the one you should pay most attention to is using only ONE gate of the 74hc14, instead of putting them all in series. If you put all in series, that just causes more delay time. This does not necessarily fix the ones that don't work, but be sure you do that just to be sure. Seeing that you used only one 555, i am guessing you used the bottom schematic, and the 555 on your board is purely for interrupter and not to start oscillations. If not, you might wanna check the bottom schematic.

I still really believe it was that dumb tantalum cap on the GDT that caused the driver to die from shorting across the GDT. Use one of those .1uf 50v ceramic capacitor you mentioned. Also use the same kind on the antenna. Film would be much better, but seeing how you do not have one (or dont know about it :P) just go with ceramic for now.

semiconductor

Mon Feb 23 2009, 06:56AM

Registered Member #1989 Joined: Fri Feb 20 2009, 05:28AM
Location: Edmonton Alberta, Canada
Posts: 17

I got sparks today!!! But then it quit... I could only get a spark every 5 seconds or so and they were only 2-3 inches long. Then it just quit. I was using a 10meg resistor and a 1uF cap on the 555. I changed all the UCC's and the 555. still nothing!!! ???

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