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Half bridge sstc revisit

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teslacoolguy

Sun Aug 10 2008, 05:33AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Today i decided to revisit Steve Wards mini half bridge sstc

. I tried to build it about 4 months back with no success whatsoever. Because i learned a ton of stuff on how sstc's work i decided to give it a revisit. I made a new low inductance bridge pc board, made a new gdt, worked out a couple glitches with the feedback circut. I powered it up and what do ya know... it worked. Although it did not work well and that is why i am posting this thread. with the primary raised about 3/4" on the secondary i was getting breakout and 1" HOT arcs to a screwdriver but nothing like Steve got. The primary started to flash over probably because of to tight coupling so i lowered it to the base of the secondary. Now i could not get breakout at all and then BOOM one of my fets blew and burned a hole in my new cement board workbench

. I am assuming that i had the coupling to low and to little energy was transfered into the secondary causing large peak primary currents. How can i fix this? I do not understand why i was not getting large sparks even when i had tighter coupling and how can i prevent flashovers. Also the primary was spaced about 1/8" away from the secondary. Here is a pic of my setup

The light bulb thing is simple ballast that i used for the first test just in case something was wired wrong i would not blow the works

Any help on how to get this thing working would be greatly appreciated.

KresoLiubov

Sun Aug 10 2008, 05:56AM

Registered Member #1153 Joined: Mon Dec 03 2007, 07:27PM
Location: Croatia
Posts: 213

Did you interrupt it? It sounds like it was running CW

Light bulb is great idea, i use it always

Mates

Sun Aug 10 2008, 08:19AM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

I think your primary is crap. You should do it in much better way. Leave bigger air gap between primary and secondary, add more turns (min 6) and make it more spreaded.Also the wire you used seems to be poor choice. (fat microphone cable you can buy in every music shop works very well for me)
Good luck!

Dr. SSTC

Sun Aug 10 2008, 08:43AM

Registered Member #1407 Joined: Fri Mar 21 2008, 07:09AM
Location:
Posts: 222

maybe you should add a top loaad on it, and try to use like some big drain pipe for the primary coil form as it will add insulation, just a thought maybe there wasnt enough current going into the primary as you have aded limiting

uzzors2k

Sun Aug 10 2008, 10:40AM

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

Mates is right, with so few primary turns and such a low frequency the magnetizing current alone will be enough to blow the fets (insufficient primary inductance, read up on reactance). Start with lot's of turns wound halfway up the coil and tune form there. Removing turns will increase power, as will covering more area on the secondary with primary turns. Don't cover more than half the coil with the primary though, it doesn't look good and leaves too little secondary inductance for resonating. A drain pipe or other form of insulation is a must.

teslacoolguy

Sun Aug 10 2008, 11:13AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

[quote]

Mates wrote ...

I think your primary is crap. You should do it in much better way. Leave bigger air gap between primary and secondary, add more turns (min 6) and make it more spreaded.Also the wire you used seems to be poor choice. (fat microphone cable you can buy in every music shop works very well for me)
Good luck!

With my setup i was just trying to follow what Steve Ward did. I do have some 6" drain pipe left from my big coil so i will start with.... lets say 10 turns and tune it from there.

teslacoolguy

Tue Aug 12 2008, 05:35AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

While i wait for more mosfets to come, I thought of a way to audio modulate it that would be worth discussing. Anyway if i disconnected the interrupter and ran it on CW, what would happen if i put the signal from a zero crossing detector to pin 3 of the ucc's? If i am correct it will audio modulate it but i am not sure if the sparks will be to big and cause allot of distortion or not? tell me what you think.

Steve Ward

Tue Aug 12 2008, 06:08AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Yeah, that will give crude audio modulation, it will be very distorted and really only useful for simple music, but is still fun to play with.

Shaun

Tue Aug 12 2008, 05:33PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

When I built my version of Steve's SSTC, I copied his design exactly, just like you have. The primary was four turns wound directly over the bottom of secondary, but I had some HDPE sheets (cut from notebook covers) between the two coils. My coil was acting exaclty how yours is: blowing FETs, flashovers, short arcs, and unreliable in general.

Now, it can hit 14" arcs on a good day and it dims the lights when I hit the "CW" button.

Here's what I did to fix it:

1. Its hard to tell from your pic, but it looks like your two FETs share a heatsink. Keep in mind that the drain is tied to the metal tab on the back of the FET, and the insulation between the tab and the heatsink is crude at best. I split mine up.

2. Primary Former. I used a red Folger's coffee can, about 6" wide. Still 4 turns but now they are spaced out over a 2" winding length. This got rid of all the flashovers.

3. Topload. Mine's an 8" steel gazing ball with the paint scraped off, then sanded smooth. For a breakout point I used a thumbtack, since it appears to achieve longer arcs than a tall breakout point, like a nail. BTW this lowered Fres to 200kHz (still no problems with 4 turn primary). It really surprised me how much a topload helped arc length.

4. Improved ground. This one I don't really understand, but it works so far. The bottom of the secondary is connected directly to about 10' of 6AWG which leads to an 8' copper spike in my backyard. I thought this would be way overkill, because after all I had used it on my 15/60 NST coil. However, I found that when I would even touch the [still grounded] secondary base, the streamers got longer and hotter and began breaking out at a much lower voltage. So I just alligator-clipped the base to the metal legs of my workbench, as well as leaving the original ground in place.

5. Oh yea, and I switched to an H-bridge, once it was all reliable. Arc length nearly doubled. 170V primary ftw

.

I know thats kind of a lot, but any one of them helps a lot.

Steve Ward

Tue Aug 12 2008, 06:40PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Shaun touches on a number of good points. Im not sure why the primary geometry is giving so much grief, it surely does limit things but should get you some decent sparks first, unless you simply have the primary up too high on the coil. In any case, spacing the primary off the secondary and inch or 2 is much better!

I think most problems are probably caused by lack of proper grounding. Mains ground works OK for me, but im usually on concrete floors, where real earth isnt far away. Otherwise, the coil needs some sort of "earth" to act as the other plate of its capacitor, and by grounding to the metal on his work-bench, Shaun created that.

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