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Steves Mini SSTC trouble shoot

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StevenCaton

Thu Jan 08 2009, 04:53AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

First of all, let me start by saying that i got this coil to work in the past. It worked great, with about 5+ inches of spark.

I know what i am going to do to fix the coil but I thought I'd throw this post out here to see if you guys agree with my logic.

Now, however, Ive been running into problems ever since I changed the primary to 15 turns and spaced it an inch away from the secondary on all sides. In fact, i can't even get the coil to make sparks at all right now.

I injected a 100khz square wave into the circuit to test the electronics. I then measured the +24v -24v swings on the MOSFETS. (the signals on the fets are definitely out of phase too) So all is well there, the electronics work properly. The 555 works also.

But when I apply power to the bridge, the variac actually blew its 6amp fuse. So I'm thinking theres a short. The mosfets were new though, and I know for sure I'm not wiring it incorrectly because I am doing the same as i did in the past. (when it worked)

I actually think one of the caps in the bridge is shorted or destroyed somehow. Afterall, what else could be causing the variac to blow its fuse, when theres new fets and the thing is wired correctly.

So Ive narrowed the problem down to three possible things
1. destroyed/blown cap or caps within the bridge
2. bad cap on the antenna (highly unlikely)
3. one of the diodes may be bad right off the antenna. (the ones that form the clipper circuit)

Has anyone ever discovered that theyve blown a capacitor within a small SSTC like this one. It doesn't seem like this should happen easily.

Do you think the fact that i now have a whole bunch of turns on the primary is preventing the oscillation from starting????

Arcstarter

Thu Jan 08 2009, 06:32AM

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

Having too many turns on the primary (within reason) doesn't sound like it would keep oscillations from starting. If i can get the same circuit to oscillate on 12 volts, i am sure 120 volts (actually 170 with rectification) would start it even with too many primary turns.

If one of the antenna clamping diodes was dead, it would not clamp the antenna to 5 volts, which would result in a dead 74hc14.

Is the capacitor on the antenna that important that it would prevent oscillations? I am pretty new to this.

Destroyed capacitors sounds possible.

Maybe the mosfets dies as soon as you put them in? If the variac fuse was fast-blow it would be easy for the mosfet to die without exploding. I have been running a ZVS driver on 40 volts and a power supply that could probably short at 50 amps, and the mosfets have died twice without asploding. I know the circuits are worlds apart, just saying that mosfet failures are possible, unless you have checked them already?

Wolfram

Thu Jan 08 2009, 07:21AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Are you sure the rectifier is OK?

Anders M.

StevenCaton

Thu Jan 08 2009, 07:26PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Are you sure the rectifier is OK?

Yes, I did check that. I disconnected the bridge and left the rectifier and cap, it gave 170VDC max with no blown fuses on the variac.

I'm starting to think that maybe the fact that I changed the # of turns from a close 4, to 17 that are spaced an inch away, might be causing the problem. I believe that a smaller number of turns generally increases the initial primary current. So maybe my 17 is preventing the initial oscillations from starting.

Well, I'll get two new caps for the bridge, and some new diodes, and then I'll get the system working with 4 turns. Then I'll play with the number of turns and see how the coil reacts. I really don't know why it wasn't working with those 17 turns.

Arcstarter

Thu Jan 08 2009, 08:08PM

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

I believe that a smaller number of turns generally increases the initial primary current. So maybe my 17 is preventing the initial oscillations from starting.

Hehe, i forgot about current :P. I was thinking that wouldn't happen because it is hard to get so off tuned that there is not enough of a pulse to start oscillations. But that mixed with low current, that sounds about right.

But if you want, you can use more turns and if you add a toroid it will pull more current. I don't mean more than 17, i just mean more than the initial amount(4 iirc). So, if you want to run with toroid without mosfets exploding or overheating or it being inefficient, add more turns to reduce the primary current.

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