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It's time for an SSTC

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LAN_403

uzzors2k

Wed Oct 24 2007, 07:33PM

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

This might be a bit far-fetched, but since no one else has answered I'll dare to speculate.

For some reason lower frequency coils give longer sparks than higher frequency coils for the same power input. I don't why, but it seems to be the norm. This would explain the better spark output since the topload would lower the TC frequency.

The reason a larger topload doesn't improve the spark size further is the increased capacitance. Energy stored in a capacitor is a function of voltage and capacitance. Higher capacitance means more energy per volt. With the same energy and more capacitance, the voltage will have to decrease.

I can't explain the increased power draw though. Maybe more power was being radiated into the environment around the TC with the larger topload, and there was therefor less to be "used" for sparks. This sort of explains the arcing too, as the spark is only a portion of the energy put in. Think of florescent tubes lighting from a distance for example.

Dr. Dark Current

Wed Oct 24 2007, 08:06PM

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

I don't know, but this just came to my head- couldn't the larger toroid act as an one big shorted turn eating the power away?

Zum Beispiel

Wed Oct 24 2007, 08:23PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

jmartis wrote ...

I don't know, but this just came to my head- couldn't the larger toroid act as an one big shorted turn eating the power away?

Yes, but so would the smaller one. Of cource a larger toroid would have higher resistance but I doubt it would make that much of a difference.

Dr. Dark Current

Fri Oct 26 2007, 04:49PM

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

I've solved many electrical mysteries in my circuits before, but this one is really hard.

I'm now running the TC in a way that it's fed with halfwave mains input and an interruptor that turns it on as the mains reaches peak value (the idea was to get the same spark length with 1/2 input power).

The first mystery is that one of my fets was getting much hotter than the other. I thought I had defective gate driver, so I replaced them both. It was still the same.

To find out where the problem was, I swapped polarity of TC primary leads and gdt primary leads (to have the feedback polarity right).
The discharge made a really funny noise, but there was HF output on the gate drivers at the resonant freq.

I put one scope probe around 1m from the topload to see the RF envelope and unlike before, I saw a HUGE negative spike on each interruptor pulse. I think this even damaged my scope

. (Update: my scope isn't damaged, one hard hit on the side repaired it. It's some 50 years old so I think it absolutely has the right to have a bad solder joint, especially considering what abuse it has survived so far!)

I would be grateful for any ideas. Why one of my FETs runs hotter? Why changing the polarity of primary and GDT did change the output so much? Why the HUGE spike? (it was showing up on my scope really hard even with all probes unplugged!)

It's also possible that the problem lies in my old breadboard, but I doubt that.

Thanks for any help!

Firnagzen

Sat Oct 27 2007, 04:30AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

Just hazarding a guess- I'm by no means an expert- maybe one FET heats up more because it has a slightly higher resistance. They can't make every FET exactly the same, so maybe some defect in that one makes it heat up more?

Ps. Why shouldn't you mix up deuterium and tritium? The only difference is that tritium is radioactive, right?

Dr. Dark Current

Sat Oct 27 2007, 08:30AM

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

Firnagzen wrote ...

Just hazarding a guess- I'm by no means an expert- maybe one FET heats up more because it has a slightly higher resistance. They can't make every FET exactly the same, so maybe some defect in that one makes it heat up more?

Ps. Why shouldn't you mix up deuterium and tritium? The only difference is that tritium is radioactive, right?

Well, it is possible that one FET has higher resistance, but I doubt it. I'll try to swap them and see what happens.

P.S maybe you didn't get the point, it's a quote from an article on heavy water, just how many people are there who have deuterium and tritium in their closet?

Firnagzen

Sat Oct 27 2007, 08:38AM

Registered Member #567 Joined: Tue Mar 06 2007, 10:55AM
Location: Singapore
Posts: 147

I wouldn't know how common either is. I've never tried obtaining 'em, have I?

By the way, if you suspect a loose solder joint in your o-scope, perhaps you should fix it? Just a suggestion.

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