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First SSTC - no breakout & magic smoke

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Kipmans

Mon Jul 28 2008, 07:00PM

Registered Member #91 Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45

Hi,

I've been working on my first SSTC for a few weeks now and finally everything is coming together. My design is basically the same as Steve Ward's mini SSTC except I use different FET drivers and different MOSFETs (IRFP460) because the mains voltage over here is higher (230 V).

I don't have a variac so I can only power the coil at 35 V from my bench power supply or directly from the mains. First, to verify everything worked as it should, using a 4-turn primary I connected the bridge to the 35 V. This produced a very small breakout at the breakout point (about 5 mm) and sparks to ground of a few centimeters.

So today I wound a primary of 12 turns and connected the coil to mains voltage. Altough this produced the usual radio frequency interference (lighting TL lamps etc..) the coil did not break out... Even the sparks I could get from toroid to ground where extremely tiny. However, when I (by accident) held my grounded rod about halfway the secondary sparks of maybe 5 or 7 cm would jump over. Another thing I noticed is that the bridge got pretty damn hot (too hot to touch at least) in a very short period.

Then the bridge decided to release its magic smoke. Since I don't have any spare parts and am virtually out of money at the moment it'll be a while before I can continue testing. During the test, the coil was drawing slightly less than two amps.

My camera appears to be broken because 99% of the time now it just shuts down when I try to take a picture, so no pictures of the completed setup yet... I do have pictures of the control board and the toroid though: board toroid. All I can say about the primary is that it are twelve turns of 18 awg wire directly over the bottom of the secondary. Raising it to up about 3 cm from the bottom didn't help matters.

Since this is my first SSTC I'm wondering what may have caused this results and of course what I can possibly do to make it work in the future! Any input will be appreciated.

Tonic

Mon Jul 28 2008, 07:44PM

Registered Member #528 Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166

Hey, I remember your site, it had a big role when I was constructing Mazilli flyback driver :)

With 4 turns you had bigger sparks than with 12 turns most probably because few turns has lesser impedance and it means bigger primary current and bigger sparks.

What you meant with helding rod about halfway?

What coupling do you have? (where is bottom of secondary coil, below, inside or above of primary coil?)

EDIT: Another questions:

What are secondary coil dimensions, what wire and how much turns?

Did you try without topload?

About camera - perhaps it's about time to recharge batteries?

Kipmans

Mon Jul 28 2008, 10:38PM

Registered Member #91 Joined: Thu Feb 09 2006, 03:03PM
Location: The Netherlands
Posts: 45

Tonic wrote ...

Hey, I remember your site, it had a big role when I was constructing Mazilli flyback driver :)

That's nice to hear.

wrote ...

With 4 turns you had bigger sparks than with 12 turns most probably because few turns has lesser impedance and it means bigger primary current and bigger sparks.

Yeah, but realize that with the 4 turn primary I ran te coil on only 35 V as opposed to 230 V with 12 turns.

wrote ...

What you meant with helding rod about halfway?

Well, just that.

When I held my grounded stick close to the middle of the secondary, sparks would jump over.

wrote ...

What coupling do you have? (where is bottom of secondary coil, below, inside or above of primary coil?)

EDIT: Another questions:

What are secondary coil dimensions, what wire and how much turns?

Right now, I have the primary in the lowest position possible, which with this amount of windings means the lower half of the primary is below the windings of the secondary, and the upper half of the primary is wound over the secondary. Dimensions of my secondary coil are about the same as Steve Ward's mini SSTC: 11 cm diameter, 25 cm winding height (about 950 - 1000 turns with 0,25 mm (30 awg) wire) and about 28 cm total height.

wrote ...

Did you try without topload?

Not yet, as my MOSFETs exploded

Someone on another forum I visit suggested that the problems might be caused by the feedback circuit picking up not the resonant frequency, but a harmonic instead. This would explain the highest voltage output being in the middle of the secondary and the FETs getting too hot (as they're switching on a much higher frequency than intended). After some searching on SSTCs oscillating on harmonics I found this thread which seems to contain a lot of useful tips... Going to try them as soon as I'm able to buy new MOSFETs.

Also I'm definitely going to put a bleeder resistor across the main buffer capacitor (the one connected to the mains), having the two leads touching each other in your hands when the cap's still charged is no fun... Luckily they didn't touch my finger instead!

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