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A small DRSSTC with a Coke can sized secondary and an Fres of only ~260kHz !

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Ash Small

Thu Aug 16 2012, 01:30PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

plazmatron wrote ...

Slightly off-topic, Les, but do think these would outgas in a vacuum chamber?

I was planning on using plates (aka vacuum capacitors) to eliminate outgassing but this 'could' be an alternative.

EDIT: The idea of adding capacitance to the secondary isn't entirely new. HF TIG welders (at least the one I built) utililize capacitors in series with the secondary and arc as part of the HF choke that prevents the HV HF from getting back into the welding current transformer/rectifier circuit.

plazmatron

Thu Aug 16 2012, 02:56PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Ash Small wrote ...

Slightly off-topic, Les, but do think these would outgas in a vacuum chamber?

I was planning on using plates (aka vacuum capacitors) to eliminate outgassing but this 'could' be an alternative.

It's funny you should mention that, since it had occurred to me that these might have some potential use within a high vacuum system. Everything out gasses to some degree, but having managed to break one of these, they appear, for all intents and purposes to be quite vitreous, so my guess is that the out gassing would be tiny, and certainly not a problem on a continuously pumped system.

I am a real fan of unsealed vacuum tubes, since you can get away with having things like polyethylene windows and so forth, which would normally be considered to leak like a sieve!

@Uspring

There is so so much I don't know about the finer design points of Tesla coiling! I'm surprised this thing worked at all, but I am very glad it did!

plazmatron

Fri Aug 17 2012, 09:04PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

An update:

I stripped down this coil today to rebuild it. Since I had it in bits, I thought I would photograph the secondary and its MMC for those who are interested.

Since this coil has worked out so well, I purchased a nice new Eddystone box off ebay measuring 125x125x57 mm, and shoehorned in, all the electronics from out of the original plastic case.

In the new shielded case, everything is far more stable, and since the whole case is metal, I cranked it up a bit to see what it could do.

This thing belted out 9 inches of spark! The best photo I got of such a strike is this:

I think the toroid could do with raising just a little, to help prevent it arcing to the box as often.
I did have a strike ring on there, but all the coil was doing was striking it all the time, so I will have to figure something out. Raising the toroid will probably help.

All in all, I'm totally stoked by this coil! If I knew it would work out so well, I would have put it in the projects section.

Les

Dr. Dark Current

Fri Aug 17 2012, 09:30PM

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

This is really intriguing, are the coils sitting directly on a metal surface? And it works so well?

I would say the eddy currents in the conductive case would eat most of the input power... Does the surface below the coils get hot?

plazmatron

Fri Aug 17 2012, 09:55PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Dr. Dark Current wrote ...

This is really intriguing, are the coils sitting directly on a metal surface? And it works so well?

I would say the eddy currents in the conductive case would eat most of the input power... Does the surface below the coils get hot?

Both primary and secondary coils sit about 5mm higher than the surface of the box, and all off the box is made of cast Aluminium.

The whole thing, including the electronics stays cool.

Yes, I have thought about eddy currents and metal cases before. It even occurred that an open ended metal case might "look like" a shorted turn to the coil too.
My last coil was mounted on an old PC power supply case, and whilst in that instance the top of the box was plastic, the sides and bottom were all mild steel, so I figured it might not make that much difference if the lid is metal either (although I have never tried a steel lid, which I imagine would heat up).

I don't know how an all metal box would fare with larger coils, but nobody I know has managed to melt a toroid yet, which is essentially the same deal.

As far as the vicinity of the Aluminium lid to the primary coil goes, the lid is about 4mm thick, and surely would act as a EM reflector (/ground plane), rather than heating.

Killa-X

Mon Aug 20 2012, 03:42AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Bens topload inspired me.

36Awg 2x8 inch coil. Going to try this secondary mmc thing myself

Ben Solon

Tue Aug 21 2012, 04:12AM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

You guys are killing me... One more to add: 34mmX130mm with 40aWg wire. An entire solenoid. And I only broke it once, then managed to solder the strands of .07mm wire Bach together with a low profile. Why did I do it? I don't know

it's fres is just under 800khz, so with a decent wireframe topload and internal caps, I could be down to 200khz...

plazmatron

Tue Aug 21 2012, 04:19PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

@ben123324

Go for it! I don't think you will be disappointed!

Something I alluded to a while back has been solved, which was:

plazmatron wrote ...

There is also an interesting quirk I noted. When the coil is powered, it takes several bursts from the interrupter before there is any output at all, if you start with a very short pulse width. However, if you start at 100Âµs or so, the thing bursts into life right away, however with each subsequent burst, the arcs get bigger and bigger! So then you have to back off the pulse width a little to keep the thing under control.

Turns out it was the interrupter all along, so I feel pretty stupid for not having considered it!

It was designed around an ATMega168, and works pretty well, however it turned out it needed some more decoupling caps at various points on the supply rails, and now it behaves properly around the coil.
It also now has a debug light, so you can tell at a glance whether it is working properly!

I'm glad I got it though, since I can hook it up to the MIDI controller now, and blast out the tunes!

Ben Solon

Tue Aug 21 2012, 06:42PM

Registered Member #3900 Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600

we all to stuff like that once in a while, so no worries. I had a theory going that there was enough capacitance, and the burst was so low duty that it took a while for the cap to get up to full amplitude in resonance! good to see I was wrong!

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