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A feasible goal for a Miniature SSTC?

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Dr. Shark

Thu Feb 09 2006, 07:31PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I have never seen a really small TC (small here meaning something like a 1" by 4" secondary), that had streamers approaching or exceeding the lengh of the secondary. I suppose a trivial reason for this is that with a higher winding resistance and lower inductance, the smaller coil is farther away from being an "ideal" transformer. But still, the maximum streamer length should be limited by flashovers to the primary or the like, if one had a "perfect" driving circuit.
I suppose that the main obstacle to this is the primary voltage, since most small coils seem to be working of 50VDC or less. Indeed it seems that small SGTCs perform much better than their SS counterparts, even though with big coils, SS seems to win.

So what do you experts out there think? Is it possible to to build a driver that gets 5" sparks from a coil of this size? If this sounds doable I'd really like to try building something like that, because for me the reason to get into tesla coiling is that I moved into a small flat so I cannot play with the big stuff.

Reaching

Thu Feb 09 2006, 07:48PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

a few days ago i build a small 2" x 6" and got around 14" upt to 16" streamers ,so i think, drsstc is the way to go. a simple fullbridge of ultrafast igbts and a feedback driver and voila.

Marko

Thu Feb 09 2006, 07:48PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

My small coil puts 5cm streamer not being in resonance and diameter of secondary is just 1,5cm, length is around 15 cm.
It was SGTC, powered by ignition coil and with bottle caps.

Its sitting now uselessly %-6

Dr. Shark

Thu Feb 09 2006, 08:14PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Reaching, that is indeed impressive. The problem I see with IGBTs (compare with the IGBT thread in the electronics board) is that they can only go so fast - and with a Vitamin C bottle coil like I was proposing you cannot get the resonant frequency much below 2MHz. So unless your IGBTs are ultra-ultra-fast, or MOSFETs with a low Rds(on) could be used for a DRSSTC, I don't see how this would scale to miniature coils. It seems that the more efficient the driving technique (OLTC, DRSSTC...), the more it depends on the resonant frequency being reasonably low.

Extreme Electronics

Thu Feb 09 2006, 08:26PM

Registered Member #74 Joined: Thu Feb 09 2006, 09:17AM
Location: Nottingham UK
Posts: 99

A 5" coil giving 5" streamers sould be possible, but probably not a DRSSTC.

There are a few issues here as the coil size decreases the frequency increases, this means that you will have to get faster transistors, IGBT's are too slow to run a coil above around 100Khz, so you have to go back to good old mosfets.

With mosfets you are severly limited in the max current they can handle, even using their pulsed spec. The other problem is that using a DRSTC at high frequency the primary cap becomes so small you cant get much of a bang building in your primary.

but

If you use decently sized mosfets (irf460) and 400 or so volts you can pump enough power at 500Khz into a 5" coil to get much more than 5" streamers, the problem is that the drive electronics becomes bigger than the coil its self..

Take a look at Roffesoft Experements with an SSTC

Smaller than 5" and you get all of the above issues, but much worse.. plus the coil size:drive size ratio becomes even worse..

take a look at
Millietess &Nano tess for fun when you get really small.

Derek

Avalanche

Thu Feb 09 2006, 08:37PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Have a look at my 'Maplin Special' coil on my website (see link in sig). This is a 2.5" diameter secondary, with a winding length of 3.5" - fairly small, but a nice performer.

I managed a consistant 4.5" output, and then on the mains it managed 7-8" from that small secondary. The coupling between primary/secondary was very tight, but well insulated.

Marko

Thu Feb 09 2006, 08:38PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

The problem I see with IGBTs (compare with the IGBT thread in the electronics board) is that they can only go so fast - and with a Vitamin C bottle coil like I was proposing you cannot get the resonant frequency much below 2MHz.

You need to wind it with very thin wire, actually keep proportions with bigger coils (smaller coil -> thinner wire) and with lots of turns freq. can be relatively low (few hundred khz)

And I think tha 2 inch (more than 5 cm) is not more a tiny coil, c-vitamin and thinner ( < 1 inch) would be more in category, maybe?

But 16" streamers from that are really cool..

Part Scavenger

Thu Feb 09 2006, 09:03PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

The reason you don't see alot of really small SSTCs is because of resonant frequency. The smaller the coil, the harder it is to bring the resonant frequency down enough to have acceptable switching losses. To to this, you have to use very small wire, which makes for a more fragile secondary, and so you can't pump as much power into it. They are getting smaller though...

Marko

Thu Feb 09 2006, 09:11PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yes I tortured myself winding that mini coil, wire was thinner than hair and I torn it once (soldered and continued :D) and preformance was not bad at all...

HV Enthusiast

Thu Feb 09 2006, 09:37PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

There was a huge thread of a micro tesla coil contest about a year ago. There was a large variety of very small tesla coils (1" secondaries and perhaps even smaller, but i can't remember for sure) created by several of the members here. If you have time, search the archives, as there is a wealth of information there.

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