Unconventional 3kW SSTC

[davekni]

Decided to pull out my first Tesla coil, an unconventional 3kW SSTC.  Built initially in 2013, then constructed new electronics in 2014.  Here are the basics:

3kW, 60mA RMS arc current, 50kV RMS with short breakouts, bit lower with long breakouts.
No bulk caps and no interrupter: Amplitude tracks rectified line voltage.
No primary coil.  Ferrite transformers feed 2.8kV RMS to bottom of "secondary".
160mm diameter by 600mm high "secondary" using ~1420 turns of 27AWG wire.
120 ohms DC at 20C, 75mH, 97kHz with top load.
680 x 700mm OD top-load (not quite round).
Ferrite plate layer between "secondary" and top-load.
Two H-Bridges on two 120VAC line cords, eight 16N40E FETs per bridge (16 total).
H-Bridges feed E55 ferrite transformers, 3T:35T, seconderies in series for 2.8kV RMS.
Gate-drive transformer on E55 core, eight outputs, two turns each.

The key advantage of ferrite transformers instead of primary winding is that the H-Bridges are switching at zero-crossings.  The magnetization current is small compared to the output power, unlike conventional SSTCs.

Some breakout points work much better on this coil than on my larger DRSSTC.  I think it's the high RMS arc current that helps.  These include ion-thrusters (spinners), arc coloring with metal salts, and making steel and aluminum wire into sparklers.  Here's a video link, followed by lots of pictures.  Scroll to the very bottom for schematics.

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Driver with dual H-Bridges:




"Secondary" showing ferrites on top:




With top load:




0.35mm diameter steel wire (stretched spring) breakout point:






Aluminum window screen breakout point:






Strontium Chloride and Ammonium Borate (soaked rags) breakout points:






Schematics:

[davekni]

A few more pictures; Didn't want the initial post to be too long.

0.35mm diameter steel wire (stretched spring) breakout point:










Aluminum window screen breakout point:

[Weston]

Interesting design!

I have been considering trying to design a primary-less tesla coil, people have taken measurements of spark impedance and from that point its just a matter of treating it as a matching network.

Is there any additional safety hazard with the base feed designs if the discharge point is touched? It seems that this could possibly supply more current under a fault condition or something.

The ferrites on top of the secondary are interesting, I assume you are using them to try to shield the topload from acting as a shorted turn? Have you tested if it has an impact on coil performance?

[davekni]

Weston,

Your question:
"The ferrites on top of the secondary are interesting, I assume you are using them to try to shield the topload from acting as a shorted turn? Have you tested if it has an impact on coil performance?"

prompted me to actually measure the inductance difference.  It's not as significant as I was guessing.  Without ferrites, the inductance drops 4.3% due to the top load.  With ferrites, inductance drops only 1.7%.  Yes, the idea is to provide a path for the winding's magnetic field below the top load.  When building this coil, the only 6" pipe I could find came in 2' sections, limiting height.  I wanted to fill it all up with winding rather than waste space at the top for a magnetic field path as is normal.  My newer DRSSTC does have space instead of ferrites.

There definitely is increased danger with base-feeding.  The danger is touching the base-feed wire, not from touching the top.  The base-feed has much higher current capability than the top, combined with sufficient voltage to arc, 2.8kV RMS in my case.  The only mitigation I've done is make the over-current latching, until power is switched off.  But the over-current doesn't trip until ~2A RMS at the base.  If it arced to a person and didn't draw quite 2A, that would make a nasty burn.  (I once got too close to 2kV 0.25A 30kHz building my plasma-jet.  That made a tiny but deep burn to my knuckle, and burnt-open the 27AWG wire.)

Touching the top shouldn't be significantly different with base-feed or a normal primary winding, for a given power level.  However, this coil is running higher average power than most SSTC's I see here.  It's also running continuously rather than pulsed, which leads to higher average current and lower average voltage.  For those reasons it may be dangerous for burns touching the top independent of the feed architecture.