Micro TC

Smaller and smaller, and as you might see, it took just 3 hours to build... It runs from a lead gel 12V 7Ah battery and is flyback (incl. internal rectifier) powered. It gives continuous 3-4cm streamers with an banjo effect that is visible by the plain eye (sadly a digi camera is too slow to capture it). The TC should run at 2 MHz  +- some 100kHz.

For size comparison, there's a student card leaned on the secondary. The topload is a ping pong ball. The spark gap consists of two halves of copper pipe (center of pic). The flyback can be seen on the left. The capacitor string is in the front.

This TC is rather undangerous as it runs off a 12V battery and has no ground connections (the PCB serves as a large capacitance, which the secondary sees as low impedance), so no prob to touch the streamers. Of course, strikes taken to a small metal object, not the skin directly - otherwise it is *ouch* and skin punch-through.

It might look rather cool if worn on top of the head... ;o) Although, a high current primary coil with all that E-M alternating field directly above the brains is maybe not all too healthy in the long run...

specs  
secondary coil side: * 2.8cm outer diameter
* coil former consists of a rolled-up transparency sheet
* winding length 10.3cm, and 0.5cm free space on top and bottom
* coated with polyurethane varnish
* wire is typical 12V 70mA mains transformer secondary wire, looks like 9 turns per millimetre (0.028" wire dia).
* bottom 0.5cm if windings is wound with slightly thicker wire, but this is not really necessary
* no idea how many turns there actually are, probably around 950
* black end cover caps ("corona inhibitor") on top and bottom made from a single camera film container (those that come with the film rolls) split in two. The bottom end is closed but has a metal screw through it to connect the secondary to PCB ground. Lower "cover cap" prevents primary=>secondary flashover.
* tuning is done with the topload - a table tennis ball covered with aluminium foil, thickness of wrapping is varied to get proper tuning

primary side: * primary is 5 turns of 1mm dia copper wire, inner&outer dia 3.5cm
* "bunch-wound": five turns + polyurethane varnish + wrapped with insulating tape

* HV capacitor is ~9.1 nanoFarad 7.5kVpeak, a single string of capacitors (Wima FKP1 type, this is important, as other caps will blow very fast). Caps connected in series. Because of what I had lying around my string consists of three 0.033uF 1250VDC and three 0.150uF 1250VDC caps in series. Possible other choice: for example eight or nine 0.068uF 1250VDC caps in series.

* spark gap: two 2cm long copper pipe sawed in half, halves screwed to PCB board, one half can be slid along the PCB to adjust gap space (never go over 2mm gap space! this will kill the capacitors...). For fast streamers & banjo effect, a gap space of about 1mm (a bit less) works nice.

* grounding: star pattern etched on PCB below the secondary&primary (sec ground plugs to center of star), otherwise PCB copper area held at maximum but with appropriate (>2cm) space/clearance to the flyback driver electronics section on the PCB. PCB ground must additionally be connected to a larger surface-area metal object.

* flyback: old flat disc shaped flyback with HV diode included. New flybacks will work also. Flyback is driven by a 555 timer + 2n3055 transistor simple oscillator setup (and not by 2x2n3055 plus extra flyback primary turns self-resonant setup, as that would be a working "solid state" tesla coil already by itself). You can find schematics in the links section.