Hi Folks...here's part 2 or my FET coil project.  As always, if you have any advice, send me an e-mail at

cwillis@guilford.edu.  Schematics will be up here shortly since I feel that this is a good safe design that

will survive well.  At the outset, I'll mention that in defiance of the commonly-believed notion that this sort

of coil is very expensive to build and requires great technical knowledge, I built this coil for literally a few

bucks- like maybe $20 total, all for PC boards, caps, and a couple ICs.  (Granted, I did get some free

MOSFET samples from www.ONsemi.com!)  There are two guys to thank for making this such an

inexpensive and relatively painless project:  Richie Burnett (www.richieburnett.co.uk) and Alan Sharp

(www.alansharp.co.uk), both of whom have volumes of helpful ino on their websites.  Thanks to the

members of the Tesla list who have been helpful as well.

Anyway, the above two shots hardly do justice to the sparks, which come off as a thrubbing cluster of

tendrils or a rushing sheet of banjo-effect streamers  8"+ long, depending on how the interrupter

oscillator is set.  The occasional 9" sparks occur as well.  Power draw is under 300 watts (I only know

this because the 2.5 A breaker on my variable transformer is not popping- some other student ganked

the last working AC ammeter from my bench :>(

Here's a shot of the sloppy setup in use.  The

secondary is the same as the one on my first SSTC,

with the same pot-lid topload.  The half-bridge circuit,

built around two NTP15N40s and MUR1560 fast

diodes, is visible in the center of the image.  This

drives the two-turn (!) primary at about 400 kHz.

Also visible are some trash vintage electrolytics to

smooth the rectified line voltage.  A new one is on its

way which will be the size of one of them and have

three times the capacity!  In the very back is the

driver board.  All the PC boards were made with

Sharpie (tm) markers and some ferric chloride etch.

This is a better view of the drive electronics and

the half-bridge, along with that darned took-me-days-

to-find-a-good-ferrite gate transformer.  As in Alan Sharp's

design, the FETs are "hot shoed" (I think that is the right

terminology) and bolt to a trace on the bottom of the PC

board.  The screws are all isolated with about 5 turns of

duct tape as they pass through the copper sheet heat

sinks.  Apparently this duct tape can survive a couple

hundred high-frequency volts!  The chip on the left

side of the driver board is a UC3825A made by TI.  No

dead time is used, neither are the error amplifier or ramp

comparing functions.  This is just a straight-up 50% duty

cycle, double ended oscillator.  It drives a UC3708

double FET driver, which is also enabled by the high

output of a 555 timer (both these chips are at right).  A

lot of dead time eventually gets into the signal after

encountering the reactances of the ferrite transformer

and other stuff, ensuring no shoot-through.  The FETs

stay lukewarm during a typical run.

• Layout is somewhat important, but a ground plane under the whole setup really did the most to keep interference in the control board down.  Interestingly enough, the 555 timer was the most susceptible part to interference.
• Sharpie markers really make a lousy PC board unless you put on several coats where you don't want the etchant.
• Interrupter ("staccato") is a necessity.  Otherwise, FETs will blow from overheat!
• Insulating the tightly coupled primary windings is a major concern- and if the secondary sparks through, it may mean goodbye to the FETs.
• Watch out for RF burns near these coils!  They are as bad as tube coils in this respect.
• SSTCs demand careful fine tuning to get out the best sparks.
• The common-mode EMI ferrite bead chokes for cables make a good core for the gate transformer.  I used about 15 turns on my trifiliar winding around a ~0.6" OD / 0.35" ID bead.