This is my earliest use of electronics to drive a solid state Tesla coil using  SIDAC’s in place of a spark gap. It is a proof of principle coil rather than a big performer.  “Continue reading” for more details and photos. 

SIDAC’s are symmetrical breakdown devices. As set up here it is driven by a MOT transformer with the associated 11 kV diode and 0.95 uF 2 KVAC cap wired as a voltage doubler to provide 5.6 KV or so. This is dumped by a SIDAC or rather 22 in series each shunted by 1 megOhm. Each SIDAC is rated to breakdown at 240 V and has a 1 A RMS or 20 A pulse capacity. The string of 22 SIDAC’s will nominally breakdown at 5280 V. SIDAC’s act similarly to a sparkgap.

Data is here. The SIDAC’s cost US$1.33 each in 10 quantity at Digikey.
Also shown running ignition coils (with much better performance) here. This might have application for small coils running low voltages where spark gaps become problematic, but without the complexity of an SSTC.

Pic shows 2 inch sparks on my 4 inch coil (5 kV in 60 kV out). I later used 47 SIDAC’s each shunted with 1 Mohm. I could get small streamers but not much more regular spark length. However, if I used a spark gap in series with the SIDAC’s the spark length increase was marked and over 6 inches. I interpret this to indicate that quenching is not really happening with the SIDAC’s alone possibly due to the speed of the devices. I had hoped the SIDAC’s recovery when the current drops would do this but apparently not well or fast enough. Di/dt is 120 A/uS.
Unfortunately they don’t produce these SIDACs any more.

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Photo date: Jan 8, 2005