Taming the SISG

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Finn Hammer <f-h@xxxx>

All

The SISG circuit caught my eyes immediately, and I liked it right off.
The simplicity of the combined trigger/crowbar action appealed to me.

One thing put me off, though: I was unable to 
understand how the voltage doubler worked (later, 
when Stephen C. had the nerve to admit he didn`t 
either, Terry broke down and admitted he didn`t understand it himself).

Then of course there was the problem that it 
worked like a static gap, which is not good if 
the intent is to use DC resonant charging.
I live in Europe, where 3-phase is the norm, so 
fairly smooth DC is as plentifull as 6 pulse rectifiers.

Therefore, some sort of trigger circuitry had to 
be applied, and since I appeared to be the one 
that wanted it the most, I became the first to 
pick up the glove and try to design it.

What I came up with was this:

http://home5.inet.tele.dk/f-hammer/sttriggedsisg.jpeg

This was heavily inspired by that 40somethingKV 
SCR stack that I`m sure you all have seen 
Alltronichs try to sell for years over at EBay.
http://search.ebay.com/scr-stack_W0QQampsspagenameZhQ3ahQ3aadvsearchQ3aUSQQcatrefZC5QQfbdZ1QQfclZ3QQflocZ1QQfromZR6QQfrppZ50QQfsooZ1QQfsopZ1QQfssZ0QQftrtZ1QQftrvZ1QQnojsprZyQQpfidZ0QQsaaffZafdefaultQQsacatZQ2d1QQsacqyopZgeQQsacurZ0QQsadisZ200QQsargnZQ2d1QQsaslcZ0QQsaslopZ1QQsofocusZbs
Of course I was worried that a SCR would not be 
able to perform the task, but since the SIDAC´s 
shouldn`t either (too high dI/dT) I gave it a go.

The main reason for trying the gate 
transformer/SCR solution, however, was this:

Any drive circuitry would have to pass it`s 
current down the gate drain resistor, and I 
didn`t want to raise the gate voltage that much.

So, transformer drive it was.

I had selected the IR 25TTS12 1200V/16A SCR for 
the job, and noted that it could be triggered by 
46mA @ 2Volts on the gate, so I figured that if a 
gate transformer cold be made to deliver 2 volts 
into a 23ohm rsistor, I`d be on the right track.

I bought a couple of cores, wound as many turns 
on them as they would take in one layer, and 
terminated the winding in 23ohms. Then I  passed 
a wire trough them, and started experimentng with 
discharging capacitors into the wire loop.

I soon found out, that I also needed some zener 
diodes to keep the voltage low enough, and that 
the capacitance and voltage across it could be 
much smaller than I had imagined.
I ended up with 22µF charged to 16 volts. This is 
then shorted into the trigger wire by a IR460 fet for 10µs.
A more appropriate solution would be to build a 
pulse forming network, maby I`l do that in the next iteration.

I found out that the zeners were not needed when 
the SCR was in connected to the transformer. by 
then it was impossible to whack the gate higher than 1 volt.

I already had a fiber optic transmitter from a 
DRSSTC project, so the receiver on the trigger 
board was used to pull the input pin on a 555 low 
trough a RC pulser, to create a 10µS pulse, which 
in turn drives a ucc37722 driver chip, which switches the fet.

This all worked all right in a 2 stage test 
board, so I built a 10 stage board and applied some power.

Since it still functions, I guess it is safe to 
declare the design sound and fit for replication, 
which I hereby invite you all to do.

there are pictures of the SISG board here:
http://home5.inet.tele.dk/f-hammer/TOTSISG.jpg

and a bigger one of the SISG itself here:
http://home5.inet.tele.dk/f-hammer/10KVSISG.jpg

A video is here:
http://drsstc.com/~piranha/PIRANHA/PIRANHA-2/FINN-SISG-MOV00628.MPG
or here:
http://www.sharebigfile.com/file/19656/MOV00628.mpg.html

Cheers, Finn Hammer