Re: [TCML] Spark gaps, Solid state switches and diodes

[Bert Hickman <bert.hickman@xxxxxxxxxx>]

Hi Gary,

Gary Peterson wrote:
> Thanks Bart.

Bert... :^)

> I understand the SISG switch is usually left closed for two or more few
> cycles in order to wring more energy out of the primary capacitor, and also
> had thought it could not be switched off any faster than that due to
> switching-speed constraints imposed by the IGBT itself.  It appears if the
> operating frequency of the Tesla coil is sufficiently low, then the IGBT can
> be switched off at the "first notch."  

Actually, the very best you can do is to quench at the first primary 
notch - see the waveforms on Richie Burnett's excellent site - between 
numbered paragraphs 9 and 10:

http://www.richieburnett.co.uk/operation.html

Note that complete energy transfer takes a number of RF cycles - the 
number of half cycles to complete the transfer is a function of the 
coupling factor between primary and secondary. For certain coupling 
factors ("magic k" values), complete transfers can occur in an integer 
number of RF half cycles. Typical 2-coil SGTC's require from 6 (k=0.18) 
to 9 (k=0.12) RF half cycles for a complete one-way energy transfer.

Allowing the primary switch to remain closed after the first primary 
notch allows the energy previously transfered to the secondary LC 
circuit to transfer back into the primary - not desirable for maximizing 
coil output. As long as the primary switch remains ON, P --> S and S --> 
P energy transfer cycles will repeat. Each will be at a lower amplitude 
than the previous cycle due to system losses. Maximum secondary voltage 
is achieved only at the first complete ringup - i.e., at the first 
primary notch. This can be seen in the first set of waveforms on 
Richie's page.

http://www.richieburnett.co.uk/operatn2.html#quenching

> Attempting to switch it off any
> sooner than that would place a strain on the device itself, correct?

Turning turn off the switch early (such as an earlier RF zero current 
crossing during secondary ringup) is virtually impossible when using a 
spark gap. And, if your IGBT's are not switched off at or very near a 
primary zero current crossing, you may overvolt (and destroy) your 
IGBT's. However, even if you were successful in interrupting the primary 
circuit prematurely, you'll retain a portion of the initial bang energy 
in the tank cap rather than completely transferring it into the 
secondary. This lowers the secondary output voltage while also placing 
your IGBT's at risk - a very poor trade-off.

> Here is another question.  I understand that Mark Dunn designed a printed
> circuit board a while back that embodies Terry's SISG circuit design (see
> http://www.teslaboys.com/SISG/SISG4PCB.pdf ).  Was this PCB ever mass
> produced, and if so is it still available?

The SISG circuitry/stage is simple enough that it can be easily built on 
a blank perf board. However, a PCB is a certainly a cleaner solution. 
I'll let Mark respond re: availability of his boards.

> G.P. 

Bert
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