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

["Gary Peterson" <g.peterson@xxxxxxxxxxxx>]

Thanks Bart.

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

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?

G.P. 

-----Original Message-----

Hi Gary,

Adding a spark gap would be superfluous and counterproductive to your 
objective of creating "undamped or partially damped oscillations in the 
helical resonator."

An SISG can be adjusted so that it transfers the initial bang energy 
within the primary to the secondary and then traps it there, permitting 
the resonator to ring down slowly (assuming no breakout).  Only a series 
of damped waves is achievable with an SISG, since the tank cap must 
still be recharged between bangs.

Referring to the the schematic of Terry's original 900 volt SISG stage 
(at http://www.teslaboys.com/SISG/SISGSchematic.pdf) it can be seen that 
reducing the value of the discharge resistor (R4) allows "tuning" the ON 
time of the IGBT so that it is slightly longer than the one-way energy 
transfer time between primary and secondary (the "first notch").  At this 
point, all of the available system energy resides within the secondary 
resonator and toroid. Switching off at this point reflects an ideal 
"quench".  Note that, with moderate coupling and no spark loading to help 
remove secondary energy, "first notch" quenching is unachievable with 
ANY kind of spark gap. The problem is that the previously conducting gap 
cannot recover sufficient dielectric strength quickly enough to prevent 
reignition and subsequent energy flow from secondary to primary.

In Tesla's day, spark gaps were the ONLY way to "disruptively" 
(suddenly) switch charged high voltage capacitors into inductive loads 
and then open the discharge loop to permit recharging for the next 
"bang."  Today, there are combinations of solid state devices that can, 
for TC system purposes, mimic or outperform spark gaps. Solid state 
switches offer lower On-state losses and improved quenching. In 
particular, solid state Tesla coil designs that store initial bang 
energy within a tank cap and suddenly switch it into the primary 
inductor (i.e., OLTC's or SISG's) provide superior output, and 
quenching, than spark gap switched systems operating at similar input 
power. Adding a spark gap (of ANY type) to a ideally timed SISG circuit 
will simply add losses without improving quenching.

Whether the above solid state switched coils are "disruptive" or not is 
more of a semantics issue than a real one.  Historically, disruptive 
systems were spark gap-switched systems.  Indeed, "disruptive" may be an 
antiquated term that we may want to consider leaving to previous 
centuries...

Bert



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