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Re: [TCML] Spark gaps, Solid state switches and diodes
Hi Stephen,
I am happy to report I have built an SISG at the voltages your thinking
about. I'm currently running 6 Mark Dunn boards in series. It is some
cash to get all the boards filled to run this high of a voltage. Terry's
beginning with this SISG was with a MOT supply. A few of us have
series'd up enough Sidac's to cause breakdown at much higher voltages.
And really, it's the Sidac's in series that raise that breakdown point.
With Mark Dunn's boards, 3600V per board (4 IGBT's per board, 3 Sidac's
per IGBT). So three 300V Sidac's x 4 = 900 x 4 = 3600V breakdown per
board, times 6 boards = 21.6kV by calculation and basic specs. In
reality, they conduct a bit less than this. I have driven it so far with
14.4kV pig power, and it has no problem firing the IGBT's. I have found
however that a breakout is necessary with my coil and top load size.
As far as reliability, the SISG has proven to me that it is extremely
reliable (far exceeding my expectations). I didn't do a lot of
calculations with the current, but concentrated simply on building the
DC bridge, the coil, etc.. First firing was a winner! Here is my very
simple circuit and how I ended up plugging the SISG into my circuit.
This may seem very familiar.
http://www.classictesla.com/sisg/sisg.gif
Here's a pic of the coil as a whole (using a ribbon primary on this one,
so there was some time involved ensuring primary inductance).
http://www.classictesla.com/sisg/sisg-10-12-08%20005.jpg
Here's a pic of the 6 Mark Dunn boards used.
http://www.classictesla.com/sisg/sisg-10-12-08%20001.jpg
At the bottom of that pic is my DC bridge and a couple high watt
resistors on a small white nylon board. Ignore the rotary gap/motor.
That was installed prior to the SISG getting up and running and really
only for a 1 time event (it's no longer part of the coil). The IGBT's
I'm using have smaller dies than what Terry originally called out, but
I've had "zero" failures with them. They are Fairchild HGTG18N120BND.
Their running $5.20ea at Mouser right now. This was significantly less
than the original IGBT which is why I gave them a try. So far, so good.
For 6 board at 4 per board, 24 IGBT's would be required. Buy 25 and the
price breaks to $4.15ea. So 25 x 4.15 = $104 in just the IGBT's (minus
all the other components).
I can't say for sure if this is due to running 60nF on this coil, but it
is likely. Most SISG lower voltage coils will run far larger caps. I'm
running very STR. So, keep that in mind. Near the bottom of this link, I
have some spark photo's.
http://www.classictesla.com/photos/sisg/sisg.html
The SISG is a great design, but cost for these high voltages is a
burden. Better to build the SISG for lower voltage TC's simply to keep
costs down. But, with the SISG, you can hook up a couple MOT's and
really have some power to using only a couple boards. This is probably
the right way to go with SISG's from a cost stand point.
For my SISG, I plan on removing the pig power and insert my 12/200 NST
(about 4 boards). I'll use the remains 2 boards for a MOT weird coil.
Take care,
Bart
Stephen Hiscock wrote:
Hey guys, my name is Stephen, I've just joined this email group.
I work as an electrical Engineer and I've been playing with Tesla
Coils and High voltage for about 15 years or so, and built a fairly
powerful Tesla coil about 10 years that made over 1 metre (3-4 feet )
sparks.
It was just Neon Sign powered (I had 6 in parallel) and had a
conventional spark gap, but it never performed as well as I would
like. I fired it up the other night after dusting it off (it had been
in my garage for a bout 5 years), and it performed dismally only
getting about 1-2 foot sparks. I played with the spark gaps etc and
got it performing a bit better, but nothing like it used to go.
I'm thinking that I'm wasting a lot of energy in my spark gap. in fact
tow nails hammer into wood seemed to perform the best (quenching
issues?) So I went online and after a while came across Terry Fritz's
plan for a solid state spark gap, but his plan only seemed to be for a
relatively low powered MOT tesla coil. (a few hundred watts)
I'm wondering if anyone who has had first hand experience with these
solid state spark gaps (SISG) could help me answer a few questions
* My transformers put out 15,000 Volts at 30mA each x 5 (or 6 if the
fuse doesn't blow ;-) ) so how many will SISG's will I need
* how reliable will his general design be with my coil. (obviously I
will need more in series - but will I need to parallel them as well??
* Is there an easy way / simply formula to calculate how much current
will be flowing peak? (800A or 8000A) - depends on resistance or
capacitance or inductance? - I currently don't have an oscilloscope at
home I can use.
* How much power gets wasted in the reverse diodes - normally the
voltage drop across a diode is 0.7 volts - so does that mean if I'm
getting 1000A peak average in the primary I will be wasting about 700
watts as heat across the devices and possibly more depending on how
many devices I have? or am I missing something? - this would seem
worse than the conventional gap - but everyone seems to rave about the
performance of SISG and DRSSTC coils.
perhaps its about peak energy levels (not average) and then 700 peak
watts compared to 15MW peak energy is not much?
I think I just answered my own question. ;-)
Sorry for so many questions, but you guys seem to like answering
questions - and I've been enjoying the discussion - so hopefully I've
added something more to chat about.
so to be simple - how easy will it be to scale his design (SISG) to my
coil or will most components need to be upgraded.
Stephen
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