Dr. Resonance
Resonance Research Corp.
www.resonanceresearch.com
----- Original Message -----
From: "Chris Swinson" <list@xxxxxxxxxxxxxxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Tuesday, November 20, 2007 6:25 PM
Subject: Re: [TCML] Spark gap Resistance
Dr. Resonance,
John has hit on a very important point here. With classic RSG type
coils 2nd thru 4th notch quenching seems to work best. As he suggested
the problem lies in deionization or interruption of the spark gap. This
is problamatic due to the very high temperatures involved in the spark
channel, especially with all the capacitive energy behind it.
High current make harder quench ? This would support john in saying high
current across the gap is bad... though it was why I mentioned about
running a much faster RSG to decrease the dwell time. I am not saying its
a fix but really can't hurt ?
So is the problem the quench, and/or actually passing more current across
the gap ?
I would have thought higher current would conduct better, but also makes
quench a lot harder. Easy example, double the current and double the
quench time, over all gain zero....
It is what brought me onto the idea that if the actual conduction of
current across the gap is not a factor, but the only factor is the higher
current is making the gap harder to quench, then it was the idea to
reduce dwell time.. use 30" dia RSG run 100bps, 1" dia electrodes, pass a
lot of current easy over the large electrodes, will help keep them cool
also... plus the large dia RSG will reduce the dwell time a lot. maybe if
you double the current you have to reduce the dwell time 4 times to make
it work.... just pluggin at ideas.....
This is one reason why experimenters are now turning to the dual
resonant solid state coil drivers to replace the less efficient spark
gap type switches. The IGBT type switches, replacing the open air spark
gaps, are able to turn on and off at higher rates providing more rapid
dI/dt rates. Due to the fact there is no open air spark, excessive heat
does not provide the shut-off conductive problems that a spark gap
endures.
But is the spark gap "really" a loss.... IGBT can turn off whenever you
want, though *IF* you could turn off the spark gap just as fast, then
what would be the results then ?
The switching off part is much more efficient, less heat and no light
energy is wasted, and the IGBT type switching allows the experimenter to
keep 30% more energy in the secondary coil to ring up without trying to
dump the energy back into the primary system. First notch quenching is
achieved with efficiency. Spark lengths are exceeding the classic 1 ft
per kVA range typical with RSG systems.
Sounds fair enough... and I agree... But how much is lost in the spark
gap heat& light wise ? which is the greater loss, heat & light or turn
off conduction problems ? (and I just know you are going to say
both...)
For this same reason, classic type coils like a coeff. of coupling in
the range of 0.18 to 0.14 while solid state coils, with much more rapid
switch-off characteristics, operate in the coeff. of coupling range of
0.16 to 0.20. At present, research suggests that 0.18 is usually a very
efficient design goal for max power transfer. Smaller coil systems can
tolerate up to 0.20 while larger coils seem to operate best around 0.18.
Tight coupling is where you get this "double hump problem" I saw some
data on this ( TCBA news maybe) it looksed to me like a lower coupling
would be better, though the frequency range seemed to be a lot narrow.. I
may have got it backwards... though the chart had like 50khz to 200khz
for a double hump 0.4K and 80khz to 120khz for 0.2K. I am not sure about
the frequency aspect of the chart but it looked to me that lower coupling
would be better.
As I was pondering also.. lower coupling would be less inductance on the
primary and will greatly increase the current... which has to be
good...... though when you factor in park gap losses, it becomes a bit of
a problem!
To me, use a low coupling to get best primary current, and solve the
longer cycle/conduction time by just using a higher frequency. for
example.
0.2K = 100khz = 10 cycles = 100uS
or
0.1K = 200khz = 10 cycles = 100uS
I know there are other factors to take into account.. though just trying
to improve on the spark gap problems... though in itself could generate
more problems. Spark gap current and conduction time was what I was
trying to indicate...
Bart Andersons excellent computer design program, JAVATC, allows one to
experiment with different sec heights and primary inner and outer radii
to obtain the desired coeff. of coupling range. It's easy to use and
very accurate. It works great with both classic RSG coil designs and
dual resonant solid state coil designs. Just note the different design
goals for
Yes, I use JavaTC all the time, I think I broke the RSG bit, email sent
to bart about it (pokes bart)
the coeff. of coupling as you adjust your inner and outer primary radii
and also the height of the sec lower winding above the primary. I
usually use 0.25 inch elevation of the sec above the horizontal plane of
the primary for high coupling. Pri-sec spacing usually around 0.5 inch.
I had a thought about that also... say if you had a 24" dia secondary and
used a 12" dia primary and raised the coil maybe an inch above the
primary.. how would that work out over a normal primary coil which is a
few inch's wider than the secodnary (normal type setup)... ?
Use as large of a dia secondary coil as practical, typically around 10
to 20 inch dia range, to obtain max. sec potential. This also helps
keep the res freq low while helps the IGBTs switch more efficiently.
They are rated at 75 kHz, but operate more efficiently below 50 kHz.
I would like to use a large dia secondary to get high inductance (which
is good, right?) but also use a large dia wire to get resistance down. I
did plot some ponders and charts on this, to my workings resistance goes
up almost exactly the same as inductance so Q factor never really
alters... I think the best coil would be a wide dia secodnary, and using
a thick cable to get resistance down to less than 1 ohm. This is my dream
coil, though it also clashes with number of turns so I am not sure if it
would even be physically possible to build one... I think it must be as
Tesla seems to use less than 1 ohm coils.
I read ( in CSN I think) that resistance was more of a factor than
inductance for getting long sparks. I do not think anyone even considers
the resisitance of the coils and just goes for high inductance all the
time ?
I also have a second ponder ( getting off topic a little ) that Tesla
said the large toroid is only there to counteract the large inductance..
While I gave up a few weeks ago trying to work that one out, It sounds to
me like Tesla says use a small dia toroid, and get the coil resistance
down to 1 ohm or below. Maybe use a larger dia tube to get the inductance
as high as possible... Though I think resistance increases pretty much at
the same rate as inductnace does... which makes it pointless and just
forces the use of a large toroid for no reason.....
Experimenters who have seen a solid state coil in action immediately
notice the very loud "mean sound" from the spark as the much higher
currents tears the air molecules apart in the spark discharge. The
spark is also white hot and not blue or purple like the lower secondary
coil currents in a classic RSG design. Keeping the energy in the
secondary coil is the name of the game for maximum performance.
I built a SSTC, though this was just a basic H-bridge. I have a video of
it, only got 6" spark output and was very poor. I tried many coils and I
found the more inductance there was the worse it was. It could have been
the inductance on the primary acting up though I was pulling about 10amps
at the time... also it was CW and still gave purple sparks. I can only
account for this for the secodnary resistance, I think it was running
60khz and DCR was 120 ohms 0.2mm wire. I think I ran another coil , 0.4mm
wire, probably 60ohms 120khz (guessing) worked better... I do remember a
small 1" spark in the early days of testing which was white hot with a
hiss.. but as things progressed to longer sparks they went purple.
some info here if you want to take a look-see
http://www.future-technologies.co.uk/IMPULSE/hotstreamer/sstc/sstc.htm
Many happy Holiday sparks!
Holiday sure, sparks maybe not ;-)
Chris
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