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Re: How low can a spark gap go?

Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Greg,
The OLTC fired at low voltage and resonated off the AC line through a reactor to boost the voltage. It had a single turn primary and giant primary caps to still get power. Steve Conner had one of the most successful ones: 

http://www.scopeboy.com/tesla/oltcfaq10.html

http://www.scopeboy.com/tesla/

http://www.scopeboy.com/tesla/t4proposal.html

Derek too:

http://www.roffesoft.com/tesla/oltc/oltc.html

The SISG is simpler than the DRSSTC if you are interested in that:

http://drsstc.com/~sisg/

http://drsstc.com/~piranha/PIRANHA/PIRANHA.pdf
There is a fundamental problem with low voltage conventional spark gap coils. It is the primary "Q" or the ability of the primary circuit to "ring". 

Qprimary = SQRT(Lp / Cp) / Rp
As we lower the voltage, we need very large primary capacitance and thus lower primary inductance. Rp for a spark gap coil is around 3 ohms. 

For example, let's try a 300Vfire   5 joule/bang  50kHz coil:

E = 1/2 x C x V^2      5 = 1/2 x C x 300^2     C == 111uF

50,000 = Fo = 1 / (2 x pi x SQRT( L x C )     L == 91.2nF
So C is giant and L is a small single turn primary coil. Primary peak current is 10466 amps!! But those numbers are still "possible"... 

The killer is the "Q".

Qp = SQRT(Lp / Cp) / Rp  ==  0.00955
The coil has NO primary oscillation!!!! Even a super low SISG section at Rp = 0.02 gives a Q of only 1.43!!
In general, the Q has to be at least 30... So the primary oscillations for many low voltage coils are super damped out and the coil will not work.
The PIRANHA SISG based coil in the link above has a Q of about 30 with Rp=0.25 (electronic gap). If Rp = 3 (standard spark gap) then the Q is only 2.5 and the coil would not work well at all. It might make a "little" spark...
So the OLTC drove all this down to about as far as it can go with solid state gaps. The high loss of conventional gaps kills the primary Q. Standard MOT coils running at say 3000V are about as low as a standard gap coil can go in voltage.
As voltages are driven lower, primary Q is becoming a very important factor to consider!!
DRSSTCs are "different" in that they resonate the primary voltage back up to high voltage again so primary Q is not such a great concern. 

Cheers,

        Terry


At 04:19 AM 11/1/2006, you wrote:

Thanks for all the feedback on spark gap behavior. The
level of expertise on this list regarding electricity,
electronics, physics, mechanics, engineering, computer
modeling, problem-solving, etc., etc. is simply
outstanding.

I've got a copy of Dan McCauly's DRSSTC book, and I've
been studying his IGBT-based coils. His designs are
elegant. However, I am dismayed by the high parts
count and the overall complexity. I don't think I'm
yet ready to tackle such a project. I appreciate
McCauly's pioneering work, but I'm more settler than
pioneer. In a few more years, big IGBT SSTCs may well
be the norm for the hobby.  Until then, I'm going to
continue using stone knives & bear skins.

Which brings me at last to the point of my post. Dan's
DRSST is using really modest tank cap charging
voltage! Only 340VDC (doubled 120VAC) or perhaps
680VDC if the doubler is followed by resonant
charging. Those are trifling potentials for someone
brought up on NST and MOT-based coils! I'm wondering,
can something similar be implemented using a triggered
spark gap? How low can the firing voltage go on one of
those? Think along the lines of doubled 240VAC
followed by a charging reactor for 1.3KVDC. Using a
fat tank cap and an air-blast quenched TSG, could such
a scheme achieve reasonable performance? I know the
gap will be lossy, but a bigger tank cap (200nf or
therabouts?) could compensate. The secondary would
have to be wound for rather low frequency in order to
have a reasonable number of primary turns, but that's
no big deal. Anyone ever tried a low voltage SGTC
before? Is this a hopeless notion?

Cheers,

Greg




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