Re: Static gap BPS, was Static Spark Gap

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Gary, Dan, Daniel and John,

Ok, here is the long explanation ;-))

Daniel and John have a 12kV/60mA NST and they are using 11 series 0.15uF 
MMC caps.  They want 200 BPS operation from an RQ style static gap (I am 
not sure why).

First we find their primary cap value which is 150nF / 11 = 13.63nF which 
is a resonant value.  This is shown in Mark's excellent Geek Group cap 
chart at the chart at:

http://hot-streamer-dot-com/temp/MMCcapSales.gif

I do assume their firing voltage is 12000 x SQRT(2) = 17.0kV.  If it were 
more or less, that would affect BPS.  But 17kV would be the normal value 
one would want to use.  Now the only thing left to do is a giant laborious 
computer study...  But I already did that a year or two ago, so it's all 
done ;-))

http://hot-streamer-dot-com/TeslaCoils/Misc/NSTStudy/NSTStudy.htm

If we look up the 12/60NST at 13.6nF we won't find it.  However, the values 
are just the 12/30 NST numbers doubled (or, 1/2 the 12/120 numbers).  Given 
some losses, the real resonant value is 13.262nF for a BPS of 
200.0454153.  Of course, in the real world, that may vary like +-10BPS.  If 
we look at the other resonant BPS rates we see:

NST             Resonant BPS
15/60           193.575
15/30           201.149
12/30           200.045
9/30            200.253
12/120  188.98

So for whatever reason, static gap resonant cap NST systems run very very 
close to 200BPS.

See, I didn't just make it up :o))

If the cap is LTR, then it is in sync with the line frequency and it should 
fire at exactly 120 BPS.

Cheers,

         Terry


At 01:10 PM 4/24/2003 -0400, you wrote:
>The BPS of a static gap is only remotely tied to the mains 
>frequency.  With a 60 Hz supply, one would achieve 120 BPS only if the gap 
>breakdown voltage was precisely set to the voltage that the cap would 
>acquire after one half-cycle of power.  That sounds fine in theory, but it 
>turns out that the actual breakdown voltage varies considerably with time, 
>airflow, temperature, time since last breakdown, etc.  Variations in 
>breakdown voltage will translate to variations in timing, i.e. no longer 
>at peak voltage.  What isn't obvious is that if following a half-cycle of 
>cap charging, if the voltage is too small to break down the gap, that 
>charge is NOT wasted (where else would it go?).  Instead the cap forms a 
>tank circuit with the power supply (NST) secondary inductance, and the 
>energy in the cap is carried into the next half-cycle in the opposite 
>polarity.
>
>In any static gap circuit, the _average_ BPS will be governed by cap size 
>and gap width.  Truthfully, I've never actually measured my gap's average 
>BPS, though I have confirmed that the waveforms are as simulations predict 
>- chaotic, and definitely not synchronized with mains peaks.  See 
>http://www.laushaus-dot-com/tesla/gapsim.htm for simulated waveforms, and 
>http://www.laushaus-dot-com/tesla/measured_waveforms.htm for a measured waveform.
>
>I was surprised to hear Terry suggest that the BPS will ultimately work 
>out to be roughly 200 BPS, or any particular number.  I know that the BPS 
>is strictly dependant on cap size and gap width, so I'm not sure how such 
>a generalization can be made.  Maybe I'll measure my minicoil's BPS tonight...
>
>Gary Lau
>MA, USA
>
> >Original poster: "Mccauley, Daniel H by way of Terry Fritz 
> <teslalist-at-qwest-dot-net>" ><daniel.h.mccauley-at-lmco-dot-com>
> >
> >How do you figure on 200 BPS???
> >I would think that with a static gap, the primary tank circuit is going to
> >charge up at 2 * 60Hz (positive and negative
> >peaks of the input 60Hz voltage) and static gap would discharge at some peak
> >of these positive and negative peaks.
> >My calculations would say 120 BPS.
> >
> >What am I missing???
> >
> >The Captain