Re: NST & Sparkgap

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Michael,

On 16 Aug 2001, at 12:56, Tesla list wrote:

> Original poster: "Michael Rhodes by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rhodes-at-fnrf.science.cmu.ac.th>
> 
> I have built a 4 gap static gap with tungsten electrodes mounted
> in machined stainless steel heat sinks.  I connected my 15kV-at-30mA
> NST to it (without cap and primary) to check the gap firing voltage.
> 
>  From my calculations using Paschen's law, breakdown in air at
> STP would be 29.83kV at 1cm.  This equates to a total gap space
> of 7.112mm (.28") based on 21.2kV peak voltage.  I set each gap
> to 1/4 of this 1.78mm (.07").  The NST rated input voltage is 230VAC
> connected to a variac.  If I turn the variac up the spark gap
> begins firing continuously at 110VAC.  What do you suspect
> is the reason for it to fire so early?.  I am in southeast Asia
> and it is now hot (30 C) and humid.

The humidity won't be helping. The shape and size of the gap 
electrodes will also play a part.
 
> I'm reluctant to increase the gaps but if I have to can I increase
> just one gap to make up the difference?  Difficult to get to the
> gaps now.  I suspect I am losing a lot of 'bang' due to this.

If you can adjust the phase of the spinning gap relative to the 
stationary electrodes this shouldn't be a problem since the gap will 
fire as soon as the electrodes are close enough for the voltage 
across them. Generally, a rotary will fire before the electrodes are 
aligned. With a resonant sized cap the transformer might be pushed 
past breaking point if you adjust the gap while running for maximum 
output. 

I hope there is some useful information in this chaff.

Regards,
Malcolm

> Oh, by the way - this was 'first light' for me today.  I will give further
> details on my coil project later after the university science fair is
> over and I have more statistics on the output.  It is not in tune yet
> due to having to rush to assemble the parts in one day (parts where
> constructed in 3 weeks) but it is already producing
> 30cm sparks and the students who assembled were surprisingly
> excited by it all.  I will be assembling a web page for it also.
> 
>    -------------------------------
>    Michael Rhodes
>    Senior Researcher
>    Fast Neutron Research Facility
>    Chiang Mai University
>    Thailand
>    -------------------------------
> 
> 
> 
>