Re: Neon Sign Transformers (was: spark length formula needed*)

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

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

Hi All,

There are some excellent technical papers on NST modeling and cap matching
from Tero Ranta at:

http://hot-streamer-dot-com/TeslaCoils/OtherPapers/TeroRanta/CurrentLimitedTrans
formers/NSTModel.htm

http://hot-streamer-dot-com/TeslaCoils/OtherPapers/TeroRanta/NSTCapMatching/Reso
nantCapacitorMatch.htm

Note that both the above should be just one line for the URL.  It is also
under:

http://hot-streamer-dot-com/TeslaCoils/OtherPapers/

In the TeroRanta section.  Open the *.htm files...

Cheers,

	Terry




At 08:19 AM 12/27/2001 -0800, you wrote:
>Tesla list wrote:
>> 
>> Original poster: "John H. Couture by way of Terry Fritz 
><twftesla-at-qwest-dot-net>" <couturejh-at-mgte-dot-com>
>> 
>> Jim, All -
>> 
>> Your description of the NST as being "a big inductor in series with the
>> output" is a good one.
>
>	An even simpler NST description would be that the transformer is
>equivalent to a perfect transformer giving the rated open-circuit
>voltage in series with a reactance equal to the rated voltage divided by
>the rated current.  Not perfect, but "good enough for government work".  
>
>	As for the series resistance, that of the secondary can be measured
>with an ohmmeter, with the reflected primary resistance being perhaps
>slightly less.  (In other words, the resistance in series with the
>leakage reactance witll be somewhere between 1.5 and 2 times the
>measured resistance of the secondary.)  I have found that most of the 12
>kV, 60 ma transformers I have have a secondary resistance of about 4500
>ohms, so the total resistance is of the order of 7500 ohms.  This means
>that the effective Q of the transformer is of the order of 20 or more,
>which means that, if the secondary were loaded with the "matched
>capacitance" (reactance equal to that of the leakage reactance and
>opposite in sign), in principal the unloaded output voltage would rise
>to 20 times the rated voltage.  Of course, this would never happen as
>core saturation and insulation breakdown would occur at a far lower
>voltage.  I suspect that most of the blown-up transformers come from
>opening up the spark gap too far in quest for longer sparks.
>
>	As far as input and output terminal currents are concerned, they will
>be in proportion to the turns ratio.  Since this is a series circuit
>this must be true.  I believe any measurements which indicate that the
>"secondary VA are greater than the primary VA" is due to defective
>instrumentation.
>
>	When the spark gap is firing the behavior is far more complex, with the
>secondary waveform approaching chaotic under many circumstances. 
>However, the primary and secondary currents are still governed by the
>turns ratio.
>
>Ed
>