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

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

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