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High frequency impedance of a neon sign transformer

Hi All,

	Due to the discussion on the effectiveness of chokes and bypass 
capacitors in protecting one's high-voltage transformer, I was motivated to 
find an answer the following question. What is the impedance of the 
secondary of a pole pig/neon transformer as a function of frequency?


	The answer I found for a neon sign transformer shows definitely 
that one cannot consider the secondary to be a fixed inductance as a 
function of frequency. I believe that the 59Henry inductance figure quoted 
by Scott Meyers and others is only a very low frequency figure. So, without 
further ado, here are the results of some measurements I made ona 15kV, 
60ma neon:

Freq.			Secondary Impedance

			Primary open	Primary Shorted
------------		------------	---------------
100Hz			1.2Mohm		.38Mohm

1kHz			600kohm		same

10kHz			35kohm		same

100kHz			4.2kohm		same

300kHz			1.8kohm		same

* The impedances are the total of  the reactive and resistive 
componets. However, "eye balling" the scope traces suggests that the 
current is always lagging the voltage by about 90 deg, so most of the 
impedance is effectively inductive.

** I used a 1volt excitation for all measurements.

*** The open and shorted primary measurements show that the mutual 
inductance between the primary and secondary has no effect of the secondary 
impedance above about 1kHz.

I suggest that there are two reasons why the impedance is so low at high 
frequencies: (1) The inter-turn capacitance in the big secondaries of these 
kinds of power transformers is so great that it effectively shorts out 
much of the secondary inductance as the frequency is raised. (2) It is well 
known that a big 60Hz iron core cannot respond to high frequencies due to 
eddy current screening. So the core "dissappears" at high frequencies and 
therefore the inductance of the secondary may decrease by a factor of 
around 1000 (approx permeability of silicon steel).


Note that the 4.2kohm impedance meaured at 100kHz is equvalent to

A choke inductor of 6.6mH

or bypass cap of 0.4nF

Based on this comparision, I would think that one would want more than 
6.6mH of choke inductance (at 100kHz) in series with the neon and more than 
.4nF of capacitance in parallel to insure that the transformer is well 
protected from the high frequency and high voltage oscillations from the 
tank circuit.

Comments? 
-Ed Harris