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Re: High frequency impedance of a neon sign transformer

Hi Ed,

> 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
>
> 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.

Great analysis Ed!

Just one more "Hmmm" in the consideration of filter circuits.  It would seem that
the value of capacitance I am using (1000 pF) might not be quite high enough.  At that
300 kHz frequency, the reactance is 530 Ohms.  So, it is going to pass about 75% of the
remaining RF, while the transformer gets the other 25%.  It would seem that 2000 pF
would be better, but the leakage of my 60 Hz supply goes up.  (Those damned trade-offs.)

More than 6.6 mH huh?  Whew!  That's quite a bit.  I guess I could do it for neon
circuits, but for pole/potential transformers, I think I might stay with about 6 mH and
let them "swallow" a little RF.  I don't think it is going to hurt them.  I would like to
see the impedance of a pole or PT transformer.  Perhaps if I come over your way soon, I
could bring a PT with me and we could give it your test.  What do you say?

Scotty