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Re: High frequency impedance of a neon sign transformer

Ed Phillips EP wrote:

Hi Ed,

EP>Ed:
EP>	I am very interested in your measurements of neon secondary
EP>impedance as a function of frequency.  One question, though.  Don't 
EP>you really mean that the high-frequency impedance is CAPACITIVE?
EP>Inductive reactance wouldn't decrease with frequency like that.


Thanks for the interest. I think its something that may need a bit
of looking into. Now for your question:

It dosn't appear to be capacitive since the measurement shows that
the time dependent voltage leads the current by about 90 degrees no
matter what frequency is used. If I understand you concern correctly,
you're saying that in a pure inductance the impedance always goes up
with frequency, right? So why does my measurement show an inductive
impedance which goes down with frequency - is that it?

The two effects which are probably important 
were in the initial post:

(1) Any inductor/transformer with a standard 60Hz (thick)  
laminated core will have an inductance about MU (read 
magnetic permeability) times bigger than for air at 60Hz.
MU might be say 1000 for silicon steel... the exact value
dosen't change the argument just the fact that MU makes the
inductance much bigger than it would have been if the iron
core was absent.
	Now, as the frequency is raised, the skin depth of the
laminations becomes less than their thcikness. When this happens
the magnetic field induced by the windings cannot fully penetrate
into the iron laminations. The effect of this is simply like
subtracting away the iron deeper than a skin depth. As the frequency
goes to infinity the skin depth goes to zero and the effective
permeability goes to that of air! So the inductance and impedance
go down with frequency even though the winding remains inductive
in character ( Voltage leads current).
	I have done some RF amp design, and it is well known that
essentially no magnetic material will raise the permeability of a
transformer when the frequency is >1Ghz. 

(2) The other effect which would lower the impedance is the effect
of capacitance between windings and capacitance between the windings
and the core or case of the inductor/transformer. This capacitance
should have the effect of shorting-out the inductive impedance of the 
indvidual windings as the frequency increases. Only the outer turns
of the secondary winding would be "isolated" enough that their
capacitance would be "low". Though this capacitance shorts out the
inductive impedance of the inner turns, I doubt that for 100kHz it
could completely cancel all the inductance.


	I am aware that my words may not be the clearest, but if they
aren't, please let me know and I'll try to beef up the weak parts.

	Also, I was hoping that someone else would try the measurements
so that we could compare results.


-Ed Harris