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Re: High frequency impedance of a neon sign transformer


Hi Malcolm,

Malcolm Watts writes:
MW>A thought on Richard Hull's comments on neon tranny reactance....

RH> I would suggest an inductive reactance of about 2 times the net secondary 
RH> reactance for protecting the transformer.  Therein comes the rub.  Any 
RH> effective shunting capacitance we might try to add effectively adds to the 
RH> resonant tank.

MW>I think this might not be a problem because at the time the tank is 
MW>in action, the spark gap leaves the transformer and any associated 
MW>protective components out of the equation. (Y-N)?

I've been thinking about this a bit and see two possible situations
depending on how you've got the primary circiut hooked up:

(1) Shunt gap

This is where the gap is parallel to the power transformer secondary.
People have decribed this hook-up in their "balanced" or "equi-drive"
circuits. Here, of course, the gap shorts out the power transformer 
temporarily while the Tesla primary oscillation occurs. Since the gap
certainly has less impedance than the transformer/choke/bypass comination
I'd guess that little of the oscillation current would flow through
these components. Also note that this may be why this configuration has
apparently higher Q's than the series gap discussed below

(2) Series gap

Here the transformer charges the tank capacitor directly, ie, the cap is in
parallel to the transformer. The gap is now in series with the tank cap and
the Tesla primary. In this configuration, the full primary 
oscillation voltage appears across the tank capacitor and the transformer/
choke/bypass combination which is in parallel with it. In this case, if
the chokes are not big enough, the bypass caps would tend to add in
parallel to the main tank capacitor. 

Here we may have a problem with the Q of the tank! Even if the choke/bypass
is sufficent to protect the power transformer secondary, this transfomer/
choke/bypass combination impedance appears directly across the tank cap. If
this network is very lossy it MIGHT lower the Q of the tank circuit. This
problem, as suggested above, should not happen in the balanced circuits 
with the gap shorting the transformer/choke/bypass impedance during the
tank oscillation.


Now, somebody, check this... :)

-Ed Harris

ps glad to see Richard and Jim got some numbers on this...