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Re: the physics of a neon sign
In a message dated 96-05-01 06:51:25 EDT, you write:
>
>This leaves me to conclude that it's not so much the conduit (be it metal
>or plastic) but the other techniques that have been discussed (though
>clearly the type of casing makes a big difference when using large
>transformers, or when a long run from the transformer to the first tubes is
>unavoidable).
>
>-Ted Pirsig
Ted, good observation, I tend to agree. It's a complex of different
situations.
I have found, for example, if I can keep the transformer voltage less than
9kV or certainly if it's 7.5 kV with 2x the number of transformers, then the
installer can do no wrong, make every stupid mistake in the book and the sign
just keeps on tickin without a lickin. Have seen with amazement signs with
both GTO transformer leads emerging from the same metal conduit, a total
no-no in my book, but it doesn't fail because you are below the corona
threshhold!!
Yes, corona has a threshold where you get none below a certain electric
field gradient, zero, nada. If you eliminate corona, you get no wire burnout.
(Check out J. D. Cobine's "Gaseous Conductors", Dover, 1941, section 8.16 and
esp Fig.8.23 for an example of Critical Corona Gradient). Remember what
corona is, it's the conduction of electric current into the surrounding air
from the wire due to ionization of the air surrounding the wire. The air
becomes positively charged by the extremely high electric fields (E=V/d) and
the wire gets bombarded by the ionic current within the air. Plasmas are more
corrosive than concentrated acids, which is why the GTO wire insulation gets
all crapped up after being exposed to a high electric field. The same field
is concentrated and made larger for a given voltage V by a smaller distance d
between the wire voltage and ground potential in the denominator of this
(E/d), so if you have a 3/8" metal conduit, you are dicing with death on wire
burnout compared with your chances when you use only 1/2", but I don't
recommend either IF you go above 7.5-9kV. The exact threshold depends on some
other conditions such as humidity, wire diameter, dirt, etc.. Dirk doesn't
have this type of problem because in Europe, they are not looney enough to
use our higher transformer voltages, so he faces a different set of rules and
he can get away with murder compared with our problems using our largest 15kV
transformer. If you are forced to go high on transformer size by a large neon
footage in your sign, use ENT, not EMT, you will find your day (and year)
will go better, much better. That's why your strategy is fine, too.
Good comments!
Jeff Golin
But all the above