Silly Question

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Original poster: "Loudner, Godfrey by way of Terry Fritz <twftesla-at-qwest-dot-net>" <gloudner-at-SINTE.EDU>

Hi Marc

I better give a more accurate description of the core of my dental x-ray
transformer. The core consist of two long ovals which are butted together
forming the approximate shape of an EI core. The two ovals are held together
by a tight metal strap. Each oval can be broken down into two "U"s.  The
core looks exactly like the oval shaped core shown in the miscellaneous
section of your web site. There are two primary windings which are butted
end to end on the center leg. I do not know if the two primaries are
connected in parallel or series. Whether you connect the two primaries in
parallel or series, the direction of the current should be in the same
direction around the core throughout the entire primary. My suggestion is to
connect the two primaries in series to lower the voltage output. Without
access to oil compatible insulation of professional grade at every stage of
insulation, I doubt if your homebrew transformer can withstand 40 kV. When
two "U"s are put back together to form an oval, gapping will introduce a
current limiting feature. Gapping between the two ovals will introduce a
current limiting feature. Any such gappings will lower the coefficient of
coupling between the primary and secondary. My suggestion is not to gap, but
to rely on ballasting for current limiting. Gapping will complicate the
magnetic circuit of the core making the use of Mike's transformer design
engine possibly unreliable. Sectioning up the high voltage winding is a good
idea. This will spread the high voltage stress among the sections. Each
section will have to be serously insulated from each other. Once you have
three or more high voltage sections, you will not be able to ground any high
voltage leads to the core. In this case, you will have to insulate the
entire high voltage section from the core. When the two ovals, are butted
together to form an approximate EI core, you can take as the height of the
core as the distance measured along the center leg from top to bottom. This
will introduce only small errors when using Mike's transformer design
engine. An excellent feature of Mike's engine is that it will inform you
when there is not enough winding space for your high voltage goals. You can
also wind a few layers on a secondary bobbin and measure the voltage output.
Then you can scale up to your goals. 

Godfrey Loudner