Re: Loosely coupled transformer windings

Steve Carroll wrote:
John Popelish wrote:

My drawing probably wasn't too clear, but there was an air gap in the middle
leg. (It looks like a washer.) I figure that I can wind the 3 windings,
then increase the air gap to reduce the coupling to the secondary until,
under load, it drops from 1000V of "turns-ratio" voltage to about 100V-200V
of "loaded" voltage, while still keeping close coupling between primary and
feedback windings, (or two halves of a centre-tapped primary for push-pull
driving).

I understand, but increasing the gap between the core halves will also increase the magnetization current of the primary because of the lowering if its inductance and also slightly lower the coupling factor to the tightly coupled secondary.

There are E core shapes made that have
all 3 legs the same cross sectional area for this purpose.

I don't quite follow what you mean here.

Most E cores have a center leg with twice the cross sectional area of the legs on each side, with the assumption that all the coils will be placed on the center leg, and the flux will split into two equal fractions and half will pass through each of the outside legs. But I have seen a few E cores that have a constant cross sectional area in all parts. These allow for some creative winding designs with various windings on different legs.

And, of course, you could add an external inductor in series
with a tightly coupled secondary and get a very similar effect.

Yep, I've also been thinking about that. It would make starting easier, too,
especially if I adopted a method of shorting across the second side of the
heater filaments to heat them, then open that circuit to dump the series
inductor's energy across the tube for ignition, similar to mains operated
fluoros.

Incidentally, in the end I'll also add heater windings, which should have
reasonably close coupling, but one thing at a time, I reckon.

Sounds like a plan.

--
Regards,

John Popelish
.

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