Professional transformers are done two ways, nowadays: One
way, the "easy way", is with nylon molded bobbins, where the primaries
and secondaries have separate "compartments" on the molded form, and the
windings are "scramble wound" in their respective spaces. These are even
infrequently Formvar soaked, once done. The wire ends are then attached
- yes, not even soldered! - to solder tags molded into the
bottom edge of the bobbin form. If you can locate those molded bobbins
for exactly the transformer size you need to make, and don't object to
"scramble winding", go for it! Makes for an easy job! And, if you bother
to Formvar soak and properly tape over the cured windings, the end result
can be very electrically proper and "professional looking"! The second
way is with lots and lots of care and fussing around like the old boys
always did it, where the windings are done in careful flat layers, with
tissue between each layer, so that the "enamel" insulation never sees more
than the potential between adjacent turns. This winding method requires
mechanical wire distribution as the bobbin spins, stopping, laydown of
a tissue piece, resumption, etc. I never saw this being done, so I don't
really have a proper picture of what steps of this old method were automatic,
and which were manual. Yep, they actually had real humans with their
fingers down in there, manually operating small soldering irons, tape dispensers,
diagonal wire cutters, and such, years ago! Data would be amazed!
Well, if you are ready to go, say, on a 500 VA transformer, but know
there is no such thing as a molded bobbin of that particular size you need,
you can "build your own" insulative bobbin on the wooden spool, by cutting
the parts shown herewith to suit, and carefully installing, folding, and
taping. There is a professional Mylar core insulation paper used for custom
transformers, which has light blue paper like outer layers, but a very
tough Mylar (plastic film) core. I have always managed to find more than
enough scraps of this in my transformer-buddy's trashcans for my needs.
He buys it on $500 rolls, I guess. It probably comes in several thicknesses.
In the old days, say, 40 years or more, ago, "Kraft paper" was used for
such as transformer windings, and once this was finally soaked in Formvar,
after final assembly, it was suitable insulation. But it required manual
manipulation, so I am fairly sure this is the reason it fell from favor!
Kraft paper? The thicker, nicer grocery bags (not the plastic ones!) are
made of similar stuff. This paper is usually amazingly sturdy stuff, and,
when carefully cut with an industrial, single edged razor blade against
a straight edge over a smooth board, into nice strips of the appropriate
widths, it makes for transformer bobbin insulation which is every bit as
good as a "professional" one of 1958!
Jan
Rowland