On 2014-01-19, Michael A. Terrell wrote:
" wrote:
On Sunday, January 19, 2014 1:05:03 AM UTC-5, PrecisionmachinisT wrote:
Or you could wind your own...really no reason to use E laminations
you have a lathe...simply trepann out the coil cavity, leaving a "post"
in the center
drop in the coils and screw on a round cover to complete the flux
gap
This is sort of like the permanent magnet assembly in at least
one speaker I destroyed as a kid. (Well, the cone was already a goner.)
The center pole and backing plate were one piece (really the pole was
staked into the plate), and the outer housing was a ring Alnico
permanent magnet). Great construction for a stable permanent magnet
field, but not at all good for a transformer.
Now -- for really high frequencies (Ultrasonic, not RF) a
ferrite pot core is really nice. Formed in pretty much the shape
defined above for lathe construction, except that there were two of
them, mounted cup to cup. The cups were available in a large number of
sizes and a number of different ferrite compositions for different
frequency ranges and maximum flux densities.
If you do decide to wind your own, I would recommend using
laminations from a power transformer before using a solid chunk of
material. Audio transformers use thinner laminations than a power
transformer, but ones from a power transformer would be better than
solid.
Agreed! Thinner laminations are better at higher frequencies.
I do remember some special transformers for really old
telephones which were wound on a core of a number of lengths of straight
iron wire bundled together.
A large, powdered iron toroid works even better. Like an old 3A
variac.
Woah! A Variac did *not* use a powedered iron toroid -- nor
did the Superior Electric "Powerstats", nor any other variable
autotransformer by anyone else that I ever saw. :-)
Take a strip of the proper alloy (permalloy? Some other alloy?),
and wind it into a hollow cylinder (with insulating coatings between
layers) and *that* is the toroid that a variable autotransformer is
wound upon. Ideally -- pot that in an insulating material, and use that
to wind the coil on.
Powdered iron (ferrites) are much better at high frequencies,
while most variable autotransformers were optimized for 60 Hz or 50 Hz.
(I have held in my hand some optimized for 400 Hz -- and for those, the
length of the cylinder is about 1/3 that of one for the same current at
60 Hz.
I needed a modulation transformer for a AM mobile radio back in
the '70s. I wound a second winding on a surplus 88 mH telco toroid, and
had excessive audio bandwidth.
:-)
Filtration time. :-)
Enjoy,
DoN.
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