Re: interesting OPT winding comments
- From: Bret Ludwig <bretldwig@xxxxxxxxx>
- Date: Mon, 30 Jul 2007 17:25:24 -0700
Site doesn't mention toroidal xfrmr winding techniques though.
Comprehensive and informative useful information
on winding OPT with toroidal cores isn't handled by anyone on the WEB.
The winding of a typical OPT with toroidal core means thousands of turns
for
which a special machine is wanted, and because of the many taps
involved,
its a complete PITFA to ever try to do unless you can invest heavily in
a decent machine
and can sell the articles.
Gorman winding machines are readily available, bulletproof, not
terribly expensive, and can be resold for what you paid if bought
used. But it's cheaper to have a wind house do them. The flaw in
toroids is that the size advantages go away quickly if you go to a
slotted core and the expense goes way up. Conventional toroids
saturate very easily with any DC offset.
"Other than that, Mr. DiMaggio....."
So the normal bobbin windings for E&I cores and C-cores, AEM Unicores
etc are
perfectly fine and we just DO NOT NEED TO KNOW what Plitron actually do
so we could copy them; its too much trouble for the low volume maker,
or DIY hobbyist.
Plitron is affordable enough but the DC offset problem means I won't
consider them. Partridge shows that a small gap is beneficial to
linearize the core even in PP transformers. Mc uses a small piece of
"fish paper" in their C-cores. For many years, Mc production people
thought it was called fish paper because of Larry Fish, a Mc engineer
of very long tenure.
There is PLENTY to worry about with winding toroidal OPT.
leakage is one thing, and so is capacitance and all the other things I
have mentioned.
I have some
cheap (surplus) Avel-Lindberg power xfmrs that appear to have been
wound this way. They have a flat frequency response to 150KHz and
resonance at 450KHz!!! This is far better than most "Audio" output
transformers!
Your'e lucky, and yes, a mains tranny can exhibit wide BW.
I measured a Hammond 1650T (a low turns ratio model)
trannie to compare and it is flat to 30KHz and resonant at 50KHz. Main
drawback of toroid outputs is that the DC current balance must be
strictly maintained to prevent saturation problems.
In practice it is not worth considering the normal toroid as an OPT.
The guy who designed the Gotham cutter head amp, an Indian I believe
(an India indian and not a Comanche or Iroquois or whatever-although
Simpson Electric, still makers of the trusty 260 VOM, are owned by an
Indian tribe) used a GR toroid-a stock item-wound on a Variac core. It
was a pain in the ass. The amp had to be manually biased before each
side was cut! The design was very good other than that and would make
a good high end design today with a different OPT.
Do not consider servo or other auto-biasing schemes. They are
troublesome and never work terribly well.
Hammond is a poor example of response flatness to compare with a
toroidal.
However, a response at full power with Fsat at below 20Hz and with
-3pole at 50kHz and with resonance at above 70kHz is OK for anyone to
use
with or without NFB - if they know what they are doing,
and most do have not a clue these days.
In my OPT for 300W with E&I, I got Fsat just below 20Hz and
-3 HF at 270kHz. At 300 watts, rated R load,
and it simply means you interleave more than the usual standards
recommended in the 1950s, ie, 6S x 5P with thick PS insulation
instead of 5S x 4P as was the case for the original Williamson of 1947.
Toroidal OPT design does not convey enough benefits tyo prompt me into
making any, but if someone were donate a winding machine to me I would
then wind more trannies, both power and OPT.
Toroids are easy to wind if you have a machine. I like them for power
transformers but it has to be realized they have some limitations and
care must be taken with them. Cores, of power grade, aren't that
expensive anymore and if you need a one off it's often cheaper to have
it done as a toroid than a conventional transformer.
I have checked out what is possible using long windowed cores and foundNo, but the ability to fit the core into the windings determines the
no great benefits at all.
The lowest losses and most efficient wide band OPT is with a square core
section of wasteless.
Most ppl baulk at the large weight and size this means.
These ppl are not ready to face the realities of good design
which is so rarely ever seen with mass made articles because all the
mass garbage made is made to
make shareholders rich by putting the least possible hi-fi in customer's
rooms.
The reason being that the leakage pathes become long thin loops
through the winding cross-section which have nearly the same path
length as the desired flux thru the steel core. Only if the leakage
path lengths are significantly shorter than the steel can they
overcome the much higher Mu of the iron. The conventional "Scrapless"
laminations give a near square winding cross-section with near
circular leakage pathes, N. G.
Leakage inductance in a properly designed OPT does not
depend on the presence of an iron core at all.
size of the wind! Most of the designers I have talked to have said
that scrapless is used for economy and a longer lam is better. Look at
the long, tall transformers in the Woo-Woo (WE 19C heterodyne
oscillator). The longer lams are not really "wasteful" because they
use the off-fall as I lams in another size in many cases.
C-cores are probably the best compromise between the EI lam and the
toroid, but the R-core may turn out to be the way to go.
.
- References:
- interesting OPT winding comments
- From: Bret Ludwig
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