The tolerances on inductors wound on toroids (even the special #31 mix) are
surprisingly wide for people used to buying components with 1% or 5% tolerances
as a matter of course - we’ve gotten spoiled with modern component
manufacturing.
Not only is there batch to batch changes, there are dimensional changes (not
such a big deal on a 2.4” toroid, but significant on a little 3mm one) - and
changes with temperature.
And how the winding is done.
I ran into this when looking at using commercial “beads” (long skinny tubes) of
31 mi as a part of a wideband wire antenna design.
On Fri, 23 Jan 2026 14:58:37 -0800, Jim Brown <jim@audiosystemsgroup.com> wrote:
<snip>
My only fault with Steve's work, and it is a critical one, is that his
choke designs failed to take manufacturing tolerances into account. The
problem with that is that the NiZn materials have such high Qs compared
to Fair-Rite's #31 material, which is a very special MnZn material,
whose characteristics DO allow repeatable designs IF those tolerances
are taken into account. My designs were done after measuring nearly 100
#31 cores, selecting four at the limits of their complex
characteristics. When I did the same with 40 #52 cores, purchased over a
month or two from four franchised industrial vendors in lots of 10, I
could not repeat his designs.
<snip>
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