EMRFD Message Archive 365

Message Date From Subject
365 2007-01-25 13:23:10 Mike Brainard Transformers wound on 6 Hole Beads
Six hole beads were used as cores for bifilar and trifilar transformers
on several of the EMRFD projects. I have not seen any discussion of
when these cores are the right choice in comparison to toroids or two
hole balun cores. I would appreciate hearing any experience or insights.

366 2007-01-25 19:31:25 Rick Re: Transformers wound on 6 Hole Beads
6 hole beads have several advantages including a offering a compact way to obtain high
inductance with low capacitance between the turns. They are available in several mixes,
including some that work well with high DC current--so you often see them as RF chokes in
power amplifiers.

The reason they are in some of my projects in EMRFD is that I was experimenting with them
to see if they would work as well as bifilar and trifilar windings on conventional toroid and
two-hole cores. They worked fine in those applications so I left them in. In critical
applications, there is no substitute for experiments with different cores. Each type will have

Best Regards,

Rick kk7b
367 2007-01-26 02:01:31 Graham Haddock Re: Transformers wound on 6 Hole Beads

As a practical matter, you can assume that there is no coupling
between different holes in the same piece of ferrite. So there is
no inductance multiplier effect going on, as there is in multiple
turns through the same hole, where the inductance rises as the
square of the number of turns.

In other words, binocular cores, or six hole cores are a mechanical
convenience, and are no different than one pass through two different
beads of the size of half of the core in the case of a binocular core,
or a single pass through six beads in the case of the six hole core.

The inductance for six holes or six beads is just the inductance of
one pass times six. Or the total inductance is just the sum of the
inductances of each pass. (Versus the inductance of one pass
times the number of turns squared in a multi-turn inductor)

When would this additive inductance behavior be better than
the multiplicative behavior of multiple turns though the same hole?

Well, if the inductance of a multi-turn inductance would be
too high, or if the flux in the core (RF or DC) would be
too high if you used a multi turn inductor instead. I think this
is why they are used mostly for chokes on power leads,
the flux in the core of the ferrite doesn't multiply like it would
with a multi turn choke.

And if you need some finer steps in the inductance available
such as 1x, 2x, 3x, 4x ... etc.
1x, 4x, 9x, 16x ... .