Topband: Effect of DC on Ferrite Toroids?
w8ji at contesting.com
Wed Jan 10 08:40:07 EST 2007
> With larger cores and KW RF power levels, saturation can
> also be
> accompanied by core heating. Severe enough heating can
> even fracture
> cores and destroy them. Hams' small core transformers
> usually get
> destroyed by lightning and not much else, because we don't
> transmitter RF power through them (intentionally).
That's all very true but it is imporant to point out heating
does not mean saturation. As a matter of fact one of the
most common myths about cores is heat = saturation. That's
While there heating occurs long before saturation in many
applications while in others saturation occurs without any
heating at all. It isn't appropriate to judge saturation by
heat....not in the least....because they are unrelated
As for allowable current, it all goes back to the
ampere-turns. In a given core many turns with a small
current is the same as a few turns with very high current.
The allowable flux density depends on core material, any air
gap, and cross sectional area.
Also related to DC current through the winding, equal and
opposite currents in interwoven or close-spaced windings
with equal turns will cancel. The problem is just because
you wind a shield and a center (coax) carrying dc power
through the core does not mean each conductor has opposing
fields. ALL of the power supply current has to pass through
each conductor, and that is almost never the case with
unbalanced lines since ground loops are involved.
As a matter of fact there can be low frequency ground loop
currents that are **not** from power applied to the
transmission line. These current could be from nearby VLF
transmitters, power line ground loops, battery between
dissimilar grounds, or other problems.
Saturation is mostly a problem with very high permeability
cores (another reason to stay away from those ui=10,000
cores some books recommend) that have many turns, especially
cores with small cross section. Remember it all comes down
to flux density and that relates to material
characteristics, cross sectional area, air gap, the number
of turns, and the current.
It's somewhat complex, because even static magnetic fields
can cause saturation.
Damage is rare, but the most common is a core fracture.
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