Topband: Effect of DC on Ferrite Toroids?

[Tom Rauch]

> 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 
> send
> 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 
just wrong.

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 
events.

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.

73 Tom