Topband: Effect of DC on Ferrite Toroids?

[David Sinclair]

Jeff Maass wrote:
> I recall reading somewhere that putting DC current through wire turns on ferrite toroids
> could result in damage to the material.
> 
>    Am I recalling correctly?
> 
>    Is there a current threshold that should be respected?
> 
> My concern is the +12Vdc on the feedline to a preamplifier, which will pass through a
> choke of 8 turns of RG-8X through four #31 2.4-inch toroids. The current is low.
> 
> 73,  Jeff  K8ND
> 
>
For almost any imaginable ham application of ferrites, we want the core
to operate in its linear region. Especially with smaller cores, DC bias
currents through any windings can be a real concern. Enough DC current,
or DC plus signal, can put the core into saturation. Remember, cause
a device to run non- linear, and you might get it to work as a crude
rectifier or mixer. Not good. A small core running near saturation could
possibly introduce signal distortion in some cases.

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).

The current in your application sounds like it is nowhere near enough
to put those cores into saturation. You might want to look up the
formulae and the core specs and actually do the calculations for your
situation. I doubt you have a problem, however. I'd worry about 100's
of mA in a tiny core, even 10's of mA, but less so with big toroids
like those. If I'm remembering right, higher permeability materials
are more of a worry for DC saturation effects than the low perm ones,
but check the manufacturers' data on that.

73, David K3KY