Graham wrote:

I am trying to make some high value toroids, about 1mH. I have a commercial
one using a half inch core with about 20turns on it to give 3.3mH. Does
anyone know what the core material/description is or an UK source please?
The normal ones such as T37-2s etc only give a few uH.

Thanks

Graham

There are two basic categories of toroid cores. Those that contain
distributed gaps between bits or sections of high permeability
material (to increase their energy storage capability) and those made
with a single material that has a high permeability (may be metal,
metal tape or ferrite ceramic).

The T series are the distributed gap type. Their claim to fame is
their ability to have stable inductance with high ampere turns
applied. So before you can choose a core material, you not only have
to specify the inductance required, but the ampere turns this
inductance must support.

Here are links to a few ferrite manufacturers:
http://www.fair-rite.com/
http://www.mag-inc.com/ (also makes the powdered iron cores)
http://www.neosid.com/fercor/toroids/tormn.htm
http://www.ferroxcube.com/prod/assets/fertor.htm
http://tciceramics.com/f_toroid.htm

Many others are available through Google. The first link includes an
inductor design tutorial at the back of their catalog that may be
helpful.

--
John Popelish