I ordered a few toroid cores today, and I used this toroids.info to find the turns and inductance for some cores. It has a basic calculator that can give the information for the cores in the list. Most of the cores were chosen for RF projects, so some very large cores are shown. The T cores such as T-50-6 are powdered iron cores, with low permeability. The FT cores are ferrite cores, the permeability is medium to high. The FB are ferrite beads, usually smaller than a toroid. The BN are baluns, also called multiaperture cores — they have more than one hole.
The -0 core is tan / tan, but it’s actually not magnetic, it’s just a core made of light brown phenolic plastic, to hold the wire in a ring.
I have used quite a few FT-37-43 cores for Joule Thiefs. They work nicely. The equivalent is Fair-Rite 2643002402 and they call it a ferrite bead, even though it looks like a toroid. The Fair-Rite 2673002402 looks the same but has a higher permeability.
These cores a small and they typically cost less than a dollar. As the cores get bigger, the cost goes up, until cores of 2 inches or 50 mm cost several dollars apiece.
