I bought some cat# TOR-74 from All Electronics for 4 for $1.00. They say these toroid cores are Magnetics ZF410057C, but Google didn’t turn up any data on them. They are coated Grey, and are 9.5mm O.D., 4.5mm I.D., and 5.0mm high, or .375 inch O.D., .178 inch I.D., and .197 inch high. This is a nice, small size for a compact Joule Thief.
I wound one with 10 turns of 24 AWG plastic insulated telephone wire, which fits easily on this core. It measured 219.6 microhenrys, which is a very good value for a Joule Thief, or the 220 uH could be as low as 100 uH with fewer turns.
For a Joule Thief, it will need a second feedback winding. This can be thin wire such as 30 AWG or 0.25mm. My “standard Joule Thief coil” used four lengths of 30 AWG quadrifilar wound, with three of the windings connected in parallel for the primary. I could wind about 10 to 12 turns or about 7 inches. But the core I used was a little bit bigger. I tried four 7 inch lengths of 30 AWG wires on this TOR-74 core and I got to 10 turns before the wire got short. There was still enough space in the hole to put several more turns. This second core of 10 turns measured 216 microhenrys, nearly the same as the first core.
For a Joule Thief, three things make this a good choice: The small size. inexpensive cost, and high permeability meaning high inductance with few turns.
FB-102
I also bought some FB-102 toroids, or ferrite beads as they call them, for $ each. These are supposed to be uncoated ferrite, but the package is rusty brown from the dust from the beads. I think that it’s possible that the material is actually powdered iron. They are more cylindrical than a toroid, more like 2 toroids stacked one on top of the other. I assume that’s why they called them ferrite beads.
I wound ten turns of 24 AWG insulated telephone wire on one and measured the inductance at 57 uH. I continued winding more turns until I got to 14, which measured 110 uH. This means the permeability is less than the TOR-74, I’m guesstimating somewhere around 1000. This also adds to my suspicion that they may be powdered iron. I also wound 14 turns of 28 AWG enameled wire over the 24 AWG, for the feedback winding. This should make a good Joule Thief coil, but I haven’t tried it yet.
Have fun making your Joule Thief, and Happy Holidays.