A few years ago the Joule Thief fans and others built many projects with a very popular toroid core that was sold from Electronic Goldmine, a surplus electronics company, for the affordable price of five for a dollar. Eventually the supply was all sold and it was no longer available. The stock # was G6683.
Today, it looks to me as if the same toroid core is now available again, although the price has gone up. It is Stock # G21322 from Electronics Goldmine for a dollar apiece.
I ordered a few of these cores last week, and received them this Memorial Day weekend. Using a calipers, I compared these new ones to the original 5/$1 cores, and found them identical in size and shape.
Both cores were 25 mm or .985 inch O.D., 12.9 mm or .51 inch I.D., and 9mm or .356 inch high. Both cores were bare, uncoated ferrite.
I wound ten turns on the new core and measured 1.3 millihenrys. This high value means these cores are high mu aka high permeability. The old cores were, too. I used a short piece of wire to put through both cores, it is equal to a single turn. I measured a few of each. Both the old and new cores measured between 3.1 and 3.35 uH, which is very close to being the same.
At the price of 5/$1, I would expect cores would come from culls or leftovers from the manufacturing process. Typically, this process receives cores after they have been baked in batches, and they are tested and sorted according to the data sheet specifications. If a core does not meet these specs, it is culled and is put into the factory rejects bin. One test parameter that it may not meet is the mu or permeability, which affects the inductance. The inductance could be too low or too high. So when the core is tested, it would not be representative of the the typical inductance and not close to the inductance of the other cores. This does not mean the cores are defective, it just means they are not within the limits of the specifications.
So those who wish to build one of the projects requiring the old core now have a reasonably priced equivalent that can be purchased from the same surplus company. You will just have to pay more. If you want to search for a rough equivalent from another electronic supplier, you might try searching for FT87-75, which is a more common designation in this industry. The FT must mean Ferrite Toroid, the 87 is the O. D. of the core, and 75 is the type of ferrite ‘mix’ that is used. SurplusSales.com also has a good selection of ferrite cores, including this one, from various manufacturers.
One other point that should be noted. For some experimenters, getting the exact part is the best choice, and allows building a project exactly to the plans. For some like me, building a project and experimenting with different parts is a fun way to go. Once you have had fun winding your toroid, and your project works great, don’t be afraid to experiment a little. Take the little toroid out of a dead CFL light bulb and try it in your project. You can find out how to do this by watching YouTube videos.