What are they commonly found in? And are those things easy to get for free?
atx power supplys will have some good sized ones. its not that hard to get a bunch of them for free.
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There are Toroidal inductors/transformers in a lot of devices (Old car stereo amplifiers etc.). But typically they are difficult to remove from the board in working condition (they are large heatsinks when you try to solder on them, plus they are typically connected to the ground plane on at least one lead.) If you cut them off you will need to strip some of the enamel from the leads and re-tin them with solder to reuse them.
If you are just looking for ferrite beads they are fairly cheap to buy in very small form, they get more expensive as they get bigger. I usually go to Digikey to buy things like that.
most switch-mode power supplies (especially easy to get: PC Power Blocks) have toroids as common-mode chokes
I had a USB cable that connected my MP3 player to the computer, but it got washed, and I tore it up to see inside it, and I found a metal tube that the wire ran through, is this a type of toroid?
It is a toroidial ferrite bead (used to limit noise/spikes). It is not a toroid inductor/transformer unless there is wire connected to it in proper windings.
i forgot: the emc- filter on the vga cable to your screen also is a nice (but rather big) toroid
It's possible to make them, if in a pinch.
The problem with salvaging and making is measuring the inductance. If it's not labeled, the best you can do is guess unless you have the tools required to measure them.
Just about any electronic device that plugs directly into the wall will have a toroid.
I usually find them at the donut shop. They are also commonly found in mah belly.