Toroids are quite commonly used in amateur radios, antenna tuners, baluns, common mode chokes and power supplies. Toroids are usually made of powdered iron oxide along with some other metals like cobalt, copper, nickel, manganese and zinc. These metals in powdered form are held together with non-conducting binding material like like epoxy. Biggest advantage of toroid over pure metallic cores for transformers is their high electrical resistivity which reduces the generation of eddy currents. Eddy currents within the core is an important source of loss of energy in transformers. This description mainly pertains to toroids used for RF applications. Toroids for mains transformers can be made of laminated iron cores. Toroids in general have the donut or ring shape.
