Toroidal core transformers
A torus is a term used in geometry for a three-dimensional figure formed when a circle is rotated by 360 degrees around a coplanar point, but it is probably better described as 'doughnut shaped'. Although usually not fully fitting this definition, a toroidal core transformer utilises a core, often formed from a long strip of silicon steel or permalloy, coiled around into a ring shape. It operates in exactly the same way as the more usual universal transformers, with energy being passed from a primary to a secondary circuit by electromagnetic induction. The major difference is that, in the case of the toroidal core transformer, the primary and secondary windings are wound concentrically around the core. This virtually eliminates any flux leakage, resulting in higher levels of efficiency. The nature of these windings also has a shielding effect, resulting in a lower external magnetic field and reduced electromagnetic interference. Toroidal core transformers are around half the size and weight of equivalent E-I format versions and use shorter lengths of wire, but are more expensive to produce, due mainly to the fact that the full length of the wire needs to be passed through the ring for each coil loop. Small PCB transformers and variable transformers may be of the toroidal type.