The variable toroidal transformer is a special type of transformer in which the magnetic core is designed as a closed toroid. This usually consists of soft iron or special ferrite materials.

The windings made of enameled copper wire are evenly distributed over the entire toroidal core and wound in an electrically insulated manner. This even distribution results in very good magnetic coupling, which means that the leakage flux is very low compared to other designs.

Due to the closed core shape, the toroidal core has virtually no air gap, which results in a very low magnetic resistance. This is reflected, among other things, in a low no-load current and a steep hysteresis curve.

One disadvantage of toroidal transformers is the occurrence of very high inrush current peaks when they are switched on. It is therefore often advisable to use an inrush current limiter to protect electrical components and prevent unwanted tripping of fuses.