The R-core transformer adopts a round-wound steel core. Primary and secondary coils are wound on it. Because of the small gap between the coil and the iron core, the losses are very low. In addition, the magnetic core can be annealed directly to make it in an optimal magnetic state, thereby reducing deformation.

　　They are more efficient than EI transformers but lower than toroidal cores (EI - about 80% on average, R-cores about 87% on average, toroidal cores - about 95% on average). Due to this fact, toroidal transformers always lead the way in no-load and load regulation, which is the smallest among toroidal transformers.

　　At the same time, due to their design, R-core transformers do not have the biggest disadvantage of toroidal cores - high inrush currents. Often the shape of the transformer also determines implementation and purchasing decisions. R core size is generally narrower - but wider. Additionally, R-core transformers benefit from lower capacitance between the primary and secondary windings and are less sensitive to DC on the power supply.