Below is a list of Toroidal Transformers manufacturers from St. Louis, Missouri. A toroidal transformer is an electro-magnetic device used to transfer electrical energy away from one electric circuit and toward another without changing its frequency. Toroidal transformers are donut-shaped transformers, which are highly space efficient and excellent for reducing electromagnetic interference. Offering quiet, efficient operation, toroidal transformers have very low stray magnetic fields because they have almost no air gaps.