Donut Transformers

Donut transformers, also called toroidal transformers, have no internal primary winding as such. The primary is the conductor in which the current is to be monitored, and is not an integral part of the transformer. The primary conductor is simply placed through the window of the current transformer. The core in a donut transformer is a band of magnetic grade steel concentrically wound upon it. This is an extremely efficient core design that has no breaks or gaps in the magnetic path. While other types of core designs may produce relative high levels of audible noise, (approximately 30-70 DB's) the toroidal core rarely produces an audible noise level.