3.11 Toroid

Toroid shaped cores realize much of the relative permeability of the core material. They also tend to confine the magnetic fields within the toroid and therefore minimize shielding requirements. The effective permeability is somewhat below the material permeability and inductance is tuned slightly by compressing or spreading the winding evenly about the core. Compressed windings yield higher inductance.

The inductance of toroid windings is approximately

(27)

where t is the core thickness in inches, N is the number of turns (passes through the center of the core), OD is the core outer diameter and ID is the core inner diameter. The turns are assumed evenly spaced around the core and the wire diameter is small and tightly conformed to the core.

The initial relative permeability of representative commercial high-frequency core materials is given in Table 3-4. Commercial toroid core dimensions in inches are given in Table 3-5.