Air-cored foil-wound inductors

The inductance value of a foil-wound inductor is equal to that of a similar wire-wound inductor at low frequencies, but as the frequency rises, the foil inductance decreases by about 10% because of current redistribution across the foil width. A table and a formula for d.c. inductance values are given. The occurrence of a first resonance frequency due to intertum capacitance is explained and an approximate formula stated. Methods of optimising the design of foil inductors are described. Compared with wire inductors, foil coils have considerable a.c. resistance.