Ferrite Core Turns Calculator

Toroids and 2-hole binocular (BN) cores

Result

15 turns

Core data: Fair-Rite per-part datasheets (A_L in nH/N², which equals mH per 1000 turns). FT and BN names shown only where Amidon publishes the same dimensions (and, for FT, the same A_L). Wire diameters: heavy-build nominal overall diameter from the BAE Wire heavy build chart.

Notes

Toroids: each pass through the hole is one turn. Spread over about 300°; compress to trim. The single-layer check assumes that 300° spread and uses the datasheet minimum ID (these Fair-Rite cores are uncoated).

Binocular (BN) cores: one turn is one pass through both holes (in one hole, back out the other), so every hole carries all N wires. The hole-fill check uses the datasheet hole diameter minus its tolerance and the densest known packing of N round wires in a circle (exact layouts up to 20 wires; above 20 wires, a 55% fill: a practical estimate for hand-wound wire). Fair-Rite gives a minimum A_L for 61-mix binoculars only; 43 and 73 binoculars are specified by impedance, so they are not listed.

Beads (FB): not listed. Fair-Rite specifies suppression beads by impedance only, with no A_L.

Ferrite A_L is published per turn squared in nH, the same number as mH per 1000 turns. For toroids it is nominal at 10 kHz with a wide tolerance (±20% or ±25%), and permeability changes with frequency, so measure to confirm.