Ferrite Core Inductor Turns
Find the turns for a target inductance from the core AL and check flux density against saturation.
Step-by-step
- Enter valid values to begin.
Defaults suit a typical FT50-43-class toroid (AL about 523 nH/N², Ae about 13.3 mm²); take AL, Ae and Bsat from the datasheet of your exact core and mix. AL tolerance of ±25% is common, and AL is a low-level, low-frequency figure that drops with frequency and DC bias. Bsat of 250 mT is a typical hot MnZn figure, and the warning threshold is 70% of it (a rule of thumb, not a standard). Copper resistivity is taken as 1.72×10−8 Ω·m at 20°C; winding capacitance, skin effect and window fill are not modelled. In Find N mode, B and wire figures use the nearest whole number of turns.
Ferrite Core Inductors: Turns from AL
Core Engineering Principles
Every ferrite core maker publishes an inductance factor, AL, in nH per turn squared. It bundles permeability and core geometry into one number. Inductance is AL times N², and that square is why the arithmetic bites: doubling the turns quadruples the inductance, and one turn too many or too few on a 14-turn coil moves you several percent. Take the AL from the datasheet of your exact core and mix, and remember the tolerance. ±25% is common for ungapped toroids, so measure the finished coil on an LCR meter.
The AL printed in the catalog is a low-frequency, low-level figure. Ferrite permeability falls with frequency, and each mix has its own useful range: a type 43 or 61 style mix is chosen for EMI chokes or RF work, a 77 style mix for power and low-frequency use. Pick by the datasheet curve, not by what is in the drawer. The other limit is DC bias. Flux density is B = L·I / (N·Ae), and once it approaches saturation the permeability collapses and your inductor stops being an inductor. MnZn ferrite saturates around 200–500 mT and loses margin when hot. An air gap lowers AL but lets the core carry much more current, which is how power-supply inductors are built.
B = L × I / (N × Ae) • Length = N × MTL • Rdc = ρ × length / (π d² / 4)
NEC & Standard References
IEC 62024-1 covers high-frequency inductors of nominal size for surface mounting. IEC 60205 gives the method for calculating the effective parameters of magnetic piece parts (effective area, path length and volume) that the AL and B calculations rely on. The manufacturer’s datasheet remains the authority for AL, tolerance and saturation. Check the current edition of each document.1. N = √(100,000 / 523) = 13.83 turns.
2. Round to 14 turns: L = 523 × 14² = 102.5 µH (+2.5%). Thirteen turns gives 88.4 µH (−11.6%), so 14 is the pick.
3. B = 102.5 µH × 0.3 A / (14 × 13.3 mm²) = 165 mT, which is 66% of Bsat and under the 70% limit (175 mT).
4. Wire length = 14 × 40 mm = 0.56 m; Rdc = 1.72×10−8 × 0.56 / 1.96×10−7 m² = 49 mΩ.
At 1 A the same coil would reach 0.55 T, far beyond saturation.
- AL tolerance. A ±25% core gives ±12.5% in turns-for-a-target.
- Count turns correctly. One turn is one pass through the centre hole. Wire outside the core is not a turn.
- Saturation under DC bias. Inductance can drop sharply well before the current looks alarming.
- Self-resonance. Winding capacitance resonates with L and turns the inductor into a capacitor above that frequency.
- Wrong mix. A low-frequency mix has high loss at RF, and an RF mix has low AL and little noise attenuation at low frequency.