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Coil and toroid winder

Inductors for filters, matching networks and transformers are wound on toroid cores or as air-core coils. Enter the inductance you need and see the winding drawn to scale.

Winding calculator

Micrometals: carbonyl iron for RF, high Q up to 40 MHz. Painted red.

MHz

Used only to show the reactance.

T-50-2Mix 2 iron powderOD 12.7 mm, ID 7.7 mm45 turns
Drawn to scale. Each line across the core is one turn, counted where the wire passes through the hole. Alternate turns are drawn as a second layer because one layer does not hold them all.

45 turns of 24 AWG on a T-50-2 give 9.92 µH, within 5 percent (9.43 µH to 10.4 µH). They do not fit in one layer: the hole holds about 44 turns of this wire. Use thinner wire or a larger core.

Turns
45
Inductance with 45 turns
9.92 µH
Wire for the winding
73.2 cm plus leads
Reactance at 7 MHz
436 Ω
FT-50-43FT-37-61T-50-2 and T-50-6target
Doubling the turns quadruples the inductance on every core, so every line has the same slope. The core's AL value only moves the line up or down.

Try this

  • About 45 turns on a T-50-2. Low permeability iron powder needs many turns but holds its value closely.

  • Only about 5 turns on a ferrite core of the same size. Ferrite needs far fewer turns, but its AL value is only held to 20 percent.

  • Twice the turns gives four times the inductance, because inductance grows with the square of the turns.

  • A 49:1 impedance transformer needs a 7:1 turns ratio, because impedance follows the square of the turns ratio.

  • Compare with the default 25 mm length: spread over 50 mm, the coil needs more turns for the same inductance. Spreading or squeezing the turns adjusts an air coil's inductance.

Apply it

  • Count turns on the inside of the core: each pass of the wire through the hole is one turn. Spread the turns evenly around the core.
  • Measure the finished inductor. Fair-Rite holds AL to 20 or 25 percent and tests it at 10 kHz; Micrometals holds RF iron powder to 5 percent, tested with an evenly spaced full single-layer winding. Bunched turns read higher, so squeeze or spread them to trim.
  • Pick the material for the job. Micrometals describes iron powder mixes 2 and 6 as high Q up to 40 MHz, and powdered iron has the most temperature-stable magnetic properties. Fair-Rite designs 61 mix for inductors up to 25 MHz and 43 mix for suppressing interference from 20 to 250 MHz and for common-mode chokes.

How it works

  • A core’s inductance factor AL is in nanohenries per turn squared, so L = AL x N² and the turns needed are N = √(L / AL). Ferrite cores need far fewer turns than iron powder for the same inductance.
  • Fair-Rite holds AL within 20 or 25 percent and measures it at 10 kHz. Micrometals holds RF iron powder within 5 percent, tested with an evenly spaced full single-layer winding; bunched turns read higher.
  • Names such as FT-240-43 and T-50-2 give the outside diameter in hundredths of an inch, then the material. Dimensions and AL values here come from the Fair-Rite and Micrometals datasheets; for the FT names, Amidon’s published dimensions and AL match the Fair-Rite parts listed.
  • Turns are counted on the inside of the core, where the wire passes through the hole, and are spread evenly around it.
  • Wheeler’s formula for a single-layer air-core coil is L = N²r² / (9r + 10l) microhenries, with mean radius r and length l in inches. It is about 1 percent accurate when the diameter is less than 3 times the length.
  • A transformer changes impedance by the square of its turns ratio: 600 ohms to 50 ohms needs about 3.5 to 1, and a 49 to 1 impedance ratio needs 7 to 1.
  • Wire sizes use the American Wire Gauge definition: No. 36 is 0.005 inch and each gauge is the 39th root of 92 times the next. Magnet wire enamel adds a little to the bare diameter.

Learn more: Electronics basics

Sources: Toroids 5943003801 (43 material, AL 1075 nH) (Fair-Rite Products); Toroids 5961003801 (61 material) (Fair-Rite Products); Inductor Core Tolerance Guide (Micrometals); Mix 2 (Micrometals); Mix 6 (Micrometals); T50-2 datasheet (Micrometals); FT-240-43 (Amidon); Workmanship Standard WS-007, Coil Wound Products (L3Harris); Inductor Sizing Equation (Lessons in Electric Circuits) (LibreTexts); Handbook 100, Copper Wire Tables (National Bureau of Standards); 2023-2027 General question pool (G5C07) (NCVEC Question Pool Committee); 2024-2028 Extra question pool (E6D05, E6D08) (NCVEC Question Pool Committee)

Practice the exam questions: G5 Electrical Principles (General), G6 Circuit Components (General), E6 Circuit Components (Amateur Extra)

Found something wrong or out of date? Report a problem with this tool on GitHub (a public issue; nothing is sent until you submit it there).