Former-Wound Coils: Design, Materials & High-Frequency Applications

A former-wound coil (also called a coil wound on a form or bobbin) is an inductor where insulated wire is wound around a rigid cylindrical or shaped form (the "former"). The former provides mechanical support, maintains coil geometry, and can serve as a magnetic core or spacer.

Construction

• Wire: Magnet wire (enameled copper or aluminum) with thermal class rating (e.g., 155°C, 180°C, 200°C).
• Former materials:
  • Plastic: Phenolic, nylon, PTFE, PEEK—good insulation, low cost, non-magnetic.
  • Ceramic: Alumina, steatite—high temperature stability, low loss at RF.
  • Ferrite/iron powder: For inductors needing high permeability.
  • Air core: Former removed after winding (self-supporting) or non-magnetic form left in place.
• Insulation: Wire enamel + interlayer insulation (tape, film) for voltage rating.

Applications

• RF/microwave circuits: Air-core or ceramic former coils for low loss, high Q.
• Power electronics: Ferrite/iron powder formers for SMPS transformers, chokes.
• Sensors: Inductive proximity, position sensors.
• Medical devices: MRI gradient coils, RF coils (often on complex formers).

Advantages

• Precise, repeatable geometry → consistent inductance.
• Mechanical robustness for vibration/shock environments.
• Facilitates automated winding.
• Former can integrate terminals, mounting features, shielding.