Project: Toroid-Ease (NexRig FPC Inductor Generator)

1. Abstract

Toroid-Ease is a Python-based design automation tool for KiCad 9. It generates manufacturing files for Flexible Printed Circuits (FPC) that serve as high-performance, repeatable toroidal inductor windings. Instead of winding magnet wire by hand around a toroid core, this tool generates a flat FPC strip that rolls into a cylinder and locks into shape, creating a precision solenoid "liner" that sits inside standard powdered-iron RF cores (e.g., T-68, T-50).

This component was created along the way to building the larger NexRig project (an Envelope Elimination and Restoration HF transceiver).

2. The Problem & Solution

• The Problem: Hand-winding toroids is tedious, inconsistent, and difficult to reproduce exactly across multiple units. High-current windings require thick wire that is hard to manage.

• The Solution: An FPC "insert" provides exact inductance, consistent stray capacitance, and high current capacity (via wide flat tracks) in a package that can be mass-manufactured.

3. Mechanical & Electrical Geometry

The generator creates a 2-layer FPC strip with the following features:

• Form Factor: A rectangular strip sized exactly to the Inner Circumference of a specific magnetic core (e.g., T-68).

• Winding Topology:

• Tracks: Vertical parallel copper tracks run from top to bottom, forming a solenoid coil when the strip is rolled into a tube.

• Turns: The script auto-calculates the pitch and track width to fit the requested number of turns () and Current () within the core's ID.

• Locking Mechanism (The "Coffee Sleeve"):

• Edge B (Top): Features Trapezoidal Tabs extending outwards.

• Edge A (Bottom): Features Rectangular Slits (slots) cut into the PCB body.

• Assembly: When rolled, the tabs slide into the slots to mechanically lock the cylinder shape before insertion into the core.

• Termination:

• Castellated Pads: The Start and End connections are essentially plated half-holes (castellations) with a specific "U-shaped" edge cut. This allows the coil to be soldered vertically onto a motherboard or wired easily.

4. The Software (toroid-ease.py)

The tool is a CLI script using the KiCad 9 Python API.

• Inputs:

• -c: Core Type (e.g., T-68).

• -t: Number of Turns (e.g., 54).

• -a: Target Current in Amps (e.g., 3.5) — used to calculate track width.

• --angle: Angular coverage (default 360°).

• Logic:

1. Physics Check: Calculates if the requested turns + required copper width fit inside the core.

2. Rejection: If the design is physically impossible (tracks would overlap), it aborts and suggests a larger core or fewer turns.

3. Generation: Draws the Board Outline, Copper Tracks, Tabs, Slits, and Castellation cuts using KiCad's geometric primitives (PCB_SHAPE, SHAPE_T_SEGMENT).

• Output:
• A .kicad_pcb board file. This format is used because the FPC is a standalone product to be manufactured, not just a footprint on another board.

5. Current Status (As of Dec 20, 2025)

• Functionality: The script works for KiCad 9. It successfully handles the tab/slot geometry, castellation cuts, and physics validation.

• Known Constraints:

• Gap: Hardcoded safety gap of 0.15mm (6 mil) for FPC fabrication.

• Density: It enforces a strict "No Overlap" rule.

• Immediate Next Step: Validating the generated design in KiCad's DRC to ensure the calculated gaps are truly manufacturable, followed by implementing Panelization (placing multiple strips on one manufacturing panel).

6. Example Usage

./toroid-ease.py -t 54 -c T-68 -a 3.5 -o t68-54t-3.5a.kicad_pcb

(If the coil is too dense, the script will reject it and tell you the required Core ID size.)