How to Wind Coils for
a Pulse Motor

Winding a coil is one of those skills that sounds intimidating but becomes almost meditative once you've done it a few times. A well-wound coil makes a noticeable difference in your pulse motor's performance. A sloppy one will frustrate you. Here's everything I know about winding coils for pulse motors, from core selection to final resistance check.

Core Selection

The core is the material at the center of your coil. It dramatically affects the coil's inductance and magnetic field shape:

• Air core — no core material at all; lowest inductance but no core losses; good for high-frequency or experimental builds
• Ferrite core — dramatically increases inductance for the same number of turns; common in high-efficiency builds; check that the ferrite is suitable for your pulse frequency
• Iron powder core — good inductance boost with lower core losses than solid iron; available in toroid shapes
• Steel bolt or rod — widely used by beginners (cheap and available); higher core losses but acceptable for learning
• PVC or plastic tube — acts as an air core former; zero permeability boost but clean and non-lossy

For a first build: a steel bolt former is fine. As you advance, try ferrite cores and notice how much inductance (and BEMF) you gain with the same turn count.

Single Winding vs Bifilar Winding

This is a key decision in pulse motor coil winding:

Single winding: One wire, wound uniformly around the former. Simple to make, easy to understand. Used in pulse motors with external Hall sensors. Full winding is the power coil.

Bifilar winding: Two wires wound together simultaneously, side by side, in the same direction. The result is two identical coils with very tight coupling. Used in Bedini SSG circuits where one winding drives and one triggers. Also used for BEMF recovery configurations.

To wind bifilar: tape two reels of wire together, tape both ends to the former, and wind both simultaneously as if they were a single thicker wire. Keep tension equal on both strands. Mark which end belongs to which wire before you tape them so you can identify them later.

Winding Direction

Winding direction (clockwise or counterclockwise) determines the polarity of the magnetic field produced. For a simple single-coil pulse motor, it doesn't matter as long as you're consistent. What matters is:

• Both ends of the coil wire are accessible
• If you add a second coil, its winding direction must be coordinated with the first
• In a bifilar coil, both strands must be wound in the same direction

If your motor runs backwards, reverse the coil connections — this effectively reverses the field polarity without rewinding.

Step-by-Step Coil Winding Process

1. Prepare your former — clean it, check that it fits in your coil mount. If using a bobbin or PVC tube, secure the ends to prevent splaying.
2. Leave a tail — leave 6–8 inches of wire before you start winding. Tape or hold this tail securely.
3. Apply tension — maintain firm, consistent tension as you wind. Loose winding creates a bulky coil with poor fill factor.
4. Wind in layers — complete one even layer before starting the next. On each layer reversal, bring the wire across as smoothly as possible.
5. Count your turns — keep count as you go; it's easy to lose track. A paper counter or clicking pen works fine.
6. Secure each layer — a thin coat of coil varnish or just electrical tape between layers prevents unwinding.
7. Leave a tail at the end — same as the start, 6–8 inches.
8. Measure resistance — use a multimeter to check DC resistance between the two tails.

Target Resistance Values

There's no single correct resistance for a pulse motor coil — it depends on your supply voltage, transistor, and goals. But here are practical ranges:

• 12V supply with 2N3055: aim for 1–5Ω coil resistance for good current flow
• 12V supply with TIP31: aim for 3–10Ω to keep current within transistor limits
• Higher voltage supplies (24V+): can tolerate higher resistance coils

Measure your coil with a multimeter and calculate peak current (V/R). Make sure this is within your transistor's Ic rating. See the transistor guide for specific ratings.

Mounting Your Coil

Mount the coil on an adjustable bracket if possible — being able to rotate and slide the coil to adjust timing and air gap without rewiring is invaluable. A 3D-printed bracket or simple plywood bracket with slotted holes works perfectly.