The Bedini SSG Circuit:
A Complete Guide for Beginners

The Bedini SSG — Simple School Girl — is the most famous pulse motor circuit in the DIY community. Its name comes from a story about a young student building it for a science fair, guided by inventor John Bedini. Whatever the origin story, the SSG design has been built by tens of thousands of experimenters around the world, making it the best-documented and most community-supported pulse motor build you can start with.

Who Was John Bedini?

John Bedini (1947–2016) was an American inventor and electronics engineer best known for his work on pulsed motors and battery chargers. He spent decades refining circuit designs for what he called "energizers" — devices that run on pulsed DC while capturing and returning back EMF to charge batteries. Whether or not his more extraordinary claims about energy efficiency hold up to rigorous measurement (see my honest take on that), his circuits are genuinely well-designed and educational.

What Makes the Bedini SSG Unique?

Unlike a simple Hall-sensor-triggered pulse motor, the Bedini SSG uses a bifilar wound coil — two wires wound together. One winding drives the motor (the power winding), and the other acts as a trigger (the feedback or trigger winding). When a rotor magnet passes the coil, the trigger winding detects it and fires the transistor, which drives the power winding. The circuit is self-triggering with no external sensor needed.

See also: How to Wind Coils for a Pulse Motor for bifilar winding technique.

The Bedini SSG Component List

• Rotor wheel with magnets — typically 8 neodymium magnets, same pole facing outward
• Bifilar coil — two strands of magnet wire (often 24–28AWG for trigger, 20–22AWG for power) wound together on a former
• NPN transistor — 2N3055 is the classic choice; TIP35, MJL21194 also work well
• Base resistor — typically 470Ω to 1kΩ, sets the transistor's bias
• Neon lamp or LED (optional) — placed across the base resistor as a visual feedback indicator and over-voltage protection
• 1N4007 diode — on the transistor's base for protection
• Fast recovery diode (UF4007 or similar) — for the back EMF recovery path
• Primary battery — 12V SLA recommended
• Secondary (recovery) battery — 6V or 12V SLA to capture back EMF
• Heatsink for transistor

How the Bedini SSG Circuit Works — Step by Step

1. A rotor magnet approaches the coil. As it does, the flux change induces a small voltage in the trigger winding.
2. This trigger voltage is applied to the base of the NPN transistor through the base resistor.
3. The transistor turns on, allowing current to flow from the primary battery through the power winding (collector to emitter).
4. The energized power winding attracts the magnet, accelerating the rotor.
5. As the magnet passes the coil center, the trigger voltage reverses, switching the transistor OFF.
6. The collapsing magnetic field in the power winding produces a high-voltage back EMF spike.
7. This spike passes through the recovery diode into the secondary battery, charging it.
8. The next magnet arrives and the cycle repeats.

It's worth emphasizing that the transistor choice is critical here. The 2N3055 is robust but slow — it works, but faster transistors can improve switching efficiency significantly.

Bedini SSG Build Tips from Papa Bale

• Wind your bifilar coil tightly and evenly — sloppy winding gives sloppy coupling between trigger and power windings
• Mount the coil so the center is at rotor magnet height — adjustable mounting is worth the extra effort
• Start with an 8-magnet rotor for smooth, regular triggering
• Use a heatsink — the transistor gets warm even at low power levels
• The neon lamp indicator glows when the circuit is firing correctly — it's a satisfying visual confirmation
• For best BEMF recovery, make the primary battery voltage higher than the recovery battery

From Here: Scaling Up

Once your basic SSG is running, you can add more coils, use multiple transistors, or experiment with different rotor configurations. The community has built SSGs ranging from palm-sized toys to multi-coil, multi-battery systems that run for extended periods. The Pulse Motors for Beginners guide has the full roadmap.