Understanding Back EMF
in Pulse Motors

Back EMF is the secret sauce of pulse motors. It's what separates a well-designed pulse motor from a simple electromagnet that spins a wheel, and it's at the heart of why experimenters like me spend hours tuning circuits looking for every last volt. Let me explain what back EMF actually is, why it matters, and how to capture it.

What Is Back EMF?

Back EMF (back electromotive force, also written BEMF) is a voltage produced opposite to the applied voltage in an inductor when the current through it changes. In a pulse motor coil, two things cause back EMF:

1. Inductive kickback — when the transistor switches off and current through the coil suddenly drops to zero, the collapsing magnetic field induces a reverse voltage spike. This spike can be many times higher than the supply voltage.
2. Generator effect (motional EMF) — as the permanent magnet on the rotor passes through the coil's field, it generates a small voltage even without any applied current.

The inductive kickback is the big one for pulse motor efficiency. A coil with significant inductance (many turns of wire) can produce spikes of 100V+ from a 12V supply when switched abruptly.

Why Does Back EMF Matter?

In a conventional circuit, that high-voltage spike is destructive — it's what kills transistors without a protection diode. The spike has to go somewhere: in a standard design, a flyback diode clamps it and the energy is dissipated as heat in the diode and resistances. That energy is lost.

In a well-designed pulse motor circuit, that energy isn't lost — it's captured. This is what makes back EMF pulse motor designs so interesting and why the Bedini SSG and similar designs became so popular. Instead of clamping the spike to ground, you route it to a secondary battery or capacitor and store it.

How BEMF Recovery Works

The BEMF recovery circuit is elegantly simple in concept:

1. The transistor switches ON → current flows through the coil → magnetic field builds → rotor magnet is attracted/repelled
2. The transistor switches OFF → magnetic field collapses → back EMF spike is generated across the coil
3. A diode (oriented to block during normal current flow) becomes forward biased by the reverse spike
4. The spike voltage charges a secondary "recovery" battery or capacitor bank

The primary battery powers the motor. The secondary battery accumulates the recovered energy. In a well-tuned system, you can run the motor for an extended period and then use the secondary battery to recharge the primary — creating a partially self-sustaining system.

I want to be clear: this does not create free energy. See my post on Pulse Motors and Free Energy for the honest breakdown. But it does demonstrate that naive circuit design wastes a lot of energy, and careful design can recover a substantial portion of it.

Measuring Your BEMF Spike

To measure your BEMF spike voltage safely:

• Use an oscilloscope — this is the best tool. Set it to AC coupling or watch for the spike on the drain/collector of your transistor.
• A multimeter won't capture the full spike (it's too fast) but can give a rough average
• Use a high-voltage probe or resistor divider if your oscilloscope input is limited to 300V
• Start with a resistive load on the recovery side to safely dissipate spikes while you characterize the circuit

Building a BEMF Recovery Circuit

To add BEMF recovery to your basic pulse motor build:

1. Remove the standard flyback diode (or keep it as a safety backup)
2. Connect a fast-recovery diode (UF4007 or similar) from the coil's high side to the positive terminal of your recovery battery
3. Connect the recovery battery negative to your main circuit ground
4. Make sure the recovery battery voltage is lower than your typical BEMF spike voltage
5. Monitor both batteries with voltmeters while running

The recovery battery should slowly rise in voltage while the motor runs. If it doesn't, your spike voltage is too low relative to the battery voltage — try a higher-inductance coil or a lower-voltage recovery battery.

Maximizing BEMF Recovery

Several factors increase the BEMF spike and thus recovery potential:

• Higher inductance coil (more turns of finer wire — see wire gauge guide)
• Faster transistor switching (sharper cutoff)
• Higher primary supply voltage
• Optimized pulse timing

This is where the rabbit hole of pulse motor efficiency begins. There's always more tuning to be done, and the numbers can get surprisingly interesting.