By Papa Bale · April 5, 2026
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.
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:
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.
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.
The BEMF recovery circuit is elegantly simple in concept:
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.
To measure your BEMF spike voltage safely:
To add BEMF recovery to your basic pulse motor build:
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.
Once you've mastered basic back EMF recovery, several advanced techniques can further improve efficiency:
Instead of a single diode, some builders use a full bridge rectifier to capture both polarities of the back EMF spike. This can recover more energy, especially in circuits where the coil rings or oscillates after the main pulse. A bridge rectifier also simplifies grounding since both coil leads can be floating relative to the main circuit.
Rather than charging a secondary battery directly, you can store recovered energy in a capacitor bank first. Capacitors can charge more quickly from brief spikes and then discharge more slowly into a battery. This "buffer" approach can improve overall recovery efficiency, especially at high pulse rates.
Advanced designs use separate recovery paths for different voltage levels. A high-voltage path captures the initial spike, while a lower-voltage path captures the "tail" of the collapsing field. This ensures more of the available energy is harvested rather than dissipated.
For maximum efficiency, some experimenters replace the recovery diode with a second MOSFET used as a synchronous rectifier. This eliminates the ~0.7V diode drop, allowing recovery of even lower voltage spikes. However, this requires precise timing control and is significantly more complex.
One of the most interesting aspects of back EMF recovery is its effect on batteries:
Many pulse motor experimenters report that batteries charged via back EMF pulses behave differently than conventionally charged batteries. Some observations include:
While the mechanisms are not fully understood and claims should be viewed skeptically, the anecdotal evidence from thousands of builders suggests that pulse charging merits further investigation.
For battery charging applications:
Several factors increase the BEMF spike and thus recovery potential:
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.
Back EMF spikes can easily reach 5-10 times the supply voltage, and in well-designed high-inductance coils, spikes of 100V or more from a 12V supply are common. The spike voltage depends on coil inductance, switching speed, and load impedance. Always use components rated for at least 3-5 times your supply voltage to ensure safety margins.
No. Back EMF recovery does not create energy from nothing — it simply captures energy that would otherwise be wasted. The energy stored in the coil's magnetic field came from the primary battery during the pulse. Recovery circuits simply return a portion of that energy rather than dissipating it as heat. The laws of thermodynamics are not violated; the system is simply more efficient than a design that wastes the back EMF.
Standard rectifier diodes like the 1N4007 have relatively slow reverse recovery times (several microseconds). During this recovery period, they don't block voltage well, allowing some of the spike to be lost. Fast recovery diodes like the UF4007 switch off in nanoseconds, capturing more of the spike energy. For maximum efficiency, especially at high pulse rates, fast recovery or Schottky diodes are preferred.
Yes — you can recover back EMF into capacitors, supercapacitors, or even back into the primary battery (though this requires more complex circuitry). Capacitors are particularly useful for experimentation because you can watch the voltage rise in real-time and easily discharge them for testing. Some advanced designs use the recovered energy to power LEDs or other low-power circuits.
Recovery efficiency varies widely depending on circuit design, coil quality, and tuning. Well-designed systems might recover 30-60% of the energy stored in the coil's magnetic field. However, this represents a fraction of the total system input — the mechanical work done on the rotor, resistive losses in the wire, and switching losses all consume energy. View back EMF recovery as an efficiency improvement, not a source of unlimited energy.
Papa Bale demonstrates back EMF recovery circuits on YouTube. Real oscilloscope readings, real builds.