Back Electro-Motive Force (EMF)

Whilst a motor usually converts electrical energy into mechanical energy in the form of shaft rotation, it can also behave as a generator and convert the mechanical energy into electrical energy. If you rotate the shaft, then the motor will produce a voltage across its terminals. A motor obeys Lenz’s Law, which states that a current flowing through a wire within a magnetic field causes it to move. However, a motor can also behave as a generator obeying Faraday’s Law of induction, that a coil passing through a magnetic field induces a current through it. Therefore, there are two electromotive forces in action, where one converts electrical energy into mechanical energy, and the other converts mechanical energy into electrical energy.

Faraday and Lenz never got on, and as you can see, when they get into your coils, they fight there as well causing opposing forces. :-) Ideally, you just need Lenz’s electro-motive force (EMF) in a motor; however, Faraday’s Law produces a back-EMF to oppose.

Protection Diodes

Back EMF usually manifests as reverse voltage spikes within a circuit, which can damage semiconductors such as transistors. Therefore, protection diodes safely ground these nuisance voltages.

Diode

BYW98-200 is a high efficiency fast recovery rectifier diode. It has low conduction losses, negligible switching losses, low forward and reverse recovery time, and it can handle a peak reverse voltage of 200 V, with a recovery time of 35 ns.

Ideally, you will want to connect the protection diodes close to the motor terminals and have their dedicated earth wires connected to a common chassis ground.

Snubber

In this circuit, there is also an RC filter network across the motor terminals consisting of a 10 Ω resistor and a 22 µF capacitor.

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