I came across this motor switching circuit. The TVS diode as a protection diode makes sense there, however I could not find any references to why there is a diode in series with the motor. What is the purpose of that diode?

 

Where did you seethe circuit?? For some folks protection diods are their religion. Even when the rate of change is slow.so that no spike is produced. A series diode may be to prevent free-wheeling generated voltages from causing damage.

 

Where did you seethe circuit?? For some folks protection diods are their religion. Even when the rate of change is slow.so that no spike is produced. A series diode may be to prevent free-wheeling generated voltages from causing damage.

It was from a motor driver schematic. I suppose the FET already has a built in series diode. Are there any cases where a low resistance diode like schottky diode is used for any sort of transient protection?

 

The only two things I see a series diode doing are:

1. Prevent reverse voltage from being applied to the motor
2. Drop the voltage by a fraction of a volt.

I see no transient protection being provided, since the motor never generates any positive transient voltages during normal operation.

 

The diode may also be to prevent the reverse voltage that can be generated, when the load spins the motor in the other direction, from damaging the driver circuit. Most DC motors will also perform as DC generators.

 

What is the purpose of that diode?

From what you've given us, it serves no purpose.