To add protection against high voltage spikes damaging the stepper motor drivers, it is recommended to add diodes to the motor output. There is documentation on this at the Smoothieware site, however, I found there were some inconsistencies that led to some confusion.
You can find the information about adding diodes to the stepper motor drivers at the bottom of this page:
That page also links to a write up on the diode installation which I provide here for convenience:
The documentation recommends using a TVS diode for protection. The first thing to note is that there are unidirectional and bidirectional TVS diodes.
First Point of Confusion
The documentation contains a link to the bidirectional diode. The pictures show a unidirectional diode (there is a cathode mark on one end).
Second Point of Confusion
The link in the documentation is for a diode on Digikey. The pictures on Digikey show a unidirectional diode on the pages for both the bidirectional and unidirectional diode. The products are correct, but the images are wrong for the bidirectional diode. To further complicate things, the description does not spell out which is the unidirectional diode and which is the bidirectional one. You have to read the datasheet and compare it to the part numbers.
Sorting it Out
After some research and testing, I concluded you need the bidirectional version. This is the version that has the "C" in the suffix. The links for Digikey and Mouser are provided below.
Digikey Original Diode
This is the link from the documentation.
Digikey Equivalent Replacement Diode
This has the same ratings as the diode in the documentation link. Its picture is correct so it is less confusing.