Do I need a flyback diode
Do I need a flyback diode for a 24 volt DC 5.4 watt coil inductive load on my PLC relay?


Yes.


  Crouzet Millennium 3. They show to add a diode for inductive loads in the minimal manual but don't specify what diode.



For your 24 V DC application the diode needs to have a reverse breakdown rating > 24 V. 50 V would be good.
At the instant of output turn off the inductance is initially going to continue to drive the current through the diode and it will then decay to zero as energy is lost to the coil resistance and voltage drop across the diode. Therefore the diode should be rated at the load current - or at least peak rated.
\$ I = \frac {P}{V} = \frac {5.4}{24} = 0.25 \ \mathrm A \$. A 1 A diode would be excellent.








simulate this circuit &ndash; Schematic created using CircuitLab

A 1N4001, 1 A, 50 V diode meets both criteria above.