PLC Relay Outputs

We have Allen-Bradley PLC's with Relay output cards.
My question is if I'm using 24VDC from the output to drive a coil, do I use an MOV or a diode? MOV's are bi-directional and we use them on our relay outputs for driving 120Vac solenoids with some success. But I don't have much experience with DC. What is the reason for using the diode? I've found that Allen-Bradley recommends a 1N4004.

Thanks.

The diode protects the transistors in the output card from the high voltage that is created when the coils magnetic field collapses. Although I am not sure that it is needed using a relay output card.

What is the reason for using the diode?

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Surge suppression.

If you are using relay output module, not transistors are involved into contactor coil energizing. So if contactors are not very big, you may go without any suppression.
Purpose of a diods across contactor's coil - to create a shirt sircuit and because of that to speed up drop time of the contactor. From a theory, when you removing power from any inductance, the inductance generate current which is directed in a manner to maintaine inductance. So, if you connect a diod across the coil in a way it connected in oppozit direction, the diod will minimize the selfinductance effect.

kfoley said:

The diode protects the transistors in the output card from the high voltage that is created when the coils magnetic field collapses. Although I am not sure that it is needed using a relay output card.

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Its considering a good design practice to use it no matter what is driving it.