I'm using this Phoenix Contact DIN rail relay, DigiKey part 277-17665-ND (data sheet). I thought it would be prudent to add a flyback diode, but looking at the data sheet, it vaguely suggests it already has protection.

In the section “Input Data”, there’s a line “Type of protection” with value “Free-wheeling diode.” In the section “Output Data,” “Type of protection” says “Reverse polarity protection” and “Surge protection.” Later it says the rated surge voltage is 4 kV.

I don’t know much about industrial controls, but it would be convenient if they could handle inductive kickback without external protection, so I wonder if they don’t.

My inductive load is a motion brake (data sheet) labeled 24 V, 0.41 A, 9.8 W (and that was borne out in measurement on the bench; the data sheet is incorrect).

Will my relay be okay without additional protection?


1 Answer 1


No, I don't think so. The diode has to go across the load so there would have to be another pin.

You'll need to add an external diode across the load.

Whatever that reference to "free wheeling diode" means, it is in the input section of the datasheet. There is also nothing shown in the schematic.

enter image description here

A 1N400x eg. 1N4004 etc. 1A diode is more than sufficient for this application.

  • \$\begingroup\$ Thanks for confirming my suspicions. It’s a bit awkward to set it up cleanly in my crowded cabinet, but I found a way to do it (and I happened to have a bunch of 1N4004 in a drawer). \$\endgroup\$
    – Rick
    Jul 11, 2020 at 2:33

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