Relay coil diode placement question.

[Chris Wilson]

If I move a PCB mounted 12V coil relay off its PCB control board into another equipment cabinet, with the relay's coil powered from the external PCB still, do I NEED to move the diode across the coil with the relay, or can it now be remote, still on the control board? Thanks.

[fcb]

The back-emf diode is typically used in a circuit to protect the driver transistor, so it won't make any difference if you put it across the relay coil or on the driver circuit directly.

It *might* matter if you placed it on a long spur however, but I can't think why anyone would do this.

[Psi]

Its good practice to keep it with the relay, you then know the high voltage is snubbed at that point and can never make it anywhere else.

otherwise, if you had multiple connections and some wires were broken, or a plug wasn't connected, it might be possible to energize the relay with no diode present

[Rerouter]

Simplest answer is to mount the diode nice and close to the relay as it will shunt its switch on current through the diode thus keeping the current loop small,

Paranoid answer  is to mount a generic diode across the relay coil and a small capacitance across the switching device to limit any spikes due to long leads

[Kremmen]

Its good practice to keep it with the relay, you then know the high voltage is snubbed at that point and can never make it anywhere else.

otherwise, if you had multiple connections and some wires were broken, or a plug wasn't connected, it might be possible to energize the relay with no diode present

I agree the best place is right next to the relay coil. Mostly for making sure the diode is not inadvertently disconnected from the circuit. There won't be any "spike" as long as the diode is there; the diode provides a path for the coil current to smoothly ramp down to zero, dissipating into heat in the resistances present in the circuit. Only if the diode is absent will there be a voltage excursion due to the fact that coil voltage is proportional to the rate of change of current passing through it. Break the current == infinite change --> infinite voltage. At least in theory if not quite in practice.