D12 - From Arduino Pin 12 turns on Q3
ALT_O - Connected to outside world to ground side of 12v Relay coil with built in flyback diode (Also connected to Cathode of an on-board LED through a 2K resistor to +12v. 2K is used so it's not real bright.

My question; Is D4 really doing anything? There is a flyback diode on the relay. I think the intention was if a relay was used that did not have a flyback diode built in (which could be several feet from the board) D4 would help handle the flyback when the relay turns off, but will it? Is there a better method? I'm trying to decide if I should just remove D4 or leave it if it has potential.

 

Here's the data sheet:
https://assets.nexperia.com/documents/data-sheet/PMEG6020EP.pdf
it would be my guess that the fused +12V through the reverse blocking diode (D4) might be a way of ensuring that whatever ALT-O is connected to (in your case a relay) is held at a specific voltage. Not that any real current would be present, just a 12V potential. When input D12 is active (or inactive - still don't fully understand FET's) ALT-O switches to a hard ground.

As for the diode (D4), my "Guess" would be that it's not doing much in your circuit - if only connected to a relay. Since the relay has its own FB diode I wouldn't think D4 is doing much. Leave it - remove it - why go through the trouble? Just leave it if it's not affecting your circuit. It must be there for a reason.

 

Thanks for your reply,

Leave it - remove it - why go through the trouble? Just leave it if it's not affecting your circuit. It must be there for a reason.

Sorry I wasn't more clear - I am re-designing the board layout, beefing up I/O protection, and generally cleaning up the design. Just started wondering about this Diode. Only thing I can see its purpose is secondary flyback protection. I'm thinking I'll leave it in because I think it does offer some protection, and it never hurts to have more.

 

The low forward voltage of a Schottky might cause the relay to turn off a bit more slowly.

 

When Q3 conducts; what is intended to happen at ALT0 and to the F3 ?

 

When Q3 conducts; what is intended to happen at ALT0 and to the F3 ?

When Q3 turns on ALT_O is providing ground to the external relay, and to the on board LED indicating it is ON. F3 protects in case someone connects +12v to the output and Q3 turns on creating a short.

Everything works, I'm just curious about D4

 

the mosfet has built in diode that can protect it, though that may not be enough to protect other things on the board.
so if the relay is soldered onto same PCB and has a diode, you don't need to add one but...
if the relay is not soldered to the PCB (external or socketed) i would say it is better to keep that diode. diodes are cheap and tiny, taking next to no space on the board. i always add one for peace of mind.
relays with built in protection and LED indication tend to be plug-in type (removable/socketed).