Ground connection between 24v powered board and 12v supply

I have a SKR2 mainboard powered by a 24v power supply, an external heated bed mosfet unit, and a dedicated 12v supply for a 12v 300x300 bed (yes, it would have been much simpler to go with an all-around 24v sistem, but I had the bed and power supply from a second hand 3d printer already and didn't want to spend more money).

I want to use some 12v fans for cooling the stepper drivers and the external mosfet's heatsink. I also want to be able to control them via gcode. Now, I know that I can just plug them in regularly and just limit the duty cycle to half, so that the fans get only "12v", but I've read that this is not really the recommended method; instead I should use a buck converter connected to the mainboard to get 12v, wire the positive wire of the fan to the converter and the negative wire to the board.

Unfortunately, I don't have any buck converters at hand, so I tried to connect the black wire of the fan to the negative terminal of the 12v power supply instead, but it didn't work. I'm guessing this is because there is no ground connection between the SKR and the 12v supply. So, my question is, can I fix this by simply connecting the grounds of the board and the supply, or would this cause some other issues?

Yes you need to connect the grounds, however the SKR board almost certainly switches the negative side of its fan output. So connect the red fan wire to +12V from the 12V PSU and connect the black fan wire to the negative pin of the fan output of the control board.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
So connect the red fan wire to +12V from the 12V PSU and connect the black fan wire to the negative pin of the fan output of the control board.

I tried that already; it didn't work.

I did a bit of reading and I think I might have to reformulate the question a bit:
Since the skr and its 24v psu, and the heatbed and the 12v psu are isolated from each other, if I were to connect the ground from the skr to the ground on the 12v psu, would the resulting ground loop be a potentially dangerous issue?

You won't create a ground loop unless the grounds are already connected, for example if both PSUs have a connection between their negative output and mains ground. ATX and other PSUs intended for desktop PCa have such a connection built in but most other PSUs don't. You can use a multimeter to check for this.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Just to confirm , here is a quick sketch of how the wiring is done for my sistem (in case it matters, I'd like to mention that the 24v psu is a 100W laptop charger). So, would it be fine to connect the gnd of the skr to the negative of the 12v psu?

This is my first time building a 3d printer and I don't want to make a change without being sure that it won't break things.

Yes, connect the negative outputs of the two PSUs together. Laptop PSUs normally use a two core mains cable (no ground wire) so it won't create a ground loop.

The ground loops to avoid are the ones created when you connect the printer to another device such as a PC through a cable that has a ground connection e.g. a USB cable, and both devices have a connection between their PSU output and mains ground. See [docs.duet3d.com].

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Ok so I have connected the grounds between the board and the psu, and now I can control the fan successfully. However, I have noticed that whenever I turn on the printer now, the fan status indicator led is always on, no matter the status of the fan. I think it's probably at half power, since if I turn on the fan, the led glows brighter. Should I be worried about this?

By the way, I have assigned one of the spare heater ports as a fan port, since I'm planning on adding a powerful 12v blower for a cpap part cooling system, and the same behavior applies to that as well.