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I'm building fan controller for my PC water cooling loop, it will consist of:

  • 1x Raspberry Pi Pico,
  • 6x 3-pins fan headers, 0.5A max each,
  • 1x water pump header, 2.5A max,
  • 1x thermistor header for sensing coolant temperature.

My idea is to connect Pi to motherboard's USB 2.0 header and use it to control fan curves and monitor fans speed and water temperature. Water pump and fans will be powered from SATA, Pi and thermistor from USB. I noticed a problem, this whole device can take 5.5A from SATA and I'm a bit worried that current may choose USB ground to return and it may exceed its max current and damage motherboard.

Am I right and should I use separate ground and optocouplers or one common ground will be just fine?

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  • \$\begingroup\$ Not an expert, but I think it should be fine as long as you connect the SATA ground to your controller. The current should choose the least resistance path. I would be more worried about creating a ground loop I think. You could check for continuity between the signal an power ground pins from a hard drive, to see how they do it. If hard drives tie their grounds together I wouldn't give it a second thought. \$\endgroup\$
    – Toun
    Commented Mar 6, 2022 at 16:54
  • \$\begingroup\$ Just checked for you: my WD Red hard drives tie their power and signal grounds together. \$\endgroup\$
    – Toun
    Commented Mar 6, 2022 at 17:02

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Yes, if the grounds are not connected unexpected results will probably plague you. SATA is a serial interface for drives and USB is a universal serial bus, both share a ground with the rest of the electronics in the system. I would suggest a different source of power,(12V # 5.5A = 66 watts) the ATX standard for SATA or Molex connections is only rated for 54 watts max. A fully implemented SATA connector has 5 Ground connections, 3 X 12V, 3 X 5V, 3 X 3.3V and a spin up pin that is only used in enterprise class drives. It is close, maybe use two or split the loads appropriately.

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  • \$\begingroup\$ When I said not connecting grounds, I meant grounds are of course connected inside PC, but not on board, so high current from SATA doesn't return through USB cable, and use optocouplers to control fans, water pump and getting speed feedback. I believe same technique is use in cars to make sure that return current doesn't choose "wrong" header. You are right about power consumption - I totally forgot about it. I don't intend to run all fans and water pump on full speed, so something like ZXCT1032 may be useful in case I messed up something and exceed max current. \$\endgroup\$
    – urhen
    Commented Mar 8, 2022 at 14:31

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