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SchematicI have 1 computer, 1 laptop and usb powered arduino ( MCU ).

Arduino is connected to laptop via USB cable. Laptop is connected to 220V wall or runs from batteries.

One PIN from arduino is connected to PWM input ( to control fan speed ) in computer to control fan speed. I have to share ground between arduino and computer to make it work.

My question is - is it correct to connect grounds that way ?

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  • \$\begingroup\$ Are the laptop and/or computer PSUs mains ground isolated? \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 12 '14 at 12:30
  • \$\begingroup\$ I don't understand - laptop and computer are connected to wall socket \$\endgroup\$ – Michal Zahor Jan 12 '14 at 12:32
  • \$\begingroup\$ laptop runs from batteries sometimes \$\endgroup\$ – Michal Zahor Jan 12 '14 at 12:33
  • \$\begingroup\$ "Connected to wall socket" essentially means nothing. This is a characteristic of the power supply, not the plug. \$\endgroup\$ – Ignacio Vazquez-Abrams Jan 12 '14 at 12:35
  • \$\begingroup\$ ok - but I don't know how power supply is internally connected - is't ATX compatible power source and laptop is macbook pro \$\endgroup\$ – Michal Zahor Jan 12 '14 at 13:38
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have 1 computer

They use a switching supply that is connected to mains ground so the ground of the USB or any of the internal supply connectors is essentially connected to the mains ground

1 laptop

Also uses a switching supply that may or may have not a connection to the mains ground, in either case the supply is isolated from the mains so a connection of the laptop ground to the mains ground will not cause a problem.

USB powered arduino ( MCU ).

This is powered from the the laptop so the ground level it uses may be the same as the mains ground or it may not, it depends on the operation mode of the laptop (battery/mains) and the plug that the mains supply of the laptop uses (2-prong/ 3-prong ). In any case connecting the ground of the PC (which is equivalent to the mains ground) to the laptop ground through the arduino will not pose a problem.

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I do not share this opinion. Laptop switching supplies are indeed troublesome and the strict definiton of ground is not true here. In fact, once connected, each of these elements are sharing the same ground. BUT, juste before connecting to laptop USB ground, hundred volts can be on this LAPTOP USB and sometimes are able to destroy ARDUINO USB section or Laptop USB port. This issue remains, even for last generation of USB ports.

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    \$\begingroup\$ In a USB plug the first thing that "touches" (gets in contact) when you connect the plugs is the ground and as soon as that happens everything is referenced to the same level. There is case of damage only if you connect a signal without having a common ground (and assuming there is no appropriate protection for the input). \$\endgroup\$ – alexan_e Jan 14 '14 at 13:43

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