Wawa:
If you connect 12volt/ground to a relay board, and the same 12volt/ground to VCC/GND of the Arduino...Well, I leave that for you to find out.
Stand back a bit though, and wear protective glasses.
Leo..
LoL you've made your point.
But, as people always state the current flow in a circuit. Why current from the power supply would bother going to the arduino VCC instead of flowing back to the negative terminal of the power supply?