Sensors with external power supply

Now it makes sense. It would work, it would just have too much extra power that it needs to get rid of which would result in it heating up quite a bit.
I wanted to experiment a bit with external power supply.
I tried 2 new versions :

  1. I connected USB-B cable to arduino and to USB power adapter plugged into a wall which resulted in everything working fine.
  2. I did the same thing except I used DC input which resulted in sensor not working.
    My best guess is that arduino has same protection diode that drops voltage so I am ending up with a lower ouput and on the other side USB-B does not do that ?