Circuit with External Power Source and USB Port Laptop Connection = Damage?


I am looking for some help regarding using an external power source connected to a circuit and Arduino Uno connected to a MacBook Air laptop via a USB port. After the circuit was complete and the power source was turned from 6 to 7.5V, the laptop went black. Once the USB cord was plugged into a desktop, the circuit worked successfully. After taking it to a repair shop, I found that there were burn marks on the logic board and display, indicative of a power surge.
Some information about the system:

  1. Circuit composed of several LEDs, servo motors, and a pressure sensor (many circuit elements)
  2. Arduino plugged into USB port for coding
  3. Power source set to 6V originally (successful) and then increased to 7.5V to increase LED brightness (laptop died)

I am looking for any type of information regarding whether or not there are specifications for USB ports or what kinds of current/voltage/power they can handle, specifically for a laptop vs. a desktop. I am trying to determine whether this problem is a result of our specific circuit or simply due to what the laptop vs. desktop can handle. Since the USB was plugged directly into the desktop after killing the laptop and the circuit worked successfully, I think it has something to do with the USB port limits.

I am happy to share the circuit diagram if necessary.

Thank you,
Annie Rumsey