Hello Dirk,
I stumbled upon this yesterday on the internet so good timing.
Q: Is a USB plug AC or DC?
A: All USB ports are 5 volts DC. The transformer (or computer) will take care of converting the 120/220 AC current to the necessary 5 volts DC. You will not fry anything. The available current can vary though.
I found this using a Google search, of course treat all stuff on the internet with a grain of salt, but it does make sense. The thing you want to know is the amperage (amps) that your device needs.
Here is the internet reference:
https://www.tripadvisor.com/ShowTopi..._and_Gear.html
Hope this helps.