I'm pretty new to Arduino, so I apologize in advance if there's a really simple answer out there that I wasn't able to find.
I've been using my Arduino mainly to test LED's and other small components, so I've kept it plugged into my laptop without any issues until now. Recently, I built a project using a receipt printer (Hello Printer), so it was necessary to plug the Arduino (connected to an ethernet shield and the printer) into a wall socket. Almost immediately, both boards began to heat up to the point where it hurt to touch. I unplugged it as quickly as I could, and there didn't appear to be any damage. My first thought was that I was using the wrong adapter, so I checked and confirmed that it is a 9v AC-DC adapter. My next thought was that the printer was using too much power, so I unplugged it and tested a simple LED blink program. That made the Arduino and the ethernet shield heat up as well. Finally, I tried the LED blink program in a different socket, only to have the same results. I am aware that it's normal for the board to heat up slightly, but this seems a little extreme.
What should I do? I'm all out of things to try.
Thanks in advance!