recently I bought the LCD keypad shield (1602) for my arduino UNO. My USB-port couldn't deliver enough power so the shield wasn't working. Therefore I looked for an external supply today. I found a 12V AC/DC supply that I connected to the arduino. Immediatly the LCD started to work fine, but I got electric shocks from the arduino and the shield. Could this be the problem of my adapter, or is the shield just not working as it should?