Are you plugging the 3A power supply into the USB jack?Is the power supply regulated?
Post a photo of a drawing of how you have everything connected. (It must be a drawing - however crude - not a photo of your Arduino)...R
How is the PSU connected to the Arduino?
Hi, RobinIt´s actually a very simple circuit.
In that case it will only take a minute or two to draw it. No amount of words is an alternative for a drawing.It sounds like you are plugging a 5v power supply into the barrel jack on your Arduino.The minimum voltage for that jack is 6v and the recommended minimum voltage is 7v. RTFM...R