I was using a 500mA 12V AC adapter (regulated) until now. I have this prototype with a 20x4 LCD, a PN2222 and two temperature sensors on an Arduino Uno R3 and it became very hot when using the AC.
So I switched to a 5V 2.0A (regulated) and something strange happened. When I use only the AC, the Arduino powers up (I can see the ON light on the board) but the LCD stays off.
If I plug the USB (with or without AC), everything is powered up fine.
I am feeding everything via the 5V, and I am getting 4.95V with the AC at the 5V pin; or 4.87V with the USB.
AFAIK, USB can only deliver 500mA so why would it be able to power anything that a 2A supply can't?
Your external 12volt power is most likely causing the on board +5vdc regulator to shutdown because of over temperature. If you used a 8vdc external supply you would probably have more luck.
LCD logic is on +5v rail, LCD backlight is PWM pin 6 because I am using a fading effect for the backlight via a PN2222 because the PWM pin doesn't deliver enough power on its own.
retrolefty:
Your external 12volt power is most likely causing the on board +5vdc regulator to shutdown because of over temperature. If you used a 8vdc external supply you would probably have more luck.
Lefty
I am not using a 12V anymore, I am running off a regulated 5V 2A adapter now