Arduino stops working when connected to an external power supply

My Arduino works fine when its connected to the computer via the USB but as soon as I disconnect it from the computer, it goes off. I am using a 12V DC adapter (500mA) (center positive) as the external power source. Not sure what is wrong. Please help! Thanks!

priyankadasgupta: My Arduino works fine when its connected to the computer via the USB but as soon as I disconnect it from the computer, it goes off. I am using a 12V DC adapter (500mA) (center positive) as the external power source. Not sure what is wrong. Please help! Thanks!

Well with just the 12V DC adapter plugged into the arduino (no USB connected) take a measurement with your digital multimeter between Vin and ground pins, then take a measurement from the 5V and ground pins. That should tell you the story, if your adapter is bad or the on-board +5vdc voltage regulator is bad.

If you don't have a DMM then your first step is to go buy one, there are very inexpensive ones available and one should not be playing with a arduino without having and knowing how to use one.

Lefty

Resurrecting this as I have the same issue. It works fine on USB but with an external power source the program runs erratically. It's hard to troubleshoot as when I plug the usb back in it's fine. I've got a a good supply and the 5v line is measuring at a steady 5v

nickjb: Resurrecting this as I have the same issue.

What is the voltage and current rating of the supply you are using?

I've got a 12v 2A wall cube and an adjustable bench power supply which is good for 5A but there is only a 40mA draw at 9V. I'm only running the on board LED and a few opto transistors.