I have an Arduino Uno board with an I2C LCD w/ backpack. I am running a temp sensor project using the DS18B20 sensor. I've uploaded the Temp sensor sketch to the board sucessfully. The sketch runs perfectly on USB power but when I switch to external power the display works properly for a few seconds and then flickers uncontrollably. I am using a 12VDC 600ma wall wort for external power. Any suggestions?
Sounds like AC ripple on the DC power, most likely a bad power supply. You could measure the ripple with an oscilloscope (best way) or use a digital voltmeter on its AC range. Ripple should be less than about 50mv (0.05 volts AC). Or, skip this step since it might damage your Uno if things are really out of wack, voltage or ripple wise.
While 12vdc is within spec, a 9vdc supply is better as it will reduce the power dissipation in the voltage regulators, the result is a cooler running board.
Use a 5V source through USB, it's less work all around.
Thanks for the information. It was a bad unregulated power supply. I checked the voltage with a digital multimeter and found the voltage on the 12vdc wall wart peaking at 19 vdc! I replaced the power supply with a higher quality one and checked the voltage with a multimeter. It appears to be better regulated. I also lowered the supply voltage to 9 vdc.