This is probably a simple matter. I have tried to find an answer on the web without success. All published data on current-limiting resistor values is for LEDs with current rating less than power source. I am putting together an array for a museum of high brightness LEDs rated at VF 4.2, IFM 100 ma. The LEDs will be lighted one at a time by an Arduino micro-controller with output pins rated max 40 ma. The question: if I use the standard resistor value will that degrade either the LEDs or Arduino? (Will the LEDs suck too much current out of the Arduino pins?) A one-second burst using either 10 or 100 ohms and the Arduino does light the LEDs. My concern is for the long term use in a museum setting.