What is thermal runaway?

When a junction gets warmer, the current through it at a given voltage will increase. The increased current in turn heats the junction further, and the problem gets worse. Eventually, if nothing limits the current, the junction will fail due to the heat. Because of thermal runaway, it’s important to use some current limiting circuit even with a regulated voltage source. A current limiting resistor is the typical solution for most hobbyists, while circuit designers may prefer to use a current source based on one or more transistors.

[prev: What kind of resistor should I use with an LED?]
[next: How are infrared LEDs different?]