What are the electrical characteristics of LEDs?
Individual LEDs are low voltage devices. Single indicator LEDs require 2 to 4 volts of direct current, with current in the range from 1 to 50 milliamperes. An illumination-grade LED containing a single semiconducting element requires the same voltage, but operating currents are much higher, typically several hundred milliamperes. A device containing multiple elements connected in series will require higher voltage corresponding to the larger number of individual elements in the device.
LEDs require a specific electrical polarity (see What is an LED?). Applying voltage in reverse polarity can destroy them. Manufacturers provide specifications about the maximum reverse voltages acceptable for LED devices; 5 volts is a typical maximum rating.