Originally Posted by rowan

LEDs are diodes but they don't have the same forward voltage as say, a rectifier or silicon diode. A typical forward voltage for a basic opaque red LED is 1.8V, green is slightly higher at about 2.0V, and white goes higher still, up past 3.5V. That's why many LED torches use 3 x 1.5V cells, to avoid the need for circuitry to step up the voltage.