Generic technical information
LED is an acronym for Light Emitting Diode. A diode is an electronic component that allows current to pass in one direction but not the other. LEDs are based on laser technology.

Any diode has a forward voltage drop (Vfd). This is effectively a minimum voltage that has to be across the LED to pass enough current to light the LED fully, although there is not a sharp cut-off. However, it does mean that when the voltage across an LED bulb falls below the total of the Vfd for all the LEDs in the series circuit, the light output drops to zero quite quickly. As a result, a car with a faulty cell in its battery may not have sufficient voltage to light some LED bulbs adequately when the alternator is not running.

Typical Vfd for white and blue LEDs are 3.7v to 4.5v, while all other colours tend towards a range of 1.7v to 2.4v. As a result, a typical LED bulb will have up to three blue or white LEDs in a series circuit, but as many as five in other colours. Current limiting is required, which is often a simple series resistor.

Because of the way that an LED switches on, and the way that a series resistor works, the current consumption and light output of an LED bulb will generally not be linear over the whole voltage range of the bulb. However, incandescent bulbs also show this effect.

Blue and white LEDs are not as efficient as other colours, so manufacturers often use narrower light angles (more focussed lenses) to give a brighter output over a smaller area.