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Automotive interior lighting does not suffer from the lifetime issues that can justify light emitting diodes (LEDs) in other applications. Power consumption usually is not an issue either, although the trend towards hybrid and electric vehicles might have an impact on this in the future. However, LED lighting opens a new world of styling, comfort and user customization choices that will likely increase deployment of LEDs for interior car lighting.
Car interior lighting has been available since the 1940s and relied mainly on incandescent bulbs. Interior lights have been used for three functions. The primary use is just for general illumination while entering or exiting the car. Another function has been to provide a reading light for passengers and driver. Lastly, lighting gives stylistic illumination to highlight certain interior design features.
In recent years dimming has been introduced to give an exclusive touch especially to the cabin lights. In a modern car this dimming is often created by a PWM (pulse width modulated) signal from a microcontroller. This PWM frequency is kept fairly low to avoid unnecessary electrical disturbances within the car.
Today more and more vehicle manufacturers are applying LED lights in the interior. It is however impractical to add on a high intensity LED with a simple dropping resistor or drop regulator without careful consideration of heat dissipation and power consumption issues.
LED characteristics
A LED does not behave like a light bulb. A light bulb is a resistive load that generates illumination as a byproduct of the heating of the coiled filament. It still shines some milliseconds after power is removed from the lamp and takes a few milliseconds to begin emitting light after power is applied. A LED in principle shines exactly to the actual current applied in that moment with only some nanoseconds delay. To attach the LED directly to a common incandescent lamp dimmer can result in an irritating light output—because of the very fast reaction of the LED, the light will flicker when dimming the light.
Dimming is an attractive and "simple" function that provides an exclusive look. But PWM dimming of LEDs can give an effect much like using light from an old TV screen. This mistake has been committed in some rear light applications of luxury car manufacturers. If you look at the cars rear light you get a strange optical sensation of an "itching" light. That is a normal human reaction on this high speed flickering kind of light. This effect, known as the stroboscopic effect, can in extreme cases trigger epilepsy. It is important for an interior designer to know about this effect and avoid it.
The goal is to create a comfortable, pleasing illumination for the occupants. So Rule Number 1: Do not use PWM dimming or use at least a relatively high frequency PWM.
LEDs are known to be efficient—which is true in most cases so long as efficient drivers are used. This aspect will be more important for electric and hybrid cars, even if it is not so much considered in today's automotive production. Also, better efficiency results in lower power dissipation and hence lower temperature of the LED lamp. Lower temperature makes it possible to use less expensive plastic and to avoid aging of the cover or light lenses. Cooler operation can also allow more efficient packaging and accommodate placing lighting in locations not presently possible.
LED drivers
LEDs require a driver that converts the battery voltage into a suitable current for their operation. The supply voltage is of a secondary importance for the function of an LED. It is important to keep the current constant.
An LED converts current into light in a nearly direct relation. In a perfect LED, each electron in the current generates a photon if the voltage is high enough. This relation explains why a normal DC/DC converter is not really suitable for driving a LED even though it "works." There are many voltage converters available on the market but only a few active current regulators. Melexis produces the MLX10801 and MLX10803 that are probably the only two that are especially developed for the automotive environment.
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