Design with LEDs

Thermal conditions in the fitting
The thermal design, or thermal conditions in the luminaire, are important issues to consider for designers, constructors and producers of luminaires that operate with LEDs. If temperatures in LEDs exceed the threshold stated by producers, their lifetime will be considerably shortened.

The direction of fitting and choice of materials has an impact on whether the thermal design is good or not. For example, metals are good heat conductors, whereas plastics dissipate heat poorly. But ventilation and fastenings are also important issues to consider during the construction process.

Many producers of LEDs provide reference points on the PCB, on which LEDs are mounted. This is where the temperature can be controlled.

Colour stability of LEDs
There can be considerable differences in the light from LEDs mounted next to each other. The human eye is extremely sensitive to small variations in the white spectrum, and is also very sensitive to variations in light intensity or luminous flux. The white colour, intensity and quantity of the light can vary a lot from LED to LED.

For this reason, you should always ensure that the light has the same colour if, for example, several LEDs are used as spotlights on a white wall. Serious producers sort white LEDs in so-called binnings or groups, which have the same light colour within narrow tolerances.

LED varieties
LEDs come in many different varieties: mounted on a PCB or in long flexible strings, in flexible damp-proof units with screw holes, and in different toy-brick-like packs. They are also available as 5 mm LEDs with 2 terminals, and as a raw chip with pre-mounted optics – ready for assembly onto a PCB. All types come in different shades of white and in coloured versions.

LEDs are supplied with many different light distributions, which are optimised for different uses. These include narrow beam, broad beam and side projecting.

Static electricity
LEDs are sensitive to static electricity. Static electricity transfer, for example from a person who has walked on a synthetic carpet, can cause LEDs to fail. When handling LEDs, it is therefore important to ensure adequate shielding, or to take precautions against static electricity.

LED drivers
LEDs are sensitive to excessive current loads and, as a rule, operate with very small amounts of current such as 350 or 750 mA. This is why LEDs require an electronic ballast, also called a driver.

The recommended current for the LED is displayed on the supplier’s data sheet for the individual LED. Many suppliers of LEDs also supply drivers that are compatible with their LEDs or LED systems.

So-called AC LEDs have also been developed within the last few years. These are powered by normal 230V current and thus do not need a ballast or driver. The lifetime of the new AC LEDs is around 30,000 hours.