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Choosing the right LED typeUpdated 2 years ago

  • What is an LED?

LED stands for Light Emitting Diode.  It is much more than just a small globe – it is a semiconductor device which conducts  DC in one direction and emits light when the voltage across it is somewhere between 1.7 and 2.7V and the current through it is between 10mA to 500mA (depending on type, power and manufacturer). It has no glowing filament like a globe and runs very much cooler for an equivalent power rating. Being a diode, some steps must be taken to prevent excess current from destroying it. (see Appendix How LED’s work).

 

  • Voltage, power, colour, environment:

Although, as we just said, LED’s light at low voltages, the kind of LED strip lights and signage units you use will generally come in pre-packaged modules designed to run at 5, 12 or 24 volts. If they are strip lights they will have a power rating in W/m, while some high power ones are packaged in arrays, e.g. for lighting, and will have a single power rating.

Some LED’s are for indoor use only, some are splash-proof and some are water-proof.  You can see this in their IP rating. The 1st digit in the IP Rating indicates the degree of protection the device has against solid objects, the 2nd digit indicates how well the device is sealed against entry of moisture or liquid. This covers anything from water drips, to spray, to total immersion in water. This is an extremely important factor if the device is going to be used outside or in areas exposed to potential spillages.


Table of IP Ratings

  

 

So, for example, if a LED (or PSU) had an IP Rating of IP65 then it would be totally protected from any dust gaining entry and will be protected against low pressure jets of water.

 

Do not assume your sign is waterproof; moisture gets in from condensation, sea air and strong winds. Sea air deposits conductive salts on your wiring and joints. Always provide drainage holes to allow condensed moisture to escape.

 

  • Cooling:

Experience has shown that if LED’s are run too hot, their life is greatly shortened and failure can be expected. How hot is too hot? No semiconductor device should be allowed to reach a junction temperature of over 100°C.  An LED on a cool day will generally have an external temperature of about 35°C. If it’s enclosed in a box in the sun the ambient temperature could be 60°C added to the LED’s temperature of 35°C and the LED is on death’s doorstep. Sensible ventilation and heat-sinking should be provided but it is no use using heat-sinking if the heat sinks are not exposed to cooling air.

 

  • Cutting and mounting strips:

Many strip LED’s have clear markings as to where you may cut them. The reason for this was mentioned above and is due to the way the strips function with LED’s in series or with built in current-limiting circuits, divided up into intrinsic units. Cutting anywhere except along the ‘cut lines’ will destroy the device.

 

Use the peel-off tape to secure the strip LED’s. DO NOT EVER use silicone sealant products to mount LED’s as there are components in the sealant which will destroy the LED in time.

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