LED Lighting

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LED streetlight 5485-2.jpg

Types of LED

LEDs used in diy fall into 3 types:

  • mains LED lamps (typ 1-11W)
  • micropower individual die LEDs (typ 40-60mW)
  • higher power lighting dice (typ 0.5-5W)


Mains LED lamps

800px-E27 with 38 LCD.jpg

These are intended as drop in replacements for other types of mains lightbulbs, and are available in whites & various colours.

Mains LED lamps typically come with the following properties:

  • Lower power, typ 1-11W
  • CRI (light quality) fairly good
  • Long claimed life times, but often much shorter life achieved in practice
  • Efficacy typically around that of CFL, but some can reach over 100 lumens/watt
  • Built in CR ballasts fry if used on a triac dimmer
  • Built in CR ballasts fry if used on an MSW invertor
  • Chip ballasts are dimmable using trailing edge dimmers
  • White LED output degrades in use
  • Operating temp is low
  • Very low powers are available

Individual die LEDs

Row LEDs2.jpg

Single die LEDs are miniature very low power solid state light sources sometimes used for low light output applications. They are commonly available in sizes from 2mm to 10mm diameter, with 5mm most common. They have niche applications, but are too low power to take over from other forms of lighting for general lighting service.

RGB-LED2.jpg
  • Typ 40-70mW
  • Available in a range of colours & whites
  • Sizes from 2mm to 10mm diameter, with 5mm & 3mm most common.
  • Only 'ultrabright' LEDs are suitable for micropower lighting
  • Good for lighting glass shelves and glass objects
  • These require a current limited low voltage DC power supply. Trying to operate them any other way usually kills them.

Ultrabright coloured LEDs are commonly available in 2mm to 20mm sizes rated 40mW - 80mW for in the region of 2p per LED (shipped from China). These LEDs are typically rated in the region of 2-4V 20mA, and must be supplied by a current controlled low voltage DC supply. Lots of these LEDs would be needed for even dim room lighting.

Their tiny size, minimal power consumption, cold operation, robustness and safe low voltage makes them fittable almost anywhere. Their very low output makes them mostly used for low power strongly coloured glows and nightlighting.

They may be used with glass shelving to give the glass edges a coloured glow, or to light glass objects. Self colour cycling LEDs may be used for this.

Bicolour LEDs contain 2 dice of different colours back to back. These may be switched between the 2 colours, or may be faded from one colour to another at will by driving with AC and controlling the current in each half cycle separately. It's easy to make a colour dial for such LEDs, so the available colours can be dialled at will.

RGB LEDs contain 3 dice in one package, and can be controlled to give almost any colour.

Higher power lighting dice

LEDs 5892-3.jpg
  • Typ 0.5 - 5W
  • Come in larger flat packages (than 5mm LEDs)
  • Otherwise the same properties as the micropower dice
  • Useful for low level lighting
  • Require correct heatsinking
  • Beware of light quality, a lot aren't good in this respect
  • Beware of short lived LEDs with long life claims

LED properties

LEDs have their uses, but much hype about LEDs exists, and many users have been disappointed as a result.

  • Available in many pure single colours, warm white & cool whites
  • Efficacy of over 100 lpw for the most efficient ones, less for most.
  • Power per lamp is limited by thermal issues.
  • Quality (CRI) of white light sometimes usually poor
  • Individual LED dice are tiny enough to feed light into the edges of glass shelves and glass objects.
  • Colour changing LEDs are available
  • LEDs are still expensive per watt
  • Indicator LEDs can manage 50,000 hour life, lighting LEDs can't.

LED vs Fluorescent

Disadvantages:

  • Lighting LEDs have the efficacy of CFL, with the best rivalling linear fluorescent lighting.
  • LEDs' upfront cost is higher for anything beyond tiny light output.
  • LEDs are not available in powers comparable to fl lights
  • Average LED life is shorter, despite the claims

Advantages:

  • LEDs are more mechanically robust
  • LEDs in very low powers are extremely small. See surface mount LEDs.
  • The lowest power LEDs (single die fractional watt) are available in sizes 2mm to 10mm.
  • LED lights are available as tape, which occupies very little space

Things will likely change, LED performance is getting better year by year. But they haven't fully got there yet, despite the hype.

Colour

LEDs are available producing single monochromatic colours or in white.

White LEDs are warm white, like filament lamps, or cool white, like daylight. White LEDs change colour balance as they age. Torch types tend to have poor CRI, meaning they don't reproduce colours well.

Colour LEDs are mostly much cheaper than whites and suffer less from deterioration. Pink LEDs are the one exception, these deteriorate badly & rapidly, and are unsuitable for lighting.

More colours can be obtained by mixing LED colours. See 'mixing colours' section to avoid a possible issue though.

Ballasts

Mains LED bulbs have their own built in ballast, so this section addresses ballasts used with the much smaller LED dice.

Micropower and the bigger single LED dice can not simply be connected to a power supply, they need external current control with a little basic external electronics. (Mains LED lamps have this already built in.)

More or less all micropower LEDs are rated at 20mA max, and this figure should not be exceeded, or lifetime reduces rapidly.

The most common option is to use series or parallel series strings, with each series string having its own current limiting resistor. For example:


0v ---|>|-----|>|-----|>|---/\/\--- +

Series String

or

     LED1    LED2    LED3    LED4   R1      
+----|>|-----|>|-----|>|-----|>|---/\/\---+ 
|                                         | 
+----|>|-----|>|-----|>|-----|>|---/\/\---+ 
|                                         | 
+----|>|-----|>|-----|>|-----|>|---/\/\---+ 
|                                         | 
+----|>|-----|>|-----|>|-----|>|---/\/\---+ 
|                                         | 
|                                         | 
o  +12v                               0v  o 

Series Parallel

How close to the supply voltage your LED string voltage should add up to depends on how stable the supply voltage is, and how much brightness variation is acceptable.

LEDs are constant voltage devices, and have the same V drop regardless of current. This then leaves the resistor in each string seeing voltage variations that are the same in absolute terms as the supply voltage variation, but much greater in percentage terms. Therefore any supply voltage variation causes several times the percentage of variation in light output.

For mains power supplies, LEDs adding upto to 2/3 - 3/4 of the supply voltage should be fine. Variation is not usually then noticeable.

Brightness variation can be avoided entirely by using a stabilised supply, but this isn't completely necessary.


LED circuits

     +-----|>|-----+              
-----|             |-----^^^^-----
     +-----|<|-----+

           LEDs         Resistor

2 LEDs on an AC supply


-----|<|>|----------^^^^-----

  Bipolar LED     Resistor

A bipolar LED on an AC supply


     +-----|>|-----^^^^----+        The output of each 
-----|                     |-----   LED is controlled by 
     +-----|<|-----^^^^----+        its own resistor value

          LEDs    Resistors

Colour mixing 2 LEDs on an AC supply


     +-----|>|------+
     |              >
-----|              ><-----^^^^----
     |              >
     +-----|<|------+

          LEDs   Resistors  Potentiometer

Colour control with 2 LEDs on an AC supply


     +-----|>|------+
     |              >
-----|              ><-----^^^^----
     |              >
     +-----|>|------+

          LEDs   Resistors  Potentiometer

Colour control with 2 LEDs on a DC supply

With adjustable colour circuits using a potentiometer, the max current is set by the fixed resistors alone, and the pot value can be in the region of twice this resistance. The user can then dial the desired colour.

LEDs are often seen run from batteries with no resistor. To do this you need to know the characteristics of the LEDs and the batteries, and only some LEDs can be run this way. If you need to read this article it's not recommended.

Dedicated mains LED ballasts are now available.

Dimming mains LEDs

A simple 100% efficient option for dimming mains LED bulbs is a capacitor in series with the LED lamp. That and a switch gives a choice of fixed brightness settings.

Dimming doesn't alter the light spectrum of LEDs.

For voltage requirements of such a capacitor, see Droppers. No class requirement exists, since in this case capacitor failure is a safe condition. Thus acceptable caps include:

  • 600v dc
  • 250v ac class X


Dimming low voltage LEDs

When run off a fixed voltage, LED current can be reduced by increasing the value of the series resistor. A potentiometer plus a fixed resistor can be used to give a brightness control with continuous variation. The fixed resistor controls current at max brightness, and the pot will have a value of anything from 2x to 10x the resistance of the resistor.

Another way is to reduce the supply voltage. If the PSU is not voltage switchable it can be reduced with a series diode or 2.

The other way is to use a dimmable ballast with a trailing edge dimmer.

When LEDs are run from a constant current ballast, its better to let the ballast do the control job. How the target current is set depends on the ballast.

Polarity

Ledmrp.jpg

LEDs must be connected with the correct polarity, as they only conduct one way, and they can only withstand a few volts in reverse (typically 5v but sometimes less).

Length of leads is not a reliable indicator of polarity, despite being often quoted as a method of determining it. The shape of the internal construction tells the polarity reliably.


Life

LEDs are famed for very long lives of 100,000 hours. But this only applies to LEDs conservatively run, such as LED indicators. When much greater output for lighting is wanted, run currents must be higher and lamp life falls greatly.

Things are only that simple for a single LED. Lifetimes are different for multiple LED assemblies, as

  • When LEDs are in parallel, if one fails the rest still work
  • When LEDs are in series, if one fails the rest go out too
  • LED lights with around 2000 hr life are often encountered.

LED assemblies are normally wired as a series-parallel arrangement.

Mixing colours

When more than one LED emitter is used to make up a light, usually all the LEDs are white (or the same colour). It's much cheaper to mix colour LEDs to make white (usually Red, Green & Blue) but an issue results. Because the LED dice can't be in the exact same place, the result is very strongly coloured edges to shadows. It looks bad, like a misaligned TV set.

There are 2 ways to get round this, plus a 3rd partial solution.

  1. Point the mixed LEDs at a white wall or ceiling to mix the outputs, rather than pointing them at objects in the room that will cast shadows.
  2. Do this mixing in the light fitting itself, by pointing the LEDs at eg a diffusing shade, or into a white bowl to mix the light.
  3. Use a large number of low power LED dice huddled together with all colours distributed evenly, and white LEDs around the outer edge of the huddle. This makes any colouration mild.
  4. Use LED tape, lots of spread out light sources minimises colouration.

See Also