LED Lighting

Types of LED

LEDs used in diy can 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

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

Mains LED lamps aren't really equivalent replacements for filament lamps, and typically come with the following properties:

Much lower light output than the filament lamp they replace
Lower power, typ 1-11w
Much lower CRI (light quality) than other domestic lighting
Long life times
Efficacy around twice that of halogen, but lower than CFL or linear fluorescent
Built in CR ballasts fry if used on a triac dimmer
Built in CR ballasts fry if used on an MSW invertor
White LEDs degrade in use
Operating temp is much lower than filament lamps,
life expectancy is long,
very low powers are available,
LEDs are a good candidate for nightlighting.

Individual die LEDs

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 not in a position to take over from other forms of lighting for general lighting service.

Typ 40-60mW
Available in a range of colours & white
Sizes from 2mm to 10mm diameter, with 5mm 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 kills them.

Ultrabright coloured LEDs are commonly available in 2mm to 20mm sizes rated 40mW - 80mW for in the region of 20p-30p per LED. 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 extremely dim room lighting.

Their tiny size, minimal power consumption, cold operation and safe low voltage makes them fittable almost anywhere. Their very low output makes them mostly used for dim but 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.

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. Its easy to make a colour dial for such LEDs, so any available colour can be dialled at will.

Higher power lighting dice

typ 0.5 - 5w
come in larger flat packages (than 5mm LEDs)
Otherwise the same properties as the micropower dice

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 as well as white
Efficacy of over 40 lpw for the most efficient ones, less for many. Expect around twice the efficacy of halogens.
Power per lamp is low due to thermal issues.
Quality (CRI) of white light is 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 expensive per watt
LED lights are usually very long lived, with 50,000 hours often quoted. Not all live upto this though.

LED vs Fluorescent

Disadvantages:

Lighting LEDs haven't reached the efficacies of fluorescent lighting.
The lowest cost lower efficacy LEDs have not reached the efficacies of low cost linear fl or CFL.
LEDs' upfront cost is much higher for anything beyond tiny light output.
LED light quality is poor compared to fluorescent.
LEDs are not available in powers comparable to fl lights

Advantages:

A lot of LEDs have much longer lives than fl tubes.
LEDs are very 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.

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

Colour

LEDs are available producing single monochromatic colours or in white.

White LEDs tend to be a cold white (even 'warm white' are more often a cool white). They have poor CRI, meaning they don't reproduce colours accurately and the white doesn't look quite right. White LEDs also change colour balance as they age, and are prone to mismatch.

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 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 much resistance. The user can then dial the desired colour.

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 is 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.

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

LEDs must be connected with the correct polarity, as they only conduct one way, and they can only withstand a few volts in reverse.

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 to around 50,000 hours for some LED lighting, sometimes significantly less.

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

However parallelled LEDs are less energy efficient than series strings. A series string only needs one energy wasting dropper per string, but parallelled LEDs need one for each LED.

In practice 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). Its 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.

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.
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.
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 doesn't eliminate the coloured shadows fully, but makes the colouration mild.