LED Lighting Technology

LEDs, light emitting diodes are being used increasingly to provide lighting solutions for domestic and commercial lighting.

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Light Emitting Diode Tutorial Includes:
Light emitting diode How does a LED work How a LED is made LED datasheet specifications LED lifetime LED packages High power / brightness LEDs LED lighting technology LED COBs Organic LEDs, OLEDs

Other diodes: Diode types

In recent years thee has been a significant move to use LED technology for general lighting.

LED lighting technology offers many advantages over more traditional lighting technologies like incandescent, compact fluorescent lights, CFLs, tungsten, sodium, and other forms of lighting.

Although LED lighting technology involves more electronics and in many respects can be more complicated than other forms of lighting, the cost of LED lighting is falling to the extent where it has become much cheaper when the expected lifetime for he light and the energy usage are considered.

Comparison of LED lighting & other technologies

LED lighting technology has some noticeable advantages in terms of the light output for a given level of energy input as detailed in the table below:

Comparison of LED and Other Lighting Technologies
Lamp type	Typical efficiency (Lumens per watt)
LED lighting	>100 and improving
Tungsten filament lamp	~18
Compact Fluorescent Lamp, CFL	~60
Sodium lamps, e.g. street lamps	~100 - 200

LED lighting technology is significantly more efficiency than the others mentioned in the table above. Operationally LEDs have some advantages as well.

Operational Characteristics Comparison between LED Lighting & Other Common Technologies
Parameter	LED	Incandescent	CFL
Sensitivity to low temperatures	No	Some	May not work below -20°C
Sensitivity to humidity	Some	Some	Yes
Effect of on/off cycling	Comparatively small, although does reduce lifetime	Some	Not very tolerant - reduces lifetime
Speed of turn on	Instant	Instant	takes time to warm up
Durability	Very durable, can withstand reasonable shock	Not particularly good. filament sensitive to shock and glass envelope can break	Not very robust - glass can break and emit toxic mercury

How do lighting LED lights work

LED lights use basic LEDs as the component within the LED lighting lamp. The light emitting diodes emit light when a current limited voltage is applied across the diode in the forward conducting sense. They diodes must be operated under the correct conditions of voltage and current - they do not like over-voltage and the current must be limited. Also they do not like being reverse biassed over a certain small voltage.

The operating voltage of a standard white-light LED is usually in the range of 3 to 3.6 volts. Although this makes LEDs easy to use within low voltage circuits, and in equipment like mobile phones, the voltages provided by light fittings are much higher voltage, typically 240 or 120 volts and they are also alternating current, AC. As a result, conversion circuitry is required to transform the AC line voltage into a form that can drive individual LEDs.

For domestic LED lighting, cost is a significant driver and therefore all the circuitry needs to be kept to an absolute minimum. This means that only the circuits that are absolutely crucial are included within the LED lighting bulb.

Accordingly the main areas of the circuit include the following, although different LED lighting bulbs from different manufacturers often use slightly different circuit configurations.

Input fuse: To prevent catastrophic disintegration in the case of a short circuit or other over-current failure an input curse is needed. Fire safety requirements can sometimes be met by using a thin printed circuit board conductor that will blow and act as a very rudimentary fuse, although this is not to be recommended - some very cheap LED lighting bulbs may use this approach. It may meet the safety tests where components are shorted or opened, but it would not protect against the overload conditions caused by leaking electrolytic capacitors on the input stage. The only real solution is the use of a proper fuse even though it might cost more. It is often best to buy lighting LEDs from established brands although this is not a complete guarantee.
Input transient protection: It is essential to use some form of transient protection, otherwise the first electrical storm in the area will end the life of the lighting LED. Movistors are typically used. Especially with price sensitive LED lights, where voltage margins may be tighter, these are essential
Bridge rectifier: As LEDs themselves require to operate from a direct voltage and not alienating current, a rectifier is required. The most effective form of rectifier for this type of circuit is a bridge rectifier - it provides full wave rectification utilising both halves of the incoming waveform for greatest efficiency, and also it does not require the use of a centre tapped transformer which would not be appropriate in these applications.
Capacitor smoothing: The waveform that emanates from the bridge rectifier consists of a series of half waveforms rising to the peak voltage and falling to zero. For the LED control circuits to work, some smoothing is required.
Current source: The drive for the LEDs themselves consists of a low cost DC-DC converter providing a constant current source for the LEDs.
High brightness LEDs: The LEDs used within he LED light are high brightness versions. There are typically several of these, sufficient to provide the required light output.

The circuits used within the LED lighting bulbs vary from one manufacturer to the next. The exact ways of achieving the same result vary considerably although the main blocks described above are typically used.