Understanding Light Emitting Diode Specifications & Characteristics

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    

When choosing LEDs, Light Emitting Diodes, it is necessary to understand the data-sheet specifications so that the optimum electronic component can be chosen for the particular circuit design.

There is a huge variety of different LEDs that are available, each type with its own data-sheet and specifications. Everything from the colour to the package, light output to the voltage drop and many more specification parameters.

Although LEDs are, in one way, just another electronic component with electronic specifications, it does help to understand how some of the specific parameters relate to their operation in electronic circuit designs.

It is also worth remembering that LED technology has developed hugely in recent years and this means that many different types of LED are used in a variety of different areas: from LED panel lamps, to alphanumeric displays, LED room lighting and many more applications, LEDs are available in a huge number of forms.

LED type of format

LEDs are available in a huge number of formats from LEDs lamps for lighting with bayonet and Edison screw fittings and with a variety of wattages and sizes to alphanumeric displays and then there are the simple panel indicator lamps.

When searching for a particular type of LED it is key to specify the type required in search terms to be able to narrow down the search and home in on the required type of LED.

The actual type of LED will obviously be dependent upon its end application, so linking the end use with the format is essential.

There are many different formats for LEDs, and a very few of the major ones are listed below:

• LED panel indicator:   The indicator lamps were the first types of LED to be manufactured. The leaded versions consist of small dome shaped encapsulations where the maximum light emanates from the dome end of the component away from the leads. Surface mount components are also available, and these make ideal indicators for mount on boards.

  A variety of sizes are available - two popular sizes for the leaded varieties are 10mm diameter and 5mm diameter for the smaller ones. However a variety of other sizes and formats are available.

  For SMD LEDs, common types are 5050 (5.0 x 5.0mm and typically about 0.25 watt dissipation and around 50 lumen), 3528 (3.5 x 2.8mm typically about 0.08 watt and 5 lumen), 2835 (2.8 x 3.5mm typically about 0.2 watt and 25 lumen). Even though some examples of SMD LED sizes, there is a huge variety of different widely used packages available.

• Alphanumeric indicator:   A variety of forms of alphanumeric LED indicators are available. Typically they have a set of seven elements in a figure of eight with individual control of each element and this creates the various alphanumeric figures that might be wanted. These displays are available in many formats: dual in line formats for leaded versions of these electronic components as well as a variety of other formats. These displays are also available in a variety of surface mount component formats.

• High brightness LED:   High brightness LEDs are available in a host of formats. As they are often required for lighting applications, they often come as surface mount components for use in lighting bulbs, lighting strips and a variety of other uses.

• LED lamp:   LED lamps typically come in the standard lamp formats that can be used with domestic and commercial lighting. Obviously more specialist formats are available now as LED lighting is the normal form of lighting used in offices, homes and many other areas in view of its much greater efficiency and reliability when compared to others forms of lighting.

• LED Chip-On-Board, COB:   This is a relative newcomer to the LED market and it shows one of the many ways in which LED technology is moving.

  COB LEDs are essentially a substrate onto which multiple LED chips (typically nine or more) bonded directly by the manufacturer and in this way it forms a single module.

These represent only a fraction of the different types of LED available. When formulating a specification, it is necessary to understand what types are available.

LED colour

The colour of an LED is obviously is a major element of any LED specification because certain colours are more applicable to some situations rather than others.

LEDS tend to provide what is effectively a single colour. In fact the light emission extends over a relatively narrow light spectrum.

The colour emitted by an LED is specified in terms of its peak wavelength (lpk) - i.e. the wavelength which has the peak light output. This is measured in nanometers (nm).

The colour of the LED, i.e. the peak wavelength of the emission from the LED governed mainly by the material used for the LED and also by the chip fabrication process. Variations in the process can tailor the peak wavelength variations up to figures of around ±10nm.

When choosing colours within the overall LED specification, it is worth remembering that the human eye is most sensitive to hue or colour variations around the yellow / orange area of the spectrum, i.e. between about 560 to 600 nm.

Even slight process variations can cause small colour variations that can be quite noticeable if orange LEDs are chosen and sit next to each other on a front panel. This may affect the choice of colour, or position of LEDs if this could be a problem.

LED light intensity value, Iv

The LED specification for light intensity is important. The light intensity is governed by a variety of factors including the LED chip itself (including the design, individual wafer, the materials, etc.) , the current level, encapsulation and other factors.

The LED light intensity specification is not of crucial importance for most indicator applications, but with LEDs being used for lighting, this parameter is needed to be able to specify exactly what is needed in many situations.

The light output from an LED is quantified in terms a single point, on-axis luminous intensity value (Iv). This is specified as millicandella, mcd.

The lv measurement for LEDs cannot easily be compared with the values of mean spherical candle power, MSCP used for incandescent lamps.

The luminous intensity value for an LED must be quoted for a given current. Many LEDs used as panel indicators and similar functions operate at currents of around 20mA, but the light output of an LED increases with increasing current.

Those LEDs used for lighting applications will be high brightness types that operate at higher current levels.

LED current / voltage specification

LEDs are current driven devices and the level of light is a function of the current - increasing the current increases the light output. It is necessary to ensure that the maximum current rating is not exceeded. This could give rise to excessive heat dissipation within the LED chip itself which could result in reduced light output and reduced operating lifetime.

In operation, LEDs will have a given voltage drop across them which is dependent upon the material used. The voltage will also be slightly dependent upon the level of current, so the current will be stated for this.

Most LEDs require an external series current limiting resistor. Some LEDs may include a series resistor and will state the overall operating voltage.

LED reverse voltage

LEDs are not tolerant to large reverse voltages. They should never be run above their stated maximum reverse voltage, which is normally quite small. If they are then permanent destruction of the device will almost certainly result.

For example a standard 20mA panel indicator lamp is likely to only have a reverse voltage rating of around 5 volts or may be even a bit less.

If there is any chance of a reverse voltage appearing across the LED, then it is always best to build in protection into the circuitry to prevent this.

Normally the addition of a single PN junction diode in series with the LED can provide adequate protection for most electronic circuit designs. Obviously the PN junction diode must be able to tolerate the expected reverse voltage with some margin for safety.

LED angle of view specification

It can be noticed that for any LED, and also for any electronic display, the light output can only be seen over a certain angle, dropping off either side of the main "beam."

This was particularly apparent on early LEDs that only had a narrow angle, but now the LEDs produced have a much better viewing angle.

The viewing angle is often quoted as the angle for which the brightness is greater than or equal to 50% of nominal. This means that for an LED with a 30° viewing angle, the light output drops by 50% at ±15° either side of the peak.

While the viewing angle specification for LEDs may not be particularly important for some applications, it is of great importance for others.

LED specification for operational life

The light intensity of a LED does diminish gradually with time. This means that a LED has an operational life.

This LED specification is of particular importance when a LED or LEDs are to be used for lighting applications. It is not normally as crucial when the LED is used as an indicator - here a catastrophic failure is of greater importance.

The LED specification for its operational life is generally defined in the following terms:

The standards state that during these times, the LED should not exhibit any major shifts in chromaticity.

The rationale behind these figures is that 70% lumen maintenance equates to a 30% reduction in light output. This is around the figure for the threshold for detecting gradual reductions in light output.

Where light output is not critical, the 50% lumen maintenance figure may be more applicable. However for applications where lights may be placed side by side any differences will be very noticeable and therefore an 80% lumen maintenance figure may be a more applicable specification.

Figures for LED operational life may be of the order of 50 000 hours or more dependent upon the lumen maintenance figure used. There is a belief that LEDs are not life’ed items, but especially where LEDs are used for lighting applications, the life of the components needs to be viewed very carefully.

These are some of the main LED specifications that are likely to be seen in the datasheets. Before choosing a particular LED it is necessary to look at all the parameters to make sure it is suitable, and allow a good margin for the spread of parameters within the specification.

More Electronic Components:
Batteries     Capacitors     Connectors     Diodes     FET     Inductors     Memory types     Phototransistor     Quartz crystals     Relays     Resistors     RF connectors     Switches     Surface mount technology     Thyristor     Transformers     Transistor     Unijunction     Valves / Tubes    
    Return to Components menu . . .