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Light Emitting Diodes (LEDs)

Function

LEDs emit light when an electric current passes through them.

Connecting and soldering

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).

LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

Tri-colour LEDs

The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.

The diagram shows the construction of a tri-colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.

Bi-colour LEDs

A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.


Sizes, Shapes and Viewing angles of LEDs

LED Clip

Photograph ©

LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular.

Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular.

As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less.

Rapid Electronics stock a wide selection of LEDs and their catalogue is a good guide to the range available.

Reading a table of technical data for LED

Suppliers' catalogues usually include tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.

The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities.

Type

Colour

IF
max.

VF
typ.

VF
max.

VR
max.

Luminous
intensity

Viewing
angle

Wavelength

Standard

Red

30mA

1.7V

2.1V

5V

5mcd @ 10mA

60°

660nm

Standard

Bright red

30mA

2.0V

2.5V

5V

80mcd @ 10mA

60°

625nm

Standard

Yellow

30mA

2.1V

2.5V

5V

32mcd @ 10mA

60°

590nm

Standard

Green

25mA

2.2V

2.5V

5V

32mcd @ 10mA

60°

565nm

High intensity

Blue

30mA

4.5V

5.5V

5V

60mcd @ 20mA

50°

430nm

Super bright

Red

30mA

1.85V

2.5V

5V

500mcd @ 20mA

60°

660nm

Low current

Red

30mA

1.7V

2.0V

5V

5mcd @ 2mA

60°

625nm

IF max.

Maximum forward current, forward just means with the LED connected correctly.

VF typ.

Typical forward voltage, VL in the LED resistor calculation.
This is about 2V, except for blue and white LEDs for which it is about 4V.

VF max.

Maximum forward voltage.

VR max.

Maximum reverse voltage
You can ignore this for LEDs connected the correct way round.

Luminous intensity

Brightness of the LED at the given current, mcd = millicandela.

Viewing angle

Standard LEDs have a viewing angle of 60°, others emit a narrower beam of about 30°.

Wavelength

The peak wavelength of the light emitted, this determines the colour of the LED.
nm = nanometre.


Flashing LEDs

Flashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, typically 3Hz (3 flashes per second). They are designed to be connected directly to a supply, usually 9 - 12V, and no series resistor is required. Their flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example our Flashing LED project which uses a 555 astable circuit.