Light emitting diodes, LEDs come in a variety of different packages.
Dependent upon their application and the type of LED, there are several standard package styles that can be obtained.
LEDs used for niche applications will obviously have their own LED package styles, but the more familiar indicator LEDs have some well established package formats.
Indicator lamp LEDs can come in a variety of configurations. These give a considerable degree of flexibility when design circuits:
- Single colour: This is the standard format or configuration for an LED. It has two leads, one a cathode and the other an anode. The LED comes on and off according to when a current is passed through the diode.
- Bi-colour LEDs : This format for LEDs uses a pair of LEDs wired in an inverse parallel formation. This enables one LED to be illuminated at a time dependent upon the polarity of the voltage applied.
LEDs do not withstand significant reverse voltages. Although the diodes in this configuration will experience a reverse bias, this is limited to the forward voltage of the other diode, and this is not sufficient to damage the reverse biased diode.
- Tri-colour LEDs : This LED configuration again uses two separate LEDs, but in a different configuration. Each LED has a different colour. There are two anode connections and a single cathode. It is therefore possible to turn each LED on separately, giving a choice of two colours - the third is provided by turning both LEDs on together and giving a third colour by addition. It is also possible to have different intensities of both LEDs to further vary the colour.
- Flashing LEDs: This form of LED is relatively easy to implement. The package contains not only the LED but also a simple IC that provides a timing function to enable the LED to flash.
Traditional LED packages
The traditional LED has been available since the early 1960s and has been produced in quantities of billions.
LEDs are available in a variety of package sizes. Possibly the most widely used is the 5mm diameter one, although a host of others are available ranging from 1.8mm, 3mm, 4mm, 8mm, and 20mm. There are also and rectangular LEDs available - almost any size to fir a wide variety of requirements.
Like any diode LEDs must be fitted into a circuit with the correct orientation. To operate, a LED must have current flowing through it, and this can only happen if it is forward biased. In the reverse direction no current will flow. Also if a LED has a high reverse voltage placed across it, then it can easily blow.
The two connections for a LED are denoted by a for the anode which is positive (+), and K for the cathode which is negative (-).
For the small indicator LED lamps that are often used in electronics equipment and electronics projects the cathode (k) usually has a shorter lead and there may a slight flat on the body of cylindrical LEDs. Sometimes it is possible to see inside the LED and again the cathode normally has the larger electrode.
If the LED has two leads with leads that are equal in length, then it is possible to look inside the LED package and normally two small metal plates can be seen. The smaller plate indicates the positive (anode) lead; the larger plate belongs to the negative (cathode) lead. Unfortunately this method is not totally reliable as some large LEDs have the plates reversed so this is not a 100% reliable method.
Another indicator can be a flat placed in the plastic encapsulation for the LED - the lead adjacent to the flat is normally the cathode of negative connection.
Surface mount diodes are more difficult to determine the polarity. If a data-sheet is to hand then this is the best approach. Also a few SMD LEDs have a negative sign "-" on the package to indicate the polarity, but this is not always present and often difficult to see.
If all else fails, then a multimeter can be used: set the multimeter to its diode / continuity setting. Often, the multimeter will supply enough current into the LED and this may even light it up all be it rather dimly. The for continuity or forward conduction, the black or common lead on the multimeter indicates the negative or cathode connection, and the red indicates the positive or anode.
Surface mount, SMT LED packages
Like all components, LEDs are used in vast quantities in LED SMT packages. They are available in the industry outline packages. However they have not been subject to quite the same level of miniaturisation as many components such as resistors and capacitors that are available in 0201, etc. This is partly because they need to be a certain size to see!
The most common LED SMT sizes with dimensions are given below.
|Length (mm)||Width (mm)||Height (mm)|
LED alphanumeric displays
Although liquid crystal and other forms of display have taken over many alphanumeric display applications, LED technology is still used in a number of applications. It has advantages that it does not need external light as in the case of an LCD. However they are less versatile and costs for customisation normally prohibit this type of use. It is often only possible to display numeric characters, and sometimes some limited graphics
Nevertheless LED displays are used in many areas, particularly where power is not an issue. Alarm clocks, test instrumentation, and other forms of mains powered electronic apparatus use LED alphanumeric displays.
The LED alphanumeric displays use a variety of approaches to display the characters:
- Dot matrix: The dot matrix display is the most flexible format. The LED dot matrix display is made up from a matrix of LEDs, each providing a dot. It can be obtained in a variety of formats although 5 rows and five columns, or 7 rows and five columns are common formats.
- Seven segment: This form of display can be used to create digiots between "0" and "9". An additional LED can also provide a decimal point, making this basic form of LED display applicable for a variety of basic numeric display applications. One example may be clock radios where power is not an issue.
- Star-burst: The starburst format for an LED display has the ability to illuminate fifteen lines and in this way it gives a considerable improvement in flexibility over that of the basic seven segment display. It can be programmed to create numeric characters as well as certain limited graphics.
The LED package is an important consideration. With a huge number of different package styles available for LEDs, it is possible to have an enormous choice, enabling the required style to be chosen to meet almost any application.