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Light Emitting Diodes

Specification and Use of Light Emitting Diodes (LEDs)

Introduction

A Light Emitting Diode (LED) is an electronic semiconductor component that emits a single colour (monochromatic) light when a DC current flows through it in a forward direction. Introduced during the early 1960s by Texas Instruments, the first LED components were dim and only available red in colour. Today LEDs produce a far brighter light source, are available in a variety of voltages and sizes, and in a range of colours including red, orange, yellow, green, blue and white.

These robust and electrically efficient components (a typical LED requires a DC current of about 10 milliamps to begin emitting light) make them ideal for use as indicator lamps on control panels.

LEDs emitting a non-visible light in the infra-red part of the radiation spectrum are also available. These LEDs are invaluable for use in detection applications when used in conjunction with infra-red detector components.

Features

Compared to incandescent lamps, LEDs offer a number of advantages including:

  • Robust construction - there is no glass to shatter or filament to break.
  • Modern LEDs are extremely efficient - they can emit light equal to a small incandescent lamp while consuming about 10 percent of the electrical power.
  • High reliability - modern LEDs have life spans of 100,000 hours (over 11 years) of continuous use.
  • Environmental - LEDs can withstand large shock and vibration far beyond that tolerated by incandescent lamps.
Structure of a LED

Construction

A discrete LED component consists of the "die" (or light emitting semi-conductor material), a lead frame to support the die, and an encapsulation epoxy which surrounds and protects the structure.

Types of LEDs

Components using LED technology are available as discrete components, packaged for specific applications, or as high intensity light source products:

Discrete LEDs
As well as the popular "standard" type manufactured in a small round dome epoxy encapsulation, LEDs are also available in a variety of other shapes and sizes. In particular, rectangular, square and triangular LEDs are available for Panel Indicator applications.
Other available types of direct connection LEDs include Low Current, High Brightness, High Voltage, Flashing and multi-colour variants. Specialised LED components include Axial Leaded LEDs, Bar Graph Displays, Tri-coloured RGB LEDs, and Surface Mount Technology components.
"Standard" type LED
Alpha-Numeric Displays LED displays (comprising 7 or more individual LEDs) were introduced around 1967. Today these displays are found in many electrical appliances and other items. Displays are arranged to form either a LED multi-segment display or a LED dot-matrix display. "Alpha-Numeric" LED Display
LED Clusters and Lights Modern individual LED components are not anywhere near as bright as large incandescent lamps. Therefore, most LED powered lights require a group or cluster of LED devices to create a bright light source. In some applications this is an advantage, as failure of just one or two individual LEDs within a group or cluster will barely reduce the brightness of the overall light source.
LED technology is increasingly being used for providing high density light sources, including Railway and Road Traffic Signals
LED high density light source

LED Specifications

In this section an explanation is provided for each of the LEDs parameters that are normally quoted in manufacturer's and supplier's literature.

Dimensions

Dimension of a "Standard" LED

Discrete LEDs are now available in a variety of shapes and sizes. The most common type used are the "standard" types available in small round dome encapsulations. The size measurement quoted for these "standard" components refers to the diameter of the body encapsulation.

Refer to the manufacturer's or supplier's literature to determine the size for the irregularly shaped LEDs.

Light Output

This parameter specifies the intensity of the light produced by a LED, and is normally quoted in units of "mcd" for a stated Forward Current (IF) flowing through the component.

The unit of light measurement is the "Candela". One Candela (or 1 cd) is defined as the light intensity of a "standard" candle viewed from a distance of 12 inches. This intensity is approximately equal to the light produced by a small 2 watt standard incandescent bulb. One Candela equals 1000 milliCandelas (mcd).

Modern LED components are available with wide ranging light outputs from 1 mcd to 500 mcd (or more).

Forward Voltage (VF)

Indicates the voltage measured across the LED when it is drawing the stated Forward Current (IF).

Forward Current (IF)

Indicates the current flowing through the LED for normal operation.

Reverse Voltage (VR)

Indicates the maximum voltage when applied in reverse polarity across the LED that the component can normally withstand.

Power Dissipation

Indicates the maximum power that the LED can dissipate without sustaining damage.

Power Dissipation = Forward Voltage (VF) x Forward Current (IF)

NOTE: Power dissipation for the LED will invariably be quoted in milliWatts (mW). 1 mW = 0.001 Watt.

Peak Wavelength

Single Coloured (monochromatic) LEDs

The colour of light is the way we perceive its wavelength. The light radiation spectrum is expressed in "nanometres" (nm) and was standardized by the Commission Internationale d'Éclairage (CIE) in 1931.

Unlike incandescent lamps that produce light over a wide spectrum (of which visible light is only a small segment), LEDs emit light over only a relatively small part of the radiation spectrum. Peak wavelength is the technical method of defining the colour emitted by the LED (the wavelength of the emitted light) and is measured in "nanometers". Typical figures range from 450nm (blue), through 535nm (green), 585nm (yellow), 620nm (orange),700nm (red), up to 950nm (infra red).

Radiation Spectrum

Generally the output is not at one precise wavelength but is distributed over a narrow range, a graph of intensity against wavelength would show a peak at the specified wavelength.

The peak wavelength for any LED component is determined by the chemical make-up of the semiconductor substrate, rather than the current or power dissipated. It makes no difference if the LED is in a coloured or clear package.

White LEDs

White LEDs are specified in a different way to single colour (monochromatic) versions due to the way they work.

These components are essentially "blue" LED semiconductors where the light excites phosphors in the epoxy casing. The resultant overall emission is a white light with a bluish tinge, produced in much the same way as a fluorescent tube works.

White LEDs are specified by reference to the 'X' and 'Y' Chromaticity Co-ordinates, and the Colour Temperature.

The 'X' and 'Y' Chromaticity Co-ordinates determine the position in the standard colour triangle to indicate the source colour. Colour Temperature is a measurement of the colour of light radiated by an object while it is being heated, and is expressed in terms of degrees Kelvin. A temperature of 2400K is red; 9300K is blue. Grey at 6504K is considered as the neutral temperature.

HINT:White LEDs produce a white light with a slightly bluish tinge. To neutralise the bluish tinge, place a piece of clear yellow plastic over the LED, or use the clear yellow paint available for decorating light bulbs.

Viewing Angle

Unlike incandescent bulbs that radiate light in all directions, LEDs emit light in one direction only. For this reason the viewing angle is specified and is the angle between which the light source emission is viewable.

Practical Considerations

This section outlines some practical aspects of using LEDs.

LED Symbol in Electrical Circuits

LED Symbol

It is good practice to prepare a circuit diagram as a record of electrical circuits for future reference. A LED is usually depicted by the symbol shown to the right

Identifying LED Leads

LEDs are polarity sensitive and must be wired correctly to enable them to emit light. For most "standard" type LEDs the "cathode" lead will be identified as follows:

LED - Side View LED - Bottom View

To avoid damaging your components, always check the suppliers or manufacturers literature for this information.

 

 

Calculation of Series Resistors for LEDs

It is recommended that a resistor always be connected in series to limit the current flowing through a LED. The resistance value required is calculated using the formula:

DC Series Resistor Circuit Formula - DC Series Resistor

Where:

Vs is the Supply Voltage

and for the LED .....

VF is the "Forward Voltage"; and

IF is the "Forward Current" (in Amps).

NOTES

1 - Forward Current IF for the LED will invariably be quoted in milliamps (mA). (10 mA = 0.01 Amps)

2 - The power dissipated (in Watts) by the resistor is calculated using the formula:

Power = ( Vs - VF) x IF

As a general rule a miniature 0.5 Watt resistor will be more than adequate.

AC Series Resistor Circuit

AC Operation

LEDs can be operated from an AC supply voltage, but a reverse diode must be connected as shown in the illustrated circuit (see right).

A 1N4148 diode is suitable for this application.

The resistor required in this instance is calculated using the same formula given above for "Calculation of Series Resistors for LEDs", but the resistance value is halved and the wattage doubled.

Multicolour LEDs

Bi-Coloured LEDs
Bi-Coloured LED

Within the LED epoxy package are two separate reverse parallel semiconductor chips, each producing a different colour.

At any one instant of time only one of the LED chips can emit light, which one depends upon the direction of current flowing through the component.

Only one series resistor is required, and is calculated using the same formula given above for "Calculation of Series Resistors for LEDs".

Bi-Coloured LEDs can produce a third colour that is a product of mixing together the two primary colours. For example, a red and green Bi-Coloured LED can produce a yellow light. The simplest method to achieve this is to operate the LED from an AC voltage source. This results in each of the primary colour chips operating during their respective half cycles of the alternating flow of current, but the human eye however perceives the rapidly flickering red and green lights as a constant yellow.

Tri-Coloured LED
Tri-Coloured LEDs

Within the LED epoxy package are two separate semiconductor chips that each produce a different colour. A common lead from the two semiconductor chips is connected internally to produce a 3 terminal component as illustrated. Both "common cathode" (see right) and "common anode" types are available.

Used simply these components provide a selectable two-coloured light source by switching the voltage between the two semi-conductor chips. Alternatively both semi-conductor chips can be operated simultaneously to mix the two primary colours.

Only one series resistor is required providing that both semi-conductor chips are never operated simultaneously. Otherwise it is essential that each chip is protected by its own separate dedicated resistor.

The resistor(s) required is calculated using the same formula given above for "Calculation of Series Resistors for LEDs".

Page author: C.J. Dadson (with contributions from other MERG members)

Last updated: 10 August 1998

Re-formatted: 28 June 2010

Copyright © 1998 Model Electronic Railway Group