Technological advances give LEDs wider use

Colin McAhren looks at the technological advances that are now moving LEDs into a wide range of new application areas now that they are available in any desired colour and are as bright as bulbs

LEDs are widely used as indicator lights and in numeric displays, but were considered to have a low light output and a lack of colour options; Colin McAhren, Marketing Manager for Control and Automation Products at Cutler-Hammer, looks at the technological advances that are now moving LEDs into a wide range of new application areas.

Across the USA, Japan and Sweden, highway authorities are replacing the lamps in their traffic lights with new bulbs that are brighter, more energy-efficient, have a longer life and provide major cost and power savings in the long-term.

However, these new traffic lights are not advanced versions of the traditional tungsten halogen lamps - the new lights are made up of rows of LEDs, or light emitting diodes.

And similar systems are already being planned for the UK, following successful testing in Westminster, London.

LEDs are solid-state semi-conductor devices that convert electrical energy directly into light.

They have long been recognised as extremely energy-efficient and long-lived, and their 'cold' light generation is more efficient than incandescent lamps because most of the energy radiates in the visible spectrum rather than as heat.

And their lack of a filament makes them less vulnerable to shock or vibration - common causes of failure in incandescent bulbs.

These particular characteristics of LEDs suit them to applications that require a low light output 24 hours per day, such as entrance and exit signage and industrial control indicator lighting.

They can also be embedded in plastic materials and this, coupled with their long life, makes them ideal for applications like path or road marking and emergency wayfinding systems.

LED applications have been limited in the past by their relative lack of brightness and restricted colour range relative to incandescent light sources.

Recent advances, however, now mean that LEDs are available in any desired colour - including white - and at levels of brightness equal to or exceeding that of standard incandescent bulbs.

The successful utilisation of LEDs in traffic lights demonstrates the potentially enormous energy savings attainable - one estimate suggests that replacing the incandescent signals in the United States with LED signals would save nearly 2.5 billion kilowatt hours annually.

With this increased flexibility and level of performance, LEDs can now perform practically all of the functions that used to require an incandescent lighting source.

And LEDs manufactured by Cutler-Hammer, the world-leading manufacturer of industrial controls and circuit protection equipment, are at the forefront of this technological advance.

The convergence of recent technological developments - single more reliable LED chips utilising the new super bright LED materials - is steadily leading to the decline of the older, less reliable technology of the multi-cluster LEDs.

The development of 6-7 cluster LEDs from the preceding 3-4 cluster versions was an attempt to improve light output and reliability.

This process of improvement has reached new levels with the development of brighter, longer-lived single chip LEDs.

Cutler-Hammer LEDs now incorporate a single high intensity LED chip that has a typical life of 100,000 hours compared to just 7,000 hours for an incandescent bulb - or 11 years as opposed to nine months.

This means the incandescent bulb has to be replaced at least 14 times during the lifetime of a single LED.

Taking the average price of an incandescent bulb to be œ1.25 and an LED to be œ5, the cost of incandescent bulbs over the average life of a œ5 LED bulb would be œ17.50.

It is apparent that buying incandescent bulbs for lower initial outlay is a false economy - and this example does not even take into account the labour costs of changing the incandescent bulbs, or the possibility of premature failure of incandescent bulbs due to shock or vibration.

Further design features add to the reliability of Cutler-Hammer's single chip LEDs, particularly with just four internal solder points, compared to the 20 to 23 typically found in multi-cluster LEDs.

As the solder points are critical stress areas where differential rates of expansion can break weak connections, the single chip design with four solder points will experience only 20 per cent or less of the failures expected in a multi-cluster LED with 20 or more solder points.

Pushing and twisting the bulbs during insertion into their holders in the field can often cause mechanical stress and lead to bulb failure.

Cutler-Hammer is the only LED manufacturer to pot the LEDs themselves with epoxy, bonding each unit securely and removing the risk of failure.

The small amount of heat generated by normal operation of the LED can cause bulb failure, as the plastic sleeve of the LED can soften and cause the relaxation of the press fit base contact point.

However, Cutler-Hammer LEDs contain two base solder points, and are epoxy-bonded in the area behind the plastic sleeve, eliminating yet another mode of failure and further increasing the reliability of Cutler-Hammer LEDs.

A case history - Norweb cuts its bills with Cutler-Hammer LEDs Norweb Distribution operates and maintains the electricity distribution network for North West England, delivering electrical power to 2.2 million customer premises through 14,000 kms of overhead lines, 45,000 kms of underground cables and 32,000 substations.

The company is an industry leader in reliability of supply, distributing 23,800 GWh of electricity annually to domestic customers, commerce and industry, and is constantly looking for ways to improve its service.

This is why it has replaced the filament indicator lamps in its control-relay panels at grid and primary sub-stations with Cutler-Hammer LEDs.

Rod Clegg, Design Engineer with Asset Management Design at Norweb Distribution, explains: "When our Relay Control Centres were refurbished we installed a new, sealed-for-life battery which couldn't be fitted with a booster.

This meant that we needed to reduce the burden on the battery.

Our relay panel manufacturer, Acrastyle Limited, suggested the use of LEDs as indicator bulbs, and recommended Cutler-Hammer as a supplier.

"Adopting Cutler-Hammer high intensity LEDs as status indicators to replace the existing filament lamps in our 11kV and 33kV Relay Control Panels has proved to be a great success.

The current drawn from the battery by the Cutler-Hammer LEDs is negligible compared to the incandescent filament bulbs, which ran at 15 watts, and this will increase battery reliability during 'off-network supply' time.

"In addition, our maintenance and bulb replacement costs have been drastically reduced.

In the past, if an indicator light wasn't illuminated our first action was to check if the bulb was faulty, but now we have full confidence in the reliability of the Cutler-Hammer LEDs.

If the indicator bulb is not lit, we can be sure there is no problem.

We have now replaced the incandescent indicator bulbs in all of our Relay Control Centres with Cutler-Hammer high intensity LEDs," Rod Clegg concluded.

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