technical talk A Dim View
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e have all heard of Light-Emitting Diodes or LEDs. LEDs present many advantages over traditional light sources including lower energy consumption, longer lifetime, improved robustness, smaller size and faster switching. However, they can be expensive and require a more precise current and heat management system than a traditional light source. The application of LEDs has been wide spread. They are used as low-energy indicators or signal devices, and more importantly are used as a replacement for traditional light sources in our homes and businesses. But how do they work? How do the manufacturers obtain the correct colour rendition, and can they be dimmed? These are questions our Technical team are often asked. What could now be called the “humble LED” is quite simply, a solid-state semiconductor device that converts electrical energy directly into light. This is achieved by allowing electrical current to pass through the semiconductor in one direction only after a threshold voltage is attained between the anode and cathode terminals, commonly known as the PN junction (as seen in Figure 1). The electrical current that passes through the PN junction releases energy in the form of photons of light. This is why LEDs can shine. The wavelength (colour) of the light depends on the materials forming the PN junction or chip. Red and green LEDs have been around for decades but they are all monochromatic in their wave form or of single hue. If Red or green LEDs are used as a light source, an illuminated object may appear to conflict with the natural colours of the object. As a result of the introduction of blue light LEDs by a Japanese scientist, Shuji Nakamura in 1993, the application of LED development and technology has rapidly spread. With the discovery of the blue LED the combination of our three basic colours was obtained, Red, Green and Blue (RGB). This has given birth to the application of various phosphors to produce white LEDs, and
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as we have seen the realisation that LEDs can replace conventional light sources. LEDs are current driven devices. Less current in means less photons out. But we don’t generally power LEDs from a true current source. The vast majority of power supplies are voltage sources, and that’s what we usually find powering LEDs. In Figure 2, (VT) represents the voltage threshold and current only flows through the PN junction once the voltage threshold has been reached. By gradually lowering the voltage source across the LED back toward the threshold level, a reduction in light may be noticed (dimming effect). Lowering below the threshold level will of course turn the light off. Unfortunately there is no really secure method of dimming an LED light source, other than by an attempt to trick the human eye. By allowing only short pulses of full voltage, the human eye may be tricked into believing the output is dimmer than the LEDs full intensity. This is known as Pulse Width Modulation. The human eye cannot distinguish this flicker of 50 times per second. The shorter the pulse width, the dimmer the light output will appear. This can be seen in Figure 3. Naturally this article only scratches the surface on dimmable LED technology. Manufacturers are generally very helpful and will usually advise on the most suitable components to use, based on your proposed application. Most dimmable fittings are now compatible with “Dynalite” and “C-Bus” type controllers thus providing ease of choice and fitment. Accredited Master Electricians and ECA members are best advised to consult a lighting specialist for specific advice and design assistance, or the Technical team for further information. Correction The Autumn 09 edition of the magazine depicted an incorrect illustration in relation to the ‘Understanding Fault Loop Impedance’ article’. The correct illustration can be found at www.masterelectricians.com.au.
ANODE
CATHODE
Figure 1
V or I VT LED on
LED off time
Figure 2
V or I 100% time 50%
10%
Figure 3