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Electric Lighting Pt.12: Pt.13:Automotive AutomotiveLighting LightingUsing UsingLEDs LEDs Light emitting diodes (LEDs) have particular advantages over incandescent lamps when used in the brake, tail and indicator lights of vehicles. They last longer, are more efficient, have better vibration resistance and they turn on faster. By JULIAN EDGAR When used in brake lights, the faster turn-on time of light-emitting diodes when compared with incandescent lamps gives drivers in following cars significantly more time to react and apply the brakes. Incandescent brake lamps have turn-on times of up to 300ms. In that time, a car travelling at 60km/h will travel 5 metres – or about one car length. By comparison, a LED has a turn-on time of 100ns (one tenth of a microsecond) which is negligible. Fig.1 gives a graphic comparison of the turn-on times for a typical incandescent brake light and the LED equivalent. Note that this assumes that the full battery voltage is available but in a typical brake light circuit significant voltage drops are often present. These make the turn-on time worse, often much worse. With a voltage drop of 4V in the braking circuit, the turn-on time of an incandescent automotive bulb can double and the brightness is greatly reduced. Both factors mean that the reaction time of the following driver is greatly increased. Studies have shown that LED brake lights provide a reduction in driver response time of between 170 and 200ms under favourable road conditions and up to 300ms under adverse conditions. In addition, practical testing has shown that the response time of a person viewing a LED brake light is actually faster than expected, even taking into account the much shorter LED switch-on time. It is thought that because it reaches April 1999  71 Fig.1: turn-on times at 12.8V for a typical incandescent brake light and the LED equivalent. Note that the LED effectively turns on instantly. (Hewlett Packard). full brilliance very quickly it is more likely to catch the eye of the following driver. Tail & marker Lights While cars have used high intensity red LEDs in rear spoiler brake light arrays since 1986, they have been little used elsewhere. Now, drop-in replacement LED tail and marker lamps for trucks and semi-trailers have been introduced. In these applications, the benefits of LEDs include shock and vibration resistance, less current drain and Fig.2: Hewlett Packard’s Super Flux LEDs are designed expressly for motor vehicle lamps. The LED body is 7.6mm square. (Hewlett Packard). 72  Silicon Chip constant light output over a wide voltage range. Voltage drop is a problem in heavy vehicles, where the rear trailer lights have a very long cable run. This is compounded where ABS systems are to be fitted. For the operation of anti-lock brakes on trailers, at least 9.5V must be available. For older B-double and triple trailer combinations being upgraded to ABS, the easiest way of making sure that 9.5V is available, short of re-wiring the trailer with heavier cable, is to reduce the total current drain by using LEDs. The longer life of LEDs is a bonus – in fact one US manufacturer is offering “the industry’s only lifetime warranty” on their LED direct replacement truck lamps. To car users, longer life in brake and tail lamps is not important; after all they seldom fail. But it has been estimated that heavy vehicle marker or clearance lamps cost about US$500 to maintain over a trailer’s life. Most of this figure consists of labour costs and it makes the adoption of LED lights in the heavy vehicle industry very attractive. American Freight-ways of Arkansas, USA is currently specifying LEDs for the red three-lamp cluster located above the rear door of 5,000 of its trailers. All-LED Lights The adoption of LEDs for all external passenger vehicle lamps (except the headlights) is expected to occur over the next few years. At only 50mm thick, LED light assemblies can be much thinner than incandescent lamps, which can be up to 150mm deep. However, the biggest advantages remain lower power consumption and the increased life. LED manufacturer Hewlett Packard recently surveyed 17 1998 US-market cars and trucks. The total power for incandescent signal lamps varied from 93 - 217W for daytime operation and from 135 - 263W for night use. They then calculated the required Fig.3: the luminous flux output characteristic of a Hewlett Packard AlInGaP LED. At 75°C the luminous flux is reduced to half of that developed at 20°C. (Hewlett Packard). Fig.4: LED current can be kept constant irrespective of battery voltage variations by the use of a constant current drive circuit. This eliminates the increased LED heating that otherwise occurs at times of high battery voltage. (Hewlett Packard). number and type of LEDs to replace these incandescent signal lamps. For the exercise, the LEDs were connected in series strings with four LEDs per string. Each string was driven at 60mA with the current set by a resistor. The potential power savings were about 80% for daytime running and 78% Fig.5: to avoid over-heating the LEDs, it is common practice to use a PTC resistor to reduce the current at high ambient temperatures. (Hewlett Packard). at night. Next, HP calculated the proportion of time that each of the lights would be on. For example, if a car is driven entirely in urban conditions, they suggest that the brake lights will be operating 25% of the time, the turn indicators 1.4% and the ‘parking’ (ie, tail lights and front marker lights) 30% of the time. From this they calculated the reduction in the power rating of the alternator for a car equipped with LED signal lights. Taken in conjunction with the lighter gauge wire that could be used in a LED installation, a very small reduction in overall vehicle mass could be made. However even this small reduction had worthwhile benefits in fuel consumption figures. Another advantage of LED turn signals is that their reduced power consumption allows much longer operation of the hazard flashers before the battery is flattened. At a 50% duty cycle, the average current hazard flashers using incandescent lamps is 4.7A. This can be reduced to 2.3A if LEDs are used. Thus the use of LEDs could more than double the length of time the hazard flashers could be operated without the engine running. Automotive LEDs The use of LEDs in centre high mount stop lamps has become common. The fast switch-on time of LEDs gives following cars significantly more time to stop. (Hewlett Packard). Hewlett Packard’s recently released Super Flux LEDs are designed expressly for automotive exterior lighting. They feature a high light output (3000 millilumens at 70mA) and have an operating temperature range of -40°C to 100°C. They also meet the colour requirements for automotive signal lighting as specified by the appropriate regulating bodies. The LEDs use AlInGaP construction and have a low profile package. Fig.2 shows an outline drawing of the new LED. There are two major design conApril 1999  73 Fig.6: the light flux distribution of a Hewlett Packard Super Flux LED is symmetrical around its optical axis. Luminous output falls to nearly zero at angles of more than 50 degrees to the optical axis. (Hewlett Packard). siderations that must be made when developing LED automotive lights. These are: • control of heat; and • management of the light output by lenses and reflectors. Heat control As discussed last month in this series, the light output of LEDs declines with increasing temperature. Fig.3 shows the output characteristics of a Hewlett Packard AlInGaP LED. It shows that light output at 75°C is half that produced at 20°C. This is important since maximum temperatures of 70°C are common within exterior high-mounted central brake lights, while interior-mounted lamps can go as high as 90°C. This temperature is due to heat build-up from the sun as well as the design of the lamp itself. In addition, a change in temperature causes a change in the colour of light emitted by LEDs. The dominant wavelength of a LED will increase by one nanometre (1nm) for every 10°C rise in junction temperature. This change in colour is not critical in brake light applications (where the allowable colour range of approximately 90nm is very broad) but in some amber signal lights the allowable colour range is much narrower at 5-10nm. Apart from the actual power dissipation, the main factor in the temperature rise of the LED lamp is the way in which the LEDs are assembled and driven. Table 1 shows various design layouts of LEDs in automotive lamps and 74  Silicon Chip their associated junction temperature rise (above ambient) versus power dissipation. The layout indicated by line 2 of Table 1 is most commonly used in high-mount centre stop lamps and line 4 is most commonly used in rear combination (ie, turn/stop/tail) lamps. Table 1 indicates that if the LEDs are densely packed on the PC board, they will need to be derated; ie, operated at a reduced current. The reduction of heat build-up within the lamp assembly can be accomplished in a number of ways. Firstly, the PC board can have broad copper tracks on the cathode side of the LEDs, to act as heatsinks. To reduce their heat contribution, the current limiting resistors can be mounted outside the lamp assembly, on a separate PC board or within the wiring loom. If required, the current limiting Fig.7: the light output of a LED both refracted and reflected-refracted light. (Hewlett Packard) resistors can be distributed evenly along the length of the PC board, to reduce the heat build-up at any one location. In addition, the LEDs can be spaced as widely as possible and lamp housings ventilated by holes and/or the PC board thermally connected to the housing so it acts as a heatsink. Mind you, in a typical Australian summer setting, the main source of temperature rise within the lamp housing will be the sun, so it won’t be much of a heatsink – more a heat source! The electrical drive circuit can also be arranged to reduce LED heating. Firstly, drive current fluctuations can be minimised and secondly, the drive circuit can be designed to dissipate the minimum amount of heat. Many drive circuits in LED high mount stop lamps consist only of a current limiting resistor and a silicon Temperature LED Lamp Design Rise  (°C/W) 1 Single row of LEDs with the current limiting resistors/drive circuitry located off PCB   325 2 Single row of LEDs with the current limiting resistors/ drive circuitry located on the same PCB as the LEDs 400 3 Multiple rows or an X-Y arrangement of LEDs with the current limiting resistors/drive circuitry located off the PCB   500 4 Multiple rows or an X-Y arrangement of LEDs with the current limiting resistors/drive circuitry located on the PCB   650 Table 1: the temperature characteristics of various combinations of LEDs used in automotive lamps. As LEDs are more densely packed on the PC board, or if the drive circuitry is included on the PC board, they need to be derated. the light (diverging optics) or gather the incoming light into a beam (collimating optics). The most common type of diverging optic used is the pillow lens, shown in Fig.8. Collimating optics can use reflecting cavities in which the LEDs are mounted. These reflectors may have a straight or parabolic profile and are often used with a pillow lens, as shown in Fig.9. Another approach is to use a collimating lens such as a Fresnel SC design, shown in Fig.10. Marker lamps for trucks now commonly use amber LEDs. Turn indicator lights on cars will soon follow this lead. (Dialight). diode to prevent reverse-polarity connection. This means that the LED current varies with battery voltage. This is avoided by using a constant current drive circuit, as shown in Fig.4. Basically, this takes the form of an LM317 (or equivalent) adjustable voltage regulator connected as a constant current source. Ambient temperature compensation can be used to allow the LEDs to be driven at a higher forward current during cooler conditions. Note that this is the opposite approach to that discussed last month with regard to traffic lights, where an increase in temperature is accompanied by an increase in current so that adequate LED brightness is maintained. Reducing the current at higher temperatures can be simply achieved by the use of a positive temperature coefficient (PTC) resistor. Fig.5 shows this approach. Fig.8: the pillow lens is commonly used in automotive LED lamps. It diverges the light from its source. (Hewlett Packard). Optical Design Even more important than heat considerations is the design of reflectors and lenses. The light distribution of a LED is symmetrical around its optical axis, as shown in Fig.6. However, unlike an incandescent lamp, a LED cannot be regarded as a point source of light. Some of the light produced in a LED chip is refracted by the LED’s epoxy dome (refracted-only light). The remainder of the light is reflected by the reflector cup and then subsequently refracted by the epoxy dome (reflected then refracted light). Fig.7 shows this effect for a Super Flux LED. The “refracted only” light appears to come from a certain location within the LED, while the “reflected-refracted” light appears to come from a different location. So the chip is not a point-source and light appears to come from a range of locations, termed the “focal smear”. In the HP Super Flux LEDs, the centre point of the focal smear is approximately 1mm below the base of the epoxy dome and this is used as an arbitrary point source for the purpose of the lens design. The optics of a LED lamp can consist of a lens or reflector or a combination of both. The optics may spread Fig.9: straight or parabolic profile multiple reflectors are often used in conjunction with a pillow lens. (Hewlett Packard). Fig.10: a LED luminaire using a combination of Fresnel and pillow lenses. (Hewlett Packard). April 1999  75