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Electric BY JULIAN EDGAR Lighting Pt.11: High Intensity Discharge Lighting For Cars The headlights in some prestige cars no longer use incandescent lamps. Instead, metal halide gas discharge lights are used and these have several advantages. High Intensity Discharge lights are widely used in industrial, commercial and outdoor environments. They include high-pressure mercury lamps, high and low-pressure sodium vapour lamps, and metal halide lamps (see earlier articles in this series). But although such lights have been in use for many years, the incandescent lamp has reigned supreme in automotive headlights until quite recently. Now, manufacturers of luxury cars such as Lexus and BMW are introducing High Intensity Discharge (HID) headlights on their vehicles. Lighting 80  Silicon Chip manufacturer Hella has also recently released the Predator auxiliary driving light, which uses the same technology. The advantages of HID lighting include: (1) a higher colour temperature, resulting in better visibility and sign recognition; (2) better efficacy; (3) a very long bulb life; and (4) a distinctive blue/white light appearance – which has some advantages for vehicle manufacturers wanting to display their technical prowess. Fig.1 shows the differences in a scene illuminated by conventional halogen incandescent illumination (top) and by Bosch High Intensity Discharge lighting (bottom). Xenon Metal Halide Lamps The new HID automotive lighting systems use metal halide lamps. These lamps are filled with mercury, metal halides and xenon gas. When a high ignition voltage is applied to the electrodes, the xenon gas in the quartz bulb emits light. The starting voltage initially applied varies from manufacturer to manufacturer – Hella use a starting pulse of 25kV, Lexus 20kV and Bosch 6-12kV. During the starting phase, the Bosch Litronic system can apply a current of up to 2.6A, which is substantially more than the continuous operating current of approximately 0.4A. This initial pulse gives the very quick start-up required in a headlight application, with the xenon gas almost immediately emitting visible light. As the temperature of the bulb rises, the mercury vaporises, allowing the discharge to occur. After that, the metal halides in the mercury arc separate and the lamp operates at full brightness. Full illumination occurs when the quartz bulb reaches its operating temperature of almost 1000°K. Fig.2 shows a High Intensity Discharge headlight, as fitted to the Lexus GS300. Performance As you might expect, the new HID lighting systems have quite a performance advantage over incandescent systems. The 35 watt ‘D-1’ bulb in the Bosch Litronic system, for example, emits a luminous flux of 3000 lumens, almost twice the intensity of an incandescent H1 halogen lamp. Hella state that their 35W Predator spotlight generates a luminance of 6000 cd/ cm2. By contrast, a 100W H1 halogen globe in the same luminaire provides a luminance of just 2500 cd/cm2. The colour temperature of HID lighting is also higher (4500°K) than for conventional incandescent halogen lamps. Relatively large components of green and blue wavelengths are emitted, giving the light an appearance very similar to sunlight. The life of the Bosch lamp is quoted at 1500 hours, which roughly equates to the total expected operating time during a vehicle’s life. Hella go even further, suggesting that their HID lamp will last for 2500 hours – approximately 50 times the life of a 100W H1 halogen bulb! In addition, if failure does occur, it doesn’t happen suddenly as with incandescent lamps. Another major advantage of the HID lamps is their lack of susceptibility to vibration. This makes the HID lights very suitable for harsh environments such as mining and off-road applications. The Hella spotlights are already being used in professional rallying. The much higher efficacy of HID lights results in a reduced current draw for the same degree of illuminance. Two 35W Hella Predators provide better illuminance than four 100W incandescent driving lights, while at the same time reducing the current drawn from 33A to 5.8A (at a nominal 13.8V). The use of HID lights in combined high/low beam applications has occurred only very recently. Bosch’s third generation Litronic system has Fig.1: these two photographs show the difference between conventional tungsten halogen lighting (top) and High Intensity Discharge (HID) lighting (above). Note the presence of the cyclist to the right in both pictures! (Bosch). Fig.2: the Lexus GS300 low-beam High Intensity Discharge headlight. high and low-beam capability within the one headlight. Headlight dipping can be achieved in two different ways. The first technique moves a shield within the luminaire, simply blocking off the high beam component. The second technique moves the bulb within the luminaire. Fig.3 February 1999  81 shows these techniques and the beam patterns that result. Electronic ballast Fig.3: the most recent Bosch Litronic HID system has the ability to operate on both high and low beams. To achieve low beam, either a shield is moved within the luminaire (top) or the bulb itself is moved (middle). The resulting beam spreads are shown at the bottom of the diagram. Note that a righthand drive perspective is used. (Bosch). The main functions of the electronic control system are to: (1) ignite the gaseous discharge; (2) regulate the current supply during the warm-up phase; (3) regulate the current supply during normal operation; (4) provide fail-safe operation. Fig.4 shows a schematic diagram of the Bosch Litronic system’s electronic control circuit. A frequency of 10kHz is used. The fail-safe functions of the controller are extensive. The Bosch system switches off the headlamp if damage occurs to the headlight’s glass or if the lamp connection is exposed. Interestingly, one reason that the lamp is extinguished with a broken lens is to reduce the chance of UV exposure. The Lexus system switches off the headlights if a voltage outside the 9-16V operating range is detected, turning them back on again if the input voltage reverts to normal. However, if the lights are already illuminated and the battery voltage falls, the lamps will stay on until there is insufficient voltage for their discharge to be maintained. If an open circuit (including a missing bulb), short circuit or flashing bulb is detected, the Light Control Computer switches off the power to the lights. In all systems, the electronic ballast is located in close proximity to the light that it controls. Fig.5 shows the layout of a first-generation Bosch Litronic system. Lamp level control Fig.4: the Bosch Litronic electronic control circuit includes several fail-safe systems. It even switches off the headlamp if damage occurs to the headlight’s glass or if the lamp connection is exposed. (Bosch). 82  Silicon Chip The very high intensity of HID lamps makes appropriate headlight level control very important. An interesting solution to this problem has been adopted on the Lexus models, which use a computer-controlled stepper motor system to automatically swivel the reflectors within their housings. Information for the “Headlight Levelling” ECU, which controls the stepper motors, is derived from a number of sources. First, height sensors are fitted to the suspension of one front wheel and one back wheel. The information from these is fed directly to the ECU, along with information on the individual wheel speeds as de- SILICON CHIP This advertisment is out of date and has been removed to prevent confusion. Fig.5: the first generation Bosch Litronic system used conventional lights for high beam. The main components of this system were: (1) electronic ballast unit with controller; (2) high voltage section; (3) HID projector (low beam); (4) conventional high beam. (Bosch). Fig.6: the most recent Bosch design integrates headlight level control into the HID system. (Bosch). Conclusion Fig.7: this is the “Predator” driving light from Hella. Two 35W Predators outperform four conventional 100W driving lights, while reducing the current consumption from 33A to just 5.8A. As with other electronic automotive innovations (eg, anti-lock brakes and airbags), the technology of HID lighting is almost certain to trickle down to medi- um-level cars in the near future. Your next car could well use HID SC headlights. • RESELLER FOR MAJOR KIT RETAILERS • • PROTOTYPING EQUIPMENT • FULL ON-SITE SERVICE AND REPAIR FACILITIES • LARGE RANGE OF ELECTRONIC DISPOSALS (COME IN AND BROWSE) CB RADIO SALES AND ACCESSORIES Croydon Ph (03) 9723 3860 Fax (03) 9725 9443 Mildura Ph (03) 5023 8138 Fax (03) 5023 8511 M W OR A EL D IL C ER O M E rived from the ABS (anti-lock braking system) sensors. As the vehicle is being driven, the Headlight Levelling ECU calculates vehicle pitch from the suspension height sensors and the model’s wheelbase. The headlight reflectors are then automatically adjusted to give the optimum beam angle. The reason that a wheel speed input is required is because the reflectors default to a predetermined initial setting if the speed is below 1.9 km/h. The most recent Litronic system from Bosch includes headlight level control as an integral part of the system. Fig.6 shows the appearance of this system. ELECTRONIC COMPONENTS & ACCESSORIES Truscott’s ELECTRONIC WORLD Pty Ltd ACN 069 935 397 30 Lacey St Croydon Vic 3136 24 Langtree Ave Mildura Vic 3500 February 1999  83