Can somebody explain me a method of dimming HID (High intensity discharge) lamps.
Metal Halide lamps are unstable enough even at normal voltages, and are riddled with problems when attempting to "architecturally dim" them. Basically, they were never designed to be dimmed, and due to the way they work there are significant problems with colour shift and lamp life when dimming is attempted. The comments below relate to "high-ish wattage" metal halides like 250-400W, in industrial applications. If you are trying to dim metal halide in retail, commercial, display or architectural applications then you have probably selected the wrong light source! To "continuously dim" them, you need a dedicated electronic ballast. The most common RELIABLE/TESTED one available is from Venture Lighting (RUUD) and this uses 0-10V Analogue control (voltage is in reverse from most fluoro ballasts). This combines with a special lamp and works very well indeed. Next way is to use a tapped step-dimming ballast. The lamps are started at full voltage, then switched to a different tapping on the ballast. This requires 2 or more relay channels and a retrigger timer and some logic so that the lamps are ALWAYS re-started at full power. The 3rd way is using Autotransformers (like Clipsal Energy Controller, but NOT the off-the-shelf fluorescent versions). The lamps are started at full voltage then after a time of (say) 15 minutes are switched down to a reduced-power mode. This would be done with a 1 channel relay and some timers. For starting, the lamps must be started at full voltage, and be allowed to run at full power for at least 8-10 minutes to stabilise. Then the output may be reduced by dimming. If power is intterupted, the lamps must cool down for about 5 mins, then be restarted in the same way. Therefore, complex control strategies are required to ensure that this restrike delay is allowed for (or the lamps will never strike properly). Generally at LEAST 15-20 minutes should be allowed before dropping them into "dim" mode. Minimum dim level needs to be carefully set, they do not suffer volttage reductions, especially sudden ones.... any dip in mains voltage will extinuish the arc in the lamps and cause massive problems due to the very slow restrike time on the lamps. By the way, the power savings are not proportional to the drop in light output. You might get 25% less light for a 15% saving in energy consumption, so dimming metal halise is NOT an effective way to save power. In a new industrial installation, it is far cheaper, easier and better to use fluorescent light sources. Unfortunately, whilst Fluorescent has significant benefits over HID in almost all applications, the average user in Australia is conditioned that HID is the only way to light warehouses and factories. This is not the case in the rest of the world, where they use the best lightsource for the application (usually fluorescent) but doing this here in Australia is an almost impossible sell! So, that's some background info to get you started.
