Technical information on reducing the wattage of high intensity discharge lamps, Feb.
1. Introduction High intensity discharge lamps generate light by exciting mercury respectively other metals in a plasma. The plasma is generated in an arc tube by current flowing between two electrodes. The electrodes are inserted into the discharge vessel so that it is sealed completely. Discharge lamps have to be operated with a ballast and are rated for a certain wattage. The ballasts consist of conventional chokes (CCG) and electronic ballasts (ECG).
To change the nominal lamp wattage of a lamp, the following physical general conditions are significant for the resulting effects: • The electrodes of discharge lamps are rated for a certain lamp current. If the current is too high, parts of the electrode melt and evaporate. If the current is too low, the electrode is operated in cold state. This changes the mechanisms of how electrons are released from the electrode so that more electrode material is eroded to the arc tube wall.
Phase control: leading edge, trailing edge Figs. 4 and 5 show the decrease in effective supply voltage by phase control with leading edge and trailing edge. There are also variations in which the supply voltage is reduced in the middle and not before or after the zero crossing. It is also possible just to reduce the supply voltage and not to take it right down to zero. same so that the voltage is still high enough to reignite the lamp.
3. Recommendations for reducing the wattage of High Intensity Discharge lamps High pressure mercury lamps These lamps can be dimmed to 50 % of the rated wattage, whereby they must start up with 100 % wattage. Dimming is possible by voltage reduction, phase control and amplitude modulation.
