Reducing the wattage of high intensity discharge lamps. Osram HQI-E 150 W/NDL CL
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5 Reducing the wattage of high intensity discharge lamps
5.1 Introduction 5.2 Wattage reduction techniques
High intensity discharge lamps generate light by exciting mercury and other metals within an arc tube into a plasma generated by the current flow between two electrodes.
The following dimming methods are generally known
(by conventional means or electronic ballast):
• Reduction in supply voltage
• Phase control: leading edge, trailing edge
Discharge lamps must be operated with a ballast and are rated for a certain lamp wattage. Either conventional chokes or electronic ballasts can be used.
• Increase in choke impedance or decrease in lamp current (amplitude modulation)
• Change in frequency for high-frequency operation
To change the nominal lamp wattage of a lamp, the following general physical conditions are significant for the resulting effects: 5.2.1 Reducing the supply voltage
• The electrodes of discharge lamps are rated for a certain lamp current. If the current is too high, parts of the electrodes melt and evaporate. If the current is too low, the electrode is operated in cold state. This changes the mechanisms for releasing electrons from the electrode with more electrode material being deposited on the tube wall. Deviations in lamp current from the nominal value in both directions can therefore cause blackening of the arc tube wall with a decline
in luminous flux, together with negative effects on the light colour and possibly also on the service life.
A reduction in supply voltage beyond recommended limits (see sections 3.1.2 and 3.1.3) will decrease the lamp wattage. Reducing lamp wattage results in decreased lamp voltage and re-ignition peak voltage, and is generally to a lesser extent than the supply voltage.
This reduction in the gap between the re-ignition peak and the supply voltage makes it more probable that the lamp will go out. This applies particularly to aged lamps where the lamp voltage and re-ignition voltage have already increased.
• The partial vapor pressure of the filling particles responsible for generating light depends on the
temperature of the arc tube wall. A change in the arc tube wall temperature resulting from a change in lamp wattage influences the composition of the filling in the plasma arc and thus the
electrical and photometric properties of the lamp.
• At higher arc tube wall temperatures, the metals do not recombine with the iodides and the pure metals can migrate into the wall (applies to quartz arc tubes).
Fig. 25 shows, as an example, the behavior of certain lamp types on reducing the supply voltage. Here the ratio of re-ignition voltage to effective supply voltage
(U
LS
/U
S
) has been standardized to 1 for 220 V supply voltage. It can be seen that when the supply voltage decreases, this ratio generally assumes values of greater than 1. This also means that the gap between
1,4
ULS/US referred to the ratio at 220 V
HCI-TM 250 W/WDL
HCI-TS 70 W/WDL
HCI-TT 150 W/WDL
HQI-TS 150 W/WDL
HQI-TS 150 W/NDL
1,3
Wattage reduction has the following side effects:
• Drop in luminous fl ux through blackening
• of thearc tube
Change in color properties
• Reduction in service life
1,2
1,1
1
0,9
140 150 160 170 180 190 200 210 220 230 240 250 260
Supply voltage US in V
Fig. 25: Relative change in the re-ignition peak (U
LS
) to supply voltage (U
S
) referred to the ratio at 220 V for various metal halide lamps
23
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