Information

Technical information on reducing the wattage of high intensity discharge lamps, Feb.2009 Page 3 of 5
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. Deviations of the lamp current
from the nominal value in both directions can
thus cause a blackening on the arc tube wall
with a decrease in luminous flux, together with
negative effects on the light color and possibly
also on the life of the lamp.
• The vapor pressure of the filling components
responsible for producing the light depends on
the temperature of the arc tube wall. A change
in the arc tube wall temperature caused by a
change in the lamp wattage influences the
composition of the filling in the plasma arc and
thus the electric and photometric properties of
the lamp.
2. Methods of reducing wattage
The following dimming methods are generally
known (by conventional means or electronic
ballast):
• Reducing the supply voltage
• Phase control: leading edge, trailing
edge
• Increasing the choke impedance
respectively reducing the lamp current
(amplitude modulation)
• Changing the frequency for high-
frequency operation
Reducing the supply voltage
A reduction in supply voltage also decreases the
lamp wattage. While reducing the lamp wattage
also decreases the lamp voltage and the re-igni-
tion peak, this is generally to a lesser extent than
the reduction in supply voltage.
This reduces the interval between re-ignition peak
and supply voltage, making it more probable that
the lamp will go out. This applies particularly to old
lamps where the lamp voltage and re-ignition
voltage have already increased.
Fig. 3 shows, as an example, the behavior of
certain lamp types on reducing the supply voltage.
It illustrates the ratio of re-ignition peak to supply
voltage compared with the value at 220 V.
Fig. 3: Relative change in re-ignition peak (ULS) to
supply voltage referred to the ratio at 220 V
Accordingly with the exception of the high pres-
sure mercury lamps, dimming must not be
brought about by reducing the supply voltage, as
the re-ignition peak can cause the lamp to go off
earlier or to flicker.