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Technical information on reducing the wattage of high intensity discharge lamps, Feb.2009 Page 2 of 5
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).
When operating discharge lamps at a choke, the
lamp wattage follows the curve shown below,
depending on the lamp voltage. The lamp voltage
is normally selected near to half the grid voltage
because the curve is flat at this point, i.e. the watt-
age only undergoes relatively slight changes when
the lamp voltage increases. The voltage of high
pressure sodium lamps and metal halide lamps
increases during the lamp life.
Fig. 1: Model of a choke curve, lamp wattage in blue,
lamp current in orange
The inductivity of the choke leads to a phase shift
between the lamp current (and choke current) and
the supply voltage, as also shown in figure 2.
Before every zero crossing, the current decreases
because of its approximate sine shape. The lower
current means that both the plasma and the elec-
trodes cool down. The recombination of electrons
with ions reduces conductivity. This is why follow-
ing the zero crossing of lamp current and voltage,
the plasma is not immediately capable of carrying
the current that the choke wants to drive.
The voltage across the lamp increases initially,
forming the so-called re-ignition peak – so the
plasma has to be re-ignited first. The higher volt-
age results in greater ionization with increased
conductivity and therefore reduced voltage again.
If the re-ignition voltage now exceeds the value of
the supply voltage at this point in time, then the
lamp extinguishes. One great advantage of elec-
tronic ballast with rectangular voltage and current
is the steep zero crossing. This means that
plasma and electrodes scarcely cool down at all
and there is almost no more formation of a re-
ignition peak.
Fig. 2: Lamp voltage and current and supply voltage in
a metal halide lamp