Information

Technical information on reducing the wattage of high intensity discharge lamps, Feb.2009 Page 4 of 5
Phase control:
leading edge, trailing edge
Figs. 4 and 5 show the decrease in effective sup-
ply 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.
Fig. 4: Principle of phase control with leading edge
(idealized diagram)
Fig. 5: Principle of phase control with trailing edge
(idealized diagram)
For phase control with leading edge, the resulting
idle intervals result in a greater cooling down of
plasma and electrodes, thus increasing the re-
ignition peak so that the lamp goes off earlier.
For phase control with trailing edge or other
methods where the supply voltage is temporarily
switched off or reduced, suitable means are re-
quired to provide an uninterrupted “smooth” lamp
current as the lamp can otherwise flicker and go
off.
Increasing choke impedance respectively
reducing lamp current
Increasing choke impedance reduces the current
through the lamp. The supply voltage remains the
same so that the voltage is still high enough to re-
ignite the lamp. The flatter zero crossing of the
current does however result in greater cooling
down of plasma and electrodes, with greater
blackening caused by the processes at the elec-
trode during re-ignition.
Fig. 6: Amplitude modulation e.g. by choke changeover
The least disadvantages are expected from
current reduction in rectangular mode. The steep
zero crossings mean that lower re-ignition peaks
and less blackening from sputtering can be
expected.
Changing the frequency in high-frequency
operation
Consideration must be given to the possible
occurrence of acoustic resonance when discharge
lamps are operating at high frequency. In the dis-
charge tube, resonances can start to oscillate
depending on arc tube geometry and plasma tem-
perature, when the high-frequency parts of the
lamp wattage meet a resonance frequency of the
lamp. This can cause flickering and, in the worst
case, destruction of the lamp. This is why the pro-
posed standard for electronic operation of metal
halide lamps limits the amount of high-frequency
oscillations.
It is therefore difficult to find reliably resonance-
free operating windows, because the resonance
frequencies change during the start-up phase and
also over the life of the lamp, and because it
should be possible to operate lamps with differing
geometries and fillings at the same operating
device. A reduction in wattage also changes the
resonance frequencies due to the change in
plasma temperature.