5.2.2 Phase control: leading edge, trailing edge. Osram HQI-E 150 W/NDL CL
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24 re-ignition voltage and the current supply voltage decreases. If the re-ignition voltage exceeds the supply voltage, the lamp goes out (see also chapter 6.2.2
“Increase in re-ignition peak”).
This means that POWERBALL HCI ® must not be
dimmed by reducing the supply voltage, as the re-ignition peak can cause earlier extinguishing of the lamp or flicker.
5.2.2 Phase control: leading edge, trailing edge
Fig. 26 and 27 show the decrease in effective supply voltage by phase control with leading edge or trailing edge. There are also variations in which the supply voltage is reduced in the middle and not before or after the zero crossing. In other versions, the supply voltage in the leading edge phase is only decreased and not reduced to zero.
5.2.3 Increasing choke impedance or decreasing 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 reignite the lamp. The flatter zero crossing of the current can however be expected to cause greater cooling down of plasma and electrodes, with greater blackening as a result of the processes at the electrode during re-ignition. The blackening therefore causes a greater drop in luminous flux compared to full-load operation.
l t
U,I
␣
U
B
U
L
U
B
U
L
Lamp voltage angle
Current flow angle t
Fig. 28: Amplitude modulation e.g. by choke changeover
The least disadvantages are to be expected by reducing current in rectangular mode. The steep zero crossings mean that lower re-ignition peaks and less blackening from sputtering can be expected.
Fig. 26: Principle of phase control with leading edge
(idealized diagram)
If a switchover to other chokes is used for dimming lamps with a wattage > 400 W, they must be left to burn at 100% for at least 1 hour.
U,I
␣
U
B
U
B
U
L
Lamp voltage angle
Current flow angle
5.2.4 Change in frequency for high-frequency mode
U
L t
A change in wattage when using an inductive ballast can also be achieved by varying the frequency of the power supply, as the inductive resistance of the choke depends on frequency. The change in choke impedance at low frequencies has been dealt with in the preceding chapter 5.2.3.
Fig. 27: Principle of phase control with trailing edge
(idealized diagram)
For the phase control with leading edge, the resulting intervals with no current result in a greater cooling down of plasma end electrodes, thus increasing the re-ignition peak, causing the lamp to go off earlier.
For the phase control with trailing edge or other methods where supply voltage is temporarily switched off or reduced, suitable means are required to provide an uninterrupted, “smooth” lamp current to prevent the lamp from flickering and going off.
If the change in impedance is caused by changing the frequency in radiofrequency operation, in discharge lamps the possible occurrence of acoustic resonances has to be considered. Resonance in the discharge tube can cause the plasma to start to oscillate depending on the arc tube geometry and plasma temperature. This can cause the lamp to flicker or go off, and in extreme cases, destruction of the lamp should the arc attach to the arc tube wall due to severe resonance. This is why a currently proposed standard for electronic operation of metal halide lamps limits the amount of high-frequency oscillations.
Increased blackening and therefore a drop in luminous flux must be expected in all versions compared to fullload operation.
It is difficult to find reliably resonance-free operating windows, for various reasons: the resonance frequencies change during the start-up and also dur-
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