3.1.1 American circuits for ballasts. Osram HQI-E 150 W/NDL CL
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This lamp behavior results from the relatively flat zero crossing for sinusoidal current. When the current approaches zero, the plasma temperature decreases and the electrodes also cool down. The recombination of electrons with ions reduces conductivity. After the zero crossing, the conductivity is too low to take up the current that the choke wants to drive. As a result, the voltage through the lamp increases again significantly until the lamp “reignites”. The higher voltage results in a higher ionization rate that increases conductivity again so that voltage falls.
As per the IEC 61167 standard, ballast units for MH lamps must by protected from overheating through rectification. This can be done e.g. with a thermal fuse
(tested according to IEC 598-1, Annex C).
3.1.1 American circuits for ballasts
By contrast, current and voltage for the rectangular waveforms of an electronic ballast change significantly faster from positive to negative half-wave, or have a faster commutation times (see chapter 3.2
“Electrical ballasts (ECG)”), so that the plasma has little chance to cool down. The instantaneous voltage required from the electronic ballast is therefore significantly lower than for the choke. This is one of the advantages of electronic ballast, as one of the failure mechanisms of metal halide lamps is to extinguish due to high re-ignition voltage. The re-ignition peak of a lamp normally increases over the service life, and when it exceeds what the supply voltage momentarily can provide, there is no “re-ignition” and the lamp goes out (see also chapter 6.2.2 “Increase of the re-ignition peak”).
When operating on a conventional choke, the lamp wattage runs through a maximum depending on the lamp voltage (see Fig. 5). The maximum occurs for a lamp voltage of slightly more than half the supply voltage. Near the maximum the lamp wattage changes only slightly with the lamp voltage. During the lamp service life, the lamp voltage increases, as also shown in chapter 6 “Lamp lifecycle, aging and failure behavior”. In order for the lamp wattage to change as little as possible, the nominal value for lamp voltage is generally chosen near the maximum, therefore at about half the supply voltage.
In this context it is important to note that the supply voltage in America has a different frequency (60 Hz).
As the inductive resistance of the choke depends on the frequency, in this case it is important to use a designated ballast for the corresponding frequency. In addition, both lamps and ballasts in the USA are standardized by ANSI, the American National Standards
Institute. To operate the systems correctly, lamps must be operated with corresponding ballasts. Ratings designations are required by ANSI to be marked clearly on all products, allowing users to clearly identify system agreement.
3.1.1.1 Autoleak transformer or high reactance auto-
If the supply voltage is smaller than around twice the lamp voltage, as is the case, for example, in the USA or Japan, then the supply voltage must first be stepped up. A good way of doing this is use an autoleak transformer. Part of the secondary windings act as lamp choke. On the one hand, this saves on material, and on the other hand, a higher voltage (open-circuit voltage) is available to start the lamp. These types of ballasts are typically more economical than a constant wattage style ballasts at the expense of wattage regulation.
U
S transformer
C
PFC
Tr
La
C
PFC
... PFC capacitor
La ... lamp
U
S
... supply voltage
Tr ... autoleak transformer
The impedance of the choke is rated at a certain supply frequency and certain supply voltage. Deviations from the nominal supply voltage will result in a different ballast curve and a related different working point for the lamp and therefore different lamp wattage. To limit the associated greater spread in the lamp parameters, a maximum deviation of 5% from the nominal values is permitted in the short term for the supply voltage, or maximum 3% in the long term. For deviations over a longer period of time, suitable choke tap must be selected. Similarly, the choke impedance must not deviate from the nominal values by more than
2% (see also chapter 3.1.3 “Influence of deviations in supply voltage”).
Fig. 7: Autoleak transformer
3.1.1.2 Constant wattage ballast
A constant wattage ballast such as those widely available in the USA consists of an autoleak transformer in series with a capacitor. The advantage of this circuit is the reduced impact of fluctuations in the supply voltage and the possibility of operating the lamp at supply voltages (110/120 V in the USA, 100 V in Japan) that lie within the range of the lamp voltage.
C
• Maximum permitted supply voltage deviations:
5% in the short-term, 3% in the long-term, use other tap on the choke if necessary.
• Maximum deviation of choke impedance 2%.
• The choke must be protected against overheating according to the standard (thermal fuse).
U
S
Tr
La
C ... capacitor
La ... lamp
U
S
... supply voltage
Tr ... autoleak transformer
Fig. 8: Constant wattage ballast
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