Specifications

ST900 Family General Handbook
667/HB/32900/000 Issue 12 Page 35 of 265
2.11.3 Selection of Dimming Transformer
In order to select the correct dimming transformer, the total average signal Lamp
Power is required. See Table 3 on page 30 for the power taken by the different
types of lamps.
When selecting the dimming transformer the peak lamp power is not used as the
transformer can withstand the overload for the 2 second Red / Amber period. Thus
for the average power consumption, it is assumed that only one lamp is illuminated
on each signal head.
For the theoretical junction (Figure 6) this would be:
One lamp per signal head @ 63W x 12 signal heads
756W
Wait indicators at 42W x 4 168W
Total average signal lamp power is: 924W
Now using Table 5 below, choose which dimming transformer is required for the
dimming voltage required.
Note this table refers to Halogen loads and is not applicable to Helios LED lanterns.
For the theoretical junction, using the figure obtained above 916W and 160V
dimming, the transformer required would be the 1kVA version. Note that in this case,
the same transformer would be chosen regardless of which dimming voltage was
required.
Maximum
Lamp
Current
Dimming Voltage Required
120V 140V 160V
1.5 kVA
20A
Load <4725W
75 Lamps
Load <3717W
59 Lamps
Load <3150W
50 Lamps
3.0 kVA
30A
Load <7200W
114 (150)* Lamps
Load <6993W
114 (111)* Lamps
Load <5733W
114 (91)* Lamps
Table 5 – Selection of ST900 Dimming Transformer
The table shows the maximum ‘average signal lamp power’ that each dimming
transformer is capable of providing at each of the dimming voltages. It also shows
the equivalent number of Tungsten Halogen High Intensity lamps (see Table 3).
* The number of lamps shown before the brackets is limited by the Controller’s
maximum lamp current (see Table 4). The number of lamps shown inside the
brackets is the maximum number that the Dimming Transformer could support if it
were not limited by the Maximum Lamp Current.