The arc interacts with mercury vapor, producing radiant energy. When radiant energy strikes the phosphor coating on the inside of the lamp, the phosphor gives off light. After the arc is established, only a low voltage is required to maintain it. The ballast is placed in the circuit to provide a high starting voltage followed by a current-limiting function. Standard Ballasts Standard ballasts use a combination of inductive and capacitive networks to control current to the lamps.
Inductors are aluminum coils wrapped around laminated iron cores.
Instead of aluminum, these ballasts have copper wire coils, and the iron cores are larger. The copper has lower resistance and the larger iron core generates less heat inside the ballast. These two factors result in efficiency improvements. In order to initially light rapid start lamps, an initial voltage must be applied to the cathode. Once the light is on, however, the 2.5 to 4.1 volts of initial cathode power is no longer needed to sustain lamp operation, and thus may be eliminated.