26 Rapid-Start Ballast
Aaron Lee
The rapid-start ballast is very popular because it eliminates several major shortcomings of the instant-start fixture, such as the need for high striking voltage and the resulting reduced lamp life, and also reduces the need for large, heavy ballasts. The rapid-start fixture takes about a second to start and the striking voltage is 30% to 50% lower than the striking voltage for instant-start lamps.
The operation of the rapid-start fixture depends on a momentary preheating of the cathodes. At the instant the switch is closed, a low-voltage windings in the ballast applies three to four volts to the lamp cathodes, which causes them to heat up, thus reducing the needed magnitude of striking voltage.
A further reduction in striking voltage is achieved by the use of a nearby grounded metal plate running the length of the lamp, which in most luminaire designs is provided by the reflector or ballast and wiring channel. The presence of this grounded plate sets up a capacitance with the lamp and helps to increase the voltage stress on the gas in the lamp.
