LPS Starting And Warm-Up

A Low Pressure Sodium lamp may be controlled either by a high reactance autotransformer ballast or a " hybrid" ballast. When an autotransformer ballast is used, the open-circuit voltage is sufficient to start the lamp. When a hybrid ballast is used, an ignitor is required to provide a starting voltage pulse. In either case, a Low Pressure Sodium lamp starts and warms up to full light output in the following sequence of steps:

1. With an autotransformer circuit, when power is turned on, the ballast is energized and the proper starting voltage is applied to the lamp. This establishes an electric field between the electrodes, resulting in the emission of electrons and ionization of the starting gases.
   With a hybrid circuit, when power is turned on, the ballast is energized and a narrow high voltage pulse is applied to the lamp, initiating an electrical discharge between the electrodes. As soon as a path of ionized molecules exists, current is supplied by the ballast to initiate a continuing arc in the lamp. If necessary, the starting pulse repeats during each cycle until the arc is established, at which time the pulse automatically ceases.

2. The initial arc occurs through the neon and argon starting gases, which begin to glow after current starts to flow in the circuit.

3. As the gas mixture gives off an increasing amount of heat, the sodium metal rapidly vaporizes in the arc tube and radiates visible energy at the characteristic yellow sodium wavelength.

4. At approximately 260°C, the arc tube reaches its optimum operating temperature and the sodium discharge achieves its maximum efficacy. This warm-up cycle usually takes 10 to 12 minutes.