Dynamic study of the time constant in HID lamp

Recent publications have established that the lamp time constant is an important factor to determine the stability of the lamp-ballast system. Different tests based on step response have been employed to obtain the dynamic characteristics of the lamp. These tests consist on applying a current or voltage pulse in order to obtain the lamp time constant. Nevertheless, a study that determines if the lamp time constant is the same in both cases has not been developed yet. On the other hand, the lamp time constants only have been tested in a single operation point, ignoring the effect of the limiting impedance (ballast). This paper presents a study about the lamp time constants and the interaction between time constants and ballast. This study concludes that the dominant dynamics of the lamp are determined by the gas thermal constant, which is extracted from the lamp current.