Electrode characterization in dimmed operation of fluorescent lamps

The working temperature of the electrodes of a fluorescent lamp is a critical issue not only during the ignition process but during operation at steady state. To keep the hot spot temperature close to its optimum value during the operation at reduced power levels, an additional heating current must be supplied to the electrodes. The SoS lines described in IEC60901 standard provides a straightforward way to determine the heating current required as a function of the discharge current. The equivalent resistance of the electrodes in dimmed operation can be helpful to design the heating circuit but they also play an important role in the determination of the lampballast stability. The negative impedance inherent to every discharge lamp is partially compensated by the electrode resistance in fluorescent lamps, thus the characterization of this resistance can be used to accurately determine the ballast stability throughout the desired dimming range. In this paper, a new procedure to determine the equivalent circuit of the electrodes in dimmed operation is presented. Using the proposed procedure, the electrode equivalent circuit of a T5-14W fluorescent lamp was obtained following the allowable operating area defined by SoS lines.