Abstract
High-frequency fluorescent lamp operation requires proper cathode preheating by electronic ballasts incorporating a preheating function. This paper examines minimum thermionic emission curves for three different ways of measuring emission times. One is based on a lamp ignition test, another on the ratio of hot to cold coil resistance (Rh/Rc), and the third on lamp voltage monitoring. The observation that the emission time measured by the lamp ignition method tends to be shorter than that measured by the lamp voltage monitoring method in a short preheat time may be understood in terms of the relaxation of the limited thermal conductivity of the emissive material due to the initial flow of the discharge current. We do not believe that the shorter emission time measured by the lamp ignition method causes premature thermionic emission. The Rh/Rc method needs much more investigation to measure emission times because it depends on the resistance of the whole cathode composed of the filament coil and the emissive material, whose specific heats differ.
This paper points out that the average resistance of a standardized cathode over the period up to the emission time should be regarded as a substitution resistor. Furthermore, the maximum preheating lines for the requirement in the present IEC should be changed into a maximum curve similar to the minimum thermionic emission curve.
