Effect of Cathode Heating on Relation between Crest Factor and Cathode Fall Voltage
2000 Volume 84 Issue 8 Pages 480-485
Details
2000 Volume 84 Issue 8 Pages 480-485
The operating life of a fluorescent lamp is determined by the life of its cathode, which depends on how quickly the cathode loses its oxide coating. There are two mechanisms, vaporizing and sputtering, in this process. The sputtering is affected by the current crest factor (CCF) because the electron current is determined by the cathode temperature and the electric field of the cathode fall: Ie a T2exp (-e/kT) exp (0.4E/T).
When CCF is high, the cathode temperature cannot follow variations in the current because of thermal inertia. As a result, the cathode fall voltage (CFV) increases in order to supply the flow of peak electron current, which leads to heavy sputtering. The equation for electron current also shows that CFV can be reduced by heating the cathode auxiliarily if CCF is high. Examination of the relations among CCF, CFV, and the cathode temperature measured by the indirect method showed that
(1) The peak CFV increases as the CCF becomes higher when the cathode is not heated.
(2) The peak CFV can be reduced by heating the cathode if the CFV is high.
(3) The heating power needed to reduce the CFV is small enough for the practical circuit.
