Why is the lamp black?

The lamp tube belongs to fluorescent lamp, and its working principle is that the filament emits electrons after heating, so that the mercury vapor in the lamp tube is ionized to emit ultraviolet rays, and the fluorescent powder on the tube wall emits visible light under the excitation of ultraviolet rays. In order to increase the ability of filament to emit electrons, barium carbonate, calcium carbonate, strontium carbonate and other emitting substances are coated on the surface of filament. After long-term use, the slow splash of luminescent substances will make both sides of the lamp black and gradually reach the service life.

In order to solve the problem of cathode preheating of fluorescent lamps, people use positive temperature coefficient thermistor (PTCR). The temperature resistance characteristic curve is shown in Figure 2. The TB point in the curve is the switching temperature of PTCR (the temperature when the resistance increases to twice the minimum value). When the temperature of PTCR is higher than the tuberculosis point, the resistance of PTCR will suddenly change to a very high value with the increase of temperature. The preheating start-up circuit designed by using this characteristic of PTCR is shown in Figure 3. At the moment when the circuit is turned on, the output voltage V0 of the high-frequency power supply is applied to both ends of the lamp, as shown in Figure 4. At this time, the Q value of the resonant circuit is very low because the thermistor PTCR shunts the resonant circuit, so that the high voltage cannot be formed at both ends of the lamp tube and the lamp tube cannot be lit. At the same time, the high-frequency current preheats the cathode through the inductor L, filament Rf and thermistor PTCR. After t 1 (the national standard stipulates that it is more than 0.4 seconds), the resistance value of PTCR increases rapidly due to the rising current, which weakens the shunt to the resonant circuit. When the resistance increases to a certain value, the resonant circuit vibrates and the amplitude V2 of the resonant voltage increases to light the lamp. When the lamp is lit (t2), the lamp presents negative resistance characteristics, that is, the lamp current increases, the voltage V3 across the lamp drops to the rated working voltage value, the preheating start-up process ends, and the lamp turns into normal operation.