T8 lamp working principle
The fluorescent tube is filled with low-pressure argon or argon-neon mixed gas and mercury vapor. The inner surface of the glass fluorescent tube is coated with a layer of phosphorous fluorescent paint, and both ends of the tube are provided with filament coils made of tungsten. . When the power is turned on, the current passes through the filament to heat and release electrons. The electrons will turn the gas in the tube into plasma and increase the current in the tube. When the voltage between the two sets of filaments exceeds a certain value, the tube starts to produce The discharge causes the mercury vapor to emit 253.7nm and 185nm wavelengths of ultraviolet light. The phosphorous fluorescent paint on the inner surface of the fluorescent tube absorbs ultraviolet light and emits longer-wavelength visible light. The color of the emitted light is controlled by the proportion of phosphorous components, and the glass tube prevents harmful ultraviolet rays and other harmful substances such as mercury from leaking out. There is a positive feedback relationship between the discharge current of the fluorescent tube and the on-resistance: when more current flows into the fluorescent tube, more gas is ionized, so that the on-resistance in the tube is continuously reduced, which will cause more current Into the fluorescent tube. If the fluorescent tube is directly connected to a fixed voltage power supply, the fluorescent tube will be burned quickly due to the rising current. Therefore, an auxiliary circuit is required to control the current entering the fluorescent tube to a fixed level, and this current control circuit is usually It is called a ballast. The ballast is actually an inductance. When the on-resistance drops to a very low level, the fixed inductance and copper loss of the ballast make the on-current approximate to a constant value and start to work stably. Ballasts are divided into inductive ballasts and electronic ballasts. Inductive ballasts need to be matched with a small element called Starter (commonly known as "Star") to make the pressure difference between the filaments reach the desired level. The fluorescent tube is sufficient to discharge, but the electronic ballast does not need a starter, because the start-up work is already contained in the ballast.
