30 May 2011
Author: Giorgos LazaridisHow Gas Discharging Lamps work (Cold Cathode CCFL and Hot Cathodes)
The story begins back in 1675, when a French astronomer named Jean-Felix Picard made a remarkable observation. He was carrying a mercury barometer, when he noticed that the empty-space glowed as the mercury jiggled. Many people tried to explain this phenomenon, among which an English scientist named Francis Hauksbee, who was the first to demonstrate a gas-discharge lamp in 1705, operated with static electricity. 100 years later, Vasily V. Petrov, a Russian self-taught electrical technician described for the first time the phenomenon of electrical arc, which was a kick start to research extensively different kinds of discharge light sources.
What is a gas discharge lamp
First of all, what is a gas discharge lamp? A gas discharge lamp is a light source that generates light by creating an electrical discharge through an ionized gas. There are many different types of lamps that operate under this principle. Commonly, we separate them in 3 basic categories: High pressure discharge lamps, Low pressure discharge lamps and High-intensity discharge lamps.
High pressure discharge lamps
These lamps have pressurized gas inside the tube, with higher pressure than the atmospheric pressure. Some examples of high pressure discharge lamps is are the metal halide lamps, the high pressure sodium lamps and the high pressure mercury-vapor lamps which are are very old, being replaced in most applications.
Low pressure discharge lamps
These lamps have gas inside the tube, with lower pressure than the atmospheric pressure. The classic fluorescent lamps are of this kind, the well known to PC modders Neon lamps also, as well as the low pressure sodium lamps which are used for street lighting. They all have very good efficiency, with the sodium lamps being the most efficient among all gas discharge lamps. The problem with this type of lamp is that it produces only an almost monochromatic yellow light.
High-intensity discharge lamps
In this category, there are those lamps which produce light by means of an electric arc between the electrodes. The electrodes are usually tungsten electrodes, housed inside a semi-transparent or transparent material. There are many different example of HID (High Intensity Discharge) lamps such as the metal halide lamps, the sodium vapor lamps, the mercury-vapor lamps, the ceramic discharge metal halide lamps, the Xenon arc lamps and the Ultra-High Performance (UHP)lamps.
How Gas Discharge Lamps work
As said before, a gas discharge lamp is a light source that generates light by creating an electrical discharge through ionized gas. Typically, these lamps use nobble gases such as argon, neon, krypton and xenon, or a mixture of these gasses. Many lamps are also filled with additional gases like sodium and mercury, while some others have metal halide additives.
When power is applied to the lamp, an electrical field is generated in the tube. This field accelerates free electrons in the ionized gas. The electrons collide with the gas and metal atoms. Some electrons orbiting around these atoms are excited by the collision to a higher energy state. When the electron of the excited atom returns to its previous energy state, it releases energy in the form of photon. This light can be anything between IR, visible or UV radiation. Some lamps have a fluorescent coating on the inside of the lamp to convert the UV radiation into visible light.
Some tubes contain some source of beta radiation to start ionization of the gas inside. in these tubes, glow discharge around the cathode is minimized, in favor of a so called positive column, filling the tube. Neon lamps is one good example.
Hot Cathode? Cold Cathode?
You have probably heard before the term CCFL which stands for Cold Cathode Fluorescent Lamp. On the other hand, the Hot Cathode term is not widely known. But what is the difference between these lamps? It all has to do with the electron emission method.
In Hot Cathodes, the electrons are generated by the electrode itself with thermionic emission, and that is why they are referred to as thermionic cathodes. The cathode is usually an electrical filament made of tungsten or tantalum. Later cathodes were covered with an emissive layer, which could produce more electrons with less heat, thus increasing the efficiency. In some applications where the AC humming is a problem, the cathode is indirectly heated by another source of heat underneath (called the heater), electrically insulated from the cathode. This method was widely used in vacuum tubes.
Hot cathodes produce significantly more electrons than cold cathodes from the same surface. They are used in electron guns for electron microscopes, cathode ray tubes (if you remember...), vacuum tubes and fluorescent lamps.
In Cold Cathode, the electron emission is not done with thermionic emission. A high voltage is applied across the electrodes generating thus a strong electric field which ionizes the gas. The surface inside the tube is capable to produce secondary electrons upon electron and ion impact higher than unity (one impact may result in more than one free electrons). The electrons are then accelerated by the electric field. Some tubes contain rare earths on the inside coating to enhance electron emission.
Another method to generate free electrons without thermionic emission, is the Field Electron Emission. Field emission happens in electric fields generated by very high voltages. This method is used in some x-ray tubes, field electric microscopes, field emission displays and other.
The term "cold cathode" does not mean that the cathode remains in ambient temperature all the time. The operation temperature of the cathode is subject to increment in some cases, but this temperature is not the method that the electrons are generated (thermionic emission). In alternating current, both electrodes become anode and cathode. Some electrons then can cause localized heat. Although this phenomenon is considered as parasitic in cold cathodes, it is often an advantage for other systems. Take for example the fluorescent lamps. Upon start up, the tungsten wire is cold. The lamp operates as a cold cathode and the above phenomenon is utilized to heat up the filament. When it reached the desired heat to start emitting electrons, the lamp operates normally as a hot cathode.
Cold cathode devices require high voltage, thus a high-voltage power supply is required. This is often called CCL inverter. What this inverter does, is to generate a high voltage to create the initial space charge and the first arc of the current within the tube. When this happens, the internal resistance of the tube is decreased, and the current eventually increases. The inverter can sense this and it decreases the voltage so that the current is also decreased. If the voltage is not decreased, either the lamp will become very hot, or the power supply will be damaged.
Cold Cathode Lamps are often met in electronic devices. CCFLs (Cold Cathode Fluorescent Lamps) are used by computer modders to spice up their mods. Also, it is widely used as an LCD backlit. Laptops for example have CCFLs to light the monitor. Another example of wide use is the Nixie tubes. Nixie tubes are also cold cathode lamps. Lately, cold cathode fluorescent lamps are used to light bigger areas. These lamps come with a built-in inverter and they can directly replace a normal lamp.