Cold Cathode - Details

Details

The cathode is the negative electrode. Any gas discharge lamp has a positive (anode) and a negative electrode. Both electrodes alternate between acting as an anode and a cathode when these devices run with alternating current.

A cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. Discharge tubes with hot cathodes have an envelope filled with low pressure gas and containing two electrodes. Examples are most common fluorescent lamps, high pressure discharge lamps and vacuum fluorescent displays.

The interior surface of cold cathodes are capable of producing secondary electrons at a ratio greater than unity (amplification) upon electron and ion impact. For acceleration of the ions to a sufficient velocity for creating free electrons from the cathode material, cold cathode discharge lamps need higher voltages than hot cathode ones, causing a strong electric field near the cathodes.

Another mechanism for generating free electrons from a cold metallic surface is field electron emission. It is used in some x-ray tubes, the field electron microscope (FEM), and field emission displays (FEDs).

Cold cathodes sometimes have a rare earth coating to enhance electron emission. Some types contain a source of beta radiation to start ionization of the gas that fills the tube. In such a tube, glow discharge around the cathode is usually minimized; instead there is a so-called positive column, filling the tube. Examples are the neon lamp and nixie tubes. Nixie tubes too are cold-cathode neon displays that are in-line, but not in-plane, display devices.

A common cold-cathode application is in neon signs and other locations where the ambient temperature is likely to drop well below freezing, The Clock Tower, Palace of Westminster (Big Ben) uses cold-cathode lighting behind the clock faces where continual striking and failure to strike in cold weather would be undesirable. Other examples include the thyratron, krytron, sprytron, and ignitron tubes. Large cold-cathode fluorescent lamps (CCFLs) have been produced in the past, and are still used today when shaped, long-life linear light sources are required. As of 2011 miniature CCFLs were extensively used as backlights for computer and television liquid crystal displays. CCFL lifespans vary in LCD televisions depending on transient voltage surges and temperature levels in usage environments.

Due to its efficiency, CCFL technology has expanded into room lighting. Costs are similar to those of fluorescent lighting, but with several advantages. The light emitted is easier on the eyes, bulbs turn on instantly to full output and are also dimmable.

In systems using alternating current but without separate anode structures, the electrodes alternate as anodes and cathodes, and the impinging electrons can cause substantial localized heating, often to red heat. The electrode may take advantage of this heating to facilitate the thermionic emission of electrons when it is acting as a cathode. (Instant start fluorescent lamps employ this aspect; they start as cold-cathode devices, but soon localized heating of the fine tungsten wire cathodes causes them to operate in the same mode as hot cathode lamps.)

This aspect is problematic in the case of backlights used for LCD TV displays. New energy efficiency regulations being proposed in many countries will require variable backlighting—this also improves the perceived contrast range highly desirable for LCD TV sets. However, CCFLs are strictly limited in the degree to which they can be dimmed, both because a lower plasma current will lower the temperature of the cathode, causing erratic operation, and because running the cathode at too low a temperature drastically shortens the life of the lamps. Much research is being directed to this problem, but high-end manufacturers are now turning to high-efficiency white LEDs as a better solution.

Cold cathode devices typically use a complex high-voltage power supply with some mechanism for limiting current. Although creating the initial space charge and the first arc of current through the tube may require a very high voltage, once the tube begins to heat up the electrical resistance drops, thus increasing the electric current through the lamp. To offset this effect and maintain normal operation, the supply voltage is gradually lowered. In the case of tubes with an ionizing gas, the gas can become a very hot plasma and electrical resistance is greatly reduced. If operated from a simple power supply without current limiting, this reduction in resistance would lead to damage to the power supply and overheating of the tube electrodes.