Cold cathode

This article is about light sources and indicators. For cold cathode ion sources, see Ion source.

Cold cathode fluorescent lamp

A cold cathode is an element used within nixie tubes, gas discharge lamps, discharge tubes, and some types of vacuum tube. The term cold cathode refers to the fact that the cathode is not independently heated, but may still operate at elevated temperature.

Terminology

Types of cold cathode lamps include the following:

• Cold Cathode Fluorescent Lamps (CCFLs)
• Neon lamps are a very common example of a cold cathode lamp.

Cold cathode lamps remain popular for LCD backlighting and enthusiast computer case modders.

In the lighting industry, “cold cathode” historically refers to luminous tubing which is larger than 15mm in diameter and operates on a minimum current of 60 milliamps. This larger diameter tubing is often used for interior cove and accent lighting. The term "neon lamp" refers to tubing that is smaller than 15 mm diameter and typically operates at approximately 40 milliamps. These lamps are commonly used for neon signs.

Electron emission

A cathode is any electrode that emits electrons. When used in electrical and electronic devices (most fluorescent lamps, vacuum tubes, etc.), the cathode is explicitly heated, creating a hot cathode. By taking advantage of thermionic emission, electrons can overcome the work function of the cathode without an electric field to pull the electrons out. But if sufficient voltage is present, electrons can still be stripped even out of a cathode operating at ambient temperature. Because it is not deliberately heated, such a cathode is referred to as a cold cathode, although several mechanisms may eventually cause the cathode to become quite hot once it is operating. Most cold cathode devices are filled with a gas which can be ionized. A few cold cathode devices contain a vacuum.

Details

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

A standard computer case fitted with blue and green cold cathode tubes.

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 a pair of cathodes. Examples are most common fluorescent lamps, high pressure discharge lamps and completely-evacuated electron tubes 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 on them 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, in favor of a so called positive column, filling the tube. The best example is the humble neon lamp. Another good example is nixie tubes. Nixie tubes too are cold cathode neon displays that also happen to be in-line, but not in-plane display devices.

A common cold cathode application is in neon signage 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 less than ideal. Other examples include the thyratron, krytron, sprytron, and ignitron tubes. Large-scale 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. Nowadays, miniature CCFLs are extensively used as backlights for computer liquid crystal displays, as well as LCD televisions. CCFLs' lifespans vary in LCD televisions depending on transient voltage surges, humidity and temperature levels in usage environments. Additionally, CCFLs are used by computer modders to light the insides of their customized computer cases.

Due to its efficiency, Cold Cathode Fluorescent Lamp (CCFL) technology has expanded into room lighting. Costs are competitive to those of fluorescent bulbs, 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 cathodes alternate as anodes and the impinging electrons can cause substantial localized heating, often to red heat. The cathode may take advantage of this heating to facilitate the thermionic emission of electrons when it is acting as a cathode. (Instant start fluorescent lamps definitely do employ this aspect; they start as cold-cathode devices but soon localized heating of the fine tungsten wire cathodes causes them to operate as ordinary 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 make variable backlighting highly desirable for LCD TV sets, as well as improving the perceived contrast range. 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 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 electrical 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 operational resistance would lead to damage to the power supply or the tube electrodes from overheating.

External links

• Everbrite neon power supplies