Cold cathode



A cold cathode is an element used within some Nixie tubes, gas discharge lamps, gas filled tubes, and vacuum tubes. Cold cathodes do not employ a cathode heater.

Cold cathode fluorescent lamps (CCFLs) are usually also called cold cathodes. Neon lamps are a very common example of a cold cathode lamp.

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

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
Before discussing cathode ray tubes of any kind, a few basic terms must be defined:

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 act alternating as anode and cathode because the 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 a pair of cathodes. Examples are 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 accelerating 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 emission. It is used in some x-ray tubes and the field electron microscope.

Cold cathodes sometimes have rare earth coating on them for enhancing 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. 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. Additionally, CCFLs are used by computer modders to light the insides of their customized computer cases.

Despite their name, cold cathodes don't necessarily remain cold as they operate; they can get painfully hot. 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 or may not 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.)

Cold cathode devices typically use a complex high-voltage power supply with some mechanism for limiting current flow. 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 resistance to current flow begins to drop, requiring a gradually lower voltage to maintain operation. In the case of tubes with an ionizing gas, the gas can become a very hot plasma where electrical resistance 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.

Patents

 * - Philo Farnsworth - Cold cathode electron discharge tube
 * - Philo Farnsworth - Cold cathode electron discharge tube
 * - Clarence W. Hansell - Cold cathode electron discharge device and circuits therefore