#Cold cathode free

In some tubes, glow discharge around the cathode is usually minimized instead there is a so-called positive column, filling the tube. Some types contain a source of beta radiation to start ionization of the gas that fills the tube. It is used in some x-ray tubes, the field-electron microscope (FEM), and field-emission displays (FEDs).Ĭold cathodes sometimes have a rare-earth coating to enhance electron emission.

#Cold cathode free

The resulting strong electric field near the cathode accelerates ions to a sufficient velocity to create free electrons from the cathode material.Īnother mechanism to generate free electrons from a cold metallic surface is field electron emission. Cold-cathode discharge lamps use higher voltages than hot-cathode ones.

External circuitry limits the discharge current. The discharge is self-sustaining when for each electron that leaves the cathode, enough positive ions hit the cathode to free, on average, another electron. Some crashing positive ions may generate a secondary electron. The result is for each electron that leaves the cathode, several positive ions are generated that eventually crash onto the cathode. The original electron and the freed electron continue toward the anode and may create more positive ions (see Townsend avalanche). The collision may just excite the molecule, but sometimes it will knock an electron free to create a positive ion. An electron that leaves the cathode will collide with neutral gas molecules. The surface of cold cathodes can emit secondary electrons at a ratio greater than unity (breakdown). Hot cathode devices include common vacuum tubes, fluorescent lamps, high-pressure discharge lamps and vacuum fluorescent displays. Discharge tubes with hot cathodes have an envelope filled with low-pressure gas and containing two electrodes. Both electrodes alternate between acting as an anode and a cathode when these devices run with alternating current.Ī cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. Any gas-discharge lamp has a positive (anode) and a negative electrode. These lamps are commonly used for neon signs. The term 'neon lamp' refers to tubing that is smaller than 15 mm in diameter and typically operates at approximately 40 milliamperes. This larger-diameter tubing is often used for interior alcove and general lighting. In the lighting industry, “cold cathode” historically refers to luminous tubing larger than 20 mm in diameter and operating on a current of 120 to 240 milliamperes. Neon lamps primarily rely on excitation of gas molecules to emit light CCFLs use a discharge in mercury vapor to develop ultraviolet light, which in turn causes a fluorescent coating on the inside of the lamp to emit visible light.Ĭold-cathode fluorescent lamps were used for backlighting of LCDs, for example computer monitors and television screens. The flash tube is a cold-cathode device filled with xenon gas, used to produce an intense short pulse of light for photography or to act as a stroboscope to examine the motion of moving parts.Ĭold-cathode lamps include cold-cathode fluorescent lamps (CCFLs) and neon lamps. Counter tubes were used widely before development of integrated circuit counter devices. Each time a pulse is applied to a control electrode, a glow discharge moves to a step electrode by providing ten electrodes in each tube and cascading the tubes, a counter system can be developed and the count observed by the position of the glow discharges. A Dekatron is a cold-cathode tube with multiple electrodes that is used for counting. Voltage regulator tubes rely on the relatively constant voltage of a glow discharge over a range of current and were used to stabilize power-supply voltages in tube-based instruments. Many types of cold-cathode switching tube were developed, including various types of thyratron, the krytron, cold-cathode displays ( Nixie tube) and others. Addition of a trigger electrode to a device allowed the glow discharge to be initiated by an external control circuit Bell Laboratories developed a 'trigger tube' cold-cathode device in 1936. Neon lamps are used both to produce light as indicators and for special-purpose illumination, and also as circuit elements displaying negative resistance. Study of the phenomena in these devices led to the discovery of the electron. Early cold-cathode devices included the Geissler tube and Plucker tube, and early cathode-ray tubes. The stacked digit arrangement in a Nixie tube is visible hereĪ cold-cathode vacuum tube does not rely on external heating of an electrode to provide thermionic emission of electrons.