Electronics Protection - Fall 2015 - (Page 14)

Feature An Unsung Hero: the Gas Discharge Tube Aldo della Coletta and Len Knight MET Laboratories The world is replete with unsung heroes, from Sisyphus of Golden Greek mythology, to nowadays equivalents - obscure, unpraised actors that find themselves engaged in undertakings requiring herculean efforts to bring to completion, while enslaved to endlessly repetitive and thankless tasks. Our unsung hero is the Gas Discharge Tube, also known as the GDT. What precisely is the GDT? It is a specialized version of the Ionization Chamber (see accompanying plot), an important tool of physics, with which it shares the basic theory. It is, essentially, a switch. That is, a voltage controlled switch, a relatively compact electrical device, exhibiting extraordinary characteristics of dynamic range, unchallengeable on-state conductivity and able to accomplish all its functions with exemplary ruggedness. Its predominant usage is as a circuitry "primary protector" from high voltage impulses. Most notable of its characteristic is that, due to the exceedingly high conductivity of the plasma formation and its exceptionally rapid transition times (not to be confused with its activation time), it is capable to commute enormous currents, up to hundreds of kiloAmperes. Even some of the smallest and inexpensive devices are rated into kiloAmperes. Also notable, its parasitic capacitance is in the neighborhood of a single picoFarad, an extraordinary feature when utilized in high frequency applications. Its control voltage starts as low as about 50 V and extends to the kiloVolt region. It is a bipolar device, with no polarity consciousness. History of the Development The parental lineage of the GDT has its roots in the sparkgap and the neon bulb. The spark gap was "discovered" (almost certainly by accident) in the early 1850's. It was discovered that if two conductors were positioned facing each other but separated by a small gap and a sufficiently elevated electrical potential was applied, a spark appeared due to the ionization of the volume of air gases interposed between electrodes. If these electrodes were sharply pointed, the sparking effect appeared at much lower potentials, due to the increased density of electrons per unit surface. Spark gaps are inexpensive and ruggedly effective. They have countless applications from internal combustion engine ignition to telephone carbon blocks, light sources, Marconi transmitters, oscillators, all sorts of electric welding, plasma cutters and so on. The very hot plasma formation (6,000 to 10,000°K), with accompanying severe Brownian agitation, exhibit very high conductivity and a seemingly imperturbable arc voltage (the low voltage appearing between the electrodes during spark duration) The Neon Bulb At the turn of the 20th century, Air Liquide, in the quest for cheaper and increased production of oxygen, developed a fractional distillation reactor that was able to extract this gas from the atmosphere. As a byproduct of this refining liquefaction 14 Fall 2015 * www.ElectronicsProtectionMagazine.com process, a lot of other gases became available in industrial quantities - among them, the whole cluster of noble gases Argon, Neon, Xenon, Krypton, and occasionally Radon. The phenomenon of light emission from the ionization of neon was only known as a scientific laboratory curiosity, due to the scarcity of this gas. Now it became possible to inexpensively mass produce this light bulb. It was found that confining this gas, under a low pressure (10 to 20 Torr) in a glass vessel with two metallic electrodes, could produce a sparkly glow light emission. The confinement of the gas permitted to securely isolate the device from environmental influences and contaminations. It exhibited stable characteristics of the breakdown voltage, permitting the realization of voltage regulators, and predictable other characteristics. At low pressure, the ionization potential appeared at low potential and, because of minimal current draws, the efficiency of this light source was very high, even by modern standards. This oddity of negative resistance characteristics permitted, among many other applications, the realization of relaxation oscillators, based solely on capacitance and resistance, without cumbersome and costly inductances. Neon bulbs are still in usage, mostly as indicators for their high impedance and sensitivity. One interesting application is as a high impedance voltage detector and qualifier: given that glowing positive ions crowd around the negative electrode, it is possible to read if the voltage is AC or DC, and its polarity. The Discharge Gas Tube This nifty device is a happy marriage of the neon bulb and the spark gap. It exhibits both the glow behavior and the negative resistance of the neon bulb and the plasma and arc voltage of the spark gap. There are a large variety of GDT devices with wide latitude of characteristics. Excluding very high power devices, most of the mass-produced GDT are physically small devices, perhaps 8 mm or so in diameter and 6 to 10 mm in length. There are two terminal devices, with a single chamber, and three terminal devices composed by a common ionization chamber constituted by two intercommunicating chambers. Typically, the GDT is constructed with a tubular (hence its name) glass or ceramic envelope with two electrodes constituted by the closure discs of the tube, carefully joined by metal to glass or ceramic brazing. This welding has to be very reliable because any violation of the weld integrity will provoke a total failure of the device as the noble gases are at a very low pressure and therefore any leaks, however small, will signify a relentless penetration of air until atmospheric equilibrium is reached. It signifies entrance of mainly nitrogen, with its much higher breakover potential and the much higher pressure (atmospheric) will bring the break-over voltage of a leaked device at least 25 times higher than originally constructed. The inside of the cavity is filled with a mixture of gases, predominantly neon and argon but with traces of other gases. It http://www.ElectronicsProtectionMagazine.com

Table of Contents for the Digital Edition of Electronics Protection - Fall 2015

Electronics Protection - Fall 2015
Contents
Editor's Choice
Beat the Heat: Six Best Practices for Protecting Your People and Your Business from Arc Flash Hazards
Data Center Design and Cooling for Sensitive Electronics
Electronic Access Solutions - Design Considerations for Your Data Center
An Unsung Hero: the Gas Discharge Tube
Innovation Demands That You Break the Rules
Enclosures
Thermal
Power
Hardware
Industry News
Calendar of Events

Electronics Protection - Fall 2015