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Neon Application Information

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Neon Application Information engineered to light up your design www.vcclite.com 1.800.522.5546 Neon Application Information Lamp Construction Basic Principles of Lamp Operation The basic construction of neon lamps is shown in figure Neon lamps have the voltage/current relationship shown 1. The bulb is made of glass sealed in two areas, the pip in Figure 3, which determines their operation. As the and the pinch. In the pinch applied voltage is increased there is virtually no current Neon Lamp Construction area, the leads protrude flow until the striking voltage is reached at point A. The Gas Pip through one end of a glass current then increases to a value A which is largely tube and the glass is heated Glass and pinched about the Envelope Voltage/Current Characteristic A A1 for Neon Lamps leads. This holds them in Electrodes C Pinch position and forms a B I B1 Voltage, V hermetic seal. The leads Leads V are made from a special Figure 1 C1 Current, I Figure 3 material known as dumet which is a low resistance material consisting of a copper sheath around a nickel determined by the impedance of the external circuit, and iron core constructed so that its expansion coefficient is the voltage across the neon lamp reduces to a lower the same as that of the glass used for the bulb. Butt value B, known as the maintaining voltage. Further welded to the lead wires are nickel electrodes of larger increasing the supply voltage (or reducing the external diameter. These electrodes are impedance) increases the lamp current with little change Based Neon coated with a special emissive Lamp Construction to the maintaining voltage until point B is reached. Then material prior to being sealed into the maintaining voltage increases with current to a point Plastic the glass bulb.This emissive coating Top Cap C when the neon lamp discharge changes from a glow to has a lower work function than the an arc and the voltage across the lamp drops to a few base material andcauses the neon Neon Lamp volts at C . Neon lamps should be operated in the range lamp to have a lower striking B to B which is called the "negative glow" region. B to C voltage than would otherwise be the is the "abnormal glow" region and from C beyond is the case. The sealed unit is evacuated Resistor "arc" region. A neon lamp operates in the following and an inert gas is introduced via manner: when the applied voltage is in excess of the the pip prior to tipping off. The neon striking voltage, electrons are emitted from the surface of Metal Base lamp is commonly used without a the negative electrode (cathode) and, accelerated by the base in "wire-terminal" form, and is Figure 2 electric field, collide with atoms of gas. One of two available with or without a resistor. Versions are reactions occur: available mounted on a metal or plastic base for 1. The traversing electron can collide with an atom of gas replaceable use in a corresponding lampholder or fitting. and elevate one of its valence electrons to a higher Lamps mounted on bases are available without a resistor energy state. At some time after being elevated, the or with a resistor incorporated inside the base for direct electron will return to a lower energy state and a photon operation from main voltage supplies. These lamps of light will be emitted. The wavelength of the light consist of a T-2 neon lamp with a series resistor welded output is determined by the difference in energy levels, on, mounted in the metal base with a plastic top cap as which will be dependent upon the type of gas used. The shown in Figure 2. total light output from a neon lamp is the result of CML and Chicago Miniature Lamp are registered trademarks of Visual Communications Company, LLC. engineered to light up your design www.vcclite.com 1.800.522.5546 Neon Application Information millions of such occurrences at a given instant. Lamp Characteristics 2. The atom can have an electron knocked off, in which The main characteristics of neon lamps are detailed as case the atom becomes a positively charged ion. This follows. In general, these characteristics are not tightly ion is attracted by the negative electrode (cathode) and controlled, as the prime function of the device is to emit accelerated towards it. On colliding with the cathode, light to act as an indicator. If a tube having a more the ion causes material to be removed, a phenomenon controlled electrical specification is required for use as a known as sputtering. The emissive coating on the circuit component, a cold cathode diode should be used. electrodes of neon lamps is removed by ionic Light Output: Neon lamps are available from Chicago bombardment so that eventually the base metal is Miniature Lamp in two light output ranges (standard and exposed and the striking voltage of the lamp increased. high brightness). The standard brightness types have a When the lamp is operated from an a.c. supply, each light output of approximately .06 lumens per mA, and the electrode is alternately positive (anode) and negative high brightness types emit approximately .15 lumens per (cathode) and material is sputtered from them during mA. The light output of neon lamps is mainly in the yellow alternate half cycles. However, when the lamp is and red regions of the spectrum between 580 and 750 operated from a d.c. supply, one electrode is constantly nanometers with a band in the infrared between 820 and the cathode and material is removed from it during the 880 nanometers. entire time that the neon lamp is operating. Striking Voltage: The voltage at which the neon lamp Consequently, the effects of sputtering become ignites is usually between 45 and 65V a.c. for standard apparent earlier during d.c. operation and the lamp life brightness types and between 70 and 95V a.c. for high expectancy is about 60% of that when operated from an brightness types. This is sometimes called the breakdown a.c. supply. or ignition voltage. For reliable operation, neon lamps require some free Maintaining Voltage: The voltage across the lamp after charged particles to be present in the tube to initiate the it has ignited. This voltage is a function of the lamp glow discharge when an operating voltage is applied. current and is usually quoted at the design current. These charged particles can be created by Nominal values are 60V for standard brightness and 75V photoemission from the emissive surface of the for high brightness lamps. electrodes, so that if the lamp is used in normal light no Extinction Voltage: The voltage at which the lamp ignition problems should be encountered. extinguishes if the supply voltage is reduced. It is normally a few volts below the maintaining voltage. Dark Effect Design Current: The current at which the lamp has been If the lamp is used in total darkness, some ignition delay designed to operate. It will be determined by the supply can occur unless a tube which contains a small amount voltage and the value of series resistance. Operation at of radioactive material is used. Operation in total lower currents can result in the glow discharge becoming darkness is normally only encountered in situations unstable (i.e., flickering) and operation at higher currents where the neon lamp is being used as a circuit can severely reduce the useful life of the lamp. It is component (i.e., its electrical characteristics are of prime therefore important to use only the recommended value importance, its indicating property being of secondary or of series resistance. no importance). In this case a cold cathode diode should Operating Temperature: Neon lamps can be used up to be specified which is "dark effect" reduced. a maximum temperature of 160°C, but for optimum life it CML and Chicago Miniature Lamp are registered trademarks of Visual Communications Company, LLC. engineered to light up your design www.vcclite.com 1.800.522.5546 Neon Application Information is recommended that wherever possible the operating The life expectancies quoted in Table1 apply to neon temperature should be within the range -50°C to +90°C. lamps operated from Table 1 However, it should be noted that carbon film resistors an a.c. supply. As Typical Life Expectancies of Neon Lamps which are recommended for use in series with neon Operated from an AC Supply explained earlier in lamps are normally derated, for maximum power Brightness Level Life Expectancy these notes, when dissipation, at temperatures above 70°C. Therefore, at Standard 25,000 hours the lamps are higher temperatures other types of resitor, such as metal High 8,000 hours operated from a d.c. oxide, may be more suitable and resistor manufacturers' supply, lifetimes are For DC life expectancies, see text. data should be consulted. approximately 60% Leakage Resistance: This is not a specified parameter, of these values. In order to obtain the maximum lifetime it but in practice values in excess of 10 ohms will apply to is recommended that ±5% carbon film series resistors be wire terminal lamps. Measurements should always be used. Variations in supply voltage will also affect the life taken in total darkness as photoemission from the of the lamp due to the resultant change in operating electrodes under ambient illumination will result in false, current. The relationship between operating current and low resistance readings. The voltage at which leakage life can be expressed as a 3.3 power law, resistance is measured should be at least 10V below the minimum striking voltage of the lamp.
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