Gas Discharge Tube Gdt Products Page 5 Gas Discharge Tube GDT Products

Gas Discharge Tubes 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/12/17 CARBON BLOCK PROTECTOR: A carbon block protector consists of a pair of carbon elements separated by a 0.003-0.004 inch air gap. When a specified potential is placed across the carbon surfaces an arc will be initiated. Like the air gap protector, the carbon block is an unsealed device and its performance suffers in the same manner as the air gap. Carbon block protectors are used mainly for telephone line protection but are being replaced, in most installations, with the more reliable and consistent gas discharge tubes. GDT GAS DISCHARGE TUBE: Littelfuse's GDT, a hermetically sealed gas filled ceramic tube with metal electrodes, is recognized for: Stable electrical parameters High insulation resistance Low capacitance High current capability Low leakage current Low arc voltages For a gas tube to begin conduction, an electron within the sealed device must gain sufficient energy to initiate the ionization of the gas. Complete ionization of the gas takes place through electron collision. The events leading up to this phenomenon occur when a gas tube is subjected to a rising voltage potential. Once the gas is ionized, breakdown occurs and the gas tube changes from a high impedance state to a virtual short circuit and thus, any transient will be diverted from and will not reach the protected circuit. The arc voltage (the voltage across the gas tube while the gas tube is conducting) will typically be 15 volts. After the transient has passed, the GDT will extinguish and again appear as an open circuit. In order to insure gas tube turn off at the zero crossing in AC applications, the current through the GDT once the transient has passed, must be less than the follow-on current rating of the gas tube. The follow-on current requirement can easily be met by placing a resistor in series with the gas tube. Littelfuse's AC series gas discharge tube surge arresters were developed specifically to protect AC power lines and normally will not require additional components to limit follow-on current. In DC applications, the gas discharge tube will extinguish as long as the device is operated within the specified holdover conditions. Holdover conditions involve the maximum bias voltage that can appear across a gas discharge tube under specified current conditions and still allow the gas discharge tube to turn off. Under normal operating conditions, the GDT shunted across a circuit, will act like an open switch with a high insulation resistance. Comgap Gas Discharge Tube Voltage vs. Current Characteristics Current (A) Voltage (V) .00001 15 Breakdown Voltage The GDT's breakdown voltage is determined by electrode spacing, gas type (usually neon and/or argon), gas pressure (less than atmospheric), and the rate of rise of the transient. Breakdown voltage is defined as that voltage at which a crowbar type of surge arrester changes from a high impedance state to a low impedance state. The GDT series is categorized by the breakdown voltage of each gas tube when a slowly rising transient is applied. For example: Littelfuse's CG2-230L gas tube will breakdown at 230V (+/- 15% to 20%) when subjected to a ramp with a rate of rise of 100V/second. The breakdown voltage response of a crowbar to transients with ramp rates of 1V/microsecond or less is referred to as the DC breakdown voltage level. Due to the nature of gas discharge tubes, the same gas tube will experience breakdown at a higher voltage as a transient's ramp rate increases. For example: At 100V/microsecond, the CG2-230L gas tube will breakdown at 600V maximum. The breakdown voltage response of a crowbar to transients with ramp rates greater than 1V/microsecond is referred to as the impulse breakdown voltage level. Due to the GDT's rugged construction, it can handle currents that far surpass other transient suppressors' capabilities - greater than 10 pulses of a 20,000 peak amperes pulse having a rise time of 8 microseconds decaying to half value in 20 microseconds (also referred to as an 8/20 wave form). The surge life of the GDT is at least 1000 shots of a 500 amperes peak 10/1000 pulse. GDT is the practical device for the protection of telephone circuits, AC power lines, modems, power supplies, CATV and almost any application where protection from large and/or unpredictable transients is desired. ) GDT Characteristics, Terms and Consideration Factors (continued)

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