Gas Discharge Tube Gdt Products Page 6 Gas Discharge Tube GDT Products

Gas Discharge Tubes 2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/12/17 SILICON CONTROLLED RECTIFIER (SCR): Unlike the crowbar devices discussed above, the SCR is a semiconductor. Like the GDT, the SCR will have a very low voltage drop across it while conducting. The SCR does require a trigger signal when a surge is present before it can begin to conduct. This trigger signal is usually supplied through the use of a zener diode. Packages that combine the SCR and zener diode are now available. These packages are monolithic devices and often contain an SCR-type thyristor with a gate region that acts like the avalanche diode. Once triggered, the SCR begins to conduct, dropping the voltage across the zener diode to a value below the zener's operating voltage and thus causing the zener to stop conducting. The SCR will conduct until the applied voltage drops to zero (zero crossing of AC) or until the current falls below a specified value (sometimes referred to as a holding current). SCR Voltage vs. Current Characteristics Current (mA) Voltage (V) 130 Zener Voltage Breakdown Voltage Although typically having a faster response time than a GDT, the SCR package is subject to higher leakage current and capacitance. The SCR package can handle currents of several hundred amperes of an 8/20 wave form and packages are available that offer bi-directional protection. CLAMPING PROTECTION: A clamping device actually limits the voltage transient to a specified level by varying its internal resistance in response to the applied voltage. A clamping device must absorb the transient's energy and therefore, cannot withstand very high current levels. Although these devices have quick response times, they are subject to leakage currents and their capacitance values are higher than those found in the GDT. ZENER (AVALANCHE) DIODE: The zener diode comes closest to modeling the ideal constant voltage clamp. It responds quickly to a fast rising voltage potential and is available for a fairly wide range of clamping voltages (from less than 10 volts up to several hundred volts). The zener is placed in parallel with the circuit to be protected and will not operate until a surge exceeds the zener's breakdown voltage. The surge, causing the zener to conduct will be clamped to the zener's rated voltage. The zener is a good protector for circuits operating at low voltages. Caution is advised when designing the device into RF circuits due to the diode's high capacitance. Also available are silcon avalanche suppressers which are referred to as transient voltage suppressers (TVS) diodes. These diodes consist of fairly large junction zeners which have been designed specifically for surge protection. The TVS diodes are rated for higher current surges than zener diodes and they can carry these currents for periods of 2-10 microseconds. For use in AC signal lines, two zeners are required. These are available as packaged devices. Avalanche diodes are often used to protect IC's from static discharge and other forms of transients in power supplies computer buses, and data lines. Avalanche Diode Voltage vs. Current Characteristics Current (A) Voltage (V) .001 Leakage Region Clamping Voltage MOV (METAL OXIDE VARISTOR): As its name suggests, the MOV is a voltage variable resistor made from sintered metal oxides. The grains produced in the sintered metal oxide material of the MOV can be thought of as a network of series and parallel diodes. As the voltage potential across the MOV increases, some of the diodes experience avalanche breakdown and begin to conduct and as a result, reduce the net resistance of the MOV. The MOV can handle current pulses of higher peak values and for a longer duration than a diode, but the MOV can experience cumulative degradation and performance changes after it is exposed to large current pulses when not properly selected. The high peak current surges tend to fuse the oxide grains and thus alter the MOV's performance. Some engineers recommend that a fuse be used with an MOV as a large current surge could damage the grain structure, fuse the grains together and result in the protected circuit being shorted out. The MOV is available in a wide range of voltages and experiences a quick turn on time when subjected to a fast rising surge. The MOV is subject to leakage current and high capacitance (10's to 1000's of picofarads). When designing with a MOV it is necessary to remember that as the current GDT Characteristics, Terms and Consideration Factors (continued)

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