In telecommunication systems, voltage surges caused by lightning or ac power faults can affect sensitive electronics from the main distribution frames to subscriber terminals. Manufacturers are increasingly using overvoltage protection components in telecom equipment such as telephones, fax machines, and modems. These components protect the susceptible devices by shunting the surge currents to ground and by limiting the transient voltages to a safe level. In modem applications (ADSL, cable boxes, fax machines, etc.), semiconductor thyristors are widely used. Thyristors are marketed under trade names such as SIDACtor (Teccor), Surgector (Littelfuse), TISP (Bourns), Trisil (STMicroelec-tronics) and SiBar (Raychem). The SIDACtor, which is the most well known of all thyristors in the U.S. market, will be referenced throughout this article. An alternative―but relatively unused―solution for modem overvoltage protection is the gas discharge tube (GDT). This article introduces a GDT and describes the benefits of this new technology for modem applications. Also included is a parametric comparison of common SIDACtors versus comparable specially developed GDTs for modem applications.
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