Thyristors can take many forms, but they have certainthings in common. All of them are solid state switcheswhich act as open circuits capable of withstanding therated voltage until triggered. When they are triggered,thyristors become low−impedance current paths andremain in that condition until the current either stops ordrops below a minimum value called the holding level.Once a thyristor has been triggered, the trigger current canbe removed without turning off the device.Silicon controlled rectifiers (SCRs) and triacs are bothmembers of the thyristor family. SCRs are unidirectionaldevices where triacs are bidirectional. An SCR isdesigned to switch load current in one direction, while atriac is designed to conduct load current in eitherdirection.Structurally, all thyristors consist of several alternatinglayers of opposite P and N silicon, with the exact structurevarying with the particular kind of device. The load isapplied across the multiple junctions and the triggercurrent is injected at one of them. The trigger currentallows the load current to flow through the device, settingup a regenerative action which keeps the current flowingeven after the trigger is removed.These characteristics make thyristors extremely usefulin control applications. Compared to a mechanical switch,a thyristor has a very long service life and very fast turnon and turn off times. Because of their fast reaction times,regenerative action and low resistance once triggered,thyristors are useful as power controllers and transientovervoltage protectors, as well as simply turning deviceson and off. Thyristors are used in motor controls,incandescent lights, home appliances, cameras, officeequipment, programmable logic controls, ground faultinterrupters, dimmer switches, power tools,telecommunication equipment, power supplies, timers,capacitor discharge ignitors, engine ignition systems, andmany other kinds of equipment.Although thyristors of all sorts are generally rugged,there are several points to keep in mind when designingcircuits using them. One of the most important is torespect the devices’ rated limits on rate of change of voltage and current (dv/dt and di/dt). If these areexceeded, the thyristor may be damaged or destroyed. Onthe other hand, it is important to provide a trigger pulselarge enough and fast enough to turn the gate on quicklyand completely. Usually the gate trigger current should beat least 50 percent greater than the maximum rated gatetrigger current. Thyristors may be driven in manydifferent ways, including directly from transistors or logicfamilies, power control integrated circuits, byoptoisolated triac drivers, programmable unijunctiontransistors (PUTs) and SIDACs. These and other designconsiderations are covered in this manual.Of interest too, is a new line of Thyristor SurgeSuppressors in the surface mount SMB package coveringsurge currents of 50, 80 and 100 amps, with breakovervoltages from 77 to 400 volts. NP Series Thyristor SurgeProtector Devices (TSPD) protect telecommunicationcircuits such as central office, access, and customerpremises equipment from overvoltage conditions. Theseare bidirectional devices so they are able to havefunctionality of 2 devices in one package, saving valuablespace on board layout. These devices will act as a crowbarwhen overvoltage occurs and will divert the energy awayfrom circuit or device that is being protected. Use of theNP Series in equipment will help meet various regulatoryrequirements including: GR−1089−CORE, IEC61000−4−5, ITU K.20/21/45, IEC 60950, TIA−968−A,FCC Part 68, EN 60950, UL 1950. See ONSemiconductor application note AND8022/D foradditional information.