The thyristor is a four-layer, three terminal semiconducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. Four-layer devices act as either open or closed switches; for this reason, they are most frequently used in control applications. They stay on once they are triggered, and will go off only if current is too low or when triggered off. The SCR is the most widely used and important member of the thyristor family. The SCR is almost universally referred to as thyristor. Common areas of application for SCRs are Time-delay circuits, regulated power suppliers, static switches, motor controls choppers, battery charger and heater controls. Etch..
Thyristors can take many forms, but they have certain things in common. All of them are solid state switches which
act as open circuits capable of withstanding the rated voltage until triggered. When they are triggered, thyristors
become low−impedance current paths and remain in that condition until the current either stops or drops below a minimum value called the holding level. Once a thyristor has been triggered, the trigger current can be removed without turning off the device. Silicon controlled rectifiers (SCRs) and triacs are both members of the thyristor family. SCRs are unidirectional devices where triacs are bidirectional. An SCR is designed to switch load current in one direction, while a triac is designed to conduct load current in either direction. Structurally, all thyristors consist of several alternating layers of opposite P and N silicon, with the exact structure varying with the particular kind of device. The load is applied across the multiple junctions and the trigger current is injected at one of them. The trigger current allows the load current to flow through the device, setting up a regenerative action which keeps the current flowing even after the trigger is removed. These characteristics make thyristors extremely useful in control applications. Compared to a mechanical switch, a thyristor has a very long service life and very fast turn on and turn off times. Because of their fast reaction times, regenerative action and low resistance once triggered, thyristors are useful as power controllers and transient overvoltage protectors, as well as simply turning devices on and off. Thyristors are used in motor controls, incandescent lights, home appliances, cameras, office equipment, programmable logic controls, ground fault interrupters, dimmer switches, power tools, telecommunication equipment, power supplies, timers, capacitor discharge ignitors, engine ignition systems, and many other kinds of equipment.
Here i give a proper format of seminar on topic Thyristor. The report file of this ppt also enclosure with the ppt at ELECTRODATA
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That is a good detailed information about the Thyristors and their types.
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