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Component Theory - Thyristor
Thyristor is a process component that is an example of a bistable latch. Bistable meaning that it is stable in both the ON and OFF state. Like a transistor it has three component legs and acts like a switch except when triggered the thyristor will remain on until reset. A thyristor has a;
The circuit symbol is similar to a diode as it is essentially two diodes in one case. When the voltage at the gate is LOW (<2V) the thyristor will not conduct between the Anode and Cathode legs. When a voltage is applied (2V>) the thyristor conducts and latches on to the current. There must be a voltage drop across the Anode and Cathode for the thyristor to be reset to OFF. Typically a push to make is used.
The circuit shown is typical Thyristor alarm and often comes up in the exam. Thyristors are ideal for alarms as unlike a transistor they will not switch off after being triggered by an intruder. If a buzzer/siren is used for the output, a resistor (1K) should be placed in parallel to prevent the buzzers oscillating causing a voltage drop and resetting the thyristor.