How thyristor get on and turn off ?

A thyristor turns on by receiving a gate pulse when the anode is positively biased relative to the cathode. This gate pulse injects charge carriers into the device, reducing the forward breakover voltage and allowing current to flow from anode to cathode. Once the thyristor is conducting, it remains on even if the gate pulse is removed, as long as the current through the device stays above the holding current level. To turn off the thyristor, the current through it must drop below this holding current level, which can be achieved by reducing the anode-cathode voltage or by commutating the current externally.

A thyristor is switched on by applying a positive voltage pulse to its gate terminal with respect to the cathode. This gate pulse initiates the flow of charge carriers in the semiconductor layers, triggering the device into conduction. The switching action relies on the internal regenerative feedback mechanism within the thyristor, where the initial gate current helps to establish the necessary conditions for current to flow through the device.

The condition for a thyristor to turn on is the presence of a forward voltage across the anode and cathode and a sufficient gate current. The forward voltage should be positive at the anode relative to the cathode, creating a potential difference that supports current flow. Additionally, a gate current must be applied to initiate the turning on process by injecting minority carriers into the thyristor’s junctions, thereby lowering the breakover voltage.

A thyristor is triggered by applying a gate current pulse when the anode is positively biased with respect to the cathode. The gate pulse initiates conduction by injecting charge carriers into the device, which creates a path for the main current to flow from anode to cathode. The gate pulse needs to be of sufficient magnitude and duration to ensure reliable triggering of the thyristor.

A thyristor latches on when the initial gate-triggered current establishes a sufficient flow of charge carriers within the device, creating a self-sustaining conduction path. Once the anode current exceeds the holding current, the thyristor maintains its conducting state independently of the gate signal. This latching effect occurs due to the internal regenerative action, where the current flow sustains the required carrier injection, keeping the thyristor on until the anode current drops below the holding level.

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