Why signal train is needed to trigger the thyristor?

To reduce gate power dissipation, SCR firing circuits generate a single pulse or a train of pulses instead of a continuous DC gate signal. This allows precise control of the point at which the SCR is fired. In addition, it is easy to provide electrical isolation between the SCR and the gate trigger circuit.

How does thyristor starts conduct with the application of a gate pulse?

Thyristors in Forward Biased State Triggering by Gate Current Pulse – When it is triggered by the gate current pulse, it starts conducting and will act as a close switch. The Thyristors remains in the ON-state, i.e. it remains in the latched state. Here the gate loses its control to turn off the device.

What is triggering in thyristor?

Triggering means turning ON of a device from its off state. Turning ON of a thyristor refers to thyristor triggering. Thyristor is turned on by increasing the anode current flowing through it. The increase in anode current can be achieved by many ways.

Which device is triggered or turned on by a narrow gate pulse of either positive or negative polarity?

Most TRIACs can be triggered by applying either a positive or negative voltage to the gate (an SCR requires a positive voltage). Once triggered, SCRs and TRIACs continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current.

Why should the gate signal be removed after turn on?

When thyristor is turned on the gate signal should be removed immediately. A continuous application of gate signal even after the triggering on and thyristor would increase the power loss in the gate junction.

Why is a thyristor not turned off immediately?

Even if the gate voltage is removed (after the SCR turns ON), the SCR doesn’t stop conducting. So, the gate has no control flow of current from anode to cathode and the SCR cannot be turned OFF through the gate terminal.