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1. The basic working principle of the SCR is that as the triggering or the biasing is applied at the terminal gate then the conduction begins. As it is a unidirectional device the current will be in a single direction. It resembles the operation of the diode but the only difference is that this can withstand the high amount of voltages and powers.
2. Commutation of SCR is defined as the process of turning off an SCR / thyristor. It is the process by which an SCR or thyristor is brought to OFF state from ON state. We know that, an SCR is turned on by applying a gate signal to a forward biased SCR. But for the purpose of power control or power conditioning, it is required to turn off SCR as and when required. Turn off of a thyristor means bringing it to forward blocking mode from forward conduction mode. Once an SCR goes in forward conduction mode, gate loses its control. This means, some external techniques / circuit must be employed to turn off SCR. This external circuit is known as commutation circuit.
To turn off an SCR / thyristor, it is required that its anode current should fall below the holding current and a reverse voltage should be applied across the SCR for the sufficient time so that it regains to forward blocking mode from forward conduction mode. Thus, to turn off an SCR some methods must be applied so that the above mentioned conditions can be met to turn off SCR as per requirement. This method of turning off an SCR / thyristor is called commutation process.
3. The conduction angles is the point in time where we have applied the gate pulse, and the Thyristor starts conducting this is known as the Triggering angle of the SCR.
Status of the TRIAC at,
position 1: The TRIAC is not conducting in this position, since there is no gate signal.
position 2: The TRIAC operates in first quadrant (During positive half cycle only) only, since it is getting only positive signal.
position 3: The TRIAC operates in both first and third quadrants (Both positive & negative half cycles), since it is getting both positive and negative signals
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