Gate Triggering methods SCRs

There are different methods of trigerring the gate of a Silicon Controlled Rectifiers (SCRs).  They are

Using DC Voltage
By applying a positive voltage to the gate with respect to the cathode, the junction J2 can be forward biased.  This will switch on the SCR.  This process requires a constant dc voltage to be applied between the Gate and the Cathode, which is a disadvantage.  Besides, there is no isolation between this triggering dc voltage and the main dc supply

Using AC voltage
In AC applications, the trigger voltage can be obtained from the AC voltage suitable reduced.  The phase of the AC voltage is modified and applied to trigger the SCR at the desired instant.  The SCR will continue to conduct till the negative half cycle. 

A separate transformer is required for the trigger circuit which increases the cost

Pulse triggering
This is the most widely used form of triggering.  In this method, a pulse of a small duration is applied to the gate to switch it ON.  Sometimes, a series of pulses are applied.  The pulse need not be continuous.  This reduces the losses in the gate.