Methods of turning on an SCR

The SCR cannot switch on on its own once its anode and cathode are connected to the positive and negative terminals respectively. 

The following are some of the methods.

Gate Triggering

This is the most popular method.  A single pulse or a train or pulses are applied to the gate terminal of the SCR.  This creates a forward bias across junction J2 and switches on the SCR.

Thermal Triggering
When the SCR is heated above a certain value, more hole-electron pairs are produced this increases the charge carriers and can cause the SCR to switch on.

Light Triggering

When light is made to fall on the junction in reverse bias, hole-electron pairs are created due to the energy of the incident light.  This can cause the SCR to fire.  Special components such as LASCR (Light activated Silicon controlled Rectifiers) and LASCS ( Light activated Silicon controlled Rectifiers) work on this principle.  This method of triggering is cheaper when designing components of higher ratings.  The light is conducted to the junction by means of optical cables.

dv/dt triggering
In this method of triggering, a rapid change in the voltage current to flow through the junction J2 which acts as a dielectric between two conductive junctions (J1 and J3).  The SCR will switch on even if the voltage is low provided the rate of change of the voltage is high.