Light Activated SCR

Photothyristors are light-activated Thyristors. 
Two common photothyristors include the Light Activated Silicon Controlled Rectifier (LASCR) and the light-activated triac.

• A Light Activated SCR (LASCR) acts like a switch that changes states whenever it is exposed to a pulse of light. Even when the light is removed, the LASCR remains ON until the anode and cathode polarities are reversed or the power is removed.
• A light-active TRIAC is similar to a LASCR but is designed to handle AC currents.

LASCR:

• The LASCR is also known as Light Triggered Thyristor(LTT).
• It may be triggered with a light source or with a gate signal. Sometimes a combination of both light source and gate signal is used to trigger an SCR.
• In this, the gate is biased with voltage or current slightly less than that required to turn it on, now a beam of light directed at the inner p-layer junction turns on the SCR.
• The light intensity required to turn-on the SCR depends upon the voltage bias given to the gate. Higher the voltage(current) bias, lower the light intensity required.
• These devices are available up to the rating of 6kV and 3.5ka, with on-state voltage drop of about 2V and with light-triggering the requirements of 5mW.
• The symbol for a LASCR is shown below.
  

Basic Operation:

• When no light is present, the LASCR is OFF; No current will flow through the load.
• However, when the Light Activated SCR (LASCR) is illuminated, it turns ON, allowing current to flow through the load.
• The resistor in this circuit is used to set the triggering level of the LASCR.

How LASCRs Work
The equivalent circuit shown here helps explain how a LASCR works.

• When photons from a light source collide with electrons within the p-type semiconductor, they gain enough energy to jump across the pn-junction energy barrier—provided the photons are of the right frequency/energy.
• When a number of photons liberate a number of electrons across the junction, a large enough current at the base is generated to turn the transistors ON. The net result is a current that flows from the anode to the cathode.
• Even when the photons are eliminated, the LASCR will remain ON until the polarities of the anode and cathode are reversed or the power is cut.

LASCR Applications:

• The primary use of light triggered Thyristors is in high-voltage high- current applications, Static reactive-power compensation etc.
• The Light activated SCRs have complete electrical isolation between the light-triggering source and the high-voltage anode-cathode circuit.
• High Voltage Direct Current (HVDC) transmission systems, several SCRs are connected in series-parallel combination and their light-triggering has the advantage of electrical isolation between power and control circuits.