A presentation on light activated-scr

1. Welcome To A presentation on Light Activated Silicon-Controlled-Rectifier(LASCR) Presented By: Sabiha Mehbuba Student ID: 110925 Year: 4 th ; Term: I ECE Discipline, Khulna University.

2. Introduction A Photothyristor. Acts like a switch. C hanges states whenever it is exposed to a pulse of light . Even when the light is removed, the Light Activated SCR (LASCR) remains ON until the anode and cathode polarities are reversed or the power is removed . A lso known as Light Triggered Thyristor (LTT).

3. Construction: Likewise the normal SCR, LASCR is also a four semiconductor layer device, consisting of three junctions J1, J2 and J3. The gate structure (the reverse-biased n-p junction) is exposed to the light and designed such that it allows the light source to trigger the device. It has an optical sensitive region in the gate and some of the LASCRs have a transparent window or region in their cases to couple the LEDs.

4. Symbol and Equivalent Circuit: Fig: Equivalent circuit of LASCR Fig: Symbol of LASCR

5. 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. Resistors can be added between the gate(G) and cathode(C) to reduce its susceptibility to noise and dv/ dt effects, but this degrades its sensitivity to light triggering.

6. Light Triggering: 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 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 Light Activated SCR (LASCR) will remain ON until the polarities of the anode and cathode are reversed or the power is cut

7. Applications: The primary use of light triggered thyristors is in high-voltage high- current applications, Static reactive-power compensation etc. In High Voltage Direct Current(HVDC) transmission systems. In relay driving circuits.

8. THANK YOU ALL

9. QUESTIONS ???