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

2. Introduction
 A Photothyristor.
 Acts like a switch.
 Changes 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.
 Also 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 LASCRFig: 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???