 Sunil Kalshatti, ASEAN Professor, Department of Electronics Engineering, Valshan Ishtarov Technology, Sulapur. Today, I am going to discuss light-activated silicon-controlled rectifier, LASR, Learning Outcome. At the end of this session, students can describe construction, characteristics and operation of LASR. It is photothyrister, it likes as a switch and opto-isolator. Opto-isolator means it provides the isolation between the control circuit and power circuit. It changes its state whenever it is exposed to pulse of light. Even when light is removed, LASR remains on until anode and cathode polarities are reversed or the power is removed. The LASR is a type of thyristor which is triggered by photons present in the light rays. It is the schematic symbol of LASR. LASR having three terminals, anode, cathode, gate, similar to the thyristor. This is the constructional view of light-activated silicon-controlled rectifier. This is the gate lead, this is the cathode lead, this is the anode, this is the silicon pallet. Figure A shows the physical layout of LASR having the three junctions J1, J2, J3. And figure B is the equivalent circuit using two transistors, construction. Like was normal LASR, LASR is also four layer, three terminal, three junction device. The gate structure that is the reverse biased NP-junction is exposed to light and designed such that it allows the light source to trigger device. The LASR is made up of silicon material and the glass lens in the LASR is used to focus the light from the external source on the semiconductor material. The silicon pallet is used in the bottom of the device and the light intensity dislodges electrons in the semiconductor crystal and contributes to conduction. It has optical sensitive region in gate and some of the LASRs have transparent window or region in their cases to couple LEDs. The light-activated is also termed as light-triggered thyristor. This is because the thyristor is triggered by the supply of light from the external source. This is the VI characteristics of LASR, which is the forward breakdown voltage. The light intensity, sorry, the forward breakdown voltage depends upon the light intensity. If light intensity is more, forward breakdown voltage is less and if light intensity is less, forward breakdown voltage is more, operation. When no light is present, LASR remains off. When forward voltage is applied across anode and cathode, J1 and J3 get forward biased and J2 get reverse biased and blocks current flow. When light is focused on LASR, the incident photons will generate electron-hole pairs in the vicinity of junction J2. The number of optically generated hole pairs is proportional to the intensity of light. As light is increased, the current in reverse biased diode will increase. This diode current acts as a base current for transistor Q2. So because of this, IC2 flows through the Q2 and this IC2 is equal to beta 2 into IB2. This IC2 acts as a base current of Q1. So because of this, the IC1 flows through the Q1 and this IC1 is equal to beta 1, beta 2, IC2 and this IC1 acts as a gate current or base current for Q2. So because of this regenerative action, LASR turns on. The LASR is most sensitive to light when its gate terminal is left open. Its sensitivity can be reduced and controlled to some extent by inserting a register between its gate and cathode terminals. It offers complete electrical isolation between the triggering source and the switching device. That's why LASR acts as an opto-isolator. The forward breakdown voltage decreases with increase in light intensity. The voltage rating of LASR is up to 6 kV at 3.5 kA with on-state voltage drop of about 2V with light triggering power about 5 mW. The typical dv by dt could be as high as 2000V per microsecond and di by dt is 250A per microsecond application. The primary use of LASR is in high voltage, high current application and static reactive power compensation. In high voltage direct current that is in the HVDC transmission system several SCRs are connected in series parallel combination and their light triggering has the advantage of electrical isolation between the power and control circuit. In relay driving circuit high power pulse generators and high voltage drives. How LASR can be used to trigger high power SCR? When voltage is applied across photodiode D1, the photodiode D1 conducts. So because of this the current flows through the input source RD1. This photodiode emits the light and because of this light LASR conducts. Because the LASR conducts the current flows through the LASR and this current acts as a gate current for the main thyristor. So because of this the main thyristor conducts and the current flows in this direction. Here the gate current is depends upon the light intensity. If light intensity is more the LASR requires less time to turn on. If light intensity is less the LASR requires the more time. How LASR can be used to control AC power? Here in this circuit two LASRs are used and these two LASR LASR1 and LASR2 are connected in anti-parallel. During positive half cycle LASR1 is more forward wise and during negative half cycle LASR2 is more forward wise. When switch is closed and input voltage is applied the photodiode conducts. It emits the light and as this supply is positive half cycle so LASR1 is conducts and when LASR conducts the current flows through the VS load LASR1. And during negative half cycle LASR2 conducts and current flows in anti-clockwise direction. So in this way these two LASRs are controls the power in positive half cycle and negative half cycle. These are references. Thank you.