Light Activated SCR (LASCR) Tutorial
Light Activated SCR: Understanding Basics and Operation:
Photothyristors have the same basic structure as conventional thyristors and function in similar modes, except that they are triggered by light instead of gate current. The advantage of light triggering is that the device is now insensitive to electrical signals, which can cause faulty operation in electrically noisy environments.
Photothyristors are nothing but 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 Light Activated SCR (LASCR) remains ON until the anode and cathode polarities are reversed or the power is removed.
- A light-active TRIAC is similar to a Light Activated SCR but is designed to handle AC currents.
Light Activated SCR [LASCR]:
- The Light Activated SCR (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.
- For 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 requirements of 5mW.
- The symbol for a Light Activated SCR (LASCR) is shown below.
Basic Operation of Light Activated SCR :
- 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.
How Light Activated SCR (LASCR) 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 Light Activated SCR (LASCR) will remain ON until the polarities of the anode and cathode are reversed or the power is cut.
Light Activated SCR (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 lihgt-triggering source and the high-voltage anode-cathode circuit.
- In 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.