Optothyristors: Light‑Activated SCRs and TRIACs for Reliable Opto‑Isolation

Optothyristors are light‑activated semiconductor switches that replace the traditional triggering voltage with an optical signal. This technology is used in high‑reliability applications where electrical isolation is critical.

There are two main types of optothyristors:

• Light‑Activated SCRs (LASCRs) – These are essentially silicon controlled rectifiers that are triggered by light rather than voltage. Their schematic symbol is shown below.
• Opto‑TRIACs – While they don’t have a separate acronym, opto‑TRIACs perform the same function as a TRIAC but are optically controlled. Their symbol is displayed below.

Light‑Activated SCR (LASCR) Symbol

Opto‑TRIAC Symbol

In practice, these devices are typically housed inside sealed opto‑isolator modules. The module contains a light source (usually a LED) and a photodetector, allowing the optothyristor to be triggered remotely while maintaining electrical isolation between input and output.

Optothyristors are widely used in motor control, power supplies, and safety interlocks because they provide:

• Fast switching times
• High voltage and current handling
• Reduced EMI and electrical noise
• Long‑term reliability in harsh environments

Because they eliminate the need for direct electrical connection to the triggering circuitry, optothyristors help prevent ground loops, reduce fire hazards, and improve overall system safety.