Optothyristors: Features & Principle of Operation

Silicon Controlled Rectifiers (SCRs), Triacs and other solid-state devices used for switching larger voltages and currents ON and OFF are referred to as thyristors. Thyristors control switching in an ON-OFF way, in the same way to a light switch which is different from a transistor that can vary the amount of current in its emitter-collector circuit by changing the bias on its base. In other words, thyristors are semiconductor devices that tend to stay ‘ON’ once turned ON and tend to stay ‘OFF’ when turned OFF. A momentary event is able to flip these devices into their ON or OFF states where they will remain that way on their own, even after the cause of the state change is taken. These devices are useful as ON/OFF switching devices per se, much like toggle switch and cannot be used as analog signal amplifiers.  The amount of current that flows through a thyristor must be controlled by adjusting the point in a sine wave where the device is turned ON. Optothyristors also referred to as photothyristors are light-activated thyristors. Just like bipolar transistors, SCRs and Triacs are also fabricated as light-sensitive devices, the action of striking light replacing the function of triggering voltage. Two common optothyristors include the light activated SCR (LASCR) and the light-activated triac or opto-TRIAC.

The symbol for LASCR is illustrated below:

The schematic symbol for opto-TRIAC is shown below:

The Principle of Operation of Light Activated SCR (LASCR)

Let’s consider the following diagram:

With reference to the above Figure 1.2, in (a) when the light strikes the LASCR p-n junction, a photon will collide with an electron in the p-semiconductor side, and an electron will be ejected across the p-n junction into the n-side. 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. Even when the photons are removed, the LASCR will remain ON until the polarities of the anode and cathode are reversed or the power is cut.

Let us now consider a case where the LASCR is switching ON-OFF current through a load as shown below:  

When no light is present, the LASCR is OFF; no current will flow through the load. But when the LASCR is illuminated, it turns ON, allowing the current to flow through the load. The resistor in this circuit is used to set the triggering level of the LASCR.

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