Triac and SCR output photocouplers are also known as triac and SCR output optocouplers. They are semiconductor devices that combine the functionality of an opto-triac or an opto-SCR with a power triac. They provide electrical isolation between a low voltage input and a high voltage output, whilst being able to switch the high voltage output. This is important in electrical equipment because isolation prevents catastrophic or dangerous faults that can occur in such scenarios where the higher voltage accidentally couples onto the low voltage circuit. This could lead to the destruction of the associated circuits or even electrocution. Such isolation is mandated in government type approval compliance requirements for certain equipment.These isolating switches contain a low voltage input connected to a simple photo diode circuit. When a small current flows through the photodiode, it generates a beam of light which travels across an electrically insulating barrier or gap and detected by a receiving photodiode. The light beam can be modulated on and off by the input current , typically less than 10mA. This current is known as the LED trigger current. An LED photodiode current is modulated by changing the voltage presented across the diode, crossing the LED Forward Voltage (typically around 0.9-1.4V) as long as the photodiode is presented with voltages that can turn it on and off. This is typically around the 1-1.4V. The sensing photodiode, in this case, is a photo-triac or a photo-SCR. Triac stands for triode for alternating current and is a device that can conduct current in either direction when it is triggered, or turned on by detecting a light beam on its trigger junction (gate). An SCR is similar to a triac, except that is conducts only in one direction. When used in AC switching applications it is typical to find dual-power SCRs, and a zero crossing detector are used instead of a single triac to allow bidirectional current switching. These devices are used in applications like high voltage AC switching, gas pumps, washing machines, microwave ovens and water heaters. They are commonly used anywhere where there is a need to protect and electrically isolate two circuits in a switching application. Read more Read less