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The SCR is controlled by the intensity of the LED. Same as like other opto-coupler the LED emit Infrared. In the upper image a Photo-SCR based opto-coupler’s internal construction is shown. SCR stand for Silicon controlled rectifier, SCR also referred as Thyristor. The photo-TRIAC based opto-coupler examples are IL420, 4N35 etc are example of TRIAC based opto-coupler. Opto-coupler provide excellent isolation in this case too. The led can be controlled using DC, and the TRIAC used to control AC. TRIAC is mainly used where AC based control or switching is needed. In the upper image the internal construction or the TRIAC based opto-coupler is shown. 4N32, 4N33, H21B1, H21B2, H21B3 are few photo-Darlington based opto-coupler example. The 6th pin which is internally connected to the base of the transistor, used to control the sensitivity of the transistor as discussed previously in photo-transistor description. This type of opto-coupler also used in DC circuit related area for the isolation. As usual the LED emits infrared led and controls the base of the pair transistor. In this configuration the Darlington Transistor provide high gain ability. In the upper image there are two types of symbol, internal construction of Photo-Darlington based opto-coupler is shown.ĭarlington Transistor is two transistor pair, where one transistor controls other transistor base. Photo – Transistor based opto-coupler is used in DC circuit related isolation. PC816, PC817, LTV817, K847PH are few widely used photo-transistor based optocoupler.

In this configuration, false triggering due to noise or electrical transients can be controlled effectively.Īlso, before using Photo-transistor based optocoupler, the user must know the maximum rating of the transistor. Often the pin is used to connect with ground or negative using a high value resistor. This pin 6 is used to control the sensitivity of the photo-transistor. On the second image on the left, there is additional pin out which is internally connected with transistor’s base. Photo-Transistor can be further of two types depending on the output pin availability. The Transistor type can be anything whether PNP or NPN. In the upper image the internal construction is shown inside a Photo-transistor Optocoupler.

Opto-coupler which use Photo Darlington Transistor.

Opto-coupler which use Photo Transistor.

Depending on the use there are mainly four types of optocouplers are available. There are many different types of Optocouplers are available commercially based on their needs and switching capabilities. The hollowed space between LED and photo-transistor can be made using Glass, air, or a transparent plastic, the electrical isolation is much higher, typically 10 kV or higher. Also, between the photo-transistor and the Infrared LED, the space is transparent and non-conductive material it is electrically isolating two different circuits. Since the LED can be controlled by a different circuitry and the photo transistor can control different circuitry so two independent circuits can be controlled by Optocoupler. Intensity of the LED directly controls the photo-transistor. The photo-transistor switches the output circuitry by its collector and emitter, same as typical BJT transistors. On the left side pin 1 and pin 2 are exposed, it is a LED (Light Emitting Diode), the LED emit infrared light to the photosensitive transistor on the right side. This the internal structure of the opto-coupler. In present commercial market, we can buy Opto-coupler with 10 kV to 20 kV input to output withstand voltage capacity, with a specification of 25 kV / uS voltage transients. Often in circuits, especially low voltage or highly noise sensitive circuits, Optocoupler is used to isolate circuitry to prevent electrical collision chances or to exclude unwanted noises.

Optocoupler also called Opto-isolator, photo coupler or optical isolator. Opto-coupler is an electronic component that transfers electrical signals between two isolated circuits.