Optocouplers/Optical Isolators: Characteristics, Types, Advantages and Disadvantages, Applications!

　　Optocouplers or optoisolators are combinations of light sources and optical detectors in the same package.

　　They are used to optically couple signals from one point to another, by providing a complete electrical isolation between them. This isolation is provided between the low power control circuit and the high power output circuit to protect the control circuit.

　　Depending on the type of light source and detector, we can get a wide variety of opto-couplers.

　　Even if an opto-coupler electrically isolates DC and low frequency signals, a pulsed input signal (one that changes abruptly) will generate a displacement current ic = Cf*dv/dt. This current can flow between the input and output due to the presence of capacitance Cf between the input and output. This allows noise to appear in the output.

　　The LED photodiode is shown in the figure, here the infrared LED is used as the light source and the photodiode as the detector.

　　The advantage of using a photodiode is the high linearity. When the pulse at the input becomes high, the LED lights up. It emits light.

　　In response to this light, photocurrent will begin to flow through the photodiode. Once the input pulse is reduced to zero, the LED will turn off & the photocurrent through the photodiode will be reduced to zero. Thus, the pulse at the input is coupled to the output.

　　The LED phototransistor optocoupler is shown in the figure. The IR LED is used as the light source and the phototransistor is used as the photodetector.

　　This is the most commonly used optocoupler because it does not require any additional amplification.

　　The LED lights up when the pulse at the input goes high and the light from the LED is focused on the CB junction of the phototransistor.

　　As a response to the light, photocurrent starts to flow as the base current of the phototransistor.

　　The collector current of the phototransistor begins to flow. Once the input pulse is reduced to zero, the LED turns off and the collector current of the phototransistor is reduced to zero. Thus, the pulse at the input is light-coupled to the output.

　　Optocouplers/Optical Isolators Advantages of photocouplers：

　　Due to the electrical isolation, the control circuit is well protected.

　　Broadband signal transmission is possible.

　　Due to the unidirectional signal transmission, noise from the output side is not coupled to the input side.

　　The interface to the logic circuit is easy to implement.

　　It is small and light weight.

　　Optocouplers/Optical Isolators Disadvantages：

　　Slow speed.

　　Signal coupling possibilities for high power signals.

　　Optocouplers/Optical Isolators Advantageous applications：

　　Optocouplers are basically used to isolate low power circuits from high power circuits.

　　At the same time, control signals are coupled from the control circuit to the high power circuit.

　　Some of these applications are.

　　AC-DC converters for DC motor speed control

　　-High power choppers

　　-High power inverters

　　One of the most important applications of photocouplers is the coupling of the base drive signal to the power transistor connected in the DC-DC chopper.

　　Note that when the output is on the collector of the phototransistor, the input and output waveforms are 180º out of phase.