Things You Need to Know about Optoisolators

Ambiguities about the Optoisolator and the Optical coupler

Today, Easybom would like to bring you some fundamental knowledge you need to know about optoisolators.

An optical coupler is abbreviated as OC, also known as an optical isolator, opto-isolator. The structure of an optoisolator is equivalent to the light-emitting diode and photosensitive (triode) tube sealed together. The light-emitting diode converts the input electrical signal into optical signal to the photosensitive tube for electrical signal output. Because there is no direct electrical connection, it not only couples the transmission signal, but also isolates the interference. 

The terms optical coupler and Optical Isolator can be used interchangeably to refer to the same function. The slight difference between the two terms is characterized by the isolated voltage. 

Optoisolator are used to transfer analog or digital information from one voltage potential to another while maintaining potential isolation below 5000V. However, the optical coupler is used to isolate the power supply system, while transmitting analog or digital data between the systems, isolating the power supply system with a voltage of 5000 to 5000V or above.

Structure of an Optoisolator 

From the optoisolators line of various manufactures which you can find on Easybom, optoisolator is mainly composed of three parts: light emission, light reception and signal amplification. The emitting part of the light is mainly composed of light-emitting devices which are generally light-emitting diodes converting electrical energy into light energy and emit light driven by DC, AC, pulse and other power supplies. But it must be used with positive voltage. The receiving part of the light is mainly composed of photosensitive devices, which are generally photosensitive transistors. Photosensitive transistors work by using the principle that the reverse resistance changes from large to small under the irradiation of light when the reverse voltage is applied by PN junction.
Most of the signal amplifiers of light are mainly composed of electronic circuits and so on. The pin of the light-emitting device is the input, while the pin of the photosensitive device is the output. When working, the electrical signal is added to the input to make the core of the light-emitting device emit light, while the photosensitive device produces photocurrent after being illuminated and magnified by the electronic circuit, so as to realize the conversion of electricity, thus realizing the electrical isolation of the input and output circuits. Thanks for that the input and output circuits of the optoisolator are isolated from each other, and the electrical signal has the advantages of unidirectional transmission, the optoisolator has good anti-electromagnetic interference ability and electrical insulation ability.

Two Kinds of Optoisolators

As concluded by Easybom, in terms of output characteristics, there are two kinds of optoisolators: nonlinear (digital) optoisolators and linear (analog) optoisolators. 

Nonlinear optoisolators are suitable for the transmission of switching signals, but not for the transmission of analog signals. The current transmission characteristic curve of the linear optoisolatoris close to the straight line, and the performance is better when the signal is small, so it can be isolated and controlled by the linear characteristic. 

The commonly used optoisolators in switching power supplies such as color TV sets and displays are linear products. If they are replaced by non-linear products, it is probable to deteriorate the oscillation waveform and cause parasitic oscillation in serious cases, which interferes with the image picture and reduces the load capacity of the power supply at the same time. Therefore, when repairing the switching power supply of household appliances, if you find that the optoisolators are damaged, you must remember to replace it with a linear optoisolator.

Christophe Rude
Christophe Rude
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