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The main function of optocoupler

elecfans | 06/07/2021

The working principle of optocoupler

The coupler uses light as the medium to transmit electrical signals. It has a good isolation effect on input and output electrical signals, so it is widely used in various circuits. At present, it has become one of the most diverse and versatile optoelectronic devices. The optical coupler generally consists of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light-emitting diode (LED) to emit light of a certain wavelength, which is received by the photodetector to generate a photocurrent, which is then further amplified and output. This completes the electrical-optical-electrical conversion, thereby playing the role of input, output, and isolation. Since the input and output of the optocoupler are isolated from each other and the electrical signal transmission has the characteristics of unidirectionality, it has good electrical insulation and anti-interference capabilities. In addition, since the input end of the optocoupler is a low-resistance element that works in current mode, it has a strong common-mode rejection capability. Therefore, it can greatly improve the signal-to-noise ratio as a terminal isolation element in long-line transmission of information. As a signal isolation interface device in computer digital communication and real-time control, it can greatly increase the reliability of computer work.

The role of optocoupler

Due to the wide variety of optocouplers, unique structure and outstanding advantages, their applications are very wide, mainly in the following occasions:

(1) Application in logic circuits

Optocouplers can form various logic circuits. As the anti-interference performance and isolation performance of optocouplers are better than transistors, the logic circuits formed by them are more reliable.

(2) As a solid switch application

In the switch circuit, it is often required to have a good electrical isolation between the control circuit and the switch, which is difficult to achieve for general electronic switches, but it is easy to achieve with a photocoupler.

(3) Application in trigger circuit

The photoelectric coupler is used in the bistable output circuit. Since the light-emitting diodes can be connected in series to the two emitter circuits, it can effectively solve the problem of output and load isolation.

(4) Application in pulse amplifier circuit

The photoelectric coupler is applied to the digital circuit, can amplify the pulse signal.

(5) Application in linear circuits

Linear optocouplers are used in linear circuits, with high linearity and excellent electrical isolation performance.

(6) Application on special occasions

The photoelectric coupler can also be used in high-voltage control, replacing transformers, replacing contact relays, and used in many occasions such as A/D circuits.

The circuit diagram of optocoupler used as solid state relay

The use of optocoupler as a solid state relay has the advantages of small size, close coupling, small driving power, fast action speed, and wide operating temperature range. Figure 1 shows the actual circuit diagram of an optocoupler used as a solid state relay. The left half of the circuit can be used to convert the input electrical signal Vi into the light signal of the light-emitting diode in the optocoupler; and the right half of the circuit uses light. The photosensitive triode in the coupler then restores the optical signal to an electrical signal, so this is a very good electro-optical and photoelectric combined conversion device. The current transfer ratio of the optocoupler is 20%, the withstand voltage is 150V, and the drive current is between 8-20mA. In actual use, because it does not have the actual contacts that are common in general electromagnetic relays, there are no phenomena such as poor contact and arcing, and it will not cause malfunctions due to external forces or mechanical shocks. Therefore, its performance is relatively reliable and its work is very stable.

The circuit diagram of using optocoupler instead of audio transformer

In linear circuits, audio transformers are often used for coupling between two-stage amplifiers. The disadvantage of this coupling is that it will lose part of the power in the transformer iron chip and may cause some distortion. And if optocoupler is used instead of audio transformer, these shortcomings can be overcome. When the input signal Vi is amplified by the front stage of the transistors BG1 and BG2, the LED on the left side of the optocoupler is driven to emit light, and it is absorbed by the photosensitive tube on the right and converted into an electrical signal. The emitter passes through the capacitor C3 and outputs an undistorted amplified signal V0. Since this circuit completely isolates the front and rear amplifiers, the possible interference caused by the ground loop is eliminated. At the same time, because the circuit also has a noise canceling function, the distortion of the signal is avoided. The total gain of the whole circuit is expected to reach more than 20dB, and the bandwidth is about 120kHz.

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