“An optocoupler is an electrical-optical-electrical conversion device that transmits electrical signals through the medium of light. It uses light as a medium to convert the electrical signal at the input end into an optical signal, couples it to the output end and then converts it into an electrical signal, so it is called an optocoupler. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectionality, so it has good electrical insulation ability and anti-interference ability. Light-emitting devices often use light-emitting diodes or laser tubes; receiving devices commonly use photodiodes, phototransistors, and optical integrated circuits.

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An optocoupler is an electrical-optical-electrical conversion device that transmits electrical signals through the medium of light. It uses light as a medium to convert the electrical signal at the input end into an optical signal, couples it to the output end and then converts it into an electrical signal, so it is called an optocoupler. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectionality, so it has good electrical insulation ability and anti-interference ability. Light-emitting devices often use light-emitting diodes or laser tubes; receiving devices commonly use photodiodes, phototransistors, and optical integrated circuits.

Optical couplers are generally composed of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light emitting diode (LED), so that it emits light of a certain wavelength, which is received by the photodetector to generate a photocurrent, which is further amplified and output. This completes the conversion of electricity-optical-electricity, thus playing the role of input, output and isolation. Since the input and output of the optocoupler are isolated from each other, the electrical signal transmission has the characteristics of unidirectional, so it has good electrical insulation ability and anti-interference ability. And because the input end of the optocoupler is a low-resistance element that works in current mode, it has a strong common mode rejection capability.

Working characteristics:

(1) The common mode rejection ratio is very high. Inside the optocoupler, since the coupling capacitance between the light-emitting tube and the light receiver is small (within 2pF), the common-mode input voltage has little effect on the output current through the inter-electrode coupling capacitance, so the common-mode rejection ratio is high.

(2) Output characteristics. The output characteristic of the optocoupler refers to the relationship between the bias voltage VCE applied to the phototransistor and the output current IC under a certain luminous current IF. When IF=0, the light-emitting diode does not emit light, and the collector output current of the phototransistor at this time is called dark current, which is generally small. When IF > 0, under the action of a certain IF, the corresponding IC is basically independent of VCE. The change between IC and IF has a linear relationship, and the output characteristics of the optocoupler measured by the semiconductor tube characteristic grapher are similar to the output characteristics of ordinary transistors.

(3) Isolation characteristics. Including isolation voltage, isolation capacitance and isolation resistance. The isolation voltage is the insulation withstand voltage value between the input end and the output end of the optocoupler; the isolation capacitance is the capacitance value between the input end and the output end of the optocoupler device; the isolation resistance is the value between the input end and the output end of the semiconductor optocoupler insulation resistance value.

(4) The transfer characteristics include the current transfer ratio rise time and fall time, etc.

(5) The optical coupler can be used as a linear coupler. A bias current is provided on the light-emitting diode, and the signal voltage is coupled to the light-emitting diode through a resistor, so that the phototransistor receives an optical signal that increases or decreases on the bias current, and its output current will follow the input signal. The voltage changes linearly. The optocoupler can also work in an on-off state to transmit pulse signals. When transmitting a pulse signal, there is a certain delay time between the input signal and the output signal. The input and output delay times of optocouplers with different structures are very different.

Optocoupler is a new type of device developed in the 1970s, and is now widely used in electrical insulation, level conversion, inter-stage coupling, drive circuit, switching circuit, chopper, multivibrator, signal isolation, inter-stage isolation, Pulse amplification circuit, digital instrument, long-distance signal transmission, pulse amplification, solid state relay (SSR), instrumentation, communication equipment and computer interface. In a single-chip switching power supply, a linear optocoupler can be used to form an optocoupler feedback circuit, and the duty cycle can be changed by adjusting the control terminal current to achieve the purpose of precise voltage regulation.