Understanding Common Electronic Components – Optocoupler (OC)

Optocoupler (OC) is also called photoelectric coupler, abbreviated as OC. It is a device that uses light as a medium to transmit electrical signals. Usually, the light emitter (infrared light-emitting diode LED) and the light receiver (photosensitive semiconductor tube) are encapsulated in the same tube shell. When an electrical signal is added to the input end, the light emitter emits light, and the light receiver generates photocurrent after receiving the light, which flows out from the output end, thus realizing the “electric-light-electric” conversion.
Optocoupler 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 widely used optoelectronic devices.

Working principle

An optocoupler generally consists of three parts: light emission, light reception and signal amplification. The input electrical signal drives the light-emitting diode The tube (LED) emits a certain wavelength of light, which is received by the photodetector to generate photocurrent, and then output after further amplification. This completes the conversion of electricity-light-electricity, 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.


There are many varieties and types of optocouplers, which can usually be classified according to the following methods:

1. According to the light path

It can be divided into external optical path optocouplers (also known as photoelectric intermittent detectors) and internal optical path optocouplers. External optical path optocouplers are further divided into transmission type and reflection type optocouplers.

2. According to the output form

a. Photosensitive device output type, including photosensitive diode output type, photosensitive Transistor output type, photocell output type, photo-thyristor output type, etc.

b. NPN transistor output type, including AC input type, DC input type, complementary output type, etc.

c. Darlington transistor output type, including AC input type, DC input type.

d. Logic gate circuit output type, including gate circuit output type, Schmitt trigger output type, three-state gate circuit output type, etc.

e. Low conduction output type (output low level millivolt order of magnitude).

f. Optical switch output type (on-resistance less than 10Ω).

g. Power output type (IGBT/MOSFET, etc. output).

3. According to the packaging form

It can be divided into coaxial type, dual in-line type, TO package type, flat package type, patch package type, and optical fiber transmission type, etc.

4. According to the transmission Signal input

It can be divided into digital optocouplers (OC gate output type, totem pole output type and tri-state gate circuit output type, etc.) and linear optocouplers (can be divided into low drift type, high linear type, broadband type, single power supply type, dual power supply type, etc.).

5. By speed

It can be divided into low-speed optocouplers (phototransistor, photocell, etc. output type) and high-speed optocouplers (photodiode with signal processing circuit or photosensitive integrated circuit output type).

6. By channel

It can be divided into single-channel, dual-channel and multi-channel optocouplers.

7. By isolation characteristics

It can be divided into ordinary isolation optocouplers (generally optical glue potting is less than 5000V, and empty seal is less than 2000V) and high-voltage isolation optocouplers (can be divided into 10kV, 20kV, 30kV, etc.).

8. By working voltage

It can be divided into low power supply voltage type optocouplers (generally 5~15V) and high power supply voltage type optocouplers (generally greater than 30V).

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