What is the function of optocouplers?

Optocoupler, also known as optocoupler, is a semiconductor device that achieves electrical isolation through optical signals. Its core structure consists of light-emitting diodes (LEDs) and photosensitive devices (such as phototransistors, photosensitive Darlington transistors, photosensitive thyristors, etc.), which achieve electrical isolation between input and output through optical signal transmission. Optocouplers play a crucial role in electronic systems, and their functions can be summarized into the following four core dimensions:
1、 Electrical isolation: blocking high voltage and protecting low voltage circuits
The core function of optocouplers is to achieve electrical isolation between input and output circuits, and their isolation voltage (Viso) can reach several thousand volts or even tens of thousands of volts. This feature is crucial in the following scenarios:
Isolation of high and low voltage systems
Industrial control: In PLC (Programmable Logic Controller), optocouplers isolate 220V AC input signals from 3.3V/5V digital circuits to prevent high-voltage impact from damaging the microcontroller.
Medical equipment: In electrocardiogram monitors, optocouplers isolate the patient's body (potential high voltage) from the signal processing circuit to ensure equipment safety.
Ground potential difference suppression
Communication system: In the RS-485 bus, optocouplers isolate the ground potential difference (up to several hundred volts) of different devices to avoid interference from ground loop currents in signal transmission.
Distributed system: In photovoltaic inverters, optocouplers isolate the DC side (high voltage) from the AC side (low voltage) to prevent DC side faults from affecting the power grid.
2、 Signal transmission: transmitting information across isolation boundaries
Optocouplers achieve signal transmission in an isolated state, supporting digital, analog, and AC signals:
Digital Signal Transmission
Industrial automation: In motor drivers, optocouplers transmit PWM control signals from MCU (microcontroller) to IGBT drive circuits to achieve motor speed regulation.
Consumer electronics: In mobile phone chargers, optocouplers provide feedback output voltage to the primary side control chip to achieve constant voltage output.
analog signal transmission
Power management: In a switching power supply, a linear optocoupler (such as IL300) feeds back the output voltage error signal to the primary side to achieve closed-loop voltage regulation.
Data collection: In the sensor interface, the optocoupler isolates the analog signal conditioning circuit from the ADC (Analog to Digital Converter) to avoid digital noise interference with measurement accuracy.
Communication signal transmission
Motor control: In the AC speed regulator, the light controlled thyristor (such as MOC3023) isolates the control signal from the AC load to achieve contactless voltage regulation.
Lighting control: In the dimmer, the optocoupler isolates the phase control signal from the AC power supply to achieve LED brightness adjustment.
3、 Anti interference: Improve system electromagnetic compatibility (EMC)
Optocouplers transmit optical signals, effectively blocking noise transmission paths and enhancing the system's anti-interference ability
Common mode interference suppression
Industrial site: When driving the motor with a frequency converter, the optocoupler isolates the control circuit from the power circuit to suppress the interference of high-frequency noise (up to MHz level) on the control signal.
Automotive Electronics: In the onboard ECU (Electronic Control Unit), optocouplers isolate the CAN bus from the sensor interface to prevent engine compartment noise from affecting communication.
Transient pulse protection
Power system: In lightning protection circuits, optocouplers isolate external inputs from internal circuits and withstand transient overvoltages (up to several thousand volts) without damage.
Consumer electronics: In smart meters, optocouplers isolate the strong and weak electrical sides to prevent grid surges from impacting the metering chip.
4、 System Protection: Fault Isolation and Safety Enhancement
Optocouplers play a role in fault isolation and safety enhancement in the system:
Overcurrent/Overvoltage Protection
Power management: In switch mode power supplies, optocouplers detect output overvoltage signals and provide feedback to the primary side, triggering protection circuits (such as turning off MOSFETs).
Motor drive: In servo drives, optocouplers isolate fault signals (such as overcurrent and short circuit) from the control circuit to achieve fast protection.
Security authentication support
Medical equipment: In medical lasers, optocouplers isolate high-voltage power supplies and control circuits, meeting the IEC 60601 medical safety standard.
Industrial equipment: In robot controllers, optocouplers isolate emergency stop signals from drive circuits to ensure the safety of operators.
5、 Technological Evolution: From Basic Isolation to Intelligent Integration
With the development of technology, the functions of optocouplers are constantly expanding:
High speed optocoupler
Communication upgrade: Supports GHz level signal transmission (such as 6N137), applied to fiber optic communication and Ethernet isolation.
Motor control: Achieve nanosecond level transmission delay (such as 75ns) to meet the high-speed response requirements of servo systems.
Intelligent optocoupler
Interface integration: Integrate I2C/SPI interfaces (such as ADUM1250) to achieve bidirectional data isolation transmission.
Functional integration: Built in LDO regulator, temperature sensor (such as ST isolator), simplifies system design.
6、 Selection and application precautions
Key parameter matching
Isolation voltage: Select according to the application scenario (such as 5kVrms for industrial scenarios and 10kVrms for medical scenarios).
Transmission latency: Choose ns level for high-speed applications (such as 6N137), and μ s level for low-speed applications (such as PC817).
Current transfer ratio (CTR): Select based on the driving capability (such as selecting CTR ≥ 100% for relay driving).
Heat dissipation and layout optimization
High power applications: equipped with heat sinks or choose low thermal resistance packaging (such as TO-220).
High frequency application: Shorten PCB routing length and reduce parasitic inductance.
summarize
Optocouplers achieve electrical isolation through optical signals and play an irreplaceable role in industrial control, communication equipment, medical electronics, and other fields. Its core values are reflected in:
Safe isolation: blocking high voltage to protect personnel and equipment safety;
Signal transmission: transmitting digital, analog, and AC signals across isolated boundaries;
Anti interference: Suppress common mode noise and transient pulses to enhance system stability;
System protection: Implement fault isolation and safety authentication support.
With the evolution of technology, optocouplers are developing from basic isolation devices towards high-speed, intelligent, and integrated directions, providing more efficient and reliable isolation solutions for electronic systems.