What kind of circuit uses optocouplers?
Date:2025-06-23
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Optocouplers are widely used in circuits that require electrical isolation, and their core value is to transmit and isolate electrical signals through optical signals, blocking interference from high voltage, noise, or grounding circuits on sensitive circuits. The application logic is analyzed from three aspects: technical principles, typical scenarios, and selection points
1、 The core function of optocouplers is to serve as a bridge for electrical isolation
Optocouplers are composed of light emitting diodes (LEDs) and photosensitive devices (such as phototransistors and phototransistors) packaged together, and achieve signal isolation and transmission through the conversion of "electricity → light → electricity". Its key features include:
Isolation voltage: Supports withstand voltage ranging from several hundred volts to several thousand volts (such as Toshiba TLP785F supporting 5000Vrms isolation).
Anti interference: Block common mode noise (typical CMTI>50kV/μ s) and suppress circulating current caused by ground potential difference.
Unidirectional transmission: It naturally blocks the electrical connection between the input and output terminals to prevent high-voltage side faults from affecting the low-voltage side.
2、 Typical application scenarios of optocouplers
1. Isolation of strong and weak electrical systems
Industrial control (PLC, frequency converter)
Isolate the 220V/380V high-voltage control circuit from the 24V logic circuit to prevent high-voltage pulses from damaging the CPU.
Typical case: Siemens S7-1200 PLC achieves 3000Vrms isolation and voltage resistance through optocoupler isolation of DI/DO modules.
Power electronic equipment (inverters, UPS)
Isolate the high-voltage DC bus (such as 800V) from the low-voltage control circuit (such as 3.3V) to avoid the risk of high-voltage breakdown.
Key application: In IGBT driver circuits, optocouplers (such as Avago ACPL-332J) transmit PWM signals and provide feedback on fault states.
2. Signal isolation in noise sensitive scenes
Communication equipment (RS-485, CAN bus)
Isolate industrial site electromagnetic interference (such as frequency converter radiation) to prevent bus communication interruption.
Typical solution: TI ISO1042 isolated transceiver integrates optocoupler isolation and bus protection, supporting 2500Vrms isolation.
Audio equipment (microphone preamplifier)
Isolate power noise from audio signals and improve signal-to-noise ratio (SNR) to above 110dB.
Case: Optocouplers (such as Vishay IL300) achieve "phantom power" isolation in professional microphones.
3. High and low voltage level conversion
Industrial sensor interface
Isolate 24V industrial sensor signals from 3.3V microcontrollers, compatible with different level standards.
Typical device: Broadcom ACPL-072L supports 5V to 3.3V level conversion with a response time of<2 μ s.
Battery Management System (BMS)
Isolate the high-voltage battery pack (400V~800V) from the low-voltage BMS motherboard to prevent leakage current from triggering safety risks.
Key application: Optocoupler monitoring of battery cell voltage while blocking high-voltage circuits.
4. Digital communication isolation
Medical equipment (electrocardiograph, defibrillator)
Isolate the patient's leads from the host circuit to prevent leakage current from endangering human safety (in accordance with IEC 60601 standard).
Typical solution: Optocouplers (such as Lite On LTV-817) achieve isolated transmission of physiological signals.
Electric vehicle charging station
Isolate the CAN communication bus from the charging control module to prevent communication failures caused by ground potential differences.
Key component: ADI ADuM1201 digital isolator (based on optocoupler technology) supports a data rate of 150Mbps.
3、 Selection and optimization of optocouplers
Isolation voltage requirement
Industrial scenario: ≥ 2500Vrms (such as Toshiba TLP521 series).
Medical scenario: ≥ 5000Vrms (such as Renesas IL610 series).
Transmission speed matching
Low speed control signal: Select optocouplers with CTR (current transfer ratio)>100% (such as Sharp PC817).
High speed digital signal: Select optocouplers with response time<1 μ s (such as Avago HCPL-060L).
Special application extension
AC load driver: bidirectional thyristor output optocoupler (such as Lite On MOC3063).
IGBT driver: High speed optocouplers (such as Infineon ACPL-339J) support 15kV/μ s common mode rejection.
4、 Technological Evolution and Alternative Solutions
Although optocouplers are still the backbone of isolation technology, new isolation technologies such as digital isolators are gradually penetrating:
Digital isolators (such as ADI iCoupler): Based on CMOS technology, power consumption is reduced by 80%, and data rate is increased to 150Mbps.
Magnetic isolation chips (such as Silicon Labs Si86xx) have stronger radiation resistance and are suitable for the aerospace industry.
Conclusion: Optocouplers are irreplaceable in circuits that require high voltage isolation, noise suppression, and level conversion, especially in industrial control, power electronics, medical equipment, and other fields. With the evolution of technology, optocouplers are complementing digital isolation technology to jointly build safer and more reliable electrical isolation solutions.