Optocoupler is a semiconductor device that couples and transmits input and output electrical signals through a light beam. It uses light-emitting diodes (LEDs) to convert electrical signals into optical signals, which are then restored to electrical signals through photodetectors such as phototransistors, photodiodes, photoresponsive thyristors, etc. This "electric optical electric" conversion process establishes a physical isolation barrier between input and output.
It is precisely this core characteristic of electrical isolation that determines the key application scenarios of optocouplers in circuits. The following are typical circuit types that require the use of optocouplers and their reasons:
Isolation of strong and weak electrical interfaces (industrial control, equipment input/output):
Scenario: Microcontrollers (such as microcontrollers, PLCs) and low-voltage digital logic circuits need to control or monitor the status of high-voltage AC (such as 220VAC) or DC (such as 24VDC, 48VDC) equipment (such as motors, relays, solenoid valves, indicator lights).
Problem: Connecting strong electrical circuits (which may contain surges, noise, and high voltage) directly to sensitive weak electrical control circuits can easily cause damage to control chips, system malfunctions, and even safety accidents.
Optocoupler function: as a "security guard". The control signal drives the LED inside the optocoupler to emit light, and the internal photodetector (such as a phototransistor) conducts or cuts off on the other side of the isolation barrier, thereby safely controlling the relay, thyristor (Triac), or solid-state relay (SSR) to switch on and off strong electrical loads. Similarly, the status signals of high-voltage equipment (such as limit switches and buttons) can also be safely transmitted to the control circuit through optocouplers. Complete electrical isolation has been achieved between the control side (weak current) and the controlled side (strong current).
Feedback control loop of switch mode power supply:
Scenario: In isolated switching power supplies such as flyback and forward, it is necessary to feed back the voltage or current sampling signal of the secondary (output side) to the PWM controller of the primary (input side) to stabilize the output voltage.
Problem: Strict electrical isolation must be maintained between the primary (usually connected to a high-voltage DC bus) and secondary (outputting low-voltage DC) to meet safety regulations (such as IEC/UL certification), while precise feedback signals need to be transmitted.
Optocoupler function: as an "isolation messenger". The error amplifier on the secondary side (such as TL431) drives the LED of the optocoupler, and its brightness reflects the error of the output voltage. The primary side phototransistor receives the light signal, converts it into a current signal, and transmits it to the PWM controller (such as UC384x) to adjust the duty cycle of the switching transistor. While maintaining primary/secondary safety isolation, closed-loop voltage regulation has been achieved.
Communication interface isolation (RS-232/485/422, CAN, Profibus, etc.):
Scenario: When communicating over long distances or connecting devices from different power supply systems, common mode noise and ground potential difference (ground loop) are easily introduced on the communication line.
Problem: The ground potential difference can generate a huge loop current, leading to data errors and damage to the interface chip. Common mode noise can also interfere with signal quality.
Optocoupler function: as a "noise blocker". Place the optocoupler in series in the communication line (such as TxD, RxD). The signal at the sending end drives the optocoupler LED, while the photodetector at the receiving end reproduces the signal. Cut off the electrical connection (including ground wire) between device A and device B, eliminate the conduction path of ground loop and common mode noise, and improve the anti-interference ability and system safety of communication.
High/low side switch drive circuit (motor drive, power device drive):
Scenario: Driving power switches such as MOSFETs and IGBTs, especially in bridge circuits (such as H-bridges).
Problem: The source voltage of high-end switching devices will fluctuate significantly with the switching state (from near ground to near bus voltage). The driving signal needs to refer to this floating source potential, which exceeds the capability of ordinary logic level converters.
Optocoupler function (specific type): as a "floating driver". Some high-speed optocouplers (or combinations of optocouplers and integrated drivers) can provide sufficient driving current and voltage swing, and their output stage can be "suspended" at the source potential of the power transistor, providing effective gate driving signals for high-end switching transistors. Solved the level shift and isolation issues of high edge driving.
Isolation barriers for safety critical systems (medical equipment, testing and measuring instruments):
Scenario: Medical equipment (such as patient monitors) is connected to the sensor part of the human body, and testing and measuring equipment is connected to the part of the high-voltage/high current circuit being tested.
Problem: It is necessary to ensure multiple reliable isolation protections between the operator or the main body of the equipment and the hazardous voltage/current.
Optocoupler function: as a "safety barrier". Optocouplers are used on critical paths where sensor signals enter the main processing system or control signals output to actuators, providing reinforced insulation or double insulation that meets safety standards (such as IEC 60601-1) to protect personnel and equipment safety.
Logic level conversion and noise suppression (early or specific requirements):
Scenario: It is necessary to connect digital circuit modules that operate in different voltage domains (such as 3.3V logic and 5V logic) or in high noise environments.
Problem: Direct connection may result in incompatible voltage levels or noise coupling.
Optocoupler function: as a "level adapter/noise filter". The input and output of the optocoupler can be powered by independent power sources. The logic signal on the input side drives the LED, while the phototransistor on the output side operates under an independent power supply, generating a new logic level signal. While converting levels, it provides isolation between power and ground, effectively blocking the conduction of power and ground noise.
Summary: The core value of optocouplers lies in providing reliable electrical isolation.
Choosing to use optocouplers in circuits is usually to address one or more of the following key issues:
Safety: Prevent high voltage/high current from endangering personnel or low voltage control circuits.
Anti interference: Block ground loops, suppress common mode noise conduction, isolate noise sources.
Level fluctuation: Power devices that drive significant changes in reference point voltage (such as high side drives).
Signal integrity: Reliable transmission of digital or analog signals in noisy environments (with appropriate circuitry).
Compliance with regulations: Complies with electrical safety standards and electromagnetic compatibility requirements.
Although modern technology also provides other isolation solutions (such as magnetic coupling based on transformer principle and capacitive coupling based on capacitor principle), optocouplers are still an indispensable key component in many circuit designs that require electrical isolation due to their simple and intuitive principle, relatively low cost, and reliable isolation performance (especially for DC and low frequency).