Optoisolators
Optoisolators, also known as optocouplers or photocouplers, are electronic components that achieve electrical isolation based on optical signals. They use a combination of a light-emitting diode (LED) and a photosensor (such as a phototransistor or photodiode) to completely isolate the input circuit from the output circuit, allowing only one-way optical signal transmission, thereby preventing high voltage, noise, or interference from affecting the low-voltage system.
1. What are the Working Principles and Structure of Optoisolators?
Core Mechanism: When input current activates the LED, it emits infrared light. This light signal is transmitted through a transparent isolation layer to the photosensor, which converts the light energy into an electrical signal for output. This “electrical-optical-electrical” process ensures no direct electrical connection between the input and output circuits. The isolation resistance can exceed 10¹²Ω, effectively blocking transient interference up to 10kV.
Internal Structure: Typically enclosed in a light-tight housing, it contains an LED light source and a photosensor (such as a phototransistor or thyristor). The isolation layer is composed of air, glass, or plastic, enabling low-loss signal transmission (typical loss ≤ 0.2dB).
2. What are the Core Functions of Optoisolators?
Electrical Isolation: Separates high-voltage and low-voltage circuits, preventing high-voltage leakage into the low-voltage side, protecting sensitive components (such as MCUs), and serving as a “safety gate” in switching power supplies, inverters, and medical devices.
Interference Immunity: The high internal resistance of interference sources prevents them from providing sufficient current to drive LEDs, blocking noise signals (such as electromagnetic interference) at the input, ensuring signal integrity and system reliability.
Signal Transmission: Supports digital and analog signal transmission. Current Transfer Ratio (CTR) is a key parameter (typical CTR is 20-30%), affecting response speed and stability. CTR decreases in high-temperature environments, requiring design margins.
3. What are the Advantages and Disadvantages of Optoisolators?
Advantages: High isolation (up to 63dB), fast response, compact size (miniaturized diameter 3-4mm), and suitable for wide operating temperatures (-20°C to +70°C).
Disadvantages: LED drive current is low, making them susceptible to low-power interference and requiring software noise filtering. Traditional devices rely on temperature control, but integrated approaches (such as magneto-optical photonic crystals) are addressing these size and dependency issues.
4. What are the Types of Optoisolators?
Common types include phototransistors (such as the 4N35, used for general signal isolation), Darlington diodes (with a CTR of up to 500%, amplifying weak signals), photothyristors (such as the MOC3071, controlling AC circuits), and optical isolators (specifically for use in the 1310nm-1550nm optical fiber communication band).
5. What are the Applications of Optoisolators?
Power Supply Isolation: Prevents power supply interference when input and output voltages are inconsistent, protecting circuit boards (such as modems and lightning arresters).
Communication Systems: Prevents optical feedback in optical fiber amplifiers and lasers, ensuring unidirectional transmission; and filters atmospheric interference in data transmission, such as in CAN buses.
Industrial Control: Isolates high dV/dt noise in inverters and PLC systems, reducing the error rate (case studies have shown this can be reduced to 0.3%).
Level Conversion: Enables passive conversion between 3.3V and 5V systems with microsecond latency.