The core function of optocouplers is to achieve the transmission of electrical signals under the premise of electrical isolation.
This seemingly simple sentence contains the two most critical value points of optocouplers:
Electrical isolation: This is the foundation of optocouplers.
Signal transmission: This is the core function of optocouplers.
Detailed explanation of its function:
Electrical isolation for signal transmission:
Core mechanism: The optocoupler contains a light-emitting device (usually an infrared LED) and a photosensitive device (such as a phototransistor, photodiode, photosensitive thyristor, etc.) inside. The two are encapsulated in an insulated casing, physically completely isolated and without direct electrical connection.
Signal transmission path: The electrical signal at the input drives the light-emitting device to emit light ->the light passes through the insulating medium ->the photosensitive device receives the light and generates the corresponding electrical signal output.
Isolation effect: There is no common ground reference point (common ground) between the input circuit and the output circuit. The potential difference (voltage) between them is effectively blocked by the insulating medium in between (usually air, plastic, or silicone). This means that voltage fluctuations, noise, and even high voltage at the input end will not be directly transmitted to the output end.
Block the transmission path of interference signals (noise):
Root cause of the problem: In complex electronic systems, the ground potential of different parts (such as digital circuits, analog circuits, strong current control, weak current sensing) may not be consistent ("ground bounce"), or there may be strong noise sources (such as motors, relays, switching power supplies).
Optocoupler scheme: Due to the physical isolation between the optocoupler input and output, the ground loop is completely cut off. The noise or ground interference at the input end cannot be directly transmitted to the sensitive receiving circuit through electrical connections, greatly improving the system's anti-interference ability and signal transmission stability. Especially in noisy environments such as industrial control and long-distance transmission, it plays a significant role.
Protect low voltage/sensitive circuits from high voltage damage:
Application scenario: In systems that require control of high voltage and high current devices such as motors and AC power supplies, low-voltage logic circuits such as microcontrollers and sensors are very fragile.
Safety barrier: The optocoupler acts as a reliable safety barrier. Even if the circuit controlling high voltage fails (such as short circuit, overvoltage), due to the existence of physical isolation, the high voltage or high current generated by the fault cannot directly impact the low-voltage control circuit at the back end, effectively protecting the safety of core control devices and operators. This is crucial in fields such as switching power supplies, motor drives, and medical equipment.
Implement signal interfaces between systems of different levels:
Level difference: Different circuit modules may operate at different voltage levels (for example, a 5V microcontroller needs to control a 24V relay coil, or a 3.3V circuit needs to communicate with a 12V circuit).
Interface Bridge: The input and output sides of the optocoupler can be powered by independent power sources. The input side uses the voltage of the source system to drive the LED, and the output side can use its own voltage to provide working bias to the photosensitive device according to the requirements of the target system. In this way, optocouplers can achieve signal transmission between circuits of different voltage levels without affecting the operating voltage of their respective systems, serving as a bridge for level conversion.
Enhance system security (meet security standards):
Safety standards: Many electrical safety standards (such as IEC/UL, etc.) require that in certain applications (such as equipment connected to AC mains), the parts accessible to users (low voltage side) must have reliable electrical isolation from the dangerous voltage parts (high voltage side).
Certification guarantee: Optocouplers (especially specific types such as reinforced insulation) are designed to meet these safety isolation requirements, and their insulation medium can withstand specific high voltage tests (such as withstand voltage tests of several thousand volts), providing the necessary isolation level to comply with safety regulations and reducing the risk of electrical contact.
In summary, the core value of optocouplers lies in their clever use of the conversion process from electricity to light to electricity, establishing a sturdy "insulation wall" between the input and output circuits. This wall not only blocks the direct transmission of harmful voltage, current, and noise, protecting personnel and equipment safety, but also allows useful control signals and information to pass through smoothly, achieving reliable communication between circuits of different potentials and levels. It is one of the key components for achieving safe, reliable, and anti-interference signal transmission in modern electronic systems.