The 4-20 mA signal is a widely used analog standard in industrial control systems, particularly for transmitting process variables over long distances. This current-based signal is preferred because it is more immune to electrical noise and voltage drops compared to voltage signals. In this system, 4 mA typically represents the zero point, while 20 mA represents the full scale. The use of current instead of voltage ensures that the signal remains stable and accurate, even over extended wiring lengths.
The 4-20 mA signal is standardized by the International Electrotechnical Commission (IEC) and has been adopted globally, including in China through the DDZ-III series of electric instruments. While some systems use 1-5 V DC as a voltage signal, the 4-20 mA standard is often used for transmission, with the receiving end converting the current into a voltage for processing. This current-voltage conversion method enhances reliability and reduces signal degradation during transmission.
One of the key advantages of the 4-20 mA signal is its resistance to electromagnetic interference. Since current is transmitted rather than voltage, the signal is less affected by external noise. Additionally, the 4 mA lower limit helps distinguish between actual zero values and potential sensor drift or mechanical issues. It also allows for two-wire systems where the same pair of wires provides both power and signal transmission, making it ideal for remote or hard-to-reach locations.
The 4-20 mA signal is commonly used in applications such as temperature, pressure, and flow measurement in industrial environments. Its robustness and simplicity make it a reliable choice for automation and monitoring systems. For instance, in a two-wire configuration, the 4 mA level can supply power to the sensor, eliminating the need for separate power lines. This design not only simplifies installation but also improves system efficiency and reduces maintenance costs.
When setting up a 4-20 mA system, it’s important to use shielded twisted-pair cables, especially for long-distance transmissions, to minimize interference. Proper grounding is also essential to ensure signal integrity. Calibration and zeroing of the system are necessary to maintain accuracy and consistency across the entire measurement range. These steps help ensure that the 4-20 mA signal delivers precise and reliable data in real-world applications.
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