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4-20 mA is an analog electrical transmission standard for industrial instrumentation.
The symbol "mA" is standard SI notation for milliampere, or 1/1000 of an ampere. The signal is a current loop where 4 mA represents zero percent signal and 20 mA represents the one hundred percent signal.
The reason zero is at 4 mA and not 0 mA is that this "live zero" allows the receiving instrumentation to differentiate between a zero signal and a broken wire or a dead instrument. This standard was developed in the 1950s and is still widely used in industry today, even though many attempts have been made to replace it with digital forms of communication such as fieldbus and Profibus. Its benefits of being a widely followed standard, low cost, its reliability and immunity to electrical noise keep it in regular use. Current loop is also much easier to understand and debug than more complicated digital fieldbuses. Using fieldbuses and solving related problems usually requires much more education and understanding than required by simple current loop solutions. Additional digital communication to the device can be added to current loop using HART Protocol.
See also
4~20mA is the industrial standard because 4mA gives a "Live Zero" just like the 3~15 psi pneumatic signal does as opposed to 0~12 psi. By using 4mA as zero it is easy to troubleshoot and check the signal transmission by being able to distinguish a "zero" signal from a dead instrument or disconnected wire. But if a 0~16mA signal is used then its not possible to check whether a signal is being transmitted at its low level or not.
External links
Further reading
- Lipták, Béla G. Instrumentation engineers' handbook. Process Measurement and Analysis. CRC Press. 2003. HB. ISBN 0-8493-1083-0
"4-20 mA"