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Advantages and disadvantages of using a 4 to 20 mA signal


The 4-20mA current loop is a common method of transmitting sensor information in many industrial process-monitoring applications – typically in systems monitoring pressure, temperature, pH, or other physical factors. These systems employ the familiar 2-wire, 4-20mA current loop, in which a single twisted-pair cable supplies power to a module and also carries the output signal.

Transmitting sensor information via a current loop is particularly useful when the signal has to travel long distances – 1,000 feet is not uncommon. 
The use of basic two-wire technology makes the installation both inexpensive and simple to wire, maintain, and troubleshoot.

The loop’s operation is straightforward: A sensor’s output signal is first converted to a proportional current, with 4mA normally representing the sensor’s zero-level output and 20mA representing the sensor’s full-scale output. A reading of 20 mA means that a valve, for example, is 100 % open, and a reading of 4mA means that it’s closed. Readings between the maximum and minimum values mean the circuit is controlling the valve.

Why is 4mA chosen as the lower threshold representing the ‘off’ or ‘closed’ position? The design takes into account that there must be a means of representing a fault caused by an open circuit or a lost feed. Therefore, a reading of 0mA is essentially “reserved” to signal a fault in the system, while a reading of approximately 4mA or 20mA would indicate a properly working circuit.

Key among the advantages of 4-20mA current loop technology is that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability.

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