Why 4-20mA signal is more usable? What is the origin of this popular range?

When pneumatics predominated in the industry and its control was referenced with ranges from 3 to 15 PSI, any value outside of that range was considered an alarm signal.

Most important events happened in relation to the industrial revolution 2.0, the integration of electronic control to pneumatics.

In this context, the easy installation and maintenance of the electrical wiring have attracted the industrialists, who suffered from the problems of pressure losses in the pneumatic lines and the energy expenditure represented by the continuous operation of those inefficient compressors.

Why 4-20 mA?

  • The electronic solution, with current loops 4 to 20 mA, quickly gained ground because of its
    1. greater precision,
    2. low power consumption, and
    3. algorithmic potential for complex controls (PID).
  • Zero signal (4mA) and fault can be easily identified.
  • Perhaps, not only this, it withstood two more revolutions in industrial technology, becoming a standard that seems to have no expiration date.
  • 4-20 mA signal and its power circuit are combined into one cable. 
  • With measuring devices, if zero offsets are found, it can be adjusted without the need for polarity change of signal.
  • The 4-20 mA range has equivalent to the 3-15 PSI range, used in pneumatics.
  • Considering Kirchhoff’s 1st law, the current measured at any point in the loop is always the same.
  • Therefore a 4-20 mA loop will have higher precision than any voltage signal.
  • 4-20 mA signal is more stable over long distances and more immune to electrical noise, electromagnetic or radio frequency interference.
  • Considering the 4 mA value as 0% of the signal, it is very easy to detect faults in the wiring or open circuit.
  • Passive 2-wire sensors are powered by the energy provided by the receiving equipment, improving the cost of installation.
  • Transmission of the HART digital data over the same cables without interference between both signals
    can be used for intrinsic safety signs in hazardous areas.
  • Easy fault measurement with a multi-meter (values less than 3.8 mA or greater than 20.5 mA).
  • The 20 mA current value is below the 30 mA threshold for electrical health risk.

 
General composition of a 4-20 mA loop:

Sensor


Measures a process variable delivering a proportional signal in the 4-20 mA range.

The sensor can be active or passive.

The active sensor has its own external power input (independent circuit) plus 2-wire 4-20 mA output that connects directly to the receiving equipment.
 
Passive 2-wire sensors (most of them) do not need an external power supply, since they are powered by the energy provided by the receiving equipment.
 

Current loop

A direct current (DC) circuit that sends signals from a sensor to a receiving device through two conductive wires.

Receiving equipment

Receives the information to analyze, store, or process variables, it can be a PLC, PID controller, SCADA, signal conditioning, etc.

Author: PSS Bapu Rao

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