The 4–20 mA convention was born in the 1950s out of the earlier highly successful 3–15 psi pneumatic control signal standard which had the same features of being able to power some remote devices and have a “live” zero. With the advent of faster, networkable protocols, why has a 1950s protocol persisted?
🔌 In so called “loop powered” devices, the loop can power the remote device, with power supplied by the controller, removing need for power cabling.
🔬 Measurement of absolute zero is difficult and expensive as it requires circuits that can measure to their negative rail accurately. Until the recent release of rail to rail amplifiers, this has been especially difficult.
⚠ It is self-monitoring with currents less than 3.8 mA, or more than 20.5 mA taken to indicate a fault.
📐 The accuracy of the signal is not affected by voltage drop in the interconnecting wiring.
🎺 It has high noise immunity, as it is low-impedance circuit, usually through twisted-pair conductors.
📡 It can be carried over long cables up to the limit of the resistance for the voltage used.
📺 Inline displays can be inserted and powered by the loop, if total allowable loop resistance is not exceeded.
⚡It is easy to convert to a voltage using a resistor.
Senquip ORBs and QUADs interface with 2-wire “loop powered” devices or 3-wire powered devices. To find out more, visit Senquip.com.
