Current loop

In electrical signalling an analog current loop is used where a device must be monitored or controlled remotely over a pair of conductors.

These loops have the advantages of simplicity and noise immunity, and have a large international user and equipment supplier base.

Various automation protocols may replace analog current loops, but 4–20 mA is still a principal industrial standard.

The key advantages of the current loop are: Field instrumentation measurements include pressure, temperature, level, flow, pH or other process variables.

The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA.

Panel mount displays and chart recorders are commonly termed "indicator devices" or "process monitors".

The transition was gradual and has extended into the 21st century, due to the huge installed base of 3–15 psi devices.

To allow the construction of hybrid systems, where the 4–20 mA is generated by the controller, but allows the use of pneumatic valves, a range of current to pressure (I to P) converters are available from manufacturers.

These circuits require end-to-end direct current (DC) continuity, and unless a dedicated wire pair was hardwired, their use ceased with the introduction of semiconductor switching.

A current loop remote control saves the cost of extra pairs of wires between the operating point and the radio transceiver.

An alternative type, the tone remote, is more complex but requires only an audio path between control point and base station.

The office would have a remote control unit that would operate the taxi company base station over a current loop circuit.

For example, imagine one possible scheme where the presence of these currents cause the base station to change state: This circuit is polarity-sensitive.

If a telephone company cable splicer accidentally reversed the conductors, selecting channel 2 would lock the transmitter on.

The voice of the user in the dispatch office would be modulated and superimposed over the DC current that caused the transmitter to operate.

Showing the evolution of analogue control loop signalling from the pneumatic era to the electronic era
Example of current loops used for sensing and control transmission. Specific example of a smart valve positioner is shown.
Control valve with pneumatic diaphragm actuator and "smart" 4–20 mA positioner which will also feed back the actual valve position and status over the current loop
A Motorola T-1300 series remote control is built in a telephone housing. The dial is replaced with a speaker and volume control. This remote control uses a two-wire circuit to control a base station .