Current loops For industrial process control Instruments
For industrial process control instruments,
analog 4–20 mA and 10–50 mA current loops are commonly used for analog
signaling, with 4 mA representing the lowest end of the range and 20 mA
the highest. The key advantages of the current loop are 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. The live-zero represented by 4 mA
allows the receiving instrument to detect some failures of the loop, and
also allows transmitter devices to be powered by the same current loop
(called two-wire transmitters). Such instruments are used to measure
pressure, temperature, flow, pH or other process variables. A current
loop can also be used to control a valve positioner or other output
actuator. An analog current loop can be converted to a voltage input
with a precision resistor. Since input terminals of instruments may have
one side of the current loop input tied to the chassis ground (earth),
analog isolators may be required when connecting several instruments in
series.
Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. (A 4-wire instrument has a power supply input separate from the current loop.) Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device).
Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).
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. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum
Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. (A 4-wire instrument has a power supply input separate from the current loop.) Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device).
Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).
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. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum
