In the world of industrial controls, one of the most common analogue signalling schemes is the 4-20mA current loop illustrated in Figure 1. A transducer, on the left, measures some quantity (temperature, pressure, flowrate etc) and transmits that value from the point of measurement to a remote receiver (display, process controller etc) by controlling the loop current.
The remote transducer on the left regulates the loop current in proportion to the quantity being measured, with 4mA representing a zero reading and 20mA full scale. The sensor converts the current back to a voltage for display or other use.
The receiver measures the current, usually by converting it to a voltage across a resistor as shown. Power is supplied to the loop by a power supply, typically at 24VDC, that may be part of the receiver or a standalone supply. The transducer zero and full-scale values are represented by currents of 4mA and 20mA respectively.
This system has been around for a long time and is very widely used for a number of reasons. Being relatively low impedance (a few hundred Ohms typically) is relatively immune from induced electrical noise. It’s also suitable for long cable runs since cable resistance (up to a point) is immaterial.
If you are new to this system, you may wonder why a zero-scale reading is represented by 4mA loop current and not zero. There are two big advantages; It allows the receiver to discriminate between a zero-reading and no reading at all – for example an open circuit or faulty transducer, and it allows for the transducer to be powered by the loop itself as we shall see below.
The system is very easy to configure and troubleshoot – a simple multi-meter is usually all that is necessary, although specialist test equipment is available (like the Fluke 787B process multi-meter) which can source and measure 4-20mA signals.
The disadvantage of the 4-20mA current loop is that there can be only one transmitter on the loop (there can of course be multiple receivers) so each transducer requires its own dedicated wiring. If there are many process variables to measure, the wiring can get complex and ground loops can become a problem.
We mentioned that 4-20mA transducers can be loop-powered. Figure 2 shows a simplified diagram of how this can work. A voltage to current converter converts the sensor output to a 0 to 16mA current. In parallel a fixed 4mA current source provides the power supply via a shunt regulator.
Using 4mA to represent a zero reading has the advantages that no signal (open circuit) can be differentiated from a zero signal and that the transducer can be loop-powered as shown here. The voltage to current converter provides a 0-16mA signal in response to the sensor value, while the 4mA current source is used to provide a power supply.
While you can build a circuit like this fairly easily, there are chips on the market which will do this for you, like for example the AD693 shown in Figure 3. In addition to powering itself, this chip can provide up to 3.5mA to power to the transducer.
The AD693 transducer chip is a typical example of the integrated solutions available for 4-20mA transducers. It provides flexible signal conditioning, voltage to current conversion and a loop-powered supply for the transducer.
On the receive side we need a difference amplifier to amplify the voltage across the load resistor as shown in Figure 4, as well as an offset to return the signal to a zero-based one. We need a difference amplifier since there is no guarantee one end of the sense resistor is connected to signal ground. Note that the input impedance, R1, of the difference amp must be much higher that the 100Ω sense resistor.
The 4-20mA receiver simply measures the voltage across a sense resistor (here 100Ω) and adds an offset to account for the 4mA zero-signal current. A difference amplifier is required because neither end of the sense resistor is likely to be at signal ground potential. R1 must be much larger than 100Ω to minimise errors.
Again, there are chips available to do this for us, such as the RCV420 from TI, shown in Figure 5. This IC has a built-in reference that can be used to offset the and can be used in a variety of configurations.
The RCV420 is a typical example of an integrated 4-20mA receiver. It uses a difference amplifier to measure the loop current across the two 75Ω the and includes a voltage reference generator and offset circuity.
The 4-20mA current loop has been the standard for analog signal transmission in the industrial environment for more than 70 years and, judging by number of transducers that support it today, will be around for a bit longer yet!
Bibliography
“Current Loop.” In Wikipedia, January 12, 2024. https://en.wikipedia.org/w/index.php?title=Current_loop&oldid=1195114747.
“Fluke 787B ProcessMeter™ Digital Multimeter & Loop Calibrator.” Accessed March 7, 2024. https://www.fluke.com/en-us/product/calibration-tools/ma-loop-calibrators/fluke-787b.
“AD693 Datasheet and Product Info | Analog Devices.” Accessed March 11, 2024. https://www.analog.com/en/products/ad693.html.
“RCV420 Data Sheet, Product Information and Support | TI.Com.” Accessed March 11, 2024. https://www.ti.com/product/RCV420?utm_source=google&utm_medium=cpc&utm_campaign=asc-null-null-GPN_EN-cpc-pf-google-soas&utm_content=RCV420&ds_k=RCV420&DCM=yes&gad_source=1&gclid=CjwKCAiA0bWvBhBjEiwAtEsoW6S8fWFw80W2mgIS6W5tEg90wtfl07iTcp9hkJs0KNW5yQ2VbUvDcxoCNesQAvD_BwE&gclsrc=aw.ds.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
