How To Make 4-20 mA Current LoopMeasurementsIt seems that at least one 4-20 mA (milliamp) measurement is required by our typical customer, andthe way to do it is a constant source of confusion for many. So I thought I’d zero in on the various 4-20 mA current loop configurations and elaborate on the specifics that you need to know to make asuccessful measurement. The following discussion is ordered from the most to least commonconfiguration, and I hope to cover all those that I have encountered in customer applications. Ifyours isn’t included, please use the comments section to fill me in.

4-20 mA Current Loop BasicsSensors or other devices with a 4-20 mA current loop output are extremely common in industrialmeasurement and control applications. They are easy to deploy, have wide power supplyrequirements, generate a low noise output, and can be transmitted without loss over great distances.We encounter them all the time in both process control and basic measurement data logger and dataacquisition applications.

The idea behind 4-20 mA current loop operation is that the sensor draws current from its powersource in direct proportion to the mechanical property it measures. Take the example of a 100 psisensor with a current loop output. With 0 psi applied, the sensor draws 4 mA from its power source.With 100 psi applied the sensor draws 20 mA. At 50 psi the sensor draws 12 mA and so on. Therelationship of mechanical property measurement to current output is almost always linear, allowingthe resulting current loop data to be scaled with a simple mx+b formula to reveal more usefulmeasurements scaled into engineering units.

How you actually measure the 4-20 mA current loop signal is a function of the sensor’s architectureand the capabilities of the instrument you’ll use for the measurement.

TerminologySo that my discussion translates well across the various kinds of 4-20 mA current loopconfigurations, I’ve opted to standardize the terminology I use to describe each. Here’s an overview:

“E” (dc excitation)Most configurations that follow will show a DC voltage excitation source that I denote as “E”. Manywho use current loop sensors for the first time are surprised to learn that they need to supply thisexcitation source. Nonetheless, unless the sensor is self-powered (i.e. AC line powered) an externaldc source is required. The good news is that this can sometimes be supplied by the instrument, andthe range of acceptable values is usually very wide, typically 10-24 V dc.

“R” (shunt resistor)Here’s a bit of trivia for you: No instruments measure current directly. They all do it indirectly bymeasuring the voltage dropped across a resistor of known value, and then they use Ohm’s Law tocalculate actual current. The resistor is referred to as a “shunt”, is absolutely required to make acurrent measurement, and is either supplied externally to, or built into the measuring instrument.

For clarity, I assume that it’s supplied externally.

“i” (current loop value ranging from 4-20 mA)This is the 4-20 mA current signal generated by the sensor. Note that some sensors may draw 0-20mA and even other values, but the vast majority of them use the 4-20 mA convention.

“v” (shunt voltage that’s proportional to current)This is the voltage actually measured by the instrument. Since our industry has standardized on ashunt value of 250 Ohms, “v” will range between 1 and 5 volts for a 4-20 mA current loop signal (v=i* resistance). Note that shunt resistor value is arbitrary as long as it’s known. You also need toensure that it doesn’t burden the loop, so lower values are better than higher. Yes, I mean lower.Remember that we’re working with current, not voltage, so the rules are inverted. Just as infinitely-high resistor loads work well for a voltage source, you can take the load all the way to zero Ohms fora current source without consequence.

Self-powered SensorsI promised to order these configurations from mostto least common, and the self-powered sensor justnoses out the first runner up. Self-powered sensorsare those that, well, power themselves. The sensormay have an integral ac power supply, therebynegating the need for an external DC power source.Or it may not be a sensor at all. It could be an outputfrom a PLC or other source that is internallypowered.

2-wire Sensors (Low-side Shunt)Okay, this can get confusing for first-time 4-20 mAcurrent loop users. Yes, it is possible to both powerthe sensor and measure the current it draws over thesame two wires. In the 2-wire examples shown here,only two wires connect the sensor to its powersupply, and the sensor draws current from it indirect proportion to the mechanical property that itmeasures. As current changes, the voltage developedacross resistor R will change, thus providing a signalthat’s suitable to connect to a measuring instrument like a data logger or data acquisition system.

In most situations, care should be taken to place the resistor in the low-side of the loop as shownhere, as opposed to the high-side. Doing so will allow non-isolated instruments to make themeasurement. In the next section, I’ll deal with a high-side shunt placement and discuss thesecautions in more detail.

2-wire Sensors (High-side Shunt)This configuration is almost exactly like the low-side,2-wire approach, but it places the shunt resistor inthe high-side of the loop. Note that while the voltageacross the resistor is proportional to the currentdrawn by the sensor (just like the low-sideapproach), there is also a common mode voltage(CMV) present on either side to ground. On one sideto ground the CMV is equal to the supply voltage. Onthe other side to ground it’s equal to the supply voltage, less the voltage dropped by the resistor (v).The presence of the CMVs places conditions on the instrument that you use to measure v. Specially,the instrument needs to have an isolated front end so it can float to the level of the CMV and stillsuccessfully make the measurement. Try this with a non-isolated, single-ended instrument and youwill short-circuit the sensor to ground. A non-isolated differential instrument will either saturate orprovide erroneous results.

3-wire SensorsThree-wire sensors with a process current outputhave a separate wire for ground, signal (4-20 mA),and the power supply. This configuration is theeasiest for current loop beginners to grasp, one inputfor power and a second for the current loop with acommon ground. The primary advantage of a 3-wiresensor over its 2-wire counterpart is its ability todrive higher resistive loads. Resistors drop voltagefor any given current in direct proportion to theirresistance value. Holding current constant, higher resistances drop more voltage. Turning back tothe 2-wire sensor and holding current constant, as the shunt resistance increases the voltage dropacross the sensor also increases. You might reach a point where the voltage dropped by the shuntlowers the voltage drop across the sensor below the minimum required for it to operate properly.

We had a customer whose 2-wire current loop measurements functioned beautifully until loopcurrent reached about 18 mA, at which point everything went haywire. Upon close examination, wedetermined that the supply voltage she used was too low by at least 0.56-V. She needed 2 mA moremeasurement to reach full scale, which translates to 0.56 V with her 250-Ohm resistor. The solutionwas to use a higher voltage power supply to ensure that the voltage drop across the sensor stayedabove the minimum level. She could have also used a 3-wire sensor, which ensures that the voltageapplied to the sensor is independent of shunt resistor voltage drop.

Watch Your Grounds (or use an isolatedinstrument)Contrary to what many believe (and have been erroneously taught in school), grounds are almost

never the same in industrial settings, exactly where most 4-20 mA current loop sensors are used.Two or more grounds that are the same means that they are at the same potential. If so, ameasurement between the grounds of the various field sensors and the instrument using a digitalvolt meter (DVM) on both its DC and AC settings will show zero volts, or very close to it. In reality,you’ll measure at least several volts, and I’ve seen as much as 75 Volts. When grounds that are notat the same potential are tied together (which you need to do to make the measurement), currentflows through them, creating several possible measurement outcomes for non-isolated instruments:

The measurement is noisy.1.The measurement is inaccurate.2.You irreparably damage the instrument.3.You saturate the instrument (it’s not damaged, but you can’t make a successful measurement,4.either.)

To remedy these problems requires the following:

Use an isolated instrument for your 4-20 mA current loop measurements. This single decision1.allows you to ignore all other grounding issues in exchange for successful measurements in anysituation. If you don’t have an isolated instrument, read on…Ensure that the loop power source is isolated. This means that its output ground (the one2.connected to the sensor) is not tied to its input ground (the one that connects to AC line power.)An isolated power source means that the output ground can be tied to another ground (like a non-isolated instrument) without consequence.In self-powered applications, ensure that the low-side of the loop is isolated from its power3.source.If you lack control over the power sources and determine that they are not isolated, then your4.only option is to power ALL devices (power supplies, self-powered sensors, the instrument, and itsconnected PC) from exactly the same power outlet. Don’t make the mistake of using outlets thatare close to each other. If you run out of receptacles on a single outlet, then use a power strip.

Again, it’s worth repeating that all of the cautions associated with proper grounding disappear if anisolated instrument is used to make the measurement.

Sensors with 4-20 mA outputs are encountered in all disciplines and in many configurations. Contactus with any questions that arise in your unique situation.

Additional Reading:4-20 mA Current Loop Products

4-20 mA Current Loop Measurement Resolution Calculations Made Easy

4-20mA Current Loop Data Acquisition

DI-8B32-01 4-20 mA Current Loop Amplifier

DI-8B42-01 2-wire Current Loop Amplifier with Power Supply