4-20mA current loop: how the signal works

A 4-20 mA current loop is the most common way for an industrial sensor to report one measurement to a controller. A pressure, level, flow or temperature transmitter sets the current flowing around a pair of wires somewhere between 4 and 20 milliamps, in proportion to what it is measuring, and the receiving device, usually a PLC analog input, measures that current and converts it back into a value. It has been the standard analog signal in process plants for decades, and it is still how a large share of field instruments are wired. This guide explains how the loop works, how to size and wire it, what the fault levels mean, and how to read a loop into an IoT system. The industrial IoT hub collects the guides on the rest of the plant data chain.

What a 4-20 mA signal is

The signal is a direct current whose size represents a measurement on a straight-line scale. The lowest value of the instrument’s configured range, its lower range value, is sent as 4 mA. The highest, its upper range value, is sent as 20 mA. Everything between maps proportionally: the 16 mA span between them represents 0 to 100 per cent of the range.

So a level transmitter set up for 0 to 5 meters sends 4 mA when the tank is empty, 12 mA at 2.5 meters and 20 mA when the level reaches 5 meters. The receiver needs to know the same range to turn the current back into meters; the current itself carries no units.

The standard grew out of pneumatic instrumentation, where a 3 to 15 psi air pressure carried the same kind of signal, and it was standardized by the International Society of Automation in the 1970s. The ratio of 4 to 20 matches the 3 to 15 psi ratio, which made the move from air to electricity easier for the people who had to read both.

Why current, and why 4 mA

Current does not change along the wire. In a series loop the same current flows through every part of it: the supply, the cable, the transmitter and the receiver. The cable’s resistance causes a voltage drop, but it cannot change the current, so the reading at the receiver is the reading the transmitter sent, whether the cable is ten meters or a few hundred. A voltage signal, by contrast, loses part of itself in every meter of cable.

Current signals resist noise. The loop has a low impedance, so electrical noise picked up from nearby motor cables produces much less error than it would on a high-impedance voltage input.

4 mA is a live zero. Because the bottom of the range is 4 mA, a reading of 0 mA can only mean something is wrong: a broken wire, a failed transmitter or a lost supply. A 0 to 20 mA signal could not tell a broken wire from a genuinely empty tank.

4 mA is enough to power the instrument. A two-wire transmitter takes its own operating current from the loop. As long as its electronics run on less than about 4 mA, it can be powered and send its signal over the same two wires, with no separate power cable.

Two-wire, three-wire and four-wire transmitters

Two-wire (loop powered). The most common arrangement for pressure, level and temperature transmitters. The transmitter sits in series in the loop, takes its power from it, and regulates how much current flows. The loop needs an external supply, usually 24 V DC, which may be inside the PLC’s input card or a separate power supply.

Three-wire. The transmitter has a supply wire, a common and a signal wire. It is used where the instrument needs more power than 4 mA can provide, such as some flow meters and analyzers, and the signal current returns through the shared common.

Four-wire. The instrument has its own mains or DC supply on two wires and a separate two-wire 4-20 mA output that it drives itself. The output is said to be active, or sourcing, because the instrument provides the loop power.

The receiving input has to match. Analog input cards and gateways describe their inputs as sourcing or sinking, or as powering the loop or not. A two-wire transmitter needs something in the loop to supply power; a four-wire instrument with an active output must not be connected to an input that also supplies power. The datasheets of both ends say which they are.

The loop power budget

A loop only works if the supply voltage covers everything the current has to pass through at its highest value, 20 mA. Three things take a share:

• The transmitter’s minimum operating voltage, from its datasheet.
• The receiver’s input resistance, often 250 ohms, sometimes lower.
• The cable’s resistance, for both conductors, there and back.

Add any other devices in series, such as a loop-powered display or a barrier. Here is a worked example with illustrative values: a transmitter that needs at least 12 volts, a 250 ohm input, and a cable whose two conductors add up to 20 ohms.

• At 20 mA, the input resistor drops 20 mA × 250 Ω = 5 volts.
• The cable drops 20 mA × 20 Ω = 0.4 volts.
• The transmitter needs 12 volts.
• Total needed: 17.4 volts, inside a 24 volt supply with margin to spare.

The same sum, rearranged, gives the largest total resistance a loop can have: subtract the transmitter’s minimum voltage from the supply and divide by 0.02 amps. With 24 volts and a 12 volt transmitter that is 600 ohms, shared between the receiver, the cable and anything else in the loop. Transmitter datasheets often show this as a load line chart. Long runs with thin cable, or adding a second display, are the usual reasons a loop that worked on the bench stops reaching 20 mA in the field.

Scaling a reading

The receiver turns current into a value with a straight-line formula:

value = lower range value + (current − 4 mA) ÷ 16 mA × (upper range value − lower range value)

For a pressure transmitter ranged 0 to 10 bar, a current of 12 mA gives 0 + (12 − 4) ÷ 16 × 10 = 5 bar, and 7.2 mA gives 2 bar. A PLC or gateway usually does this scaling in its input configuration, where you enter the range once.

The range is set in the transmitter, not in the signal, so the two ends must agree. When a transmitter is re-ranged in the field and the PLC configuration is not updated, the loop keeps working and every reading is wrong. Recording the range on the loop drawing and in the gateway’s tag names avoids that.

Fault levels: NAMUR NE 43

A current outside the normal range is information too. Many transmitters follow NAMUR recommendation NE 43, from the user association of the process industry, which sets aside the currents just outside 4 to 20 mA for specific meanings.

Between 3.8 and 20.5 mA the transmitter is measuring; readings slightly below 4 or above 20 mean the process is a little outside the configured range. At or below 3.6 mA, or at or above 21 mA, the transmitter is reporting that it has failed. Most transmitters let you choose whether a fault drives the output low or high, and the control system should treat both as faults rather than as real readings.

For an IoT system, that is useful information to keep. A gateway that scales every current blindly will report a broken wire as a tank 25 per cent below empty. Pass the raw current, or a fault flag, through with the scaled value.

HART: digital data on the same wires

Many 4-20 mA transmitters also speak HART, a protocol maintained by the FieldComm Group that adds a digital signal on top of the analog current. It uses two tones, 1,200 and 2,200 hertz, at 1,200 bits per second. The tones average out to zero, so they do not disturb the analog reading.

HART lets a handheld communicator or an asset management system read the transmitter’s tag, range, diagnostics and extra variables, and change its configuration, without breaking the loop. For IoT projects, HART multiplexers and HART-capable gateways can collect those diagnostics from instruments that are already installed, which is one way to add condition monitoring without new sensors.

Isolation and ground loops

A 4-20 mA loop should be grounded at one point only. When the transmitter’s body and the receiver are both connected to ground at different places, and those grounds sit at slightly different voltages, an unwanted current can flow through the signal wiring and shift the reading. This is a ground loop, and it shows up as an offset or as readings that change when large machines start.

The fixes are to ground the loop at one end, usually at the receiver, to use isolated analog inputs, or to add a signal isolator in the loop. Isolators are also used to split one signal to two receivers, such as a PLC and a separate recorder. The cable is normally a shielded twisted pair, with the shield grounded at one end.

Reading a 4-20 mA loop into an IoT system

There are four common ways to get a 4-20 mA reading into a dashboard or historian.

1. From the PLC. If the transmitter already feeds a PLC analog input, read the scaled value from the PLC over OPC UA, MQTT or Modbus TCP rather than wiring anything new. The PLC guide covers how PLC data reaches an IoT system.
2. An analog input on a gateway. Many industrial gateways have 4-20 mA inputs. The gateway measures the current, scales it and publishes the value, as described in the IoT gateway guide.
3. A 4-20 mA to Modbus converter. A small module measures one or several loops and presents the values as Modbus registers on RS-485, which a gateway then polls. The Modbus RTU guide explains the registers it will expose.
4. A wireless transmitter. Battery or loop-powered modules read a 4-20 mA signal and send it over LoRaWAN or cellular, which suits remote tanks and pumps with no cable to the control room. See the LoRaWAN guide.

To read a loop with a microcontroller for a prototype, put a precision resistor in series with the loop and measure the voltage across it with the board’s analog-to-digital converter. With a 3.3 volt converter, a 150 ohm resistor turns 4 to 20 mA into 0.6 to 3.0 volts, which stays inside the input range. Use a resistor with a tight tolerance, check the converter’s real accuracy, and remember the board must never become the loop’s path to ground at a second point. A permanent installation is better served by an isolated input module. The IoT sensors guide covers analog sensor outputs more generally, and the microcontroller comparison the boards.

When you add monitoring to an existing loop, be careful not to disturb it. Inserting a new resistor adds to the loop budget, and opening the loop to wire it in interrupts the signal the control system is using. A clamp-on milliamp meter reads the current without breaking the circuit, and HART or the PLC often has the value already.

4-20 mA vs 0-10 V

0-10 V is the other common analog signal, especially in building automation, where cables are shorter. It needs three wires for a powered sensor, loses accuracy with cable length, picks up noise more easily, and has no live zero, so 0 V could be a real reading or a broken wire. It is simpler and cheaper for short runs inside a panel or a room. For anything that travels across a plant, or where a broken wire must be detected, 4-20 mA is the safer choice.

For the parts on either side of the loop, read what an industrial IoT system is made of, and browse the rest of the industrial IoT guides.