US What a 4–20 mA signal is
4–20 mA is the standard analog signal used to send a measurement from a field instrument to a controller as a DC current. The instrument, called a transmitter, sets the current in the loop between 4 mA, which represents the bottom of its range, and 20 mA, which represents the top. A pressure transmitter ranged 0–10 bar, for example, sends 4 mA at 0 bar, 12 mA at 5 bar and 20 mA at 10 bar.
It is one of the most common analog signals in industry and is used for pressure, level, flow, temperature and position, as well as for sending setpoints to valves and drives. The range is defined in ISA-50.00.01 and IEC 60381-1, so a 4–20 mA transmitter from one maker works with an input from any other.
The loop is a simple series circuit: a DC power supply, usually 24 V, the transmitter and the receiver’s input are connected in a ring by a twisted pair. The receiver, a PLC, DCS, data logger or analog input card, measures the current and scales it back into engineering values.
Why industry uses current instead of voltage
A current loop has advantages that a 0–10 V signal does not, which is why it has been the default for process instrumentation since the 1970s, when it replaced 3–15 psi pneumatic signals.
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Cable resistance does not matter. In a series loop the same current flows everywhere, so the reading is unaffected by voltage drop along a long cable. Runs of hundreds of meters are routine.
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Noise immunity. Interference from motors and drives induces small voltages, which barely change a low-impedance current loop. Twisted, screened cable improves this further.
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Live zero. Because the bottom of the range is 4 mA, not 0 mA, a reading of 0 mA can only mean a broken wire or a dead transmitter. Faults are obvious.
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Two wires do everything. The 4 mA floor leaves enough current to power the transmitter’s electronics from the loop itself, so a two-wire, loop-powered transmitter needs no separate supply.
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Low energy. Small currents at 24 V suit intrinsically safe circuits in hazardous areas, with barriers or isolators limiting the energy.
Scaling a 4–20 mA signal
The signal is linear, so the conversion is a straight line: value = (current − 4) ÷ 16 × span + minimum. The table shows a 0–10 bar transmitter read through a 250 Ω resistor, which turns the loop into a 1–5 V signal for a voltage input.
| Loop current | Percent of range | Voltage across 250 Ω | Pressure (0–10 bar) |
|---|---|---|---|
| Below 3.6 mA | Fault (NAMUR NE 43) | Below 0.9 V | Sensor or wiring fault |
| 4 mA | 0% | 1.0 V | 0 bar |
| 8 mA | 25% | 2.0 V | 2.5 bar |
| 12 mA | 50% | 3.0 V | 5.0 bar |
| 16 mA | 75% | 4.0 V | 7.5 bar |
| 20 mA | 100% | 5.0 V | 10.0 bar |
| 21 mA and above | Fault (NAMUR NE 43) | Above 5.25 V | Over-range or transmitter fault |
NAMUR recommendation NE 43 sets the fault limits that many transmitters follow: normal readings stay between 3.8 and 20.5 mA, and the transmitter drives the loop to 3.6 mA or less, or 21 mA or more, to signal a fault.
Two-wire, three-wire and four-wire transmitters
Transmitters are classed by how they are powered. A two-wire transmitter is loop-powered: the same pair carries power and signal, which is the most common arrangement in process plants. A three-wire transmitter has a separate supply wire and a shared 0 V, usual for devices that need more than 4 mA to run. A four-wire transmitter has its own mains or DC supply and an isolated current output, used for analyzers and flowmeters with higher power needs.
For a loop-powered circuit, check the loop budget. The supply voltage must cover the transmitter’s minimum operating voltage plus the drop across the input resistor and the cable at 20 mA. With a 24 V supply, a transmitter needing 12 V and a 250 Ω input, which drops 5 V, there is around 7 V left, enough for roughly 350 Ω of cable.
HART and reading 4–20 mA into a computer
HART adds digital communication on top of the analog signal. It superimposes a small frequency-shift-keyed tone, 1,200 Hz and 2,200 Hz at 1,200 bit/s, on the loop current, so a handheld or host can read diagnostics and change the range without disturbing the 4–20 mA value. The protocol is now maintained by the FieldComm Group.
To bring 4–20 mA signals into a SCADA system or an edge computer, you need an analog input with a current range or a precision shunt, ideally isolated to break ground loops between instruments. In the other direction, several PC/104 analog output modules in our range have 16-bit outputs that can each be set to 4–20 mA, 0–20 mA or voltage ranges for valve and drive setpoints. Industrial IoT gateways then pass the scaled values on to SCADA or the cloud.
Quick answers on 4–20 mA
4–20 mA current loops: common questions
Why does 4–20 mA start at 4 and not 0?
To give a live zero. A 4 mA floor separates a genuine zero reading from a broken wire, and it leaves enough current to power a two-wire transmitter from the loop.
Why does it stop at 20 mA?
Mainly convention and low power. A 4:20 span keeps the same 1:5 ratio as the 3–15 psi pneumatic signals it replaced, and keeping the current low limits heating and suits intrinsically safe circuits.
Is 4–20 mA AC or DC?
DC. The loop runs on direct current, usually from a 24 V DC supply. HART adds a small AC tone on top, but the measurement itself is the DC level.
How do you convert 4–20 mA to 0–10 V?
With a resistor or a signal conditioner. A 250 Ω resistor gives 1–5 V and a 500 Ω resistor gives 2–10 V. For a true 0–10 V output, use a signal conditioner that removes the 4 mA offset.
How far can a 4–20 mA signal travel?
Often several hundred meters or more. Distance is limited by the loop budget, not by signal loss. As long as the supply can overcome the cable resistance at 20 mA, the reading at the far end is unchanged.
We supply analog I/O, gateways and industrial computers with the inputs and drivers configured as standard, and we can scope the I/O count and ranges from your instrument list.