How a 4–20 mA current loop carries one reading from an instrument to a controller.
The instrument sets the current flowing around a loop of two wires, and the controller measures that current. The same current flows through every part of a series loop, so the reading does not change with cable length, wire resistance, or small voltage drops at terminals, as long as the loop supply has enough voltage to drive it. A voltage signal, by contrast, loses accuracy with every ohm of wire and picks up noise more easily.
The controller usually reads the current by passing it through a small resistor and measuring the voltage across it. A 250 Ω resistor turns 4–20 mA into 1–5 V.
The range starts at 4 mA, not 0, for two reasons:
The instrument maps its measuring range onto 4–20 mA, and the controller must be set to the same span. For example, a PTSA probe scaled 0–200 PPB reads 4 mA at 0 PPB, 12 mA at 100 PPB, and 20 mA at 200 PPB — see Configuring a PTSA Probe.
The relationship is a straight line: reading = low end + (current − 4 mA) ÷ 16 mA × (high end − low end). If the instrument and the controller are set to different spans, the reading is wrong everywhere except where the two lines happen to cross, and nothing on the loop will flag it.
Something in the loop has to supply the voltage that drives the current. Instruments come in three wiring types:
Check whether the controller’s input supplies loop power (an active or sourcing input) or only measures current (a passive input). Wiring a four-wire instrument into an input that also supplies power, or a two-wire instrument into a passive input with no supply in the loop, gives no reading.
Many instruments need an external supply, commonly 24 V DC — check the instrument’s specification.
Sizing the loop supply. The supply must cover the instrument’s minimum operating voltage plus the voltage dropped across everything else in the loop at full current:
Allow some headroom above 20 mA: an instrument signalling a fault may drive the loop to about 21–22 mA, and it must still have enough voltage to do so. Instrument datasheets often state the maximum loop resistance (“load” or “burden”) for a given supply voltage.
A healthy loop never goes below 4 mA. A reading below about 3 mA means there is no good signal: check the wiring and the instrument’s power supply.
Many instruments follow NAMUR NE 43, which sets aside currents outside the measuring range to signal faults:
| Current | Meaning |
|---|---|
| 3.8–20.5 mA | Measurement, including a small over- and under-range |
| 3.6 mA or less | Instrument fault — “failure low” |
| 21 mA or more | Instrument fault — “failure high” |
| About 0 mA | No loop current: broken wire, blown fuse, or no loop supply |
Whether an instrument fails low or high is usually a setting on the instrument. Set the controller to treat the same currents as a fault, so that an instrument reporting a failure is not read as a very low or very high measurement.