4-20 mA Current Loops

Why current and not voltage

Run a 0-10 V signal 800 feet through 18 AWG wire in a wet vault and the voltage arriving at the panel is not the voltage the transmitter sent. Wire resistance and the receiving input impedance form a divider, and the reading drops. Add temperature change, a corroded splice, or a longer run than the drawing showed, and the error moves around.

Current does not behave that way. In a series circuit the current is the same at every point. The transmitter is a current regulator: it adjusts its own internal resistance to force exactly the current it intends through the loop, regardless of how much wire is in the way, right up until it runs out of voltage to do so. Add 400 feet of wire and the transmitter simply drops a little less across itself. The receiving device still sees the same milliamps.

The live zero

The choice of 4 mA rather than 0 mA for the bottom of the range is the quietly brilliant part of the standard. If 0% were 0 mA, a cut wire and a genuinely empty tank would look identical. With a live zero, 0 mA is not a valid measurement at all. It means the loop is open: a broken conductor, a failed transmitter, a tripped loop power supply, or a terminal that was never landed.

This is the single most useful diagnostic property of the standard, and it is worth building into your PLC logic explicitly rather than letting a 0 mA reading scale to 0 engineering units and quietly become a believable number.

Two-wire, three-wire, four-wire

Two-wire (loop powered)

The most common arrangement. One pair carries both power and signal. The transmitter draws its operating power from the loop, which is why it must function on less than 4 mA — it never gets more than the minimum signal current to run on. Requires an external supply, usually 24 VDC.

Three-wire

Separate power and signal share a common return. Used where the device needs more power than a loop can provide, such as some analyzers.

Four-wire

Fully separate power input and signal output. The device is line or 24 VDC powered and sources the 4-20 mA itself. Common on analyzers, flow computers, and larger instruments.

Loop resistance: the calculation people skip

A two-wire transmitter needs a minimum voltage across its own terminals to operate, called its compliance or lift-off voltage. Whatever the supply provides beyond that is available to drive current through everything else in the loop: the wire, the PLC input resistance, any indicators, any barriers.

With a 24 V supply and a transmitter needing 11 V, you have 13 V to spend, which allows 650 ohms of total loop resistance. A 250 ohm PLC input leaves 400 ohms for wire and everything else — comfortable. Add a panel indicator at 100 ohms, a surge protector, and a long run, and comfortable becomes marginal. The symptom of running out of headroom is a loop that tracks correctly at low readings and clips or goes nonlinear near full scale.

NAMUR NE 43 and what the out-of-range values mean

NAMUR NE 43 defines what a transmitter should do when it knows its measurement is not valid. Rather than reporting a plausible number, it drives the current outside the measurement range: below 3.6 mA or above 21 mA. Most modern transmitters let you configure which direction a fault drives.

This turns a sensor diagnostic into something your PLC can act on. A radar level transmitter that has lost echo can report 3.5 mA instead of a level of zero, and your logic can hold the last good value and alarm rather than starting a pump against an empty well.

Reading a loop with a meter

There are two ways to measure a loop, and one of them causes an outage if you do it wrong.

1. 1

   Series measurement — accurate, interrupts the loop

   Set the meter to DC milliamps, break the loop, and insert the meter in series. You now read exactly what the transmitter is sending. The loop is open while you move the leads, so the receiving device sees 0 mA and may alarm or trip logic. Notify operations first, and put the affected control in manual.

2. 2

   Voltage across a known resistor — non-intrusive

   Measure DC volts across the analog input sense resistor or a dedicated test resistor. With 250 ohms, 1.000 V is 4 mA and 5.000 V is 20 mA. Nothing is interrupted. This is the preferred method on a running process.

3. 3

   Loop test jacks and clamp meters

   Many transmitters include test terminals that let a meter read the loop current without breaking it. A true milliamp clamp meter also works, at lower resolution. Both are worth having.

What the numbers tell you

HART on the same pair

HART superimposes a low-level frequency-shift-keyed digital signal on the same 4-20 mA loop. Because it averages to zero current, it does not disturb the analog reading, so a HART transmitter delivers a conventional analog value to the PLC and a digital channel to a handheld communicator or a HART-capable input card at the same time.

That digital channel carries the device tag, its calibrated range, its diagnostics, and often several secondary variables. On a stubborn loop it will frequently tell you in ten seconds what a meter takes an hour to narrow down. HART needs a minimum loop resistance, typically 230 ohms, to work — which the standard 250 ohm input satisfies.