Why does an analog loop use 4 mA, not 0 mA, for the bottom of the range?

B. So a broken wire (0 mA) can be told apart from a real bottom-of-range reading, and so a two-wire transmitter always has current to run on

A. Because 0 mA would damage the transmitter

C. Because meters cannot read below 4 mA

Where does a two-wire (loop-powered) transmitter get its power?

B. From the loop: a DC supply in series drives the loop, and the transmitter runs on the current it regulates

You need to read the loop current without upsetting the process. Which method avoids breaking the loop?

B. Put the meter on mA across the transmitter’s test terminals (the test diode), or use a milliamp clamp

A. Open the loop and put the meter in series

C. Measure the voltage across the transmitter on the ohms range

D. Measure resistance across the loop

A loop reads 0.0 mA. The most likely cause is…

B. an open loop or no loop supply

A. the process is at the bottom of its range

Quatrième année Lesson 5 of 12 · about 15 min

Instrumentation and the 4–20 mA loop

What you’ll learn

Why industry sends measurements as a current, and why the range starts at 4 mA.
How a two-wire transmitter is powered by the same loop that carries its signal.
How to scale any loop current to a process value — and back — by hand.
How to measure a loop without opening it, and what 0 mA, 3.6 mA or 21 mA mean.

Before this: Intro to PLCs · Percent in the trade

A transmitter measures something — pressure, temperature, level, flow — and reports it as a current between 4 and 20 milliamps. 4 mA is the bottom of its range, 20 mA the top, and everything in between is a straight line. The PLC or controller at the other end turns the current back into the number. Almost every plant you walk into runs on these loops.

A two-wire loop: one series circuit. The supply pushes, the transmitter regulates the current to match its measurement, and the resistor at the input turns that current into a voltage the PLC reads.

Why a current, and why 4

In a series loop the current is the same everywhere. Long wire runs, a little corrosion on a terminal, or noise picked up on the way hardly change it — the transmitter simply regulates harder. That is why a current signal survives distances a voltage signal would not.

Starting at 4 mA instead of zero is called live zero. A healthy loop never reads 0 mA, so zero means something is broken — an open wire or a dead supply — and can never be mistaken for “the tank is empty.” And that 4 mA is always available to power a two-wire transmitter. Many transmitters also drive the loop deliberately low (below about 3.6 mA) or high (above about 21 mA) to signal their own failure.

Scaling by hand

The transmitter is ranged with a lower range value (LRV, at 4 mA) and upper range value (URV, at 20 mA). The 16 mA between is the span. Subtract the 4 first — always — then take the fraction of 16.

fraction = (mA − 4) ÷ 16how far up the range
value = LRV + fraction × (URV − LRV)current to process value
mA = 4 + 16 × (value − LRV) ÷ spanvalue to current
V = mA × Ω ÷ 1000across the input resistor

Worked: a pressure transmitter

Ranged 0–150 psi (in a metric plant, 0–1,000 kPa). The loop reads 13.6 mA.
Fraction = (13.6 − 4) ÷ 16 = 9.6 ÷ 16 = 0.60 — 60% of range.
Value = 0 + 0.60 × 150 = 90 psi (or 0.60 × 1,000 = 600 kPa).
Across a 250 Ω input: 13.6 × 250 ÷ 1000 = 3.4 V.
Going the other way: the operator says the gauge reads 75 psi. Expected current = 4 + 16 × 75 ÷ 150 = 12 mA. If you measure 12 mA, the transmitter and gauge agree.

Worked: will the loop have enough voltage?

24 V DC supply; 250 Ω input resistor; about 20 Ω of wire round trip.
At the worst case, 20 mA: resistor drop 0.020 × 250 = 5 V; wire drop 0.020 × 20 = 0.4 V.
Left for the transmitter: 24 − 5 − 0.4 = 18.6 V.
Compare with the minimum voltage on the transmitter’s data sheet (commonly somewhere around 10–12 V for a two-wire unit — use yours). Add a second 250 Ω device in the loop and you lose another 5 V.

Measuring a loop

Test terminals (test diode). Many transmitters have a pair of terminals across a diode in the loop. A meter on mA there is a lower-resistance path, so the loop current goes through the meter — and the loop never opens.
Milliamp clamp. Clamps around one loop conductor and reads the current without touching it.
Voltage across the input resistor. 1–5 V across 250 Ω is 4–20 mA. No disconnection needed.
In series. Open the loop and put the meter (on mA, leads in the mA jacks) in the gap. The controller sees the loop open while you do — warn operations first, and put the process in manual if it is controlling something.

Sur le chantier

Tips

Memorise the quarters: 4, 8, 12, 16, 20 mA = 0, 25, 50, 75, 100%.
Check polarity: a two-wire transmitter wired backwards usually reads nothing at all.
Know whether each input supplies loop power or expects a powered (four-wire) signal.
Shield grounded at one end only, usually the control panel end.

Common mistakes

Forgetting to subtract the 4 mA before scaling.
Opening a loop that is controlling a live process without warning anyone.
Leaving the meter leads in the mA jacks and then measuring voltage — that is a short.
Two loop supplies in one loop (a powered transmitter into a powered input).
Scaling in the PLC not matching the range set in the transmitter.

Check yourself

0 right of 0 answered 8 questions · pick one answer each

1.Why does an analog loop use 4 mA, not 0 mA, for the bottom of the range?
2.A 4–20 mA signal reads 12 mA. What percent of the range is that?
3.A temperature transmitter is ranged 0–200 °C. The loop reads 8 mA. What temperature is it reporting?
4.A controller input has a 250 Ω resistor across it. What voltage range does the 4–20 mA signal make across it?
5.Where does a two-wire (loop-powered) transmitter get its power?
6.You need to read the loop current without upsetting the process. Which method avoids breaking the loop?
7.A loop reads 0.0 mA. The most likely cause is…
8.A 24 V DC supply drives a loop with a 250 Ω input resistor. At 20 mA, how much voltage is left for the transmitter and the wiring?

Answers

B. So a broken wire (0 mA) can be told apart from a real bottom-of-range reading, and so a two-wire transmitter always has current to run on — That is "live zero." A healthy loop never goes to 0 mA, so zero means a fault — and the 4 mA that always flows is what powers a two-wire transmitter.
B. 50% — (12 − 4) ÷ 16 = 8 ÷ 16 = 0.5, so 50%. Always subtract the live zero first; 12 ÷ 20 = 60% is the trap.
B. 50 °C — (8 − 4) ÷ 16 = 0.25 of the span; 0.25 × 200 = 50 °C above the 0 °C bottom.
B. 1–5 V — V = I × R: 0.004 × 250 = 1 V and 0.020 × 250 = 5 V. That is why 1–5 V inputs and 250 Ω resistors go together.
B. From the loop: a DC supply in series drives the loop, and the transmitter runs on the current it regulates — The same two wires carry the power and the signal. The transmitter acts like a variable resistor that regulates the loop current between 4 and 20 mA, running itself on what flows.
B. Put the meter on mA across the transmitter’s test terminals (the test diode), or use a milliamp clamp — Test terminals sit across a diode in the loop; a meter on mA there offers a lower-resistance path, so the current flows through the meter instead without the loop ever opening. A mA clamp reads it without touching the circuit at all.
B. an open loop or no loop supply — A working loop sits at 4 mA or above even with the process at zero. 0 mA means nothing is flowing: a broken wire, a loose terminal, a blown loop fuse or the power supply off.
B. 19 V — The resistor drops 0.020 × 250 = 5 V, leaving 24 − 5 = 19 V. The transmitter’s data sheet gives the minimum voltage it needs; the wire resistance takes a little of the rest.

Questions fréquentes

Why current and not voltage?

In a series loop the current is the same at every point, so the resistance of a long run of wire, a terminal that has corroded a little, or electrical noise picked up along the way barely changes it — the transmitter just pushes a little harder. A voltage signal would lose some of itself in every ohm of wire. That makes 4–20 mA reliable over hundreds of metres.

What is the difference between two-wire, three-wire and four-wire transmitters?

A two-wire transmitter is powered by the loop itself. A four-wire transmitter has its own separate power supply (often 120 V AC or 24 V DC) and puts out an active 4–20 mA signal on two more wires. Three-wire types share a common between power and signal. Wiring a four-wire (active) transmitter into an input that also supplies loop power is a common commissioning mistake.

What is HART?

A digital signal superimposed on the 4–20 mA as a small tone, which lets a handheld communicator or the control system read and set the transmitter’s range, units and diagnostics over the same two wires, without disturbing the analog value.

Voir aussi

4–20 mA scaling calculator — loop current to value and back
Percent in the trade — the arithmetic under scaling
Using a multimeter — measuring current safely
Troubleshooting industrial controls
Intro to PLCs — where the analog input lands

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