Loading…
Loading…
Resources · Interactive Guide
One loop, seen from every angle — why it’s wired this way, what a weak supply really does, and how the gateway turns milliamps into a reading. Drive it yourself.
One current, one loop — 24 V supply → sensor → gateway → and back. Follow the dots; the same current flows at every point.
Jump to the live loop ↓A 4–20 mA sensor isn’t a passive switch — it’s a small computer that measures the process, converts it, and then actively regulates the current onto the loop. No supply, no measurement, no signal. In a 2-wire (loop-powered) transmitter the same two wires carry power in and signal out.
A loop-powered transmitter runs on the same pair it signals on, so it lives on a tight budget — it can only draw down to about 3.5 mA at 0%. It’s a regulated current sink, not a resistor or a switch.
Most transmitters run on 18–30 V DC; 24 V is the standard because it leaves headroom to push full current across a long cable. The sensor regulates the loop to the process value as long as the supply clears the compliance voltage (transmitter minimum + I × loop resistance). Below that, only the top of the range clips; below the transmitter minimum it stops at 0 mA. Extra volts don’t raise the current — they drop across the transmitter as heat.
| Supply | Max current it can push | At 100% (wants 20 mA) | |
|---|---|---|---|
| 24 V | ≥ 20 mA | full 20.0 mA | OK |
| 18 V | ≥ 20 mA | full 20.0 mA | OK |
| 16 V | 16 mA | clips at 16.0 mA | CLIP |
| 14 V | 8 mA | clips at 8.0 mA | CLIP |
| 12 V | 0 mA | dead — 0 mA | DEAD |
The honest version of the usual table. The reading does not slide down with voltage — the loop holds the true value until the supply can’t push the demanded current, then only the high end clips. Min supply for a full 20 mA here = 17 V; the sensor still lights (live-zero 4 mA) down to ~13 V.
The scale is 4 mA = 0%, 12 mA = 50%, 20 mA = 100%. If zero process were 0 mA, a dead loop and a true 0% reading would look identical. Anchoring the live end at 4 mA makes 0 mA unambiguous — broken wire, dead supply, or failed sensor. That one choice is why the loop is self-diagnosing.
baseline sits on 4 mA — alive
flat on 0 mA — motionless, a fault
NAMUR NE43 formalises it: 3.8–20.5 mA valid; ≤ 3.6 mA fails low, ≥ 21 mA fails high (transmitters park at ~3.5 / ~22). Reverse-polarity and open-wire below both land at 0 mA — and live-zero is exactly why the gateway catches them.
Push 12.00 mA in and 12.00 mA comes out the far end — series current is identical at every point, whatever the wire resistance. A 0–10 V signal instead sags with distance and reads wrong. That immunity to cable drop is why the field standard is current.
Immune — until the cable eats the compliance headroom, then the top clips too.
The complete circuit: 24 V → sensor → gateway AI+ → AI− → back. Drive the tank level to set the current, move the supply to feel the compliance limit, and ✂ cut the wire to watch the loop die to 0 mA. A valid reading proves the entire path is intact.
One linear formula converts current to any engineering unit. Change the range in your gateway config and only the endpoints move — the 4–20 mA stays the universal carrier.
| Current | % span | 0–150 °C | 0–10 bar | Across 250 Ω |
|---|---|---|---|---|
| 4 mA | 0% | 0 °C | 0.0 bar | 1.0 V |
| 8 mA | 25% | 38 °C | 2.5 bar | 2.0 V |
| 12 mA | 50% | 75 °C | 5.0 bar | 3.0 V |
| 16 mA | 75% | 113 °C | 7.5 bar | 4.0 V |
| 20 mA | 100% | 150 °C | 10.0 bar | 5.0 V |
The 250 Ω burden (→ 1–5 V) is the classic, HART-compatible choice and is used throughout this page; a small ~20 Ω burden (→ 0.08–0.40 V) trades signal level for compliance margin. The row nearest the live current above is highlighted.
A 2-wire sensor has just two terminals — the same pair carries power in and the 4–20 mA out. The whole job is three wires, so wire it yourself: the panel below starts unconnected. Land ①, ②, ③ in order and watch the meter — nothing flows until the last wire closes the lap. That moment is the entire lesson of a series loop.
↳ inside the sensor: measures, then regulates the 4–20 mA
↳ inside the gateway: through the 250 Ω burden to AI−
Tap ① to land the first wire →
Only ①②③ are wires you land — the dashed paths are internal. Simpler still: if your Wiman gateway has a built-in 24 V loop supply, the supply is the gateway — ① and ③ vanish and it’s just two wires: Gateway V+ → Sensor+, and Sensor− → Gateway AI+. Earth the loop at one point only (supply −).
Other sensor types (not loop-powered)
Powered on its own: V+→24 V, COM→0 V, OUT→AI+. The signal shares the common return — it is not loop-powered.
Its own power pair, plus a separate (often isolated) output pair to AI+/AI−. Power and signal are fully split.
A passive (2-wire) device must be powered by the gateway; an active output powers itself — so don’t feed it loop power. Getting this backwards is the most common miswire.
Notice the pattern — a broken or starved loop reads 0 mA, which live-zero flags instantly. (Two of these correct the common infographic: a short doesn’t spike, and “no common ground” on a 2-wire loop is just an open loop.)
Loop + and − swapped. The transmitter’s protection diode blocks reverse current. → 0 mA. Usually non-destructive, but check the datasheet.
Any break — cut wire, loose terminal, no power. Current can’t flow. → 0 mA, a clean detectable fault.
Not a spike. Shorting the input just shorts the burden; the transmitter keeps regulating 4–20 mA into ≈ 0 Ω, so the gateway reads 0 V → under-range. → reads low, no damage.
The real high-current case: 24 V placed straight across the burden with no transmitter in series. 24 V / 250 Ω = 96 mA. → cooks the burden.
Below compliance the top of the range clips first (reads low), then drops to 0 only below the transmitter dropout — not random noise. → under-reads.
On a 2-wire loop a missing return is simply an open loop (0 mA). Floating reference & ground loops (shield earthed both ends) bite 3/4-wire systems. → drift, noise.
Every loop is electrically independent into AI1–AI4, yet all share a single 24 V+ / 0 V rail. Independent signals, common power — that’s how one gateway scales cleanly.
Tap a sensor to energise its loop →
All share one 24 V+ / 0 V rail. Size the supply for the sum of the loop currents, keep 0 V a genuine single common, and isolated inputs avoid inter-channel ground loops.
The sensor datasheet is the final authority — it overrides every rule of thumb here.