Know-How
Completed
4-20 mA input reading wrong: check it step by step
Short answer: most wrong 4-20 mA readings come from the loop wiring or from a mismatch between the signal and the input type, not from the sensor. Check in this order: transmitter type, loop power, loop current, input type, common reference and, last, the scaling in the program. A multimeter is enough for almost all of them.
Download the checklist (PDF, 1 page)
Descarga la lista de comprobación (PDF, 1 página)
These checks work for any PLC or I/O module with analog inputs. At the end you will find the page for your PLC range.
The idea in one minute
A 4-20 mA transmitter sets a current in the loop: 4 mA at the bottom of its range and 20 mA at the top. The current is the same at every point of the loop, so cable length and connections barely change the value. A current below 4 mA is not a measurement: it tells you that something is wrong in the loop.
If the PLC input measures voltage, a precision resistor turns the current into a voltage. With 500 Ω, 4 to 20 mA becomes 2 to 10 V.
Six checks, in order
| Check | What to do | Good result |
|---|---|---|
| 1. Transmitter type | Read the label: 2-wire (loop powered), 3-wire or 4-wire | You know where the loop gets its power |
| 2. Loop power | With the loop closed, measure the voltage at the transmitter terminals | Within the range on the transmitter label |
| 3. Loop current | Open the loop and measure in series with the multimeter in mA | Between 4 and 20 mA, and consistent with the process |
| 4. Input type | Check that the input accepts current, or that it has the right resistor if it only measures voltage | Current mode selected, or a 500 Ω precision resistor on a 0-10 V input |
| 5. Common reference | Check that the 0 V of the loop supply reaches the analog 0 V of the PLC | One common 0 V, unless the input is isolated |
| 6. Scaling | Compare the value in the program at 4 mA and at 20 mA | 4 mA reads as 0 % and 20 mA as 100 % of the range |
What the readings tell you
- 0 mA or close to 0: open loop, broken wire or no loop power.
- Below 3.6 mA or above 21 mA: many transmitters use these values to signal a fault (NAMUR NE 43). Check the transmitter before the PLC.
- Correct mA but wrong value in the PLC: input type, channel or scaling.
- Reading stuck at the maximum: current signal on a voltage input without resistor.
- Value that jumps: noise. Use shielded twisted pair away from drive outputs, and average several samples.
If the input has never worked
- Check the polarity: the + of the loop goes to the + of the transmitter.
- A 2-wire transmitter needs a supply in series in the loop. Do not assume the PLC input powers it: check the datasheet.
- Check that the program reads the right channel and the right input mode. This depends on your PLC range.
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