4–20 mA Explained: Scaling, Live Zero and the 250 Ω Resistor
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Why is 4–20 mA everywhere? This one-minute video explains the series current loop, the scaling formula I = 4 + 16 × value ÷ span (6 bar on a 0–10 bar range is 13.6 mA), why the live zero starts at 4 mA, and how a 250 Ω resistor turns 4–20 mA into 1–5 V in the PLC.
Chapters
Key points
- Transmitter, power supply and PLC input form one series loop; the same current flows through every part.
- 4 mA is the bottom of the range and 20 mA is full scale (here 0–10 bar).
- I = 4 + 16 × value ÷ span: 6 bar on a 0–10 bar range gives 13.6 mA (calculated value).
- The live zero shows a broken wire as 0 mA, can power a 2-wire transmitter, and cable resistance does not change the signal.
- A 250 Ω resistor in the PLC turns 4–20 mA into 1–5 V.
Transcript
4–20 mA: why is it everywhere?
A 4–20 mA transmitter, its power supply and the PLC input form one series loop. The same current flows through every part of it.
The current represents the measured value. 4 mA is the bottom of the range, 20 mA is full scale. Here, 0–10 bar.
The formula: I = 4 + 16 × value ÷ span. At 6 bar, that is 13.6 mA.
Why start at 4? A live zero shows a broken wire as 0 mA, it can power a 2-wire transmitter, and cable resistance does not change the signal.
In the PLC, a 250 Ω resistor turns 4–20 mA into 1–5 V.
SEGMENsensor pressure transmitters with 4–20 mA output. Try our free calculator at segmensensor.com.