GGSTech Tools4–20 mA scaler
Why 4 mA is not zero current
The 4 mA live-zero allows a powered two-wire transmitter to operate while representing the bottom of the calibrated range. A value near 0 mA can therefore indicate an open circuit or loss of loop power rather than a valid zero process value.
Example
For 0–10 bar, 12 mA is halfway between 4 and 20 mA, so the process value is 5 bar.
PLC raw counts
Many modules convert current to integer counts. Use the exact module documentation for raw minimum/maximum values; do not assume 0–27648 or any other range across all PLC families.
What you should understand first
Linear scaling maps one range to another. The underlying principle is percentage of span: first determine how far the input is between its lower and upper limits, then apply the same fraction to the engineering range.
Signal and energy boundaries
A PLC is a controller, not the power source for a motor. Separate the low-energy logic side from the field power circuit. Digital outputs often command an interposing relay, contactor coil, valve solenoid or VFD input. The motor power still passes through the MCC/MCCB/contactor/VFD power circuit.
Fail-safe thinking
For critical shutdowns, do not rely only on software logic. Emergency-stop, trip and safety functions may require dedicated safety-rated hardware and a risk assessment. A normal PLC output should not be assumed to provide a safety function unless the architecture is specifically designed and certified for it.
Common mistakes
- Assuming 0 mA = LRV on a 4–20 mA loop
- Ignoring transmitter underrange/overrange diagnostics
- Using the wrong raw-count limits for the PLC module
- Failing to clamp or alarm impossible values
- No filtering strategy for noisy signals
Commissioning checklist
- Verify transmitter LRV/URV.
- Measure loop current with an appropriate method.
- Check PLC raw counts at 4 mA and 20 mA.
- Test underrange/overrange and broken-wire behavior.
- Document scaling constants and engineering units.
Frequently asked questions
What does 12 mA mean?
It is 50% of the 4–20 mA span, so the process value is halfway between LRV and URV for a linear transmitter.
Can every transmitter be scaled linearly?
Many are linear in their configured output, but special extraction such as square-root flow may be performed in the transmitter or PLC. Verify configuration.
What is a live zero?
The lower measurement limit is represented by nonzero current, typically 4 mA, which helps distinguish zero process from loss of loop current.