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4-20mA Analog Scaling Calculator

Map raw analog signals to engineering units with slope/intercept formulas.

Analog Parameters

mA

Scaled Value

50.00 EU
0% 100%
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Disclaimer of Liability: This calculator is provided for educational and estimation purposes only. By using this tool, you agree to discharge AutomationView of any liability for direct, indirect, or consequential damages resulting from its use. It is your strict responsibility to independently verify all calculations, validate the results against official manufacturer documentation, and ensure compliance with all applicable safety and engineering standards before implementing any parameters in a production environment.

Understanding 4-20mA Analog Scaling

In industrial automation, the 4-20mA current loop is the most robust and widely used standard for transmitting analog signals. However, PLCs and controllers do not understand current directly; they understand raw digital counts. Analog scaling is the mathematical process of converting that 4-20mA signal into meaningful Engineering Units (e.g., bar, °C, GPM) or translating raw ADC counts into physical measurements.

The Core Linear Scaling Formula

Scaling a 4-20mA signal relies on a simple linear interpolation equation ($y = mx + b$). To convert a measured current ($I$) directly into a Process Value ($PV$), use the following formula:

PV = Low Limit + ((I – 4) / 16) × (High Limit – Low Limit)

  • PV: The Process Value in engineering units.
  • I: The measured current in mA.
  • Low Limit: The engineering value corresponding to 4mA.
  • High Limit: The engineering value corresponding to 20mA.
  • 16: The active span of the signal (20mA – 4mA).

PLC Scaling (Raw Digital Counts)

When dealing with PLCs, the analog input card converts the 4-20mA current into a digital integer (Raw Counts). The scaling formula must account for this raw range. For instance, a Siemens S7-1200 typically uses 5530 to 27648, while an Allen-Bradley PLC might use 3277 to 16384.

The modified formula for PLC raw counts is:

PV = Low Limit + ((Raw – Raw_min) / (Raw_max – Raw_min)) × (High Limit – Low Limit)

Reverse Scaling: Engineering Units to mA

If you are programming an analog output (e.g., controlling a VFD or proportional valve), you must calculate the required current for a specific target value. The reverse formula is:

I = 4 + ((PV – Low Limit) / (High Limit – Low Limit)) × 16

Why 4mA and Not 0mA?

The standard uses 4mA as the “live zero” rather than 0mA. This allows the control system to easily detect a wire break, sensor failure, or loss of power. If the signal drops to 0mA, the PLC recognizes a fault condition, whereas a valid measurement will always maintain at least 4mA of current loop flow.