Beginner's Guide to 4-20 mA Temperature Sensor Scaling in Siemens PLCs

Analog-to-digital conversion takes a continuous 4-20 mA current signal from a temperature sensor and turns it into precise digital numbers, then scales those into real-world temperature readings like 0-100°C using a Siemens PLC analog input module.

Explanation

Picture a temperature sensor wired to a Siemens analog input module on your PLC. This sensor works like a messenger: at 0°C, it sends exactly 4 milliamps (mA) of current; as temperature climbs to 100°C, the current steadily rises to 20 mA. The module acts as a translator, capturing this tiny electrical current and converting it into a raw digital value simple integer between 0 and 27648. Zero current (or a broken wire) gives 0; full 20 mA delivers the maximum 27648. This raw number is what the PLC sees before any further processing.

Scaling or Conversion of the value

The process unfolds in clear stages. First comes the analog-to-digital (A/D) conversion inside the module: it samples the current thousands of times per second, measures its strength, and outputs that raw digital count (0-27648). Next, normalization squeezes this range into a simple fraction from 0.0 to 1.0. Did the raw value by 27648, so 4 mA (around 5530 raw) becomes about 0.2, and 20 mA becomes 1.0. Finally, scaling stretches this fraction across your engineering units: multiply by the span (100°C - 0°C = 100) and add the starting point (0°C). The result? A usable temperature value ready for alarms, displays, or control logic.

The Exact Scaling Equation

Boil it all down to one straightforward formula for temperature in °C:
Temperature = (Raw Value / 27648) × 100

·       At 4 mA: Raw ≈ 5530, so (5530 / 27648) × 100 ≈ 20% of range = 0°C (or very close, accounting for module precision).

·       At 12 mA: Raw ≈ 16590, so (16590 / 27648) × 100 = 60% = 60°C.

·       At 20 mA: Raw = 27648, so (27648 / 27648) × 100 = 100°C.
This linear math ensures every degree maps perfectly. In Siemens TIA Portal software, blocks like NORM_X and SCALE_X automate this: NORM_X gives the 0-1 fraction, SCALE_X applies your min/max (0 and 100).

Wiring and Configuration Guide

Start with solid wiring: connect the sensor's positive (+) to the module's mA+ terminal and negative (-) to mA-. Use twisted-pair shielded cable for noisy environments, and power the sensor correctly (often 24V DC loop-powered). In TIA Portal, add the analog module to your hardware config, set its range to "0(4)-20 mA," and assign input addresses (like IW64 for the first channel). Enable scaling in the module's parameters: low scale = 0, high scale = 27648 for raw; or directly to 0-100 for engineering units. Download to the PLC, go online, and watch live values update.

Testing and Troubleshooting Tips

Grab a 4-20 mA calibrator or simulator—set it to 4 mA and confirm raw reads ~5530 and temperature shows 0°C. Bump to 20 mA for 27648/100°C; test midway at 12 mA for 60°C. Common issues? Undershoot at 4 mA means offset error—recalibrate or check wiring. Noisy readings? Add filtering in the module settings (like 50 ms time constant). Broken wire shows 0 raw—add logic to detect and alarm below 3000. Always verify against a thermometer for accuracy, and log values in a trend view to spot drifts over time.

Why This Matters in Practice

This setup powers real automation: monitor a tank, trigger fans above 80°C, or log data for reports. It's reliable because 4-20 mA ignores voltage drops over long wires (unlike 0-10V), and Siemens modules handle noise with 16-bit resolution for smooth, precise control down to 0.1°C steps. Master this, and you're set for pressure, level, or flow sensors too—just tweak the min/max in the formula.