Load Cell to PLC Wiring: Bridge Inputs, 4-20 mA and Modbus RTU

Load cell to PLC wiring starts with identifying the sensor output. A strain-gauge load cell marked in mV/V is a bridge sensor: it needs excitation and an input designed to measure a small differential signal. Connecting it to an ordinary 4-20 mA input does not provide either function. Choose where excitation, calibration and weight conversion will occur before assigning PLC terminals.

Choose where the bridge signal becomes a weight value

A dedicated weighing input is different from a general analogue input. Confirm the exact module specification rather than assuming that an input with a millivolt range can also excite a bridge and meet the required noise, common-mode and calibration conditions. For an external instrument, start with the METRAVON weighing indicator range and verify the specific configuration. The category name does not guarantee any particular protocol.

Keep one clear definition of the transmitted quantity. If the indicator performs tare and sends net weight, the PLC should not subtract the same tare again. Record where zero, span, unit conversion and filtering are applied so a later replacement does not introduce a second conversion.

Calculate the bridge signal and excitation demand

The nominal full-scale signal change is sensitivity in mV/V multiplied by excitation in volts. The MV-LC0030 has nominal sensitivity of 2 mV/V. At 10 V excitation, that gives a calculated 20 mV change from zero to rated load. The measuring input also needs allowance for zero offset, tolerance and any permitted signal overrange.

Several bridges in a parallel junction-box arrangement increase excitation current. Four nominal 400-ohm input bridges present approximately 100 ohms to the supply and draw approximately 100 mA at 10 V. This is a circuit calculation, not a measured installation result. Check resistance tolerances, cable voltage drop and the instrument's excitation capability. The available current must suit the actual connected sensors.

Do not add the nominal 20 mV outputs of four parallel cells and configure an 80 mV input range on that basis. A passive parallel summing arrangement combines the bridge outputs according to its circuit; it is not a series voltage connection. Follow the junction-box instructions and calibrate the completed system.

For the MV-LC0190, nominal sensitivity is 0.5 mV/V at 1 kg and 1.0 mV/V at 5, 20 and 50 kg. At 10 V, the corresponding calculated full-scale changes are 5 mV and 10 mV. A common model-family name is therefore not enough to set gain. Use the ordered capacity and its calibration information.

Keep low-level wiring separate from power wiring

Use the documented terminal functions for excitation, signal and any sense leads. A four-wire bridge does not provide separate remote-sense conductors; a six-wire arrangement can provide them when supported by the measuring instrument. Follow the instrument's connection instructions, including any links required for the selected wiring scheme. Wire colours alone are not a pinout.

Route the sensor cable away from motor, variable-frequency-drive, heater and switching-power conductors. Provide strain relief and protect junctions against moisture. Shield termination should follow the complete grounding design and instrument instructions. Do not use the cable screen as a substitute signal return or protective conductor.

Keep cable extensions and junction boxes accessible for inspection. Record the final cable arrangement because changes in length, resistance or shielding can affect the measurement. When diagnosing noise, compare operating states such as a motor stopped and running; do not remove protective grounding to make a reading quieter.

Load cell to PLC wiring through a 4-20 mA transmitter

Specify the weight assigned to each end of the analogue output. For a configured 0-10,000 kg span, 4 mA represents 0 kg and 20 mA represents 10,000 kg. The conversion is weight = (current in mA - 4) / 16 x 10,000 kg. Under that example configuration, 12 mA corresponds to 5,000 kg. These are commissioning examples, not default settings for a METRAVON instrument.

An electrical loop test checks the transmitter-output-to-PLC portion. It does not calibrate the load cells or prove the mounting is correct. After the loop conversion is verified, apply suitable known loads through the mechanical system and compare the transmitter, PLC and HMI readings.

Check whether the transmitter output is active or loop-powered and whether the receiving input is passive or powered. Verify supply voltage, input burden and cable loss against the actual devices. Define how negative net values, overrange and fault conditions are represented. Do not assume every transmitter uses the same fault-current convention.

For RS485, obtain the register map before programming

RS485 identifies an electrical interface; Modbus RTU specifies a communication format. Both must be supported by the selected instrument. Agree the serial settings, address and cable arrangement with its documentation. A bridge sensor without digital electronics does not acquire Modbus support merely because the PLC has an RS485 port.

The Modbus application protocol specification defines 16-bit registers. The device register map must identify the weight address, read function, signedness, scaling and any multi-register representation. Check the address notation used by the PLC driver; a documentation reference and an on-wire offset may use different conventions.

Test a negative value where the application permits it, a value that exposes the decimal scaling, and any multi-register word order. Obtain these details from the selected device's map rather than guessing until a plausible number appears. The reading must also have a validity or freshness assessment appropriate to the control task.

Commission the value, its timing and its failure behavior

Follow a known load through the whole chain: mechanical input, bridge measurement, transmitter value, PLC tag and operator display. Confirm units and tare at each stage. Save the device settings and PLC conversion with the installed calibration results.

Measure response during the process action that matters, such as the end of filling. Filters in the transmitter, PLC and HMI can each add delay. A smooth display is not evidence that a cutoff is timely. Test the final settings under representative conditions and retain separate raw and filtered values when the control design needs them.

Use an approved test plan to simulate communication loss and other supported faults with the plant in a safe state. A retained last value must not silently remain a valid fresh measurement. Verify alarm handling, control inhibition and recovery. Remove simulation modes and forced values before handover.

Questions from PLC commissioning

Q1. Why does the display show half the expected weight?

Check sensitivity, excitation, transmitter span and PLC conversion before adjusting the mechanics. Different sensor sensitivities can produce a factor-of-two error when one configuration is reused. Trace the value stage by stage with a known load to locate where the discrepancy first appears.

Q2. Does increasing excitation always improve the result?

No. Excitation must stay within the sensor and instrument specifications. More voltage increases bridge current and may exceed the supply or create additional heating. Assess noise and signal range with the permitted excitation; do not use excess voltage to compensate for unsuitable electronics.

Q3. Can four cells share one input?

Compatible cells can be combined through an appropriate junction box when the instrument supports the total bridge load. Confirm the summing and trim arrangement and the required position tests. Independent support diagnostics may require separate channels rather than one combined signal.

Q4. What belongs in a transmitter quotation request?

List the sensor models, capacities, mV/V sensitivities, bridge resistances, sensor count and cable lengths. Add the PLC input type, required update behavior, output quantity and environment. Send these details through the METRAVON enquiry page so the proposed instrument can be checked against the complete measuring chain.

Summary

Give the bridge a compatible measuring input, define one weight conversion, and test the entire route to the PLC. A plausible number is insufficient: the installed system must also report the correct units, respond in time and make a lost or invalid measurement visible to the control logic.