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A Wheatstone bridge load cell produces a differential millivolt signal, not a ready-to-use 4-20 mA or RS485 output. Excitation energises the bridge, and strain changes the balance between its resistive arms. To choose an instrument or investigate a bad reading, start with the actual sensitivity, excitation and zero offset. Assuming that every cell produces 20 mV can result in the wrong input range, poor use of resolution or saturation when the force reverses.
Calculate the Wheatstone Bridge Load Cell Signal
The nominal signal change at rated load is sensitivity in mV/V multiplied by excitation in volts. For a linear approximation below rated load, multiply again by the applied-load fraction. Add the actual zero offset when checking whether the total output stays within the instrument's input range. Use the excitation at the sensor, not merely the value selected in an instrument menu.
| Example at 5 V excitation | Calculated nominal full-scale change |
|---|---|
| MV-LC0190, 1 kg | 0.5 mV/V x 5 V = 2.5 mV. The low-force option needs an input that can resolve the useful fraction of this signal. |
| MV-LC0190, 5-50 kg | 1.0 mV/V x 5 V = 5 mV. This is a different gain and calibration case from the 1 kg option. |
| MV-LC0149, 2 mV/V version | 2 mV/V x 5 V = 10 mV. Confirm the ordered output rather than using the model family alone. |
| MV-LC0149, 3 mV/V version | 3 mV/V x 5 V = 15 mV. An unchanged 10 mV input setting would not cover the nominal full-scale signal. |
These calculations use nominal sensitivity; they are not calibration data and exclude sensitivity tolerance, installed preload and zero balance. For example, the MV-LC0190 1 kg reference is 0.5 +/-0.1 mV/V, so its sensitivity tolerance alone gives a calculated 2-3 mV full-scale change at 5 V. Leave appropriate input headroom and calibrate the delivered sensor rather than treating 2.5 mV as exact.
The smallest useful force deserves a separate calculation. On a nominal 5 kg, 1 mV/V sensor excited at 5 V, a 10 g increment corresponds to 0.01 mV, or 10 microvolts, of signal change. This arithmetic does not promise a 10 g usable resolution: amplifier noise, mechanical effects, temperature and the test method can obscure that increment.
Check Negative Range Before a Reversible Test
An input's positive limit is only half the story. The METRAVON MV-WI0009 specifies 5 V excitation and a -3 to 30 mV input range. A sensor that develops approximately -10 to +10 mV over the required tension/compression cycle cannot be assumed to fit that range simply because +10 mV is below +30 mV.
Define polarity, preload and the expected extremes before selecting the electronics. Resetting displayed zero does not necessarily move the analogue input's saturation limits. For a reversible force test, obtain confirmation of the full bipolar input capability and calibration method. Use the amplifier selection guide when noise, transient peaks or sampling speed affect the result.
Identify Conductors by Function, Not Colour
A four-wire bridge normally has two excitation and two signal conductors. A six-wire arrangement adds sense leads so a compatible instrument can monitor excitation at their connection point. Those leads are not extra force channels. A shield is a separate connection and must not be counted as one of the measurement pairs.
The MV-LC0149 reference diagram assigns red/black to excitation, green/white to signal and yellow to shield. The documented MV-LC0099 wiring uses red/blue for excitation and white/yellow for signal. In one diagram yellow is a shield; in the other it is a signal conductor. This is why even apparently familiar colours must be checked against the exact model's wiring information.
Follow the instrument manual for sense connections when using a four-wire sensor on a six-wire input. Connecting sense at a junction box monitors excitation there, not beyond it at each bridge. It cannot compensate every voltage drop or repair a loose terminal downstream. Record the location of those connections with the cable drawing.
Resistance Is a Diagnostic Measurement, Not a Weight Reading
With power removed and the cell safely disconnected from the electronics, compare resistance across the identified excitation pair and signal pair with the exact specification. MV-LC0149 lists 385 +/-10 ohm input and 351 +/-2 ohm output; MV-LC0190 lists 350 +/-5 ohm for both. A different value is not automatically a fault when the models differ.
Parallel cells reduce the excitation load resistance. As an illustrative calculation, four identical 385 ohm inputs in parallel present 96.25 ohm; at 5 V they draw approximately 52 mA before cable effects. Use the minimum specified resistance and the instrument's drive limit when checking an actual installation. Output resistance is not the value to substitute into that excitation-current calculation.
A normal resistance result does not establish insulation condition, calibration accuracy or freedom from mechanical damage. Do not apply an insulation tester through an attached amplifier. Any insulation test voltage and procedure must be approved for the sensor and disconnected components; the consequences of using an unsuitable tester can be more serious than the original fault.
Questions at the Instrument Terminals
Q1. Can I connect the bridge directly to a PLC analogue input?
Only if that input is designed for the bridge and provides or accepts the required excitation and millivolt measurement. A conventional 4-20 mA channel is not such an input. Otherwise use suitable signal conditioning, and select the controller-facing output separately. The load-cell-to-PLC guide covers those connection choices.
Q2. Why does the display stop increasing even though force is rising?
Check for input saturation, an exceeded configured range or a mechanical stop carrying additional force. Compare raw signal, excitation and displayed value within an approved test procedure. Do not continue adding load to find the limit. A clipped reading cannot be used to show that the sensor remains below its rated capacity.
Q3. Is a bridge simulator enough to sign off the scale?
It can help check the instrument and, depending on where it is connected, part of the cable path. It does not apply force to the sensor or exercise the mount. Final acceptance still needs a suitable loaded test of the installed system. Keep simulator results separate from the mechanical calibration record.
Q4. Should I increase excitation to get a larger signal?
Only within the sensor and instrument specifications, after recalculating the input limits and drive current. For a nominal 3 mV/V cell, changing from 5 V to 10 V doubles the calculated full-scale change from 15 to 30 mV. A 30 mV input limit would then leave no allowance for positive zero offset or sensitivity tolerance. More signal is not useful if the input clips.
Summary
Use mV/V to calculate the expected signal, check both ends of the instrument range and identify every conductor from its function. When requesting METRAVON technical assistance, include the exact cell version, excitation, wiring sketch and measured input/output readings. Those details help distinguish an incompatible instrument from a damaged cable or a mechanical problem.
