Wheatstone Bridge Load Cells: Signal, Wiring and Fault Diagnosis

Most strain-gauge load cells use a Wheatstone bridge: four resistive arms arranged so small strain-related resistance changes produce a differential output. Excitation is applied across one bridge diagonal and signal is measured across the other. Output is commonly specified in millivolts per volt at rated load. A 2 mV/V cell excited by 10 V produces about 20 mV at full scale, so stable excitation, shielding and a suitable low-noise amplifier are essential. The bridge measures strain in the elastic element; mounting, temperature and overload determine whether that strain represents the intended force.

Bridge balance and sensitivity

With no load, the bridge should be near balance but manufacturing tolerances create a specified zero offset. Applied force changes tensile and compressive gauges in opposite directions, increasing differential signal. Rated output tolerance matters when several cells are connected in parallel or when replacement without recalibration is considered.

Four-wire and six-wire connections

Four-wire cells provide excitation and signal pairs. Six-wire cells add sense conductors so the transmitter can compensate voltage drop along the excitation cable. A shield is normally separate from the bridge. Follow the exact colour code because wire colours are not universal across manufacturers.

Temperature compensation

Gauge patterns and compensating resistors reduce zero and span change with temperature, but they do not eliminate mechanical effects. Thermal expansion in mounts, piping or structures can create real parasitic force. Specify compensated and operating ranges and evaluate the complete installed assembly.

Parallel connection and junction boxes

Multiple cells on a platform are often connected in parallel through a junction box. Cells should have compatible rated output and impedance. Corner trimming adjusts small sensitivity differences but cannot correct binding, severe misalignment or unequal capacity. Preserve individual outputs before summing for diagnostics.

Electrical checks

With power removed, compare input and output resistance against data-sheet tolerance and check insulation to the body using an approved test voltage. Under excitation, verify voltage at the cell, unloaded millivolt output and polarity under a small known force. A multimeter cannot prove calibration accuracy.

Amplification and calibration

Select an amplifier with suitable excitation, input range, noise, bandwidth and overload margin. Dynamic applications need sampling and anti-alias filtering; static weighing prioritizes stability. Calibrate the complete chain with traceable loads, record raw counts and verify PLC units, scaling, alarms and fault response.

Engineering checklist

Define duty and operating limits.
Provide mechanical and wiring drawings.
Specify accuracy and fault behaviour.
Plan traceable commissioning tests.
Retain as-found and as-left records.

Frequently asked questions

What does mV/V mean?

It is full-scale bridge output divided by excitation voltage.

Can four-wire and six-wire cells share one transmitter?

Only when the transmitter, wiring and system design explicitly support the combination.

Why is output unstable near a drive?

Electromagnetic interference, grounding, shielding, excitation or cable damage may affect the low-level bridge signal.

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When troubleshooting, isolate one variable at a time. Substitute a known reference or simulator at the transmitter, then test cable and cell separately. Do not repeatedly rezero a drifting system before preserving as-found data, because this can conceal moisture, mechanical restraint or progressive overload damage.

Cable resistance matters because excitation and signal operate at low levels. Six-wire sense compensation can correct excitation drop at the bridge, but it does not repair a poor terminal or shield. For long runs, calculate voltage, confirm transmitter input limits and inspect common-mode range. In multi-cell systems, compare bridge resistance and unloaded output cell by cell before connecting the junction box. A single wet cable or unstable bridge can affect the summed value. Document terminal mapping and never assume conductor colours match a previous manufacturer.