PLC Simulator
← Back to all lessons
Fault 19
Pro

Fault 19 — Load Cell Signal Pair Reversed

What you'll learn

**Reported symptom:** The weighing value moves negative when test weight is applied, while excitation and zero stability are normal.

Lab time: ~14 minutes.

Lesson briefing

Load Cell Signal Pair Reversed

Reported symptom: The weighing value moves negative when test weight is applied, while excitation and zero stability are normal.

The circuit was commissioned from a known-good drawing. Treat every terminal as suspect, but test from source to load instead of guessing. Use voltage mode on an energised control circuit and continuity only after the simulated circuit is isolated.

Your task

Verify the millivolt signal-pair polarity from the load-cell certificate to the transmitter terminals.

Choose the fault category only when the measurements support it.

Hints

Hint 1

Normal excitation confirms the EXC pair is not the problem.

Hint 2

Sign up to unlock the full hint sequence.

Hint 3

Sign up to unlock the full hint sequence.

This lesson uses 6 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.

Sign up to preview

Create a free account to preview this Pro lab; upgrade to open the interactive circuit.

Sign up to preview →

You might also like

Wiring 1A — Power the PLC
Wiring 1 — E-Stop Control Circuit
Wiring 2 — NPN / PNP Sensors (Sinking & Sourcing)

Competency and practice field guide

Reversed load-cell signal fault lesson: practice plan

Direct answer

The learner can distinguish excitation and signal pairs, compare direction at raw and engineering layers and prove correction with zero and known loads.

Written for instrumentation learners diagnosing a scale value that decreases under added load or moves in the opposite direction after maintenance.

an instrumentation calibration bench connecting pressure, temperature, load, level and smart sensors to PLC input channels and reference measurements while studying load-cell excitation, signal polarity and reversed-weight diagnosis
System map / 02

NODE 01observable

Scope

Load-cell type, excitation positive and negative, signal positive and negative, shield, junction box, amplifier, input range, zero, span, direction and mechanical installation.

NODE 02observable

Signal path

Applied force through strain elements and bridge output to signal conductors, amplifier, raw input, scaled weight, quality and independent reference load.

NODE 03observable

Baseline practice

Zero and at least two increasing known loads produce stable values in the correct direction within the declared tolerance.

NODE 04observable

Edge cases

Reversed signal pair, reversed excitation, unloaded offset, creep, side load, loose terminal, shield issue, saturation, broken conductor and restart.

NODE 05observable

Fault practice

A force, mechanics, bridge, excitation, signal, wiring, amplifier, input, scaling or reference mismatch.

NODE 06observable

Transfer to the job

The mechanical installation and measurement chain inspected and calibrated with approved known loads.

Answer surface / 07

Why does a load-cell reading go negative when weight is added?

The signal pair may be reversed, the configured direction may be inverted or the mechanical force may be applied contrary to the intended axis.

How should a repaired load-cell circuit be verified?

Check stable zero, multiple known loads, increasing and decreasing direction, repeatability and any affected alarms or control decisions.

Motor, wiring and VFD path

Connect the control command to physical motor behavior

Move from safe control wiring and contactors into VFD parameters, measurements, faults and PLC command paths.