What you'll learn
Terminate a six-wire strain-gauge load cell on a weighing transmitter, including excitation, millivolt signal, sense conductors and shield. Send the transmitter’s conditioned 4–20 mA output to a PLC analog input.
Lab time: ~22 minutes.
Lesson briefing
Six-Wire Load Cell and Transmitter
Terminate a six-wire strain-gauge load cell on a weighing transmitter, including excitation, millivolt signal, sense conductors and shield. Send the transmitter’s conditioned 4–20 mA output to a PLC analog input.
The device in the real world
A load cell is a metal block with strain gauges bonded to it, so a few millivolts appear on its signal wires when weight is applied. Weighing systems for hoppers, silos and platform scales feed these cells with a precise excitation voltage and read the output with a transmitter. Six-wire cells add two sense wires so the transmitter can measure the real excitation voltage at the cell, after the cable losses.
Why it is wired this way
Remote-sense wires let the transmitter compensate excitation voltage drop along the cable. The SIG pair carries only millivolts and is highly noise-sensitive. The shield is bonded at one end and must not carry operating current.
Worked example
Suppose a cell is rated at 2 mV per volt of excitation and the transmitter supplies 10 V. At full load the signal is only 20 mV. If a long cable drops 0.2 V in the excitation leads, the cell actually sees 9.8 V and the signal is 2 percent low, unless the sense wires report that drop back and the transmitter corrects it. The conditioned 4 to 20 mA output then goes to the PLC analog input. Wire the lab by matching the six cell conductors one for one, then powering the transmitter, then the output pair.
Common mistakes
- Swapping SIG+ and SIG-, so the weight reads backwards.
- Landing a SENSE wire on an EXC terminal at the transmitter, which removes the cable-loss correction.
- Earthing the shield at both ends.
- Routing the signal pair next to motor cables.
What you can now do
You can terminate a six-wire load cell, say what the sense wires are for and send the conditioned output to the PLC.
Commissioning standard
Check the empty-scale output first, then add a known test weight and compare the reading with the expected value.
Hints
Match EXC+, EXC−, SIG+, SIG− and both SENSE conductors one-for-one.
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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.
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