What Is a Load Cell? How It Works and How to Wire One to a PLC
A load cell explained — how the strain-gauge Wheatstone bridge works, the types, mV/V output, and how it wires through an amplifier to a PLC analog input.
A load cell is a transducer that converts a mechanical force — weight, tension, compression, or torque — into a proportional electrical signal. Inside nearly every modern weighing scale, hopper, conveyor, and tension-control system, a load cell is the sensor that answers the question: how much force is being applied right now?
The output is a small DC voltage, typically expressed in millivolts per volt (mV/V) of excitation, that scales linearly with applied load from zero to the rated capacity of the cell.
What Is a Load Cell?
A load cell is a force-measurement sensor built around a precision metal spring element — commonly called the flexure or elastic element — that deflects predictably under load. Electrical strain gauges bonded to that element detect the microscopic deformation and convert it to a voltage change. An onboard or external signal conditioner then scales that voltage to a usable output for a PLC, controller, or display.
Load cells sit at the core of almost every weighing and force-measurement system in industry: platform scales, tank weighing frames, conveyor belt scales, tensile testers, press-force monitors, and batching hoppers. They are classified as industrial sensors that produce an analog output proportional to applied force.
Key facts at a glance:
- Output signal: millivolts per volt (mV/V) of excitation — typically 1 to 3 mV/V at rated capacity
- Excitation voltage: 5 VDC or 10 VDC (some cells accept up to 15 VDC)
- Accuracy: OIML R 60 accuracy classes C3 (3,000 verification intervals) and C6 are common in legal-for-trade systems
- Operating principle: elastic deformation of a metallic body sensed by bonded strain gauges in a Wheatstone bridge circuit
How a Load Cell Works: Strain Gauges and the Wheatstone Bridge
The Elastic Element
When a force is applied to a load cell body — aluminium, alloy steel, or stainless steel depending on the type — the body bends or compresses by a tiny, controlled amount. This elastic deformation is the same every time for the same force, which is what makes the sensor repeatable and accurate.
The Strain Gauge
A strain gauge is a resistive element, typically a metal foil grid bonded to a polyimide backing, that is adhesively cemented to the elastic element. When the element stretches (tensile strain), the foil grid elongates and its electrical resistance increases. When it compresses, resistance decreases. The resistance change is small — on the order of milliohms — but measurable.
A single strain gauge on its own is temperature-sensitive and only responds to one direction of strain. Load cells therefore use four strain gauges arranged in a full Wheatstone bridge to cancel temperature drift and double the sensitivity.
The Wheatstone Bridge
The Wheatstone bridge is the heart of every foil-gauge load cell. Four resistors — in a load cell, four strain gauges — are arranged in a diamond (bridge) circuit. Two gauges are bonded where the element is in tension (resistance increases under load) and two where it is in compression (resistance decreases under load). This arrangement is called a full-bridge configuration.
When the bridge is balanced (no load), the output voltage is zero. As load is applied the bridge goes out of balance and a small differential voltage appears across the output terminals. That differential voltage, divided by the excitation voltage, gives the mV/V sensitivity figure stamped on the load cell datasheet.
Example: a 200 kg load cell with a 2 mV/V rating powered by 10 VDC excitation will produce 0 mV at 0 kg and 20 mV at 200 kg. That 20 mV full-scale signal is why an amplifier is always required before a standard PLC analog input.
Why the Full Bridge Matters
The full-bridge configuration gives load cells two critical advantages over simpler sensors:
- Temperature compensation — opposite arms change resistance in the same direction with temperature, so the differential output cancels thermal drift.
- Double sensitivity — all four gauges actively contribute, producing twice the output of a half-bridge arrangement.
Types of Load Cells
Load cell geometry is chosen based on the direction and magnitude of force being measured, available mounting space, and whether the installation is legal for trade.
Compression Load Cells
Compression load cells accept forces pushing axially along the cell body. They are the most common type in tank and hopper weighing, where legs or mounting feet sit on top of the cell and the cell body rests on a base plate. Capacities range from a few kilograms up to several hundred tonnes for structural and testing applications.
Tension Load Cells (Tension Links)
Tension load cells are designed to be pulled apart. They are threaded at both ends and installed inline with a cable, chain, or lifting eye. Common uses include crane scales, overhead hoppers suspended from load pins, and tensile testing machines.
S-Beam Load Cells
The S-beam combines compression and tension capability in a single cell with an S-shaped flexure. They accept both upward and downward forces and are versatile for general-purpose weighing, conveyor belt tension, and hanging hopper applications.
Shear Beam Load Cells
Shear beam cells use a cantilevered bar where shear stress — rather than bending — is the primary strain mode. They are flat, robust, and excel in floor scale platforms and platform-scale frames. Multiple shear beam cells are typically connected in parallel under a single scale deck.
Single-Point Load Cells
Single-point (or off-centre) load cells maintain accuracy even when the load is placed anywhere across the platform, without needing four-cell corner correction. They are the default choice for small bench scales and checkweighers.
Hydraulic and Pneumatic Load Cells
Hydraulic load cells use a fluid-filled chamber: applied force increases fluid pressure, which is read by a pressure gauge or transducer. They require no electrical excitation and are used in hazardous areas where electronic sensors are impractical. Pneumatic load cells work on the same principle with compressed air instead of oil. Both types connect to the instrumentation hub via pressure transmitters rather than mV/V amplifiers.
Load Cell Specifications You Need to Know
Understanding a load cell datasheet is essential before selecting a cell or configuring a PLC system.
Rated Capacity
The maximum force the cell is designed to measure accurately. Exceeding the rated capacity risks permanent zero shift (overload damage). Most load cells can survive short-term overloads of 150% capacity, with safe overload ratings up to 200–300%.
Sensitivity (mV/V)
The full-scale output expressed as millivolts per volt of excitation. A 2 mV/V cell at 10 V excitation produces 20 mV at full load. This value is measured and laser-trimmed at the factory; the actual figure is printed on the cell's calibration certificate.
Excitation Voltage
The DC supply voltage that energises the Wheatstone bridge. Most load cells accept 5–15 VDC. Higher excitation increases output signal magnitude but also increases self-heating of the bridge resistors, which can degrade zero stability. Amplifiers and transmitters typically provide a regulated 5 VDC or 10 VDC excitation output.
Accuracy Class (OIML R 60)
For trade-approved weighing, load cells are classified under OIML R 60 (International Organization of Legal Metrology). Class C3 supports up to 3,000 verification intervals; Class C6 supports 6,000. The class determines the number of display divisions a legal-for-trade scale can use.
Input/Output Impedance
Datasheets specify the bridge input impedance (excitation circuit) and output impedance (signal circuit), typically 350 Ω or 1,000 Ω for standard cells. Multiple cells wired in parallel require impedance matching to ensure equal load sharing.
Non-Linearity and Hysteresis
Non-linearity is the maximum deviation of the output curve from a straight line, expressed as a percentage of rated output. Hysteresis is the difference in output for the same load approached from above versus below. Both are typically ±0.02% to ±0.05% of rated output for industrial cells.
IP Rating
Most industrial load cells carry IP67 or IP68 ratings for water and dust ingress protection. Stainless steel cells in food and pharmaceutical environments often comply with IP69K (high-pressure wash-down).
Load Cell vs Strain Gauge: What Is the Difference?
These two terms are often confused.
| Strain Gauge | Load Cell | |
|---|---|---|
| What it is | A resistive element that changes resistance with strain | A complete transducer assembly |
| Output | Resistance change (ohms) | Voltage (mV/V), 4-20mA, or digital |
| Needs spring element? | Bonded directly to any structure | Built-in precision elastic element |
| Temperature compensation | Requires external circuit | Built-in full bridge with compensation |
| Calibration | Requires in-situ calibration | Factory-calibrated to OIML or ASTM |
| Typical use | Structural health monitoring, custom measurement | Industrial weighing, force testing |
A strain gauge is the sensing component inside a load cell. You can also bond strain gauges directly to beams, columns, or frames to measure structural strain — but the result is not a calibrated load cell because there is no precision elastic element with a defined cross-section. A load cell combines a precision spring element, multiple temperature-compensated strain gauges, and a factory calibration into a single device with a guaranteed accuracy specification.
When your application requires legal-for-trade weight measurement or certified force data, you need a load cell, not bare strain gauges. For structural strain monitoring or custom transducer development, bare strain gauges bonded to your own structure may be the right choice.
How a Load Cell Connects to a PLC
This is the section most tutorials skip. A load cell on its own cannot connect directly to a standard PLC analog input card because:
- The mV/V output signal is too small (typically 0–20 mV) for a PLC analog input designed for 4-20mA or 0-10V signals.
- The load cell requires a stable, regulated excitation supply that the PLC analog input card does not provide.
A load cell amplifier (also called a signal conditioner or load cell transmitter) sits between the load cell and the PLC to solve both problems.
4-Wire vs 6-Wire Connection
Load cells are supplied with either a 4-wire or 6-wire cable.
4-wire load cells have:
- Red: +EXC (excitation positive)
- Black: −EXC (excitation negative)
- Green: +SIG (signal positive)
- White: −SIG (signal negative)
6-wire load cells add two sense wires:
- Blue: +SENSE
- Yellow: −SENSE
The sense wires connect back to the excitation supply terminals of the amplifier. The amplifier compares the excitation voltage it intended to deliver (at its output terminals) with the voltage actually present at the load cell bridge (at the sense terminals), and adjusts its output to compensate for voltage drop along the cable. For cable runs longer than about 5 metres, always use a 6-wire load cell with a sense-compensating amplifier.
Load Cell Amplifier to PLC: The Signal Chain
Once the amplifier converts the mV/V signal to a standard 4-20mA current loop or 0-10V output, wiring to the PLC analog input follows the same rules as any process transmitter. Refer to the analog vs digital signals guide for a full treatment of signal types.
4-20mA output (most common):
- Connect amplifier output+ to PLC channel input terminal
- Connect PLC channel common to amplifier output−
- The PLC analog input card provides or the loop requires 24VDC loop supply
- 4mA represents zero (empty scale or no tension); 20mA represents full-scale (rated capacity)
0-10V output:
- Connect amplifier output+ to PLC channel V+ terminal
- Connect amplifier GND to PLC channel GND
- Keep cable runs short (voltage signals are more susceptible to cable resistance and noise)
- 0V represents zero load; 10V represents rated capacity
Scaling and Taring in PLC Logic
After wiring, three programming tasks remain. The PLC analog input scaling guide covers the mathematics in detail; the summary for load cells is:
1. Raw count to milliamps
The PLC analog input card digitises the signal to a raw integer count (typically 0–27,648 on Siemens S7, 0–32,767 on Allen-Bradley). Scale this raw count to the 4-20mA range first.
2. Engineering unit scaling
Map the 4mA value (0% span) to 0 kg and the 20mA value (100% span) to the load cell rated capacity. Use a linear scaling function block (SCALE in Siemens TIA Portal, SCL in RSLogix 5000).
Weight_kg = ((Raw_mA - 4.0) / 16.0) * Rated_Capacity_kg
3. Tare and filter
- Tare — capture the current weight reading when the scale is known to be empty and subtract it from all subsequent readings to zero the display.
- Digital filter — apply a moving average or first-order low-pass filter (IIR) to remove mechanical vibration and electrical noise. A filter time constant of 50–200 ms is typical for platform scales.
Multiple Load Cells in a Junction Box
Platform scales and tank-weighing frames use multiple load cells (typically 3 or 4) wired in parallel at a summing junction box. The junction box contains trim potentiometers that allow corner-correction: adjusting each cell's contribution so the scale reads correctly regardless of where the load is placed on the platform. The summed output of the junction box connects to a single amplifier channel and then to the PLC as a single analog signal representing total weight.
Frequently Asked Questions
What is a load cell?
A load cell is a transducer that converts a mechanical force — compression, tension, or shear — into a proportional electrical signal. It consists of a precision metal elastic element with bonded strain gauges arranged in a Wheatstone bridge. The output is a small differential voltage proportional to the applied force, expressed in millivolts per volt (mV/V) of excitation supply voltage.
How does a load cell work?
When force is applied to a load cell, the metal body deflects by a tiny, controlled amount. Strain gauges bonded to the body change resistance in proportion to that deflection. Arranged in a full Wheatstone bridge, two gauges increase resistance (tension side) and two decrease resistance (compression side). The bridge imbalance produces a differential output voltage proportional to the applied force. A signal conditioner amplifies this mV/V signal to a usable 4-20mA or 0-10V output for a PLC or indicator.
What are the 4 wires on a load cell?
The four wires on a standard load cell are:
- Red — +EXC (excitation positive)
- Black — −EXC (excitation negative)
- Green — +SIG (signal positive, output)
- White — −SIG (signal negative, output)
6-wire load cells add Blue (+SENSE) and Yellow (−SENSE) for remote sense compensation, which corrects for voltage drop along long cable runs.
What is the difference between a load cell and a strain gauge?
A strain gauge is a resistive sensing element — a foil grid that changes electrical resistance when stretched or compressed. A load cell is a complete, factory-calibrated transducer that contains multiple strain gauges bonded to a precision machined elastic element, forming a temperature-compensated Wheatstone bridge with a guaranteed mV/V sensitivity specification. Every load cell contains strain gauges, but a bare strain gauge bonded to a structural member is not a load cell — it lacks the precision spring element, calibration, and environmental sealing that define a calibrated force transducer.
