Load cells are common. You find them in industrial machines, cranes, and even home weighing scales. They are essential for measuring force and weight with incredible precision. To maintain or troubleshoot systems that use load cells, it’s crucial to understand how they work, how to test them, and how to connect them. Let’s dive deep into the world of load cells.

How Load Single Point Cells Work

At their core, load cells are force sensors. They operate on the idea that applying a force or load changes the cell’s electrical resistance. Most modern load cells rely on strain gauges. These tiny devices stretch when you apply force. This change affects their resistance. These changes are extremely small, but when amplified and processed, they provide a very accurate measurement of the load.

 

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A basic load cell usually forms part of a Wheatstone bridge circuit, which is excellent at detecting small changes in resistance. When the load is applied, the strain gauges inside deform slightly. This deformation alters their resistance, causing a change in the output voltage of the bridge. This voltage is then read by a display or controller, showing the weight or force.

How Do Compression Load Cells Work?

Compression load cells measure compressive forces. These forces push or press down on the load cell. When a weight presses down on a compression load cell, the internal strain gauges experience compression (shortening), leading to a change in their electrical resistance.

Compression load cells are usually cylindrical or disk-shaped. They measure force in vertical applications.

You’ll commonly find them in:

• Material testing machines
• Structural monitoring
• Industrial scales

Their design makes sure the load is applied correctly. This keeps the readings accurate over time.

How Does a Crane Load Cell Work?

Cranes deal with massive forces, so crane load cells are specially designed to measure large loads safely and reliably. Usually, a crane load cell is installed between the crane’s hook and the load. It might be a load shackle (a shackle with a built-in load cell) or a load link.

In a crane, when a load is lifted, the force travels through the load cell. The strain gauges inside sense tension, not compression like compression cells. They turn this tension into an electrical signal. This allows the crane operator to monitor the load in real-time and ensure that they’re not exceeding the crane’s safe working load limits. It’s a critical safety feature that can prevent dangerous overloads.

How Load Cell Sensors Work

Every load cell sensor relies on the same basic idea: deformation under force. Strain gauges are firmly attached to a structure inside the load cell. This structure is usually a metal body. When the structure bends or deforms due to an applied load, the strain gauges also deform.

The physical deformation is tiny, often just microns. Still, the strain gauge’s resistance changes enough to create a readable voltage output. This tiny voltage change is what gets amplified and displayed as weight or force.

High-quality load cell sensors reduce errors. They help with temperature drift, creep, and hysteresis. Creep means changes over time with a steady load. Hysteresis refers to differences between loading and unloading. Some load cells even have temperature compensation features built in.

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How to Test a Load Cell

Testing a load cell helps ensure it’s working correctly. Here’s a simple way to do it:

1. Visual Inspection: Check for any physical damage — dents, corrosion, broken wires.
2. Zero Balance Check: Disconnect the load cell from the system. Use a multimeter to measure the voltage between the signal lines (usually green and white wires). With no load applied, the output should be close to zero (millivolts).
3. Bridge Resistance Test: Check the resistance between the excitation wires, usually red and black. Also, measure the resistance across the signal wires. Compare the results to the manufacturer’s specifications. Significant deviations could mean internal damage.
4. Apply a Known Load: If possible, apply a known weight to the load cell and check whether the output matches the expected value.

If anything looks off during these steps, the load cell may be faulty.

How to Check a 4-Wire Load Cell

A 4-wire load cell has two wires for power (excitation) and two wires for signal. To check a 4-wire load cell:

1. Identify the Wires: Typically, red is for excitation (+), black is for excitation (-), green is for signal (+), and white is for signal (-).
2. Check Excitation Resistance: Measure resistance between the red and black wires. It should match the manufacturer’s specified input resistance (e.g., 350 ohms).
3. Check Signal Resistance: Measure the output resistance between the green and white wires. It should be close to the input resistance.
4. Check Isolation: Ensure no continuity between any of the wires and the body of the load cell (no short circuits).

4-wire load cells are easy to use. However, they lack sense wires. This means they can’t adjust for voltage drops in cables. As a result, accuracy may suffer with long cables.

How to Check a 6-Wire Load Cell

A 6-wire load cell includes two extra wires called sense wires. These help compensate for changes in cable resistance or temperature.

To check a 6-wire load cell:

1. Identify the Wires: Red (excitation +), black (excitation -), green (signal +), white (signal -), blue (sense +), brown (sense -).
2. Measure Excitation and Signal Resistance: As with 4-wire, check red-black and green-white resistances.
3. Check Sense Wires: Measure between sense wires and excitation wires; the voltage difference should be tiny when powered.
4. Verify Isolation: Confirm that none of the wires are shorted to the body.

6-wire load cells are more accurate, especially in tough environments, because they adjust for small variations automatically.

How to Connect a Load Cell

Connecting a load cell correctly is crucial for getting accurate readings.

• Step 1: Identify the wires: As explained above, 4-wire and 6-wire load cells have standard color codes.
• Step 2: Connect Excitation: Connect the excitation+ (red) and excitation- (black) wires to the power supply of your amplifier or indicator.
• Step 3: Connect Signal: Connect signal+ (green) and signal- (white) to the signal input terminals.
• Step 4: Connect Sense Wires (if 6-wire): Connect sense+ (blue) and sense- (brown) to the sense inputs if your amplifier has them. If not, tie sense+ to excitation+ and sense- to excitation-.

Always double-check the wiring diagram provided by your load cell manufacturer, as sometimes colors vary slightly.

Conclusion

Load cells are amazing devices that combine mechanical engineering with precise electronics. Whether it’s testing a simple bathroom scale, monitoring an industrial crane, or setting up an advanced weighing system, understanding how load cells work and how to properly test and connect them is essential. Taking the time to check them carefully can save you from costly errors and dangerous accidents. With a little knowledge and a multimeter, you can diagnose and solve many load cell issues yourself.