Why Load Cells Use Four Strain Gauges in a Wheatstone Bridge

This article explains why four metallic foil strain gauges are wired in a full Wheatstone bridge configuration within load-cell weighing electronics. By using four active gauges, engineers can effectively cancel out temperature-induced resistance changes while simultaneously maximizing the electrical signal generated by an applied weight. The following sections break down the mechanics of temperature drift, the electrical layout of the Wheatstone bridge, and how the two combine to produce highly accurate and stable weight measurements.

The Problem of Temperature Drift in Strain Gauges

A metallic foil strain gauge works by changing its electrical resistance when it is physically stretched (tension) or compressed (compression). However, mechanical force is not the only thing that alters the resistance of a strain gauge. Temperature fluctuations cause two significant issues known as thermal drift:

1. Thermal Coefficient of Resistance: The metal foil inside the gauge naturally changes resistance as it heats up or cools down, regardless of whether a load is applied.
2. Thermal Expansion: The metal body of the load cell expands when heated and contracts when cooled. The strain gauge bonded to this metal cannot distinguish between physical deformation caused by an applied weight and deformation caused by thermal expansion.

Because the resistance change from a physical weight is extremely small—often measured in fractions of an ohm—uncompensated temperature changes would easily overwhelm the actual weight signal, leading to false readings.

How the Wheatstone Bridge Works

A Wheatstone bridge is an electrical circuit arranged in a diamond shape with four resistive "legs" connected to an excitation voltage source and a voltage-measuring output. The output voltage is determined by the ratio of resistances between the adjacent legs of the bridge.

In a load cell using a "full-bridge" configuration, all four legs of the circuit are active metallic foil strain gauges bonded to the same metal element:

• Two gauges are placed in tension (their resistance increases when weight is applied).
• Two gauges are placed in compression (their resistance decreases when weight is applied).

Gauges experiencing the same type of force are wired into opposite legs of the bridge, while gauges experiencing opposite forces are wired into adjacent legs.

Canceling Temperature Drift

The magic of the Wheatstone bridge lies in how it handles resistance ratios between adjacent legs. Because all four gauges are manufactured from the same batch of foil and bonded to the same thermally conductive metal body, they experience the exact same temperature changes at the exact same time.

When the ambient temperature rises, the electrical resistance of all four strain gauges changes by the exact same amount and in the same direction. Because the Wheatstone bridge measures the difference in ratios between adjacent legs, identical changes across all four gauges cancel each other out mathematically. The bridge remains balanced, and the output voltage remains at zero despite the temperature shift.

Maximizing the Weight Signal

While temperature changes affect all four gauges equally, an applied physical load affects them differently due to their physical placement on the load cell.

When weight is applied:

• The two tension gauges stretch, causing their resistance to increase.
• The two compression gauges contract, causing their resistance to decrease.

Because the tension and compression gauges are on adjacent legs of the Wheatstone bridge, their opposing resistance changes do not cancel out. Instead, they add together. This differential effect unbalances the bridge and produces an output voltage that is four times stronger than what a single strain gauge could produce on its own. This simultaneously boosts the signal-to-noise ratio and eliminates thermal drift, resulting in precise, stable weighing electronics.