modelWheatstoneBridge

Information

Full Wheatstone bridge made of four strain gauges bonded to the load cell body (LoadCell). The resistance of a gauge changes with the strain it undergoes: R = R0·(1 ± K·eps), where K is the gauge factor (about 2 for a metal gauge).

Two gauges are stretched (j1, j4) and two compressed (j2, j3), mounted on the diagonals: the two half-bridges move away from E/2 in opposite directions, and the output voltage is vOut = S+ − S− = E·K·eps. It is four times larger than with a single gauge, and proportional to the excitation voltage E: this is why the HX711 compares the bridge output with this same excitation (ratiometric measurement), which makes the measurement insensitive to supply variations.

Order of magnitude: 1 mV/V at full load (K·eps = 2 × 500 µm/m), i.e. 4.3 mV for a 4.3 V excitation. This very weak signal explains the gain of 128 of the HX711.

The internal schematic (Diagram view) shows the four gauges, to relate the component to a bridge drawn in class.

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Components