The Wheatstone Bridge

The Wheatstone bridge circuit in its simplest form (Fig. 516.1) consists of four resistive elements, or bridge arms (R ₁ , R ₂ , R ₃ , R ₄ ), connected in a series-parallel arrangement, and an excitation voltage source (E). The electrical connections where pairs of bridge arms are joined to the leadwires from the excitation voltage source are referred to as input corners of the bridge. A differential output voltage (e _o ) is measured at the two remaining bridge corners, referred to as output or signal corners.

Fig. 516.1 - Basic Wheatstone Bridge Circuit.

While a mathematical proof is beyond the scope of this publication, it can be shown that if the arm resistances are chosen such that the bridge is resistively symmetrical about an imaginary line drawn through the bridge output corners (as is the case with most commercially available strain gage instrumentation and as assumed in this publication) the differential output voltage (e _o ) will be identically zero regardless of the value of the excitation supply voltage. In this condition, the bridge is said to be resistively balanced. If the bridge is not in balance, a differential voltage will be present at the output corners of the bridge, and the magnitude of this output voltage will be proportional to the amount of unbalance.

Page 2 of 4