Bridge Circuits

A bridge circuit consists of two voltage dividers connected in parallel as illustrated below.

If the upper voltage divider (comprising resistors R₁ and R₂) divides the supply voltage in the same ratio as the lower voltage divider (comprising resistors R₃ and R₄), the potential between points C and D is zero (U_D = 0). In this state, the bridge is said to be balanced. The balancing condition is

If resistors R₃ and R₄ are replaced by an adjustable resistor, the bridge circuit can be used to measure resistances; this kind of circuit is termed Wheatstone bridge after the English physicist of the same name (see the diagram below).

R_x is the unknown resistance, R_N a (usually switchable) standard resistance for comparison. To perform the measurement, the bridge is balanced (U_D = 0) and R_x determined using the following relationship: