A bridge circuit is a type of electrical circuit in which two circuit branches (usually in parallel with each other) are "bridged" by a third branch connected between the first two branches at some intermediate point along them. The bridge was originally developed for laboratory measurement purposes and one of the intermediate bridging points is often adjustable when so used. Bridge circuits now find many applications, both linear and non-linear, including in instrumentation, filtering and power conversion.
The best-known bridge circuit, the Wheatstone bridge, was invented by Samuel Hunter Christie and popularized by Charles Wheatstone, and is used for measuring resistance. It is constructed from four resistors, one of which has an unknown value (Rx), one of which is variable (R2), and two of which are fixed and equal (R1 and R3), connected as the sides of a square. Two opposite corners of the square are connected to a source of electric current, such as a battery. A galvanometer is connected across the other two opposite corners. The variable resistor is adjusted until the galvanometer reads zero. It is then known that the ratio between the variable resistor and its neighbor is equal to the ratio between the unknown resistor and its neighbor, and this enables the value of the unknown resistor to be calculated.
The Wheatstone bridge has also been developed to measure impedance in AC circuits, resulting in designs such as the Wien bridge, the Maxwell bridge and the Heaviside bridge. All are based on the same principle, which is to compare the output of two potentiometers sharing a common source.
In power supply design, a bridge circuit or bridge rectifier is an arrangement of diodes or similar devices used to rectify an electric current, i.e. to convert it from an unknown or alternating polarity to a direct current of known polarity.
In some motor controllers, a H-bridge is used to control the direction the motor turns.