Abridge circuit is atopology ofelectrical circuitry 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 ininstrumentation,filtering andpower conversion.^[1][2]

The best-known bridge circuit, theWheatstone bridge, was invented bySamuel Hunter Christie and popularized byCharles Wheatstone, and is used for measuringresistance. It is constructed from four resistors, two of known valuesR₁ andR₃ (see diagram), one whose resistance is to be determinedR_x, and one which is variable and calibratedR₂. Two opposite vertices are connected to a source of electric current, such as a battery, and agalvanometer is connected across the other two vertices. The variable resistor is adjusted until the galvanometer reads zero. It is then known that the ratio between the variable resistor and its neighbour R1 is equal to the ratio between the unknown resistor and its neighbour R3, which enables the value of the unknown resistor to be calculated.

The Wheatstone bridge has also been generalised to measureimpedance in AC circuits, and to measure resistance,inductance,capacitance, anddissipation factor separately. Variants are known as theWien bridge,Maxwell bridge, andHeaviside bridge (used to measure the effect of mutual inductance).^[3] All are based on the same principle, which is to compare the output of twopotential dividers sharing a common source.

In power supply design, a bridge circuit orbridge rectifier is an arrangement ofdiodes 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 somemotor controllers, anH-bridge is used to control the direction the motor turns.

From the figure to the right, the bridge current is represented asI₅

PerThévenin's theorem, finding the Thévenin equivalent circuit which is connected to the bridge loadR₅ and using the arbitrary current flowI₅, we have:

Thevenin Source (V_th) is given by the formula:

${\displaystyle V_{th}=\left(\frac{R_2}{R_1+R_2}-\frac{R_4}{R_3+R_4}\right)\times U}$

and the Thevenin resistance (R_th):

${\displaystyle R_{th}=\frac{R_1\times R_2}{R_1+R_2}+\frac{R_3\times R_4}{R_3+R_4}}$

Therefore, the current flow (I₅) through the bridge is given byOhm's law:

${\displaystyle I_5=\frac{V_{th}}{R_{th}+R_5}}$

and the voltage (V₅) across the load (R₅) is given by thevoltage divider formula:

${\displaystyle V_5=\frac{R_5}{R_{th}+R_5}\times V_{th}}$

• Phantom circuit - a use of balanced bridge circuits in telephony
• Lattice filter - an application of bridge topology to all-pass filters

• Jim Williams,"Bridge circuits: Marrying gain and balance", Linear Technology Application Note 43, June 1990.
• Bridge circuits - Chapter 8 from an online book.

• Bridge circuits

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