Editing Bridge circuit

{{Short description|Type of electrical circuit}}
A '''bridge circuit''' is a [[Topology (electrical circuits)|topology]] of [[electrical network|electrical 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 in [[instrumentation]], [[electronic filter|filter]]ing and [[power converter|power conversion]].<ref>Bureau of Naval Personnel, ''Basic Electricity'', p.114, Courier Dover Publications, 1970 {{ISBN|0-486-20973-3}}.</ref><ref>Clarence W. De Silva, ''Vibration monitoring, testing, and instrumentation'', pp.2.43-2.49, CRC Press, 2007 {{ISBN|1-4200-5319-1}}</ref>

The best-known bridge circuit, the [[Wheatstone bridge]], was invented by [[Samuel Hunter Christie]] and popularized by [[Charles Wheatstone]], and is used for measuring [[electrical resistance|resistance]]. It is constructed from four resistors, two of known values ''R''<sub>1</sub> and ''R''<sub>3</sub> (see diagram), one whose resistance is to be determined ''R''<sub>x</sub>, and one which is variable and calibrated ''R''<sub>2</sub>.  Two opposite vertices are connected to a source of electric current, such as a battery, and a [[galvanometer]] 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 measure [[electrical impedance|impedance]] in AC circuits, and to measure resistance, [[inductance]], [[capacitance]], and [[dissipation factor]] separately. Variants are known as the [[Wien bridge]], [[Maxwell bridge]], and [[Heaviside bridge]] (used to measure the effect of mutual inductance).<ref>[http://www.allaboutcircuits.com/vol_2/chpt_12/5.html All about circuits: AC bridge circuits]</ref>  All are based on the same principle, which is to compare the output of two [[potential divider]]s sharing a common source.

In power supply design, a bridge circuit or [[bridge rectifier]] is an arrangement of [[diode]]s 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 controller]]s, an [[H-bridge]] is used to control the direction the motor turns.

==Bridge current equation==
[[File:Mesh Analysis-bridge circuit.svg|alt=bridge network current flow|thumb|359x359px|Analysis of bridge current]]
From the figure to the right, the bridge current is represented as ''I''<sub>5</sub>

Per [[Thévenin's theorem]], finding the Thévenin equivalent circuit which is connected to the bridge load ''R''<sub>5</sub> and using the arbitrary current flow ''I''<sub>5</sub>, we have:

Thevenin Source (''V''<sub>th</sub>) is given by the formula:

<math>V_{th}=\left(\frac{R_2}{R_1+R_2}-\frac{R_4}{R_3+R_4}\right)\times U</math>

and the Thevenin resistance (''R''<sub>th</sub>):

<math>R_{th}=\frac{(R_1 + R_3) \times (R_2 + R_4)}{R_1 + R_3 + R_2 + R_4}</math>

Therefore, the current flow (''I''<sub>5</sub>) through the bridge is given by [[Ohm's law]]:

<math>I_5=\frac{V_{th}}{R_{th}+R_5}</math>

and the voltage (''V''<sub>5</sub>) across the load (''R''<sub>5</sub>) is given by the [[voltage divider]] formula:

<math>V_5=\frac{R_5}{R_{th} + R_5} \times V_{th}</math>

== See also ==
* [[Phantom circuit]] - a use of balanced bridge circuits in telephony
* [[Lattice filter]] -  an application of bridge topology to all-pass filters

==Gallery==
{{gallery
|Image:Wheatstonebridge.svg|[[Wheatstone bridge]]
|Image:Mostek Wiena.svg| [[Wien bridge]]
|Image:Maxwell_bridge.svg| [[Maxwell bridge]]
|Image:H_bridge.svg| [[H bridge]]
|Image:Fontanabridge.png| [[Fontana bridge]]
|Image:Diode_bridge_alt_1.svg| [[Diode bridge|Full wave rectifier]]
|Image:KelvinBridge.gif| [[Kelvin bridge]]
|Image:Lattice_filter%2C_general.svg| [[Lattice filter|Lattice bridge]]
|Image:Zobel_%283%29_Bridge_T.svg| [[Bridged T circuit]]
|Image:Carey Foster bridge.svg| [[Carey Foster bridge]]
|Image:Ring_Modulator.PNG| [[Ring modulation|Ring Modulator]]
}}

==References==
{{Reflist}}

==External links==
*[[Jim Williams (analog designer)|Jim Williams]], [http://cds.linear.com/docs/en/application-note/an43f.pdf "Bridge circuits: Marrying gain and balance"], Linear Technology Application Note 43, June 1990.
*[http://www.allaboutcircuits.com/vol_1/chpt_8/10.html Bridge circuits] - Chapter 8 from an online book.

{{Bridge circuits}}

{{Authority control}}

[[Category:Bridge circuits| ]]