Bridge circuit

(electricity) An electrical network consisting basically of four impedances connected in series to form a rectangle, with one pair of diagonally opposite corners connected to an input device and the other pair to an output device.

A circuit composed of a source and four impedances that is used in the measurement of a wide range of physical quantities. The bridge circuit is useful in measuring impedances (resistors, capacitors, and inductors) and in converting signals from transducers to related voltage or current signals. See also Capacitor; Inductor; Resistor; Transducer.

The bridge impedances Z₁, Z₂, Z₃, Z₄, shown in the illustration may be single impedances (resistor, capacitor, or inductor), combinations of impedances, or a transducer with varying impedance. For example, strain gages are resistive transducers whose resistance changes when they are deformed.

Bridge circuit with source and impedances.

Bridge circuits are often used with transducers to convert physical quantities (temperature, displacement, pressure) to electrical quantities (voltage and current). High-accuracy voltmeters and ammeters are relatively inexpensive, and the voltage form of a signal is usually most convenient for information display, control decisions, and data storage. Another important advantage of the bridge circuit is that it provides greater measurement sensitivity than the transducer.

The bridge circuit is balanced when the output read by the meter is zero. In this condition the voltages on both sides of the meter are identical. The bridge is used in two forms. The null adjustment method requires adjustment of a calibrated impedance to balance it. In this case the meter is usually a highly sensitive current-measuring galvanometer. The null adjustment method is often used to measure impedances, with the output read from a dial attached to the adjustable impedance. The deflection method requires on accurate meter in the bridge to measure the deviation from the balance condition. The deviation is proportional to the quantity being measured.

There are many special forms of the bridge circuit. When all of the impedances are resistive, it is commonly called a Wheatstone bridge. Other common forms use a current source in place of the voltage source, a sinusoidal source in place of a constant (dc) source, or branch impedances which are specific combinations of single passive impedances. The bridge circuit is also used in a variety of electrical applications varying from oscillators to instrumentation amplifier circuits for extremely accurate measurements. See also Instrumentation amplifier; Oscillator; Wheatstone bridge.

The noun has one meaning:

Meaning #1: a circuit consisting of two branches (4 arms arranged in a diamond configuration) across which a meter is connected
  Synonym: bridge

A bridge circuit is a type of electrical circuit in which the current in a conductor splits into two parallel paths and then recombines into a single conductor, thereby enclosing a loop. It was originally used for measurement purposes, but can also be used in power supplies.

Schematic of a Wheatstone bridge

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 (R_x), 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 electrical 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 neighbour is equal to the ratio between the unknown resistor and its neighbour, 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.

See also

• Fontana bridge
• Graetz bridge
• Kelvin bridge
• Lattice filter
• Maxwell bridge
• Phantom circuit - a use of balanced bridge circuits
• Wheatstone bridge
• Wien bridge oscillator
• Bridge T circuit

Gallery

Wheatstone bridge   Wien bridge   Maxwell bridge   H bridge   H bridge   Fontana bridge   Diode bridge   Diode bridge   Diode bridge   Kelvin bridge   Lattice bridge   Bridge T circuit   Bridge T circuit (Zobel bridge)   Bridge T circuit   Bridge T circuit   Bode network

External links

• Bridge circuits - Chapter 8 from an online book.
• Wien, Shering, Hay, Owen, Maxwell, and Resonance Bridges - With balance equations

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