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Bridge circuit

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A bridge circuit is a topology of 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, filtering and power conversion.^[1]^[2]

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, two of known values R₁ and R₃ (see diagram), one whose resistance is to be determined R_x, and one which is variable and calibrated R₂. 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 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).^[3] All are based on the same principle, which is to compare the output of two potential dividers 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, an H-bridge is used to control the direction the motor turns.

Bridge current equation

[edit]

From the figure to the right, the bridge current is represented as I₅

Per Thévenin's theorem, finding the Thévenin equivalent circuit which is connected to the bridge load R₅ and using the arbitrary current flow I₅, we have:

Thevenin Source (V_th) is given by the formula:

$V_{th}=\left({\frac {R_{2}}{R_{1}+R_{2}}}-{\frac {R_{4}}{R_{3}+R_{4}}}\right)\times U$

and the Thevenin resistance (R_th):

$R_{th}={\frac {R_{1}\times R_{2}}{R_{1}+R_{2}}}+{\frac {R_{3}\times R_{4}}{R_{3}+R_{4}}}$

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

$I_{5}={\frac {V_{th}}{R_{th}+R_{5}}}$

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

$V_{5}={\frac {R_{5}}{R_{th}+R_{5}}}\times V_{th}$

Gallery

[edit]

References

[edit]

^ Bureau of Naval Personnel, Basic Electricity, p.114, Courier Dover Publications, 1970 ISBN 0-486-20973-3.
^ Clarence W. De Silva, Vibration monitoring, testing, and instrumentation, pp.2.43-2.49, CRC Press, 2007 ISBN 1-4200-5319-1
^ All about circuits: AC bridge circuits

External links

[edit]

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

v t e Bridge circuits
Resistance	Kelvin bridge Wheatstone bridge
Resistance–Capacitance	Schering bridge Wien bridge
General	Bridged T circuit Carey Foster bridge Fontana bridge Lattice filter Murray loop bridge Transformer ratio arm bridge
Inductance	Maxwell bridge Anderson's bridge Hay's bridge
Other	Diode bridge H-bridge

Revisions and contributors Edit on Wikipedia Read on Wikipedia

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from Grokipedia

A bridge circuit is an electrical arrangement consisting of two parallel branches connected by a bridging network, typically used to measure unknown electrical parameters such as resistance, inductance, or capacitance by achieving a balance condition that nullifies the output voltage.^[1] This configuration, analogous to a mechanical balance scale, compares the outputs of two voltage dividers sharing a common excitation source, enabling precise differential measurements with minimal interference from supply variations or nonlinearities.^[2]^[3] The most foundational form, the Wheatstone bridge, was first demonstrated by Samuel Hunter Christie in 1833 for DC resistance measurement and later popularized by Charles Wheatstone in 1843, who adapted it for practical use in determining unknown resistances through ratio comparisons of four arms.^[3]^[1] Balance occurs when the ratio of resistances in one branch equals that in the other (e.g.,

R_1 / R_2 = R_3 / R_x

), yielding zero differential voltage across the detector, independent of the excitation voltage.^[2] This null-balance principle provides high accuracy, often up to ±0.05% with sensitive detectors, making it essential for calibration labs and instrumentation.^[2] Beyond resistance, bridge circuits extend to AC applications, including the Wien bridge for capacitance-inductance measurements (developed by Max Wien in 1891) and the Maxwell bridge for inductance, each tailored to specific impedance parameters.^[3]^[1] They are integral in transducers like strain gauges (pioneered by Lord Kelvin in 1856) for sensing physical quantities such as pressure, temperature, and force, where small resistance changes produce measurable imbalances amplified for output.^[3] Additionally, bridge topologies underpin oscillators, such as the Wien bridge oscillator invented by William R. Hewlett in 1939, and switching circuits for signal processing across DC to GHz frequencies.^[3] Modern implementations often incorporate instrumentation amplifiers with high common-mode rejection (e.g., 120 dB) to enhance sensitivity and noise immunity in industrial and precision applications.^[3]

Introduction

Definition and Basic Concept

A bridge circuit is a fundamental electrical configuration designed for precise measurement of unknown electrical parameters, such as resistance in direct current (DC) applications or impedance in alternating current (AC) setups, by employing a null detection method.^[4] This method involves comparing two circuit branches until equilibrium is reached, where the detector indicates zero output, enabling high-accuracy determinations without direct calibration against absolute standards.^[3] Bridge circuits function as versatile tools in instrumentation, applicable to sensing physical quantities like strain or temperature through associated transducers.^[2] In its basic form, a bridge circuit comprises four arms—typically labeled as resistors R1, R2, R3, and the unknown Rx—connected in a diamond-shaped topology.^[5] An excitation voltage source is applied across one diagonal of the diamond (between the junctions of R1-Rx and R2-R3), while a detector, such as a galvanometer for DC or a sensitive voltmeter for AC, is placed across the opposite diagonal (between the junctions of R1-R2 and Rx-R3).^[4] This arrangement creates two parallel voltage divider networks, with the output derived from the potential difference between the midpoints, allowing for the isolation and quantification of the unknown component when balance is achieved.^[2] The primary purpose of a bridge circuit is to measure unknown electrical components with exceptional precision by adjusting the circuit until the detector registers zero current or voltage, signifying balanced conditions across the arms.^[3] This null-balance approach minimizes errors from source variations and detector sensitivities, achieving accuracies on the order of 0.1% or better in well-designed setups.^[4] Over time, bridge circuits have evolved from traditional analog null methods relying on manual adjustments to modern digital variants that integrate analog-to-digital converters, microprocessors, and instrumentation amplifiers for automated balancing and improved stability in complex measurement environments.^[6]

Historical Development

The Wheatstone bridge, a foundational bridge circuit, was originally invented by British scientist Samuel Hunter Christie in 1833 as a method for measuring unknown electrical resistances through a balanced network of resistors.^[7] Christie described the configuration in a paper presented to the Royal Society, though it received limited attention initially. In 1843, Charles Wheatstone, a professor at King's College London, independently refined and popularized the design, demonstrating its practical utility in a lecture that led to widespread adoption; the circuit became eponymously known as the Wheatstone bridge despite Christie's prior invention.^[7]^[8] During the 19th-century Industrial Revolution, bridge circuits like the Wheatstone found early applications in precision electrical measurements and telegraphy systems, enabling accurate assessment of wire resistances over long distances to improve signal transmission reliability.^[8] A key milestone came in 1861 when William Thomson (Lord Kelvin) developed the Kelvin double bridge, an extension designed specifically for measuring very low resistances by compensating for lead wire effects, which was essential for applications in electrical engineering and metrology. The transition to alternating current (AC) bridges began in the late 19th century, with James Clerk Maxwell introducing an inductance-capacitance bridge in 1873 to measure unknown inductances using AC signals, marking an important evolution for handling reactive components.^[9] In 1891, Max Wien devised the Wien bridge, adapted for precise frequency measurements in AC circuits, further expanding bridge techniques to audio and impedance analysis.^[10] Post-World War II advancements in the mid-20th century integrated bridge circuits with emerging electronics, particularly through instrumentation amplifiers in the 1960s, which amplified small differential signals from unbalanced bridges for enhanced sensitivity in sensors and transducers. By the 1970s, the development of digital and automatic bridges, featuring LED readouts and computerized balancing, revolutionized measurement accuracy and speed, replacing manual null detection in laboratory and industrial settings.^[11] These innovations, driven by transistor and integrated circuit technologies, solidified bridge circuits as core elements in modern instrumentation.

Principles of Operation

Balanced and Unbalanced Conditions

In a bridge circuit, the balanced condition is achieved when the potential difference across the detector is zero, indicating that the ratio of impedances in the opposing arms satisfies

Z_1 / Z_2 = Z_3 / Z_4

. This equality ensures that the voltage dividers formed by the arms produce identical potentials at the detector points, regardless of the excitation source amplitude.^[12]^[13] Under unbalanced conditions, any mismatch in the arm impedances results in a non-zero voltage or current through the detector, with the magnitude proportional to the deviation in the unknown component's value. This output signal reflects the imbalance and serves as the basis for measuring changes in the circuit parameters. The Wheatstone bridge provides a classic illustration of these states in DC applications.^[14]^[12] Achieving and maintaining balance can be influenced by several factors, including temperature variations that alter component resistances—such as a 0.385% per °C change in copper wiring—leading to offset errors. Component manufacturing tolerances introduce initial mismatches, potentially causing up to ±10% full-scale zero balance errors. In AC bridges, the excitation frequency affects balance by modifying the reactive components of impedances, necessitating adjustments for magnitude and phase alignment.^[12]^[14] The null method exploits the balanced condition by iteratively adjusting one arm until the detector reading reaches zero, providing a precise measurement independent of the excitation voltage level and minimizing errors from source fluctuations.^[12]^[14]

Sensitivity and Detection Methods

Sensitivity in bridge circuits is defined as the ratio of the change in detector output (such as voltage or current) to a small change in the unknown component value, often normalized to the excitation voltage, with this ratio reaching its maximum near the balanced condition where arm resistances are equal.^[12]^[14] This measure quantifies the circuit's ability to detect minute imbalances, typically expressed in mV/V, with values ranging from 1 mV/V to 10 mV/V in standard configurations.^[12] Traditional detection methods rely on null indicators to identify balance. For DC bridges, a galvanometer serves as the detector, measuring the small current flowing through the unbalanced bridge arms with high precision, enabling detection of zero current to confirm balance.^[15] In setups requiring voltage measurement, a voltmeter is employed across the detector points, though its input impedance must be high to avoid loading the circuit.^[12] Modern detection techniques enhance sensitivity through amplification and noise rejection. Operational amplifiers (op-amps), often configured as instrumentation amplifiers, provide high gain for amplifying the low-level unbalanced output voltage, achieving resolutions suitable for precision applications.^[12]^[14] In noisy environments, particularly with AC excitation, lock-in amplifiers employ phase-sensitive detection to isolate the bridge signal from broadband noise, improving the signal-to-noise ratio by orders of magnitude.^[16] Several factors influence the overall sensitivity of bridge circuits. The ratios of the arm resistances play a critical role, as equal arm values near balance maximize the output response to perturbations in the unknown component.^[12] Source impedance affects performance by introducing bias currents that can unbalance the bridge or cause voltage drops, necessitating low-impedance sources for optimal results.^[14] Bridge resolution, determined by the minimum detectable imbalance, can reach 0.1% accuracy in precision setups through careful component selection and amplification.^[12]^[14] Despite these capabilities, limitations arise from practical implementations. Detector loading effects, such as those from finite op-amp input impedance, can alter the bridge balance and reduce measured sensitivity by introducing errors up to several percent.^[14] Additionally, stray capacitances in wiring or components couple noise and cause frequency-dependent imbalances, particularly in high-resolution AC bridges, requiring shielding and guarding techniques to mitigate.^[12]

Types of Bridge Circuits

DC Bridge Circuits

DC bridge circuits employ direct current (DC) excitation to measure resistances by arranging four resistors in a diamond-shaped network, with balance achieved when the voltage across the bridge's detector arm is zero, indicating equal potential division in the opposing branches. This configuration, often powered by a stable DC voltage source, relies on the null method where adjustments to one or more arms nullify the detector current, allowing precise resistance determination without direct current measurement through the unknown resistor.^[17] These circuits are particularly suitable for purely resistive networks, as DC excitation avoids phase shifts that arise from reactive elements like inductors or capacitors, ensuring accurate steady-state measurements focused solely on ohmic values. Common configurations feature series-parallel arm setups, exemplified by the Wheatstone bridge, which enables straightforward DC balance for comparing resistances in the range of 5 Ω to 10 MΩ with accuracies exceeding 0.01%.^[17] Advantages of DC bridge circuits include their simplicity in design and operation, utilizing basic components without the need for frequency-specific equipment, resulting in low cost and high stability for steady-state applications such as strain gauge instrumentation where temperature compensation and signal augmentation enhance reliability. However, limitations arise when measuring components with inductance or capacitance, as these introduce transient errors that prevent true DC balance, and in low-value resistance measurements, contact and lead resistances can significantly distort results unless mitigated by four-terminal techniques like the Kelvin configuration.^[18]^[17] A representative example is the use of DC bridges for basic resistance comparison in sensor applications, such as monitoring static strains via resistive elements, where the absence of frequency dependence ensures consistent performance independent of excitation variations.^[18]

AC Bridge Circuits

AC bridge circuits employ alternating current excitation, typically from sinusoidal voltage sources, to measure complex impedances involving resistance (R), inductance (L), and capacitance (C). This approach extends the utility of bridge configurations beyond purely resistive elements by accounting for both real and imaginary components of impedance, enabling precise determination of reactive parameters in components like inductors and capacitors.^[19]^[12] The balance condition in AC bridges requires that both the magnitude and phase of the voltages across the detector be nulled simultaneously, expressed in phasor form as

\frac{Z_1}{Z_2} = \frac{Z_3}{Z_4}

, where

Z_1, Z_2, Z_3, Z_4

represent the complex impedances of the four arms. This equality ensures zero current through the detector, analogous to the resistive ratio in DC bridges but incorporating phase alignment for reactive elements.^[20]^[19] Key advantages of AC bridges include their versatility in handling non-resistive components, allowing measurement of L and C alongside R, and the ability to tune the excitation frequency for enhanced selectivity in distinguishing between similar impedances. Frequency adjustment can isolate specific reactive behaviors, improving accuracy in applications like component characterization. Additionally, the bridge balance is independent of the source voltage magnitude, facilitating high-precision measurements with stable sinusoidal drives.^[21]^[22]^[19] Despite these benefits, AC bridges face challenges such as the skin effect, which increases effective resistance in inductors at higher frequencies by concentrating current near conductor surfaces, and dielectric losses in capacitors that introduce phase errors due to energy dissipation in insulating materials. Parasitic capacitances and inductances from wiring and environment can also disrupt balance, necessitating shielded enclosures and careful grounding techniques like the Wagner ground to minimize these effects.^[23]^[19]^[24] Typical operating frequencies for AC bridges range from the audio spectrum, often around 1 kHz for general-purpose measurements where detector sensitivity is optimal with headphones or simple null indicators, to several MHz for high-frequency component testing using click-based detection or advanced instrumentation. This tunability supports applications from low-speed laboratory setups to rapid, precise evaluations of RF elements.^[19]

Specific Bridge Configurations

Wheatstone Bridge

The Wheatstone bridge is a fundamental configuration in DC bridge circuits, specifically designed for precise measurement of medium-range electrical resistances. It consists of four resistors arranged in a diamond-shaped network, forming two parallel voltage dividers. The resistors are typically labeled as R1 and R2 in one arm, connected in series, and R3 and Rx (the unknown resistance) in the other arm, also in series. A DC voltage source, such as a battery, is applied across the top and bottom nodes (the junction between R1 and R3, and the junction between R2 and Rx), providing excitation to the circuit. A sensitive null detector, traditionally a galvanometer but often a high-impedance voltmeter in modern setups, is connected across the left and right nodes (the junction between R1 and R2, and the junction between R3 and Rx) to measure any differential voltage.^[3] The bridge operates on the principle of null balance, where the circuit is balanced when the galvanometer indicates zero current or voltage, meaning the voltage drops across the two dividers are equal. This occurs when the ratio of the resistors satisfies the condition:

\frac{R_1}{R_2} = \frac{R_3}{R_x}

Solving for the unknown resistance gives

R_x = R_3 \cdot \frac{R_2}{R_1}

. In practice, R1 and R2 are fixed known resistors forming a ratio arm (often R1 = R2 for simplicity, making the ratio 1:1), while R3 is a variable resistor, such as a decade box, adjusted during measurement. To measure Rx, the operator applies the excitation voltage, monitors the galvanometer, and incrementally adjusts R3 until null deflection is achieved, at which point Rx is calculated directly from the known values without needing to measure currents or voltages during the balanced state. This null method provides high precision because it relies on the sensitivity of the detector rather than absolute measurements.^[3]^[25] The accuracy of the Wheatstone bridge can reach up to 0.01% when using precision decade resistance boxes for R1, R2, and R3, with tolerances better than 0.01% and stable temperature coefficients. However, several error sources can limit performance, including lead resistance, which adds unintended resistance in series with Rx and introduces systematic errors, particularly for values below 1 Ω where lead drops can exceed 0.1% of the total. Other errors arise from thermal effects on resistors, galvanometer loading if not high-impedance, and contact resistance at connections, though these are minimized by using four-wire techniques or guarding in advanced setups.^[26] Although the bridge is named after Sir Charles Wheatstone, who popularized it through a public demonstration and paper in 1843, the original circuit was invented by Samuel Hunter Christie in 1833 as a method for measuring unknown resistances by balancing currents in a network of wires. Wheatstone acknowledged Christie's priority in his work, but the name persisted due to Wheatstone's improvements, such as incorporating a rheostat for finer adjustments.^[7]

Kelvin Double Bridge

The Kelvin double bridge is an advanced configuration of the bridge circuit specifically engineered for accurate measurement of very low resistances, typically in the range below 1 ohm, where conventional methods like the Wheatstone bridge suffer from significant errors due to lead and contact resistances.^[27] This setup employs a four-terminal sensing approach for both the unknown resistance and the standard, ensuring that the potential leads carry negligible current and thus isolating the measurement from extraneous resistances.^[28] In terms of configuration, the Kelvin double bridge features two interconnected Wheatstone bridges: a primary bridge with ratio arms

R_1

and

R_2

, and an auxiliary bridge with smaller ratio arms

r_1

and

r_2

, linked by a low-impedance network (often denoted as

r_l

) that connects the midpoints. The unknown low resistance

R_x

and a standard resistor

R_s

(or

R_3

in some notations) are each connected via separate pairs of current-carrying and voltage-sensing terminals, totaling eight terminals overall to prevent lead resistance interference.^[27] A galvanometer or null detector is placed across the bridge junction to indicate balance. The balance condition for the Kelvin double bridge is achieved when the galvanometer current is zero, leading to the equation:

R_x = \frac{R_3}{R_1} R_2 + \text{corrections for lead resistances } r_1, r_2

where the corrections account for unequal ratios between the main and auxiliary arms, typically expressed as small terms involving

r_l

(the linking resistance) to adjust for any residual lead effects; in the ideal case where

\frac{r_1}{r_2} = \frac{R_1}{R_2}

, the lead resistances are fully eliminated, simplifying to

R_x = \frac{R_3}{R_1} R_2

.^[27] This condition ensures high precision, with accuracies often reaching ±0.05% for resistances down to 0.00001 ohms.^[28] The measurement procedure requires two sequential balance adjustments to null the detector. First, the auxiliary bridge is balanced by varying

r_1

and

r_2

to match the ratio of the main arms

R_1

and

R_2

, minimizing the impact of lead resistances

r_1

and

r_2

. Second, the main bridge is balanced by adjusting

R_2

R_3

until the galvanometer shows no deflection, at which point

R_x

is directly computed from the known arm values.^[27] A stable DC current source is used throughout to maintain consistent voltage drops. In applications, the Kelvin double bridge is widely employed in metrology and calibration laboratories for verifying shunt resistors used in precision ammeters and standard low-value resistors essential for electrical standards.^[29] Its ability to handle resistances as low as milliohms with minimal error makes it indispensable for high-impact tasks like calibrating current shunts in national measurement institutes.

Maxwell Bridge

The Maxwell bridge is an AC bridge circuit designed for the precise measurement of unknown inductance and its associated series resistance, particularly suitable for medium-Q inductors. It consists of three resistors and one variable capacitor arranged in a configuration that compensates for the inductive reactance without requiring a variable inductor. Specifically, the unknown inductance

L_x

in series with resistance

R_x

forms one arm of the bridge, while the adjacent arm contains resistor

R_2

, the opposite arm has resistor

R_3

, and the remaining arm features the variable capacitor

C_4

in parallel with

R_4

. This setup allows for balance by adjusting the capacitance and one resistor, leveraging the phase opposition between the inductive and capacitive arms to achieve null detection.^[30]^[19] At balance, the bridge satisfies the condition where the product of opposite arm impedances are equal, resulting in magnitude-only equations independent of frequency and angular velocity

\omega

. The inductance is given by

L_x = R_2 R_3 C_4

and the series resistance by

R_x = \frac{R_2 R_3}{R_4}.

The phase balance occurs automatically due to the capacitive arm counteracting the inductive phase shift, eliminating the need for frequency-specific adjustments. This configuration ensures that both magnitude and phase conditions are met simultaneously through simple variation of

C_4

and

R_4

.^[30]^[31] Key advantages of the Maxwell bridge include its simplicity in adjustment using a readily available variable capacitor, which avoids the complexities and cost of variable inductors, and its independence from source frequency, making it robust against variations in the AC supply. It is particularly effective for inductors with quality factors

Q

between 1 and 10, where

Q = \omega L_x / R_x

, as the balance equations remain stable without residual inductance errors from mutual coupling. However, it is less suitable for very low-Q or high-Q coils due to sensitivity limitations in those ranges.^[19]^[31] The bridge typically operates in the audio frequency range of 1 to 10 kHz, ideal for measuring medium inductances from millihenries to henries with high accuracy, as higher frequencies can introduce parasitic effects while lower ones reduce sensitivity.^[30] A variation known as the inductance comparison bridge adapts the Maxwell configuration for comparing an unknown inductance to a standard one, often used in calibrations involving mutual inductance effects by incorporating a reference inductor in place of the capacitive arm to directly ratio inductances while minimizing lead and ground errors.^[30]

Wien Bridge

The Wien bridge is an AC bridge circuit invented by Max Wien in 1891 and originally developed for precision measurement of capacitances, particularly in the context of dielectric testing to determine capacitance and conductance for calculating loss angles in materials like electric cables.^[32]^[10] This configuration extends the Wheatstone bridge principle to handle frequency-dependent impedances, enabling accurate assessment of capacitive components under sinusoidal excitation.^[10] The circuit features a series RC arm (typically resistor

R_1

in series with known capacitor

C_1

) and an adjacent parallel RC arm (resistor

R_2

in parallel with unknown capacitor

C_x

), completed by two ratio arms consisting of resistors

R_3

and

R_4

.^[10] At balance, the voltage across the detector is zero, allowing the unknown capacitance

C_x

or the operating frequency to be determined by adjusting the bridge elements while applying an AC source.^[10] The balance conditions derive from equating the impedances of opposing arms, yielding two independent equations: one for the resistive components and one for the reactive components. For capacitance measurement, the unknown

C_x

satisfies

C_x = C_1 \cdot \frac{R_3}{R_4}

, assuming specific symmetry in the RC arms.^[33] Alternatively, for frequency determination with known components, the balance occurs at

f = \frac{1}{2\pi \sqrt{R_1 R_2 C_1 C_x}}

f=2πR1R2C1Cx

1.^[33] In its standard form, the Wien bridge serves as a null detector for precise component measurement by achieving balance through adjustments. A variant employs positive feedback by integrating the bridge network into an amplifier circuit with a gain of approximately 3, transforming it into an oscillator that sustains sinusoidal output at the balance frequency.^[10]^[34] The Wien bridge exhibits high sensitivity to changes in capacitance, making it suitable for detecting small variations in dielectric properties, and operates effectively in the audio frequency range of approximately 100 Hz to 100 kHz.^[35]^[10]

Applications

Electrical Component Measurement

Bridge circuits are fundamental tools for precisely measuring electrical components such as resistances, capacitances, and inductances by achieving a null balance condition, where the voltage across a detector is zero, indicating equality between circuit arms.^[36] This null method minimizes errors from detector sensitivity and supply variations, enabling high accuracy in laboratory and industrial settings.^[37] Measurements typically involve comparing the unknown component against known standards, with balance achieved through adjustable resistors, capacitors, or inductors.^[4] For resistance measurement, the Wheatstone bridge serves as the standard configuration for general-purpose applications, determining unknown resistances from a few ohms to megaohms by balancing DC ratios in its arms.^[38] It provides precise results when lead resistances are negligible, commonly used in educational labs and basic instrumentation for verifying resistor values.^[36] For low-value resistances, such as milliohms in cables or shunts, the Kelvin double bridge is employed to eliminate errors from contact and lead resistances through its four-terminal configuration and linked ratio arms.^[37] This setup ensures accuracy in conductivity samples and precision components, where standard Wheatstone methods would introduce significant offsets.^[39] Capacitance measurements utilize AC bridge circuits to account for reactive impedances. The De Sauty bridge, consisting of two resistors and two capacitors, compares unknown capacitance against a standard by balancing at a specific frequency, ideal for non-lossy dielectrics.^[4] For capacitors with dielectric losses, such as those in cables or insulators, the Schering bridge measures both capacitance and the loss angle (dissipation factor) using a configuration of resistors and capacitors that remains frequency-independent for enhanced precision.^[4] This bridge supports high-voltage applications, quantifying phase differences to assess material quality.^[36] Inductance evaluation relies on AC bridges tailored to coil quality factors. The Maxwell bridge measures self-inductance in low-Q inductors (Q factor 1-10) by balancing resistance and inductance arms against a standard capacitor, suitable for audio-frequency coils in transformers.^[4] For high-Q inductors (Q > 10), the Hay bridge provides accurate determination of self- and mutual inductance, incorporating frequency-dependent balance conditions to handle reactive dominance in RF components.^[4] To achieve high precision, bridge circuits are calibrated using traceable standard components from national laboratories, with error analysis focusing on component tolerances and environmental factors like temperature.^[37] For instance, resistor tolerances of 0.05% enable overall measurement accuracies approaching that level in well-controlled setups, though lead resistances and leakage currents can introduce errors up to several percent in high-resistance cases without guards.^[36] Laboratory techniques include the direct comparison method, where the unknown is balanced against a standard, and the substitution method, which alternates between standard and unknown for differential balancing to reduce systematic errors and improve precision in challenging measurements.^[37] The substitution approach is preferred for its ability to nullify bridge imbalances more accurately, especially in AC circuits operating from 40 Hz to 125 kHz.^[40]

Sensor and Instrumentation Uses

Bridge circuits, particularly the Wheatstone configuration, are integral to sensor applications where they convert mechanical, thermal, or physical changes into measurable electrical signals through resistance variations. In these setups, the bridge's unbalance produces a differential voltage proportional to the sensed parameter, enabling precise detection in instrumentation systems.^[41] Strain gauge bridges commonly employ a full Wheatstone bridge with four active gauges to measure deformation, providing compensation for temperature fluctuations and bending effects by arranging gauges such that opposing arms experience similar environmental influences. This configuration enhances sensitivity compared to half-bridge setups, as all four elements contribute to the output signal, allowing detection of microstrain levels in structural monitoring. For instance, in load cells, the full-bridge arrangement stabilizes output against thermal variations, ensuring reliable force measurements.^[42]^[43]^[44]^[45] In transducers, bridge circuits detect unbalance to output signals from various physical phenomena. Pressure transducers often use piezoresistive elements in a Wheatstone bridge, where applied pressure alters resistor values to produce a voltage change; full-bridge designs in MEMS sensors automatically compensate for temperature, achieving high accuracy in cardiovascular or industrial applications. Temperature transducers incorporate resistance temperature detectors (RTDs) in a Wheatstone bridge, where the RTD's resistance shift with temperature unbalances the circuit for precise readout, typically using a voltage divider variant for linearization. Displacement sensors leverage bridge-based strain gauges or inductive elements, correlating position changes to resistance or impedance variations for non-contact measurements in automation systems.^[46]^[47]^[48]^[49]^[50]^[51] Instrumentation amplifiers are frequently integrated with bridge circuits to condition low-level differential signals from sensors, amplifying the output while rejecting common-mode noise before feeding into analog-to-digital converters in devices like digital multimeters. These amplifiers provide high input impedance and low offset, ensuring the bridge's ratiometric nature preserves measurement integrity across varying supply voltages.^[52]^[53]^[54] Modern applications include automotive anti-lock braking systems (ABS), where magneto-resistive bridges in wheel speed sensors detect rotational variations to prevent skidding, doubling signal output and reducing temperature drift for reliable performance. In medical instrumentation, bridge circuits measure bioimpedance for applications like electrocardiograms (ECGs) and blood pressure monitoring, using four-electrode setups to assess impedance waveforms at sites such as the wrist, aiding in non-invasive vital sign detection.^[55]^[56]^[57]^[58] The advantages of bridge circuits in sensors stem from their ratiometric measurement principle, which inherently cancels supply voltage fluctuations and common-mode noise, yielding high resolution—such as microstrain detection—without needing precise reference sources. This approach supports noise-immune, high-accuracy systems in demanding environments.^[15]^[59]^[60]

Mathematical Analysis

General Bridge Balance Equation

In bridge circuits, the balance condition occurs when the potential difference across the detector is zero, resulting in no current flow through it. This universal principle applies to both DC and AC configurations, serving as the foundation for measuring unknown electrical parameters by achieving null detection. The derivation of this condition typically involves applying Kirchhoff's voltage law (KVL) and current law (KCL) to the circuit nodes and loops, assuming the detector has infinite impedance or zero current, which equates the voltages at the bridge's output junctions.^[61] For DC bridge circuits composed of resistive arms, the balance equation simplifies to the ratio of resistances in parallel branches being equal:

\frac{R_1}{R_2} = \frac{R_3}{R_x}

, where

R_1

and

R_2

form one voltage divider, and

R_3

and

R_x

(the unknown resistance) form the other. This relation arises from setting the node voltages equal via KCL at the detector points, ensuring current conservation without flow across the bridge diagonal. The condition holds regardless of the DC source voltage, as the ratios are scale-invariant.^[2]^[62] In AC bridge circuits, where arms may include inductors, capacitors, or resistors, the balance condition generalizes to impedances using complex notation:

\frac{Z_1}{Z_2} = \frac{Z_3}{Z_4}

Here, each impedance is

Z_i = R_i + j \left( \omega L_i - \frac{1}{\omega C_i} \right)

, with

\omega

as the angular frequency,

L_i

the inductance,

C_i

the capacitance, and

j

the imaginary unit. The derivation mirrors the DC case but employs phasor analysis: Kirchhoff's laws are applied in the frequency domain to node voltages, setting the phasor current through the detector to zero, which requires both magnitude and phase equality in the ratios. This ensures the real and imaginary components of the voltage difference nullify simultaneously. Like the DC case, the balance is independent of the AC source amplitude, depending solely on the impedance ratios at the operating frequency.^[19]^[63]

Current and Voltage Derivations

In direct current (DC) bridge circuits, such as the Wheatstone bridge, the output voltage across the detector for an unbalanced condition is derived from voltage division in the two arms. Assuming a standard configuration with excitation voltage

V_s

, resistors

R_1

and

R_2

in one arm, and

R_3

and unknown

R_x

in the other, the voltage at the junction of

R_1

and

R_2

V_s \cdot \frac{R_2}{R_1 + R_2}

, while at the junction of

R_3

and

R_x

it is

V_s \cdot \frac{R_x}{R_3 + R_x}

. The differential output voltage is thus

V_\text{out} = V_s \left[ \frac{R_2}{R_1 + R_2} - \frac{R_x}{R_3 + R_x} \right].

^[64] To analyze the effect of a detector with finite impedance, the Thévenin equivalent circuit is applied to the bridge output terminals. The Thévenin voltage

V_\text{th}

is the open-circuit output voltage

V_\text{out}

given above. The Thévenin resistance

R_\text{th}

is found by deactivating the source (shorting

V_s

) and computing the equivalent resistance across the detector terminals, yielding the series combination of the parallel impedances of the two voltage dividers:

R_\text{th} = \frac{R_1 R_2}{R_1 + R_2} + \frac{R_3 R_x}{R_3 + R_x}.

^[65] With a galvanometer of resistance

R_g

connected as the detector, the current through it is then

I_g = \frac{V_\text{th}}{R_\text{th} + R_g}.

This follows directly from the Thévenin equivalent, where the galvanometer loads the simplified circuit. For alternating current (AC) bridges, the analysis extends the DC case by replacing resistances with complex impedances

Z_1, Z_2, Z_3, Z_x

. The output voltage becomes a phasor given by

\mathbf{V}_\text{out} = V_s \left[ \frac{Z_2}{Z_1 + Z_2} - \frac{Z_x}{Z_3 + Z_x} \right],

where reactive elements contribute imaginary components (

Z_L = j \omega L

Z_C = 1/(j \omega C)

). The Thévenin equivalent follows analogously, with

\mathbf{V}_\text{th}

as the open-circuit phasor and

Z_\text{th}

computed similarly using deactivated sources.^[19] The sensitivity of the bridge, defined as the rate of change of output voltage with respect to the unknown resistance at balance (

d V_\text{out} / d R_x \big|_{\text{balance}}

dVout/dRx

balance), quantifies detection capability. For the quarter-bridge case (one variable arm, others equal to $R$ ), near balance with small $\Delta R = R_x - R$ , $\frac{d V_\text{out}}{d (\Delta R)} \approx \frac{V_s}{4 R},$ derived by differentiating $V_\text{out} \approx V_s \cdot \frac{\Delta R}{4 R}$ for $\Delta R \ll R$ . This sensitivity is maximum when all arms are equal, as unequal ratios reduce the slope; for example, in a general configuration, it scales with $V_s / (R_1 + R_2)^2$ times arm ratios, peaking at equality.^[14]

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Bridge circuit

Introduction

Definition and Basic Concept

Historical Development

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Balanced and Unbalanced Conditions

Sensitivity and Detection Methods

Types of Bridge Circuits

DC Bridge Circuits

AC Bridge Circuits

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Wheatstone Bridge

Kelvin Double Bridge

Maxwell Bridge

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Mathematical Analysis

General Bridge Balance Equation

Current and Voltage Derivations

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