Physics
Bridge Circuit
A bridge circuit is a configuration of electrical components used to measure resistance, capacitance, or inductance. It typically consists of four arms, with a voltage source and a detector connected to form a bridge. When the bridge is balanced, the detector indicates zero voltage, allowing for precise measurements of the unknown component. Bridge circuits are commonly used in electronic instrumentation and sensor applications.
Written by Perlego with AI-assistance
Related key terms
1 of 5
8 Key excerpts on "Bridge Circuit"
- R. M. Marston(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
4 Bridges and C-R boxes A bridge is a passive network that, when used in conjunction with a suitable energizing generator and a balance detector, enables values of inductance (L), capacitance (C), or resistance (R) to be accurately and inexpensively measured or matched. Bridges were the instruments for measuring R , C and L values up until the late 1970s, when they were replaced in many applications by the digital multimeter and its derivatives. Nowadays they are used mainly as either cheap-and-simple 'service' type instruments that give an approximate (within a couple of per cent) reading of a component value, or, at the opposite end of the scale, as 'laboratory' type instruments that make measurements with very high precision. Both types of instrument are described in this chapter. Closely associated with the conventional bridge are the resist-ance-matching bridge, which enables resistors to be matched to within 0.003 per cent or better, and the C or R 'substitution box', which enables component values to be determined with good precision, using either comparison or substitution techniques. This chapter looks at the theory, practical circuitry, and usage techniques of all of these types of instrument. Bridge basics Modern component-measuring bridges come in two popular gen-eral classes, these being the d.c.-energized type which can accu-rately measure resistance values from a few ohms to a few megohms, and the low-frequency (usually 1kHz) a.c.-energized type which, as well as measuring resistance, can measure capaci-tance from about lOpF to ΙΟΟμΡ, and inductance from about Bridges and C -R boxes 73 ΙΟμΗ to ÎOOH. Both of these types of instrument are derived from the ancient (1843) Wheatstone Bridge Circuit, and it is well worth studying this in order to learn the finer points of bridge design.
eBook - PDF- Charles A. Gross, Thaddeus A. Roppel(Authors)
- 2012(Publication Date)
- CRC Press(Publisher)
349 Sensors and Instrumentation 7.5.1 Bridge Circuits Bridge Circuits are widely used as sensor interface circuits . The principle employed is to measure the offset that results from the deviation of a sensor element (e .g ., resistance) from its nominal value . Bridges can form part of a null-seeking circuit, in which feedback is used to force the sensor element back to its nominal value . In this case, the output is measured as the amount of feedback required to null the bridge . Such null-seeking circuits are employed, for example, in accelerometers, where the feedback is used to keep the mov-ing element near zero deflection which maximizes linearity while helping to prevent damage from overdriving the moving element . The resistor Wheatstone Bridge Circuit consists of four circuit branches, each containing one resistor, as well as an ac or dc voltage bias . Each leg contains one resistor . If just one branch contains a variable resistor, the bridge is called ¼-active (“quarter-active”) . If there are two branches with varying resistors, the bridge is called ½-active (“half-active”) . Bridges with all four branches containing varying resistors are called full-active . Figure 7 .8 shows a ¼-active resistive bridge powered by a dc bias source V DC . The output of the bridge is the difference between the voltages V A and V B indicated in Figure 7 .8 . The resistor labeled R X is the resistor whose value is desired to be measured . This is often a sensor element, for example a thermistor (a temperature-sensitive resistor), a photoresistor (resistance changes with light intensity), a piezoresis-tor (a force-sensitive resistor used in pressure sensors and accelerometers), or a strain gauge (in which resistance depends on the amount of stretching) . The other three resistors labeled R f are fixed resistors . The value of R f is chosen V DC V B V A + + – – V OUT R f R f R f R x = R f + Δ R FIGURE 7.8 Wheatstone Bridge Circuit used to interface a resistive sensor.
eBook - PDF- Alan S. Morris(Author)
- 2001(Publication Date)
- Butterworth-Heinemann(Publisher)
Bridge Circuits are a particularly important type of variable conversion element, and these will be covered in some detail. Following this, the various alternative techniques for transducing the outputs of a measurement sensor will be covered. 7.1 Bridge Circuits Bridge Circuits are used very commonly as a variable conversion element in measure-ment systems and produce an output in the form of a voltage level that changes as the measured physical quantity changes. They provide an accurate method of measuring resistance, inductance and capacitance values, and enable the detection of very small changes in these quantities about a nominal value. They are of immense importance in measurement system technology because so many transducers measuring physical quantities have an output that is expressed as a change in resistance, inductance or capacitance. The displacement-measuring strain gauge, which has a varying resistance output, is but one example of this class of transducers. Normally, excitation of the bridge is by a d.c. voltage for resistance measurement and by an a.c. voltage for inductance or capacitance measurement. Both null and deflection types of bridge exist, and, in a like manner to instruments in general, null types are mainly employed for calibration purposes and deflection types are used within closed-loop automatic control schemes. 120 Variable conversion elements 7.1.1 Null-type, d.c. bridge (Wheatstone bridge) A null-type bridge with d.c. excitation, commonly known as a Wheatstone bridge, has the form shown in Figure 7.1. The four arms of the bridge consist of the unknown resistance R u , two equal value resistors R 2 and R 3 and a variable resistor R v (usually a decade resistance box). A d.c. voltage V i is applied across the points AC and the resistance R v is varied until the voltage measured across points BD is zero. This null point is usually measured with a high sensitivity galvanometer.
eBook - ePubMeasurement and Instrumentation
Theory and Application
- Alan S. Morris, Reza Langari(Authors)
- 2011(Publication Date)
- Butterworth-Heinemann(Publisher)
We therefore need to have a means of converting sensor outputs that are initially in some nonvoltage form into a more convenient form. This can be achieved by putting various types of variable conversion elements into the measurement system. We consider these in this chapter. First, we will see that Bridge Circuits are a particularly important type of variable conversion element; these are covered in some detail. Following this, we look at various alternative techniques for transducing the outputs of a measurement sensor into a form that is measured more readily.9.2. Bridge Circuits
Bridge Circuits are used very commonly as a variable conversion element in measurement systems and produce an output in the form of a voltage level that changes as the measured physical quantity changes. They provide an accurate method of measuring resistance, inductance, and capacitance values and enable the detection of very small changes in these quantities about a nominal value. They are of immense importance in measurement system technology because so many transducers measuring physical quantities have an output that is expressed as a change in resistance, inductance, or capacitance. A displacement-measuring strain gauge, which has a varying resistance output, is but one example of this class of transducers. Normally, excitation of the bridge is by a d.c. voltage for resistance measurement and by an a.c. voltage for inductance or capacitance measurement. Both null and deflection types of bridges exist, and, in a like manner to instruments in general, null types are employed mainly for calibration purposes and deflection types are used within closed loop automatic control schemes.9.2.1. Null-Type d.c. Bridge (Wheatstone Bridge)
A null-type bridge with d.c. excitation, known commonly as a Wheatstone bridge, has the form shown in Figure 9.1 . The four arms of the bridge consist of the unknown resistance Ru, two equal value resistors R2 and R3 , and variable resistor R v (usually a decade resistance box). A d.c. voltage V i is applied across the points AC, and resistance R v
eBook - ePubTechnology and Practical Use of Strain Gages
With Particular Consideration of Stress Analysis Using Strain Gages
- Stefan Keil(Author)
- 2017(Publication Date)
- Ernst & Sohn(Publisher)
4 The Wheatstone Bridge Circuit4.1 Circuit Principle
Even about 100 years before the invention of the strain gage the English scientists Hunter-Christie and Wheatstone were working on the measurement of the resistance of metal connecting wires, resulting in the invention of the Bridge Circuit with which it was possible to measure resistances even with unstable voltage sources [4.1,4.2]. A brief account of the historical background was given in Chapter 1 . The circuit which was later named after Wheatstone became the most important basic circuit in widespread use for strain gage measurements.Figure 4.1 shows schematically the basic structure of the Wheatstone Bridge Circuit in the two graphical forms usually found today. The two forms are electrically identical. The Bridge Circuit consists of four bridge arms with the resistances R1 to R4 which are here arranged on the four sides of a parallelogram. The effect of the circuit is that if it is excited with a voltage applied to opposite points, the voltage on the other two corner points, that is the output voltage, is dependent on the ratios of the resistances in the four arms of the bridge. For example, if all four resistances in the arms of the bridge are identical, then the output voltage is equal to zero. If a change occurs in one resistance, the effect on the output voltage is a measure of this change in resistance.The usual forms of representing the Wheatstone bridge for measurements using strain gagesFig. 4.1This effect is used for measurements with strain gages by measuring the change of resistance of a strain gage connected in one arm of the bridge in the manner described above. The output voltage of the circuit is then a measure of the strain.The supply voltage VB is normally between 1 V and 10 V for strain gage measurements. If this voltage is applied to points 2 and 3 of the Bridge Circuit illustrated in Figure 4.1 , then the bridge output voltage VM between points 1 and 4 is zero when identical resistances are present in all four arms of the bridge, that is if R1 = R2 = R3 = R4 . The bridge is therefore said to be balanced when the output voltage VM = 0 for an applied supply voltage of VB
eBook - PDF- Walt Boyes(Author)
- 2002(Publication Date)
- Butterworth-Heinemann(Publisher)
Windings can be added to the bridge which enable the bridge to measure the difference between a standard and the unknown. 20.7.5 High-frequency impedance measurement As the frequency of measurement is increased, the parasitic elements associated with real ( 3 e t e c t o r Figure 20.69 Effect of stray impedances on balance condition. The measurement of resistance, capacitance, and inductance 477 Voltage transformer Current transformer Voltage transformei vs (Z2 -Z1) 2 (Z, +Z,) (Z, +zl +Z,Zz/Zc) z1 llZZ ____ - Detector , ”; f 2 2 Current transformer ,ut Z0”t 7; U t Equivalent circuit Zc=jwL,: L, is inductance of ratio arms L, = M, mutual inductance of ratio arms Figure 20.70 Unbalanced inductivelycoupled bridge components begin to dominate the measurement. Therefore rf bridges employ variable capacitors (typically less than 1000pF) as the adjustable elem- ents in bridges and fixed resistors whose phys- ical dimensions are small. A bridge which can be constructed using these elements is the Schering bridge, shown in Figure 20.60. Great care has to be taken with shielding and wiring layout in rf bridges to avoid large coupling loops. The imped- ance range covered by such bridges decreases as the frequency is raised. At microwave frequencies all the wiring is coaxial, discrete components are no longer used, and impedance measurements can only be undertaken for impedances close to the characteristic impedance of the system. Further details of high-frequency measurements can be found in Oliver and Cage (1971) and Somlo and Hunter (1985). The bridged T and parallel T circuits (shown in F:gure 70.73 together with their balance condi- tions) can be used for measurements at rf fre- quencies. The parallel T measurement technique has the advantage that the balance can be achieved using two grounded variable capacitors. Resonance methods can also be used for the measurement of components at high frequencies.
- John G. Webster(Author)
- 2003(Publication Date)
- CRC Press(Publisher)
Bridges are FIGURE 15.11 Impedance measurement by bridge methods. The bridge is balanced when the voltage V d across the diagonal branch is adjusted to zero by tuning Z 1 , Z 2 , or Z 3 . Z Z Z Z Y Z Z Y x x or = = 1 3 2 3 1 2 Z Z Z Z x x = = -+ 1 3 2 1 3 2 , j j j j e = ∂ ∂ = + ( ) V Z V Z Z Z d x x 2 2 2 e = + ( ) V Z Z Z Z 3 2 2 1 3 2 15 -12 Electrical Measurement, Signal Processing, and Displays therefore principally used to measure capacitances and inductances as well as loss and quality factors of capacitors and coils. Since magnitude and phase conditions must be matched simultaneously, two ele-ments must be tuned. To obtain a wide measurement range, the variable elements are designed as combinations of switchable and tunable capacitors and resistors. The sensitivity of the zero indicator can be changed for global search and final adjustment. Unfortunately, magnitude and phase cannot be adjusted independently of each other. If the balancing is performed by hand, a suitable strategy is to search the minimum voltage by tuning each element successively. Frequently used bridges are the Wheatstone bridge (Figure 15.12) for the measurement of lossy capac-itances, and the Hay bridge (Figure 15.13) to determine inductivity and quality factor of coils. Because of its symmetrical structure, the balancing condition for the Wheatstone bridge is simply: (15.36) which is independent of frequency. The measurement of a coil with the Hay bridge requires that: (15.37) from which the quality factor is obtained as: (15.38) The inductance of high-Q coils can be determined frequency independent since (15.39) FIGURE 15.12 Wheatstone bridge for the capacitance and dissipation factor measurement of capacitors. The bal-ancing condition is frequency independent. The resistor R 1 and the capacitor C 1 must be tuned successively until the bridge is balanced.
- Stephen A. Dyer(Author)
- 2004(Publication Date)
- Wiley-IEEE Press(Publisher)
However, some of them are still correctly operating, with higher uncertainties, slightly above the 1 MHz range. Ac Networks Most of the existing circuit components and even most of the impedance standards to be measured with ac bridges are neither purely resistive nor purely reactive. This means that unwanted capacitance and inductance are present in resistors, unwanted resistance and inductance in capacitors as well as unwanted resistance and capacitance in inductors. The energy flow at sufficiently low frequencies in these real-world devices must be treated like a flow in a RLC network. When driven with an ac voltage, each RLC network will dissipate heat and store the energy of the generated magnetic and electric fields. If no precaution is taken in the construction and the use of this RLC network, the magnetic and electric fields generated by this network will be modified by any change with respect to its immediate environment. The final measured value of this RLC network will be altered. As in ac bridges, the com- parison is made in the energy flow through two or more sets of impedance standards; special care in order to limit the in- fluence of these fields is the first criterion to be fulfilled in the design of these impedance standards. In theory, for a standard of capacitance, the only energy should be that associated with the electrical field E, and for a standard of inductance that as- sociated with the magnetic field H. For a standard of resistance, the only energy should be dissipated in heat. Ac Measured Devices The definitions given for dc resistors are, even for the most so- phisticated of them, not satisfactory in low-frequency ac mea- surements mainly due to the frequency dependence of the 318 BRIDGE INSTRUMENTS Y 1 Z 2 Y 2 Z 4 Y 4 Y 3 i 3 1 a b 2 4 3 Z 1 Z 3 Z e Y H Y L Figure 11. Four terminal-pair definition. measured value, the series impedances, the shunt admittances, and the stray fields present in each device.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
