Bridge Circuit

3 Key excerpts on "Bridge Circuit"

• 

  eBook - ePub

  System and Measurements

  • Yong Sang(Author)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  A Bridge Circuit is a circuit topology where two circuit branches (usually in parallel with each other) are “bridged” along with some intermediate points by a third branch connected between the first two branches. The bridge was originally developed for laboratory measurements. Bridge Circuits are often used with transducers for converting physical quantities (temperature, displacement and pressure) into electrical quantities (voltage and current). In addition, the Bridge Circuit in the power supply design is a structure of a diode to rectify the current, that is, it is converted from an alternating or unknown polarity into a known polarity of direct current.

  The most famous Bridge Circuit is a Wheatstone bridge, which was invented by Samuel Hunter Christie and promoted by Charles Wheatstone to measure resistance. It consists of four resistors, two of which are known values

  R 1

  and

  R 3

  , one of which is to be determined as

  R x

  and the other is calibrated and variable

  R 2

  . Wheatstone bridges are also widely used for measuring impedance in AC circuits, including resistance, capacitance, inductance and dissipation factor.

  4.2.1  Classification

  In general, a circuit consists of one source and four impedances for measuring various physical quantities. The Bridge Circuit is very useful in measuring impedance (resistors, capacitors and inductors) and converting the signals of the sensors into relevant voltage or current signals. According to the different excitation voltages, the Bridge Circuit can be divided into DC Bridge Circuit and AC Bridge Circuit. Similarly, depending on the output, the Bridge Circuit can be divided into an unbalanced Bridge Circuit and a balanced Bridge Circuit.

  The bridge impedance (

  Z 1

  ,

  Z 2

  ,

  Z 3

  or

  Z 4

  ) shown in Figure 4.1

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  eBook - ePub

  Introduction to Sensors for Electrical and Mechanical Engineers

  • Martin Novák(Author)
  • 2020(Publication Date)
  • CRC Press

    (Publisher)

  Bridge Circuits can be divided into many categories. Based on the power supply voltage, there are DC, AC and impulse bridges. DC bridges are powered with DC voltage, AC bridges are powered with AC voltage and impulse bridges are powered with impulse voltage. Based on the output voltage detection, there are balanced and unbalanced bridges. In an unbalanced bridge, the output voltage is measured directly with a voltmeter. In a balanced bridge, the elements in a bridge are adjusted until zero voltage on the output is obtained.

  2.3.1DC bridges

  DC bridges are used for sensors that change electrical resistance. Capacitive and inductive components are not present in DC circuits. The bridge acts as an amplifier, and transfers the small changes in resistance to a small voltage. The voltage typically needs to be amplified with a DC amplifier.

  2.3.1.1Wheatstone bridge

  The bridge is formed by four resistances (one or more of those can be the sensors) and powered with a power supply V. Bridge output is V0 and is usually connected to a voltmeter or some other processing circuit, such as a DC or instrumentation amplifier. Power supply resistance R

  i

  is usually small compared to R1 through R4 , so we can neglect it in a simplified circuit solution.

  The Wheatstone bridge is shown in figure 2.5 . The goal of the circuit analysis is to replace this circuit with a simplification for calculations. According to Thevenin’s theorem we can replace any circuit with a combination of its substitution resistance R

  k

  and a voltage source with voltage V0 connected in series, see figure 2.6 .

  FIGURE 2.5: Wheatstone bridge

  FIGURE 2.6: Bridge equivalent circuit diagram

  Bridges can be balanced manually or automatically. The advantage of a balanced bridge is low error (we are looking for 0), but the measurement takes longer as we have to balance the bridge. It is mainly used for precise laboratory experiments. Faster reading can be achieved with an unbalanced bridge, but the precision is lower and the output is NON-LINEAR

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  eBook - ePub

  Measurement, Instrumentation, and Sensors Handbook

  Electromagnetic, Optical, Radiation, Chemical, and Biomedical Measurement

  • John G. Webster, Halit Eren, John G. Webster, Halit Eren(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Figure 30.6 .

  The procedure referred to as bridge balancing is based on a proper selection of the reference values of the bridge so as to reduce the differential voltage to zero (as referred to the output signal of the balance indicator). It can be done manually or automatically.

  The condition of the balanced bridge v 0 = 0 leads to the following relation between the impedances of the bridge branches; one of them (e.g., Z 1 ) is the measured impedance:

  Z 1 Z 3 = Z 2 Z 4 → ( R 1 + j X 1 ) ( R 3 + j X 3 ) = ( R 2 + j X 2 ) ( R 4 + j X 4 )

  (30.19)

  Putting Equation 30.19 into complex form and using expressions for the impedances of each branch, two algebraic equations are obtained by comparing the real and imaginary components. They are used to determine the values of the equivalent circuit elements of the measured impedance. In the simplest case, they are L and R elements connected in series. More complicated equivalent circuits need more equations to determine the equivalent circuit parameters. Additional equations can be obtained from measurements made at different frequencies.

  In self-balancing bridges, vector voltmeters preceded by an amplifier of the out-of-balance voltage v 0 are used as “zero” detectors. The detector output is coupled with variable standard bridge components.

  The Maxwell Wien bridge shown in Figure 30.6a is one of the most popular ac bridges. Its range of measurement values is large and the relative error of measurement is about 0.1% of the measured value. It is used in the wide frequency band of 20 Hz–1 MHz. The bridge is balanced by varying the R 2 and R 3 resistors or by varying R 3 and capacitor C 3 . Some difficulties can be expected when balancing a bridge with inductors with high time constants.

  The Hay bridge presented in Figure 30.6b is also used for measurement of inductors, particularly those with high time constants. The balance conditions of the bridge depend on the frequency value, so the frequency should be kept constant during the measurements, and the bridge supply voltage should be free from harmonic distortions. The dependence of bridge balance conditions on frequency also limits the measurement ranges. The bridge is balanced by varying R 3 and R 4 resistors and by switching capacitor C 3

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