Bridges: Electronic Instrumentation 15EC35
Bridges: Electronic Instrumentation 15EC35
MODULE 4
                                        BRIDGES
Introduction:
       Bridges is a circuit which is used for measuring various components like R, C and L
       Bridge as a simple circuit consists of having 4 resistance arms in a closed loop, with
        dc current source applied to 2 opposite junction and current detector connected to other
        2 junction as shown in Fig. 4.1.
       In this the unknown component is measured in comparison with known component
        called as standard.
    
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        This method of measurement is very accurate and the accuracy of measurement is
        directly proportional to the bridge component.
                                                        .c
            ac bridge – impedances consisting of C and L
            dc bridges – measure resistance
The dc bridge used for measuring resistance is called Wheatstone’s bridge.
                                             op
Wheatstone’s Bridge:
   It is the most accurate method for measuring resistance and a common method used
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     in laboratory.
   The circuit is shown in Fig 4.1.
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      Bridge is balanced when current through G is ‘0’ i.e potential difference at C and D
       should be equal.
i.e                                𝐼1𝑅1 = 𝐼2𝑅2 ------------ (1)
For galvanometer current to be zero, I1=I3 and I2=I4
Thus under balanced condition,
                                             𝐸
                              𝐼1 = 𝐼3 =                  (𝑎)
                                          𝑅1 + 𝑅3
And
                                                      𝐸
                                       𝐼2 = 𝐼4 =             (𝑏)
                                                   𝑅2 + 𝑅4
Using (a) and (b) in equation (1), we get
                                   𝐸 ∗ 𝑅1   𝐸 ∗ 𝑅2
                                                                   om
                                          =
                                  𝑅1 + 𝑅3 𝑅2 + 𝑅4
Simplifying the above equation we get,
        𝑅1(𝑅2 + 𝑅4) = 𝑅2(𝑅1 + 𝑅3)
                                                             .c
        𝑅1𝑅2 + 𝑅1𝑅4 = 𝑅1𝑅2 + 𝑅2𝑅3
        𝑅1𝑅4 = 𝑅2𝑅3
                                                   op
                        𝑅2 𝑅3
Now           𝑅4 =
                             𝑅1
For balancing one of the resistance will be made adjustable and if R4 is the unknown
resistance then
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                             𝑅2 𝑅3
                 𝑅𝑥 =
                                  𝑅1
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        If the G is more sensitive then, the deflection is more for the same amount of current.
        Thus sensitivity is considered as deflection/unit current. i.e S = D/I , D = deflection
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        and I = current in µA
       Sensitivity can be expressed in linear or angular with the units as S = mm/µA
        (Linear) and S = degree/µA or S = radian/µA (Angular)
       Thus total deflection D = S * I
Since the interest is to find the current through G under unbalanced condition we need to
find the Thevenin’s equivalent circuit as seen by G
The first step is to remove G and find open circuit voltage between terminals a and b as
shown in fig 4.2
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Applying voltage divider at point ‘a’ and ‘b’, we get
                                                 op   𝑅3 𝐸
                                           𝐸𝑎 =
                                                     𝑅1 + 𝑅3
                                                      𝑅4 𝐸
                                         lo
                                           𝐸𝑏 =
                                                     𝑅2 + 𝑅4
Voltage between a and b is the difference between Ea and Eb and this represents the
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                                                     𝑅1 + 𝑅3 𝑅2 + 𝑅4
                                         𝑅3 𝐸         𝑅4 𝐸
Thus                         𝐸𝑡ℎ = 𝐸 (           −           )
                w
𝑅1+𝑅3 𝑅2+𝑅4
internal impedance or with a short and looking into a and b as shown in fig 4.3.,
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If G is connected between a and b in the above circuit and its original circuit then both
experencies same deflection.
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The magnitude of current is limited by Rth and the resistance seen with G i.e Rg (internal
resistance of G)
Thus the deflection of current in galvanometer is given by
                                                         .c
                                                𝐸𝑡ℎ
                                       𝐼𝑔 =
                                              𝑅𝑡ℎ + 𝑅𝑔
                                              op
Slightly unbalanced Wheatstone’s bridge:
If three of the four resistor in a bridge are equal to R and the fourth differs by 5% or less, we
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can develop an approximate but accurate expression for Thevenin’s equivalent voltage
and resistance as follows. The circuit is shown in Fig 4.5
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                                  𝐸𝑡ℎ = 𝐸𝑎 − 𝐸𝑏 = 𝐸𝑏 − 𝐸𝑎
                                           𝐸 (𝑅 + 𝛥𝑟) 𝐸
                                   𝐸𝑡ℎ =             −
                                            2𝑅 + 𝛥𝑟    2
Simplifying this,
                                      2𝑅 + 2𝛥𝑟 − 2𝑅 − 𝛥𝑟
                             𝐸𝑡ℎ = 𝐸 (                   )
                                          2(2𝑅 + 𝛥𝑟)
                                                𝛥𝑟
                                    𝐸𝑡ℎ = 𝐸 (         )
                                             4𝑅 + 2𝛥𝑟
Now if 𝛥𝑟 is 5% of R or less, then 𝛥𝑟 can be neglected at the denominator without
                                                               om
appreciable error. Thus Eth now is
                                                   𝛥𝑟
                                           𝐸𝑡ℎ =
                                                   4𝑅
                                                         .c
The equivalent resistance can be calculated by replacing the voltage source with its internal
impedance,
                                             op
                                      𝑅𝑅    𝑅 (𝑅 + 𝛥𝑟)
                             𝑅𝑡ℎ =        +
                                     𝑅 + 𝑅 𝑅 + (𝑅 + 𝛥𝑟)
                                     lo
Simplifying the above equation,
                                           𝑅   𝑅 (𝑅 + 𝛥𝑟)
                                  𝑅𝑡ℎ =      +
                             tu
                                           2    2𝑅 + 𝛥𝑟
If 𝛥𝑟 is small compared to R, then it can be neglected
                        .v
                                      𝑅    𝑅 𝑅)
                                     𝑅𝑡ℎ =
                                         +
                                      2     2𝑅
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                                   𝑅  𝑅     2𝑅
                              𝑅𝑡ℎ = +    =      =𝑅
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                                   2   2     2
Thus the Thevenin’s equivalent circuit is shown in Fig 4.6
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                                                                om
    While measuring high resistance, the resistance presented by the bridge becomes so
      large that the galvanometer will be insensitive to imbalance. Thus for high resistance
      measurements in mega ohms, the Wheatstones bridge cannot be used.
    Another problem in Wheatstone Bridge Circuit is the change in resistance of the bridge
      arms due to the heating effect of current through the resistance. The rise in temperature
                                                         .c
      causes a change in the value of the resistance, and sometimes high current may cause
      a permanent change in value.
                                             op
Kelvin’s bridge
    When the resistance to be measured is of the order of magnitude of bridge contact and
       lead resistance, a modified form of Wheatstone’s bridge, the Kelvin's bridge is used.
                                     lo
    Kelvin's bridge is used to measure values of resistance below 1 Ω. In low resistance
       measurement, the resistance of the leads connecting the unknown resistance to the
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       the unknown resistance Rx, resulting in too high indication for Rx.
    When the connection is made to point c, R3, is added to the bridge arm R3 and
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       that the ratio of the resistance from c to b and that from a to b equals the ratio of
       resistances R1 and R2, then
                                          𝑅𝑐𝑏   𝑅1
                                              =
                                          𝑅𝑎𝑏   𝑅2
                                                                       om
From the Fig 4.7,
                             Rab + Rcb = Ry ----------------- (A)
                                                                .c
and
                               𝑅𝑐𝑏     𝑅1
                                    =        − − − − − (𝐵)
                              𝑅𝑎𝑏      𝑅2
Adding 1 to both sides of equation (B), we get
                                                  op
                                          𝑅𝑐𝑏      𝑅1
                                              +1 =    +1
                                          𝑅𝑎𝑏      𝑅2
                                          lo
                                  𝑅𝑐𝑏 + 𝑅𝑎𝑏      𝑅1 + 𝑅2
                                              =
                                  tu
                                     𝑅𝑎𝑏           𝑅2
using equation (A) in the above equation, we get
                                    𝑅𝑐𝑏 + 𝑅𝑎𝑏   𝑅𝑦    𝑅1 + 𝑅2
                        .v
                                              =     =
                                       𝑅𝑎𝑏      𝑅𝑎𝑏     𝑅2
and Rab is now
                w
                                             𝑅2 ∗ 𝑅𝑦
                                             𝑅𝑎𝑏 =
                                            𝑅1 + 𝑅2
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                                             Rcb = Ry − Rab
w
                                                             𝑅2 ∗ 𝑅𝑦
                                         Rcb = Ry −
                                                             𝑅1 + 𝑅2
                                             𝑅1𝑅𝑦 + 𝑅2𝑅𝑦 − 𝑅2𝑅𝑦
                                     𝑅𝑐𝑏 =
                                                   𝑅1 + 𝑅2
                                                      𝑅1 𝑅𝑦
                                             𝑅𝑐𝑏 =
                                                     𝑅1 + 𝑅2
Substituting Rab and Rcb in equation (1)
                      𝑅2 𝑅𝑦                      𝑅1 𝑅𝑦
        R1 * (R3 +              ) = R2 * (Rx +           )
                      𝑅1+𝑅2                      𝑅1+𝑅2
                                         𝑅1 𝑅𝑦    𝑅1        𝑅2 𝑅𝑦
                                 𝑅𝑥 +           =    (R3 +         )
                                        𝑅1 + 𝑅2   𝑅2       𝑅1 + 𝑅2
                                        𝑅1 𝑅𝑦    𝑅1 𝑅3    𝑅2 𝑅𝑦   𝑅1
                                𝑅𝑥 +           =       +        ∗
                                       𝑅1 + 𝑅2    𝑅2     𝑅1 + 𝑅2 𝑅2
                                             𝑅1 𝑅𝑦    𝑅1 𝑅3    𝑅1 𝑅𝑦
                                     𝑅𝑥 +           =       +
                                            𝑅1 + 𝑅2    𝑅2     𝑅1 + 𝑅2
        Upon simplification, we get
                                                    𝑅1 𝑅3
                                                𝑅𝑥 =
                                                       𝑅2
       The above equation is the normal Wheatstone’s bridge under balanced condition.
       Also the effect of lead resistance connecting from a to c is eliminated by connecting
                                                                    om
        galvanometer to intermediate position ‘b’.
       This is the principle of constructing Kelvin’s double bridge also known as Kelvin’s bridge. It
        is called double bridge as it incorporates 2nd set of resistance ratio arms.
       The schematic of Kelvin’s double bridge is shown in Fig 4.8
       In this 2nd set of arms a and b connect galvanometer to point c
                                                             .c
       The galvanometer gives null indication when potential at k and c are equal i.e 𝐸𝑙𝑘 = 𝐸𝑙𝑚𝑐
                                                   op
                                             lo
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                                                        𝑅2 𝐸
                                               𝐸𝑙𝑘 =
                                                       𝑅1 + 𝑅2
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and E is given by
                                              E=I*R
                                                    𝑅2
                                            𝐸𝑙𝑘 =         ∗𝐼∗𝑅
                                                  𝑅1 + 𝑅2
                                       𝑅2                     𝑅𝑦 (𝑎 + 𝑏)
                             𝐸𝑙𝑘 =           ∗ 𝐼 ∗ [𝑅3 + 𝑅𝑥 +            ]
                                     𝑅1 + 𝑅2                  𝑎 + 𝑏 + 𝑅𝑦
Dept. of ECE, SVIT                                                                           2017-18
Electronic Instrumentation                                                 15EC35
Elm = I R3
            𝑏 𝐸𝑚𝑛
𝐸𝑚𝑐 =                          using voltage divider rule
             𝑎+𝑏
                                  𝑅𝑦 (𝑎+𝑏)
𝐸𝑚𝑛 = (𝑎 + 𝑏)‖ 𝑅𝑦 ∗ 𝐼 =                      *I
                                  𝑎+𝑏+𝑅𝑦
Thus now
                                               𝑏       𝑅𝑦 (𝑎 + 𝑏)
                                   𝐸𝑚𝑐 =          ∗ 𝐼∗
                                              𝑎+𝑏      𝑎 + 𝑏 + 𝑅𝑦
Therefor
                                                    𝑏       𝑅𝑦 (𝑎 + 𝑏)
                                                                    om
                               𝐸𝑙𝑚𝑐 = 𝐼 𝑅3 +           ∗ 𝐼∗
                                                   𝑎+𝑏      𝑎 + 𝑏 + 𝑅𝑦
                                                       𝑏    𝑅𝑦 (𝑎 + 𝑏)
                                𝐸𝑙𝑚𝑐 = 𝐼 [𝑅3 +            ∗            ]
                                                     𝑎 + 𝑏 𝑎 + 𝑏 + 𝑅𝑦
                                                               .c
But under balanced condition,
                                                  𝐸𝑙𝑘 = 𝐸𝑙𝑚𝑐
                                                    op
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                                                             om
       This bridge can measure resistance in the range of 1Ω - 10μΩ with accuracy of ±0.05%
        to ±0.02%.
AC Bridges:
   The ac bridges are similar to dc bridge except that the bridge arms have impedances
                                                      .c
      and the bridge is excited by ac source rather than dc source.
   Impedances at audio frequency and radio frequency can be determined by means of
      ac bridges.
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