1.Electrical Instrumentation lab manual for diploma electrical engineering 3rd sem as per GTU

PRACTICAL 1

AIM: - TEST THE MEDIUM RESISTANCE USING WHEATSTONE   BRIDGE.

v Specific Objective:-

        Student will be able to find out resistance of the above bridge.

v Apparatus:-

1.      Adtran’s trainer kit

2.      C.R.O

3.      Patch cords

4.      Digital multimeter

v Theory:-

    A very important device used in the measurement of medium resistances is the Wheatstone bridge. A whetstone bridge has been in use longer than almost any electrical measuring instrument. It is still an accurate and reliable instrument and is extensively used in industry. The Wheatstone bridge is an instrument for making comparison measurements and operates upon a null calibration principle. This means the indication is independent of calibration of the null indicating instrument or any of its characteristics. For this reason, very high degrees of accuracy can be achieved using Wheatstone bridge. Accuracy of 0.1% is quite common with a Wheatstone bridge as opposed to accuracies of 3% to 5% with an ordinary ohmmeter for measurement of medium resistances.

     Fig shows the basic circuit of a Wheatstone bridge/ it has four resistive arms, consisting of resistances P,Q,R, and S together with a source of emf (a  battery ) and a null detector, usually a galvanometer G or other sensitive current meter. The current through the galvanometer depends on the potential difference between points c and d. the bridge is said to be balanced when there is no current through the galvanometer or when the potential difference across the galvanometer is zero. This occurs when the voltage from point ‘b’ to point ‘a’ equals the voltage from point ‘d’ to point ‘b’; or, by referring to the other battery terminal, when the voltage from point ‘d’ to point ‘c’ equals the voltage from point ‘b’ to point ‘c’.

                 For bridge balance, we can write:

                                          I₁P = I₂R…………………………………………. (1)

 For the galvanometer current to be zero, the following conditions also exist:

                                          I₁ = I₃ =E/(P+Q)

                                          I₂ = I₄ =E/(R+S)…………………………… (3)

Where

        Combining Eqns. (1), (2), and (3) and simplifying, we obtain:

                                     P/(P+Q) =   R/(R+S)  …………………………. (4)

                                        QR = PS …………………………………………   (5)

Eqn.(5) is the well known expression for the balance of Wheatstone bridge. If three of the resistances are known, the fourth may be determined fro eqn. (5) and we obtain:

                               R = S P/Q…………………………... (6)

 Where R is the unknown resistance S is called the ‘standard arm’ of the bridge and P and Q are called the ‘ratio arm’.

v Procedure :-

1.      Connect the required supply and switch ON the unit. See that the supply LED glows.

2.      Observed the sine wave at the respective terminals.

3.      Now connects the C.R.O between the ground and the terminals marked ‘detector’.

4.      Connect the unknown resistance to the terminal marked S.

5.      Select one multiplier arm by connecting link.

6.      Vary P for minimum position.

7.      Similarly vary R  For minimum position.

8.      If the selection of the multiplier is correct the balance point can be observed on the C.R.O.

9.      Substitution the same in the formula and the value of S.

v Observation Table:-

SR.NO	P	Q	S (Multiplier arm)	R = S P/Q

v Calculation :-

v Conclusion :-

FOR CIRCUIT DIAGRAM......

CLICK ON THE BELOW LINK

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