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
