Design Lab 01 ME
WHEATSTONE BRIDGE
Experiment on Wheatstone Bridge
AIM:
Measurement of resistance using wheatstone bridge
INTRODUCTION:
For measuring accurately any electrical resistance Wheatstone bridge is
widely used. There are two known resistors, one variable resistor and one
is unknown resistor connected in bridge form as shown below by
adjusting the electric current through the galvanometer is made zero.
When the electric current through the galvanometer becomes zero, the
ratio of two known resistors is exactly equal to the ratio of adjusted value
of variable resistance and the value of unknown resistance. In this way
the value of unknown electrical resistance can easily be measured by
using a Wheatstone Bridge.
Figure .1 shows the basic Wheatstone Bridge circuit, consisting of four resistors
and a sensitive center zero meter connected to a DC source.
A B
D
Figure1
R1, R2 & R3 are accurate, close tolerance, resistors. R3 is variable and
calibrated
over its full range. R4 is the unknown resistor to be measured.
THEORY:
A schematic of a Wheatstone bridge , shown in Figure 1: let R4(Rx) is an unknown
resistor, the resistor R3 is known and adjustable , and the resistors R1 and R2 have
a known ratio of R1/R2, although their individual values may not be known.
When a voltage Vi is impressed upon the circuit at point C and D , a galvanometer G
indicates the voltage difference VAB between point A and B.
Either the known resistor R3 is adjusted, or the ratio R1/R2 is adjusted until the
voltage difference VAB is zero and no current flows through the galvanometer and
VAB=0. when the VAB=0 or null, the bridge is said to be balanced .
Since VAB=0, the voltage drop from C to A must be equal to the voltage drop from c
to b, i.e.. VAC=VCB. Likewise, we must have VAD=VBD. Or we can say that With no
current in the galvanometer, the current in R1 must be the same as that in R3 and
the current in R2 must equal that in R4.
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If current I1 flows in R1 & R3 and current I2 flows in R2 & R4. Thus from ohms law,
we can write,
The unknown resistance R4 depends on the ratio R2:R1 and the value of
R3 at
balance. The resistors R1 and R2 are normally referred to as the "ratio
arms" of the bridge.
Figure.2 shows the Wheatstone Bridge layout provided with the DIGIAC
1750 unit.
Figure 2
A high quality 10-turn potentiometer fulfills the functions of the resistors R1 &
R3 for resistance, or a potentiometer for voltage measurements. The track
resistance of 10kΩ has a maximum non-linearity of 0.25%. The "Fine" dial is
calibrated 0 - 100 in steps of 2, and the "Coarse" reading is calibrated 0 - 10,
thus enabling readings to be estimated from the dial with discrimination of
1:1000, representing a resolution of 10Ω.
Reading the dial:
If the number in the window (coarse setting) is 3 and the fine setting is on 74, then the
dial reading is 374. The resistance between the 0V terminal and A (the wiper) is 10Ω x
374 = 3.74kΩ .
A close-tolerance 12kΩresistor (R2) and an unknown resistor Rx (R4) are
provided for resistance Measurement.
A switch open circuits the unknown resistor Rx to allow the measurement of other
unknown resistors which can be connected between socket C and the 0V terminal.
An accurate standard voltage of 1V is available at socket B.
The moving coil meter can be used as a center zero indicating instrument.
Since it is arranged as a 10V voltmeter its sensitivity is insufficient for a direct application
as a galvanometer. This problem can be overcome by using a differential amplifier
followed by a high gain DC amplifier from the signal conditioning circuits.
Figure.3 shows the layout diagram required for setting up the null detector.
Fig.3
Figure .4 shows the complete circuitry of Wheatstone Bridge to find out
the unknown resistance
OBSERVATION TABLE 1:
THANK YOU
