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

PRACTICAL 2

AIM: - TEST THE LOW RESISTANCE USING KELVIN BRIDGE.

APPARATUS:-

• Regulated dc supply-1no
• Standard resistance coil-1no
• Kelvin’s double bridge kit.
• Digital multimeter-1no,
• Patch codes.

CIRCUIT DIAGRAM:-

CLICK ON GIVEN LINK

THEORY:-

               Kelvin’s bridge is a modification of whetstone’s bridge and always used in measurement of low resistance. It uses two sets of ratio arms and the four terminals resistances for the low resistance consider the ckt. As shown in fig. The first set of ratio P

And Q. The second set of ratio arms are p and q is used to connected to galvanometer to a Pt dat an Approx. potential between points m and n to eliminate the effects of connecting Lead of resistance r between the known std. resistance‘s’ and unknown resistance R .The ratio P/Q is made equal to p/q. under balanced condition there is no current flowing Through galvanometer which means voltage drop between a and b, Eab equal to the Voltage drop between a and c, Eamd.

Now

Ead=p/(p+q)

Eab=I[R+S+ ((p+q)r)/(p+q+r)]

Eamd=I[R+ p/p+q [(p+q) r/p+q+r]] 

For zero deflection->

Eac=Ead

[P/P+Q]I[R+S+ {(p+q) r/p+q+r}] =I[R+pr/p+q+r] —______________________(3)

Now, if

P/Q=p/q

Then equation… (3) Becomes

R=P/Q=S-_________ (4)

Equation (4) is the usual working equation. For the Kelvin’s Double Bridge .It indicates

The resistance of connecting lead r. It has no effect on measurement provided that the two Sets of ratio arms have equal ratios. Equation (3) is useful however as it shows the error that is introduced in case the ratios are not exactly equal. It indicates that it is desirable to keep as small as possible in order to minimize the error in case there is a diff. between

The ratio P/Q and p/q.

R=P/QS

OBSERVATION TABLE:-

Sr no.	P                         (Ratio Arm Resistor)	Q  (Ratio Arm Resistor)	S Standard Resistor	R Measured Value	R Actual

PROCEDURE:-

1) The circuit configuration on the panel is studied.

2) Supply is switched on and increased up to 5v.

3) The unknown resistance is connected as shown.

4) The value of P,Q was selected such that a. P/Q=p/q

5) S was adjusted for proper balance and balance value of s was balanced.

6) The value of known resistance was calculated.

PRECAUTIONS:-

1) Check all the connections before turning ON the power supply.

2) Do not exceed the value of 5v.

3) Note the readings accurately.

CONCLUSION:-

FOR CIRCUIT DIAGRAM 

PLEASE CLICK ON THE BELOW LINK.....

DOWNLOAD FILE for practical 2

Share

Blogger Widget