Measurement of Parameters-Different methods of measuring low, medium and high resistances, measurement of inductance & capacitance with the help of AC Bridges, Q meter

1. ELECTRICAL MEASUREMENT & MEASURING INSTRUMENTS (NEE-302)
Md Irshad Ahmad
irshad.ahmad@jit.edu.in
Electrical Engineering Department
LECTURE
on
UNIT –III
Measurement of Parameters

2. CONTENTS
UNIT III
Measurement of Parameters-
 Different methods of measuring low
 medium and high resistances
 measurement of inductance & capacitance with the help of AC
Bridges
 Q meter.

3. Classification of Resistance
Measurements
• Low resistances (0-1) Ω
• Medium resistances (1-0.1M)Ω
• High resistances (≥ 0.1MΩ)
1

4. Medium Resistance
Measurement
Methods
(1) Voltmeter-Ammeter
(3) Ohmmeter
(2) Substitution
(4) Bridge

5. 3
Voltmeter-Ammeter
MethodknowAs we
internal resistances
that the
of the
voltmeter
are large
and ammeter
and small,
respectively.

6. 4

7. 5

8. Voltmeter-Ammeter
Method
(1) Simple and requires only two meters
(2) Subject to error

9. Substitution Method
Keep
vary
value
at position
the Rh up
of current
1 and
to finite
flow
through the circuit.
Shift to position 2 with the
same value of Rh in
changeprevious
the S
case,
value
same/previous
until the
current
flow through the circuit.
7

10. Ohmmeter Method
(1) Only one meter
(2) Non-linear scale

11. 9
principle of comparison
• Bridge Circuit is a null method, operates on the
• A known (standard) value is adjusted until it is equal
to the unknown value
Using Bridge Circuit

12. Wheatstone Bridge
10
Ratio arms (R1 , R2) & Standard arm (R3 )

13. Wheatstone Bridge Apparatus
switches theThe Ratio Control
ratio in decade steps
The remaining four step switches
set the resistance of the standard
arm
R1 , R2
R3 Min. step 1 Ω, Max value 11110 Ω
11
Example
10 Ω, 100 Ω, 1 kΩ 10 kΩ

14. 12

15. 13

16. 14
Measurement Errors

17. Thermal EMF
15
Creates error in measurement of low resistance

18. 16
Contact Resistance
On the micro scale, surface roughness
limits surface to surface contact
Measurement using four terminals
to avoid contact resistance
Standard resistance model with contact
resistance

19. 17

20. 18

21. 19

22. Problem
Figure below show the schematic diagram of a Wheatstone
bridge with the bridge elements. The battery is 5 V and its
internal resistance negligible. The galvanometer has a
current sensitivity of 10 mm/ µA and an internal resistance
of 100 Ω. Calculate the deflection of the galvanometer
caused by the 5 Ω unbalance in arm BC.
Ans: 33.2 mm

23. Thank You