Download Problem Set 5

EE11A – ELECTRICAL MEASUREMENTS
Problem Sets
PROBLEM SET 5
Review questions
Q1.
(a)
The manual for a 3½ digit DMM describes it as having a
frequency response of 20 Hz to 20 kHz, a 100 dB dynamic range,
high sensitivity and a low input threshold.
(i)
What is meant by the “1/2” digit specification?
(ii)
What is meant by the terms?:
1.
Frequency response
2.
Dynamic range
3.
Sensitivity
4.
Input threshold
(iii)
What is the resolution of the DMM on the 100V range?
(iv)
The accuracy of the DMM on the 10V range is given
as:
 (0.05% range + 0.2% reading + 2 digits). What
answer would be displayed and what is the error if the DMM is
used to measure a standard cell rated at 1.0128 VDC?
Q2.
(a)
Design a dc ammeter with an Ayrton Shunt using a 50 A
PMMC meter movement with internal resistance 3 k. The
ammeter is to have three ranges: 100 A; 1mA; and 10 mA.
(b)
Comment on the following statement: “An ammeter using
an Ayrton Shunt requires a make-before-break selection
switch”
(c)
The PMMC meter movement in part (a) is to be used to
design a dc voltmeter having ranges of 150 mV, 5V, 15V and 50V.
Determine
(i)
The sensitivity of the voltmeter;
(ii)
The values of the resistors needed to create a series
type multiplier analogous to the Ayrton Shunt used in
(a).
Q3.
Figure Q3(a) shows a simple AC voltmeter using a 50A PMMC
movement with internal resistance 250.
(a)
What series resistance must be used to produce a full-scale
reading of 10V rms? Assume that V D1 = 0.6V and that the forward
resistance of D1 is negligible.
Prepared by: Mr. Fasil Muddeen
1
August 29, 2000
EE11A – ELECTRICAL MEASUREMENTS
Problem Sets
D1
R
1
AC In
A
50A

Fig. Q3(a)
(b)
The meter is used to measure the voltage across the resistors
in the following circuit:
R1
33k
15V
100Hz
R2
56k
~
Fig. Q3(b)
(i)
(ii)
Q4.
What voltage does it indicate across each resistor?
What is the percentage error due to voltmeter loading
(a)
(i)
Derive the balance equations for a Wheatstone bridge.
(ii)
What are the sources of error associated with bridge
measurements?
(iii)
What factors determine the accuracy of a bridge?
(b)
A Wheatstone bridge, using a 10V emf source has the
following components in its arms:
R1 = 100; R2 = 200 and R3 = 5k. R4 is unknown.
(i)
Calculate the value of R4.
Prepared by: Mr. Fasil Muddeen
2
August 29, 2000
EE11A – ELECTRICAL MEASUREMENTS
Problem Sets
(ii)
R4 changes by 1%. Given that the null detector is a 1000,
1mm/A galvanometer, calculate the deflection in the
galvanometer produced by this unbalanced condition.
Q5.
(a)
A cathode ray oscilloscope may be described in terms of four
subsystems:
(i)
The Vertical subsystem;
(ii)
The Horizontal subsystem
(iii)
The Trigger subsystem; and
(iv)
The Display subsystem.
Explain the functions of each subsystem and the main user
controls available in each subsystem.
(b)
Describe the operation of a 10:1 compensated attenuating
probe.
(c)
A 10:1 probe with a 2.0 m coaxial lead, Cc = 60 pF/m, is
connected to an oscilloscope with an input impedance of 1
M shunted by 18 pF. What is the input impedance of the
probe at:
(i)
dc;
(ii)
A frequency of 10 MHz.
Prepared by: Mr. Fasil Muddeen
3
August 29, 2000