Download Problem Set 4

EE11A – ELECTRICAL MEASUREMENTS
Problem Sets
PROBLEM SET 4
Q1
The standard arm of a Wheatstone bridge has a range of 0 to 100 with a resolution of
0.001. The galvanometer has an internal resistance of 100 and can be read to 0.5A.
R1 and R2 are both 1k and the battery emf is 10V. If the unknown resistance is 50:
(i)
What is the resolution of the bridge in ohms and percent of the unknown; and
(ii)
What is the galvanometer current for a change of 5% in the value of the unknown
resistance?
Q2
Compute the deflection sensitivity of an electrostatic CRT in which the accelerating
voltage is Va=1000V, L=24cm, d=1.2 cm and the length of the deflection plates is 4.0 cm.
What deflection voltage is requires to deflect the beam 3.0 cm from the center of the
CRT.
Q3
Explain the difference between the trigger LEVEL control and the trigger SLOPE control.
Q4
Compare the CHOP and ALTERNATE modes. When would you use one or the other?
Q5
Why is it necessary to compensate an oscilloscope probe?
Q6.
(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?
Prepared by: Mr. Fasil Muddeen
1
© 2001
EE11A – ELECTRICAL MEASUREMENTS
Problem Sets
Q7.
(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).
Q8.
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.
D1
R1
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)
Q9.
(a)
(i)
(ii)
What voltage does it indicate across each resistor?
What is the percentage error due to voltmeter loading
(i)
(ii)
Derive the balance equations for a Wheatstone bridge.
What are the sources of error associated with bridge
Prepared by: Mr. Fasil Muddeen
2
© 2001
EE11A – ELECTRICAL MEASUREMENTS
Problem Sets
(ii)
Q10.
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.
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.
(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
© 2001