Electrical Instrumentation and Measurement /AN

Answer ALL questions (Total:100 marks)
Q1. Two voltage waveforms V1 and V2 are displayed on CH1 and CH2 of an
oscilloscope respectively as shown in Fig. Q1.
Fig. Q1
Determine:
(a) the period and frequency of V1 and V2;
(b) the peak-to-peak value of V1 and V2;
(c) the rms value of V1 and V2;
(d) the phase angle of V2 relative to V1.
(e) which switching mode (CHOP/ALT) is to be used. Why?
(6 marks)
(6 marks)
(4 marks)
(7 marks)
(2 marks)
Q2. (a) An oscilloscope has a bandwidth of 20 MHz. Is the oscilloscope
capable to display a signal with a rise time of 20 ns? Justify your
answer.
(b) With the aid of a diagram, explain the use of the input coupling
switch when selected as:
(i) DC
(ii) AC
(iii) GND
(6 marks)
(10 marks)
(c) A 10:1 attenuation probe is connected to a CRO input. The cable
capacitance of the probe Ccc is 20 pF. The input resistance Ri and
capacitance Ci of the CRO are 1 M and 25 pF respectively.
(i) Draw an equivalent circuit diagram showing the connection
between the probe and the CRO.
(ii) What should be the resistance R1 and capacitance C1 of the
probe head to give correct compensation?
(4 marks)
(5 marks)
Vertical Sensitivity:
CH1: 2 V/div
CH2: 5 V/div.
Timebase: 20 ms/div
Electrical Instrumentation and Measurement (EEE4113)
Assignment 2 (20-21) Page 2 of 2
Q3. Fig. Q3 shows a Wheatstone bridge circuit. The strain gauge forms one arm while three
fixed value resistors (R1, R2 and R3) mounted on the other arms. The bridge is in
balance when the strain gauge resistance is of its nominal value. The strain gauge has a
nominal length of 5.2 cm and gauge factor of 2. A tensile strain of 0.5×10-3 is applied
to the gauge. The resistance change of the strain gauge is measured in the form of the
output voltage Vo.
(a) State two possible errors of the measurement. (4 marks)
(b) Determine the nominal resistance of the strain gauge. (2 marks)
(c) Determine the change in resistance (R) when the force is applied
to the gauge.
(3 marks)
(d) Determine the output voltage Vo and state whether the potential at
node A is higher or lower than node B.
(8 marks)
(e) Determine the resultant length of the strain gauge. (4 marks)
(f) Suggest one method to improve the accuracy and one method to
improve the sensitivity of the bridge.
(4 marks)
Q4. A platinum resistance thermometer has a resistance of 160  at 25oC
forms one arm of a Wheatstone bridge as shown in Fig. Q4. At this
temperature the bridge is balanced with each other arms setting at the
same resistance (i.e. R = 160 ). The temperature coefficient of
resistance of the platinum at 25oC is 0.0039oC
-1
.
[Given that : R1 = R0 [ 1 + 0 (T1 – T0)]
(a) What will be the output voltage Vo of the bridge if the temperature
rises to 50oC?
(b) Determine the temperature of the thermometer if the output voltage
of the bridge is 0.3 V.