INDUSTRIAL ELECTRONICS II - B. ENG. IS PART-TIME – AUGUST 2012
Candidates are required to answer any FOUR (4) questions
Question 1
a)
b)
Define
(i)
hand switch
[2 marks]
(ii)
limit switch
[2 marks]
Discus the operation of the following switches and draw the schematic diagram
of each:
(i)
(ii)
(iii)
(iv)
(v)
c)
Toggle switch
Pushbutton switch
Proximity Switch
Pressure Switch
Temperature Switch
[3
[3
[3
[3
[3
marks]
marks]
marks]
marks]
marks]
Draw schematic diagrams of the following switches:
(i)
Double pole, 5- throw switch assembly
[2 marks]
(ii)
Double pole, single throw
[2 marks]
(iii)
Single-pole, double throw
[2 marks]
Page 1 of 9
Question 2
a)
In the field of Industrial Control Systems, sensors and transducers are often use
as a apart of control systems. Explain what is a
(i)
(ii)
b)
Sensor
Transducer
[2 marks]
[2 marks]
Explain the difference between a sensor and a transducer.
[2 marks]
c)
d)
Explain how the following sensors work and give an example of each type:
(i)
Thermostat
[3 marks]
(ii)
Thermistor
[3 marks]
(iii)
Resistive Temperature Detector (RTD)
[3 marks]
A lamp is red in series with a thermistor the themistor is used to adjust the
lighting of the lamp. Does the lamp get brighter if the thermistor is cooled when
the thermistor is:
(i)
an “n.t.c.” ? Support your answer
[3 marks]
(ii)
a “p.t.c.” type? Support your answer
[3 marks]
Page 2 of 9
Question 2 (Cont’d.)
The circuit below (Figure 2.1) and the calibration curve (Figure 2.2) can be used
for temperature measurement.
1
R
3
R
VS
R
T
VG
2
R
e)
Figure 2.1
Figure 2.2
Suppose R1 10 k , R 2 1k and R3 11k (the value of variable
resistor). What is temperature of the thermistor?
[4 marks]
Page 3 of 9
Question 3
a)
Draw a diagram showing how a permanent bar magnet is used to close a single
pole single throw (SPST) reed switch. The diagram should indicate the magnetic
flux lines, magnetic poles and induced poles on switch.
[5 marks]
b)
Draw a diagram showing how a coil wound in copper insulate wire is used to
close a single pole single throw (SPST) reed switch. The diagram should indicate
the magnetic flux lines, magnetic poles and induced poles on switch.
[5 marks]
In this door ajar alarm/beeper circuit (Figure 3.1), a bar magnet-reed switch
combination is used as the door sensor. When the door is closed, as per
mechanical arrangement, the bar magnet near the reed switch close its switch
contacts.
R = 10kΩ
(9v,60mA)
Rb = 1kΩ
Reed Switch
c)
Vi
9V
Vbe
Figure 3.1
Page 4 of 9
Question 3 (c) (Cont’d)
i)
Explain how the potential divider is used to turn on and off the speaker.
[5 marks]
ii)
Assuming the resistance of the reed switch is negligible, what is value of Vi when
the speaker is off.
[2 marks]
iii)
Assuming the resistance of the reed switch is negligible, what is value of Vi when
the speaker is on
[6 marks]
iv)
State the value of VBE when the transistor is on.
[2 marks]
Question 4
Rf
+Vs
copper
Hot
junction
thermocouple
X
V
constantan
VIN
cold
junction
Ra
Y
copper
Figure 4.1
Page 5 of 9
-Vs
Vout
0 – 1V
VOLTMETER
Question 4 (Cont’d.)
The circuit (Figure 4.1) represents an electric thermomemeter. The thermocouple
generates an emf between the terminals “X” and “Y” of 50µV per Kelvin temperature
difference between its junctions. The thermocouple is connected to the non inverting
input of an operational amplifier and feedback is provided by the resistor of the
resistance Rf to the inverting input.
a)
State what is meant by inverting input and non inverting input
[2 marks]
b)
State the type of feedback used in the circuit and explain how it functions
[5 marks]
c)
The voltage gain of the amplifier is given by:
Where R f
100 k n and Ra
Vout
Vin
1
Rf
Ra
1k
Calculate the reading of the voltmeter` when there is a temperature difference
of 100 kelvin between the junctions and the thermocouple.
[6 marks]
d)
The thermocouple is used to measure the body temperature over the range of
350C to 450C, the voltmeter giving zero deflection 350C and full scale deflection
of 1.0V at 450C. State two ways the thermocouple can be set up so that the
voltmeter reads zero when the temperature of the hot junction is 350C.
[6 marks]
e)
Describe and name the principle by which the thermocouple generates 50µV/K
between its junctions.
[6 marks]
Page 6 of 9
Question 5
a)
A simple circuit which swicthes on a lamp L when it gets dark is shown in Figure
5.1. Analyze the circuit below and explain how it works.
[6 marks]
Calculate and/or state both the typical potential at “X” and the potential
difference between the base and the emitter when the lamp is:
i.
on and the resistance of the LDR is 100MΩ,
[4 marks]
ii.
off and the resistance of the LDR is 100Ω
[4 marks]
R = 10kΩ
LAMP
(6v,60mA)
Rb = 1kΩ
X
Vi
6V
Vbe
LDR
Thermistor
Figure 5.1
RELAY
Contacts
Normally
open
Rb = 1kΩ
X
Vi
6V
Vbe
Bell
R=100Ω
b)
Figure 5.2
Page 7 of 9
Question 5 (cont’d.)
c)
Explain the operation of the high temperature alarm circuit in Figure 5.2.
[7 marks]
d)
Calculate the typical voltage potential at “X” , voltage difference “Vi” and voltage
difference “Vbe” when the bell is:
i.
ii.
on and the resistance of the thermistor is 100Ω,
off and the resistance of the thermistor is 100MΩ,
[4 marks]
[4 marks]
Question 6
a)
Describe how a relay operates and state two uses of a relay.
[4 marks]
b)
Describe how a reed switch operates and indicate one (1) advanatage of a reed
switch over a relay.
[3 marks]
c)
What voltage is required to operate a relay having a coil resistance of 1 kΩ and a
pull in current of 10ma?
[2 marks]
d)
Calculate the pull in current of a reed switch designed to operate with a coil
resistance of 700Ω on a 6V battery.
[2 marks]
e)
In the circuit of Figure 6.1, the operation of a relay is delayed intentionally for a
certain time by using a large capacitor shown.
Explain why it is only a few seconds after S is closed that the normally open
contacts of the relay close and the lamp comes on (and stays on so long as S is
on).
[2 marks]
Why is R included in the circuit?
[2 marks]
Page 8 of 9
Question 6 (Cont’d.)
R
6V
6V,
60mA
C,
2200µF
S
6V
Figure 6.1
f)
Assuming the d.c. supply is sufficient, analyze the circuits given below containing
light dependednt resistors (LDRs), bells and relays in Figures 1.2 and 1.3.
Compare and contrast the operations of the circuits.
[10 marks]
RELAY
COIL
d.c supply
LDR
BELL
Figure 6.2
RELAY
COIL
d.c supply
LDR
Figure 6.3
Page 9 of 9
BELL