Download DT002_1_Industrial_Electronics_summer_2006_ans

T1
D1
D2
D4
D3
Vmains
RLoad
Figure 1
Figure 1 shows the circuit diagram for a simple d.c. power supply.
Identify the type of rectifier circuit represented in figure 1 and explain the
operation of the circuit with reference to the function of each component
within the circuit.
This is a bridge rectifier circuit.
The mains voltage is applied to the primary winding of the transformer
T1. This typically produces a reduced amplitude voltage on the
secondary winding – closer to the desired d.c. output voltage than the
original mains voltage. The transformer also provides electrical
isolation between the mains supply and the load.
The diodes D1 to D4 perform rectification.
On positive half cycles of the secondary voltage, D2 and D4 are
forward biased and conduct – connecting positive half cycles across
RLoad, with the top of RLoad positive.
On negative half cycles of the secondary voltage, D1 and D3 are
forward biased and conduct – connecting negative half cycles across
RLoad, with the top of RLoad still positive.
RLoad represents the circuit to which it is desired to supply d.c.
electricity.
Sketch the voltage across RLoad as a function of time showing its
relationship to the secondary voltage from the transformer.
Secondary Voltage
Load Voltage
The rectifier circuit shown in figure 1 requires the addition of a filter to
produce a near constant d.c. voltage across RLoad. Redraw figure 1
showing where a smoothing capacitor should be connected.
T1
D1
D2
D4
D3
Vmains
RLoad
+
C
Explain how the smoothing capacitor sustains a d.c. voltage across RLoad,
despite the pulsating nature of the rectifier output.
The capacitor charges rapidly from the transformer secondary voltage
via the diodes in the bridge when the a.c. rises towards its peak
voltage. When the a.c. has reached its peak and starts to drop again, the
capacitor holds on to its voltage and the diodes in the bridge become
reverse biased. Current can only now be delivered to the load by
discharge of the capacitor. The capacitor voltage will drop gradually
under these circumstances at a rate dependent on the capacitor value
and the load resistance value. After a short time, the a.c. starts to rise
back towards its peak and will forward bias the bridge diodes when it
exceeds the voltage to which the capacitor has dropped. The capacitor
charge will be ‘topped-up’ ready for the next half cycle of a.c.
Provided that the capacitor is large enough, its terminal voltage will
not drop substantially between peaks of the a.c. Thus, the load voltage
is held almost constant.
Calculate a value for the smoothing capacitor in order to keep the
percentage ripple voltage across RLoad below 5%. Assume a value of 500
Ω for RLoad and a mains frequency of 50 Hz.
Peak-peak ripple voltage is given by –
VLOAD NOM
VRIPPLE 
2  RLOAD  C  f
Where:
VLOAD NOM = nominal output voltage from PSU
C = the value of the smoothing capacitor
f = the frequency of the a.c. supply
VRIPPLE
1
 0.05 
VLOADNOM
2  RLOAD  C  f
1
C
 400F
2  500  0.05  50
for 5% ripple, =>
=>
The power supply shown in figure 1 is said to be unregulated. Explain the
meaning of this term and show how a three terminal regulator chip may
be used to provide a regulated output voltage.
An unregulated power supply is one where the output voltage may
change substantially from the stated nominal output voltage under
specific operating conditions. In particular, change in output voltage
may arise from –
 Fluctuations in the mains supply voltage
 Change in the amount of current drawn by the load
A three terminal regulator chip is an integrated circuit designed to
prevent changes in the output voltage occurring – within design limits.
It is used as shown in the circuit below
T1
D1
D2
U1
In
Vmains
D4
D3
Out
Gnd
RLoad
+
C
Figure 2 shows a bipolar junction transistor (BJT) used to switch a motor
on and off in response to switch S1 closing and opening. The BJT is
specified with βDC = 100 and BVCEO = 40 V.
The base current may be calculated from –
V
V
5V  0.6V
I B  Control BE 
 93mA when S1 is closed
RB
47
V
 VBE
0V
I B  Control

 0 A when S1 is opened
RB
47
Sketch to scale an approximation to the collector characteristic curve for
the BJT used in this circuit and identify on the characteristic the active
region, the saturation region and the breakdown region.
Collector Curve
20
18
16
14
IC
12
10
8
6
4
2
0
0
5
10
15
20
25
30
35
40
45
VCE
Set out the load-line equation for the circuit and plot this on the same
graph as the collector curve. Hence or otherwise determine the load
current flowing when the BJT is switched on.
The load line equation is –
VCC  VCE 24V  VCE

RLoad
3
This may be plotted by finding two points and joining up – see above.
IC 
The load current is got by finding the intersection of the load line with
the characteristic curve – i.e. 8 A
With reference to typical values where necessary, estimate the power
dissipation in the BJT when switched on.
Typically we might expect about 0.3 volts between collector and
emitter when turned fully on. Given 8 A of load current, power is –
P  0.3V  8 A  2.4W
Explain the consequences for the BJT if a large amount of power is
dissipated and describe a technique that may be used to minimise the
effect.
Excessive power dissipation in a BJT will lead to the device heating up
to a point where the temperature of the device exceeds the maximum
rated operating temperature. In this case the device may fail or at least
the operating lifetime of the device may be shortened.
To reduce the heating, the device may be fitted with a heatsink to
conduct heat away from the device and radiate this heat to the
surrounding air.
Explain the following terms as used in relation to measurement systems –
Linearity
The transfer function of the measurement system should ideally be a
straight-line relationship. I.e. if M represents the measurand, and the
measurement system reading of the value of M is R, then -
Where f is some function. If it is possible to write –
where k and c are constants, then the measurement system is said to be
linear. The term linearity is used to describe the extent to which the
actual transfer characteristic of a measurement system approximates to
this ideal.
Accuracy
An ideal measurement system would produce readings of measurand
value, which contain no error. This is not possible due to the
cumulative effect of many phenomena. There will always be some
uncertainty about the true value of the measurand – i.e. the reading of
the measurement system is wrong by some amount. Accuracy is a
term used to describe the extent of the error in the reading compared to
the true measurand value.
Resolution
Describes the smallest change in the value being measured for which
the measurement instrument will indicate a change in value
Resistance change transducers are often used in measurement systems.
State the type of measurement that would be performed using each of the
following transducers and give an indication of the sensitivity and
linearity of each –
RTD
A RTD is a Resistance Temperature Device. It is used for measuring
temperature. It is quite linear and produces modest changes in
resistance value over the useful range of operation.
Thermistor
A thermistor is used for measuring temperature. It is highly non-linear
and produces large resistance changes over the useful range of
operation.
Strain Gauge
A strain gauge is used for measuring mechanical strain. It is reasonably
linear and produces small resistance changes over the useful range of
operation.
Draw a circuit diagram showing how a Wheatstone bridge may be used in
conjunction with a thermistor to measure temperature. Explain how the
circuit works as the temperature of the transducer changes and comment
on the linearity of the circuit.
R1
R3
Vout
E
R2
R4
R5
The transducer resistance is represented in this diagram by Rtransducer =
R4 + R5 where
R4 = transducer resistance at zero measurand value
R5 = the change in transducer resistance caused by the change
in measurand value above/below the zero value.
This is a Wheatestone bridge circuit. The voltage drop across the
transducer resistance will rise/fall as the value of the transducer
resistance changes. The output voltage is given by:
 R4  R5 
 R2 
Vout  E.

  E.
 R2  R1 
 R4  R5  R3 
Vout is approximately linear with changes in transducer resistance
provided the change in resistance is small compared to its original
value..
List and explain the signal conditioning requirements for a resistance
change transducer if the output signal from the transducer is to be
applied to an Analogue to Digital Converter (ADC) with a full-scale input
of 5 volts
 The transducer must be put into a circuit that converts the
change in resistance to a change in voltage. A Wheatstone
bridge circuit may be used for this purpose.
 The output voltage from the Wheatstone bridge circuit is too
small to send directly to an ADC. It must be amplified to boost
its level to that required by the ADC.
 The signal must be filtered prior to sending it to the ADC to
prevent aliasing.
Current signals are sometimes used in measurement systems in
preference to voltage signals. State the advantages associated with using
current signals. Specify commonly used current ranges


Current based signals are not degraded by the resistance of long
connecting wires.
They are less susceptible to unwanted radiated electromagnetic
signals – e.g. radio transmissions