Download Power_Conditioning_January_2007

R1
27 Ω
E
30 V
R2
10 Ω
Figure 1
1.
a.
Write out the equation for Ohms law showing the relationship between voltage,
current and resistance in an electrical circuit.
[3 marks]
b.
Given a resistance of 220 Ω connected to an e.m.f. of 10 V
i. Calculate the expected current flow through the resistor.
ii. Explain the likely consequences of increasing the value of the resistor to 270
Ω.
iii. The resistor is replaced by one with a value of 22 Ω. Calculate the new
value of current flow and the required power rating for the resistor.
[6 marks]
c.
With reference to figure 1:
i. Calculate the current flow in resistor R1
ii. Calculate the voltage drop across resistor R2
iii. Calculate the power dissipation in resistor R1
iv. Calculate the power dissipation in resistor R2
[10 marks]
d.
Comment on the consequences of using a 10 Ω resistor with a power rating of 5 watts
as R2 in figure 1
[4 marks]
e.
State what a heatsink is and explain why R2 may need one if a 10 Ω 10 watt
component were used in the construction of this circuit.
[5 marks]
f.
Explain what the term de-rating means and describe how it may be used in order to
improve the long-term reliability of an electronic component.
[5 marks]
T
220 Vrms
50 Hz
B
C
10:1
R
Figure 2
2.
Figure 2 shows the circuit diagram for a simple d.c. power supply.
a. Identify the type of rectifier circuit represented in figure 2 and explain the operation of the
circuit with reference to the function of each component within the circuit.
[10 marks]
b.
Sketch to scale the voltage across R as a function of time showing its relationship to the
secondary voltage from the transformer.
[6 marks]
c.
Calculate a value for the capacitor, C in order to keep the percentage ripple voltage across
R below 5%. Assume a value of 500 Ω for R and a mains frequency of 50 Hz.
[5 marks]
d.
The power supply unit shown in figure 2 is said to be unregulated i. Explain the meaning of the term unregulated as used in relation to power
supplies. Give one example where an unregulated power supply may be
acceptable and one example where a regulated power supply is required.
ii. Show how a three terminal regulator chip may be used to provide a regulated
output voltage and explain how a regulated output voltage of 9 V may be
obtained.
iii. In relation to power supply units, explain the meaning of the terms percentage
load regulation and percentage line regulation.
[12 marks]
R1
L1
220 V rms
50 Hz
T1
R2
C1
Figure 3
3.
a.
Identify the type of electronic component represented by each of the symbols shown
in figure 3 above and state the function of the circuit.
[6 marks]
b.
Briefly describe the principle of operation of the device T1 including an explanation
of how the device is made to turn on and off.
[6 marks]
c.
Sketch the typical shape of the voltage waveform that would be measured across L1
in this circuit given that R2 is set to approximately half of its maximum value.
[7 marks]
d.
Explain how the circuit operates to reduce the r.m.s. voltage across L1.
[7 marks]
e.
State the likely effect of removing component C1 from the circuit and explain the
reason why the behaviour of the circuit is modified.
[7 marks]
D1
S1
M
R2
Q1
V1
5.6k
Vcc
24V
5V
Figure 4
4.
Figure 4 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 motor draws 1 Amp from a 24 volt supply.
a. Calculate the base current in the BJT when
i. S1 is closed.
ii. S1 is opened.
[6 marks]
b.
Sketch to scale approximate collector characteristic curves for the BJT used in this
circuit when
i. S1 is open
ii. S1 is closed
iii. Identify on the characteristics the active region, the saturation region and
the breakdown region.
[10 marks]
c.
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.
[8 marks]
d.
With reference to typical values where necessary, estimate the power dissipation in
the BJT when switched on.
[5 marks]
e.
State the reason for including device D1 in this circuit.
[4 marks]