Download DT002_1 Review Questions

1. Explain the terms power rating and tolerance in relation to resistors.
2. A 10 Ω resistor has a current of 5 amps flowing through it.
(a) Calculate the power dissipated in the resistor.
(b) State whether the resistor should be of wire-wound or metal film construction.
Explain your answer.
3. Draw a simple labelled diagram to show the structure of a P-N junction diode.
4. Explain the meaning of the terms forward bias and reverse bias in relation to the
operation of a diode. Illustrate each of these terms using a simple circuit diagram.
5. Explain in one sentence the difference in the behaviour of a diode when subjected
to forward and reverse bias.
6. The diode is often described as a voltage controlled switch. Explain how the
electrical characteristics of a diode can be viewed in this way. Illustrate with a
typical V-I characteristic curve for a diode.
7. State a typical value for the forward voltage drop across a silicon diode when
operated under forward bias.
8. Explain the term Peak Inverse Voltage as used in relation to rectifier diodes.
9. State the function of a rectifier circuit.
10. A mains transformer produces a 10 volt rms secondary voltage at 50 Hz. The
secondary voltage is connected to a rectifier circuit. Sketch to scale the output
voltage waveform from (a) A half-wave rectifier
(b) A bridge rectifier
Assume that silicon diodes are used in each case.
11. Explain why a smoothing capacitor is used in conjunction with rectifier circuits.
Sketch the circuit of a simple half-wave rectifier showing how the smoothing
capacitor is connected.
12. Explain the term ripple voltage as used in relation to power supply units. Illustrate
with a diagram showing supply output voltage as a function of time.
13. State how the ripple voltage may be reduced in the output from a power supply
unit.
14. Smoothing capacitors usually have relatively large values.
(a) State the type of capacitor that is used in order to provide such large values.
(b) Identify an important characteristic of this type of capacitor that influences the
way that it may be connected into the power supply circuit.
15. The percentage ripple voltage present in the output from a 12 volt power supply is
required to be no more than 1%. The power supply must provide power to a 500 Ω
load resistance, RLOAD. Given a mains frequency, f, of 50 Hz, determine a suitable
value for a smoothing capacitor, C, to be used in the supply. The ripple voltage is
approximated by the equation –
Vripp 
VLOAD
2.RLOAD .C . f
16. A voltage regulator is often used in power supply units. State the function of the
voltage regulator.
17. In relation to power supply units, explain the terms –
(a) Line regulation.
(b) Load regulation.
18. A regulated power supply produces a nominal output voltage of 12 V. The
maximum load current is 1 A. On measurement, the output voltage is found to be
12.1 V with no load and 11.8 V on full load. Calculate the percentage load
regulation for the PSU.
19. A LM7805 voltage regulator is used to produce a 5 V regulated output voltage at a
load current of up to 1 A. The unregulated input voltage to the regulator may be
up to 8 V when supplying the 1 A output current.
(a) Calculate the power dissipation in the regulator.
(b) Comment on the consequences of this level of power dissipation in the device.
(c) Explain briefly how the effects of excessive power dissipation in a device may
be ameliorated.
20. Give one application of each of the following –
(a) Silicon diode
(b) Zener diode
(c) Light emitting diode
21. 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.
T1
D1
D2
D4
D3
Vmains
RLoad
Figure 1
22. Sketch the voltage across RLoad in figure 1 as a function of time showing its relationship
to the secondary voltage from the transformer.
23. 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.
24. Explain how the smoothing capacitor sustains a d.c. voltage across RLoad, despite the
pulsating nature of the rectifier output.
25. 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
26. 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.