Power supplies 1
QUESTION SET-1 ON POWER SUPPLIES
1. What is a power supply? Why do you need a DC power supply?
2. What are the critical factors effecting the choice of a power supply?
3. How a laboratory power supply differs from an instrument power supply?
4. Define the following terms related to the power supplies:
a. Ripple factor b. Load regulation c. Input regulation d. Efficiency
5. Draw the block diagram of a linear regulated power supply and describe the major function each block briefly.
6. Explain the function of the fuse in power supplies. What type of a fuse is preferred in power supplies?
7. What is the meaning of the voltage rating of a fuse?
8. What is a feasible link?
9. What is the transient suppressor and why it is used at the input section of a power supply?
10. What is the function of the line filter in power supplies?
11. What is the snubber, what is its function and in what position you expect to see it in a power supply?
12. What are the components of a snubber and what are their important properties?
13. Explain shortly the function of a transformer in a power supply with a simple circuit symbol and input/output waveforms.
14. What are the critical factors in selecting the transformer for a power supply?
15. Define the efficiency of the transformer in a power supply.
16. Discuss how to select a transformer for a given power supply application with an example.
17. Discuss the function of the rectifier diode and the difference between rectifier diodes and other types of diodes that you know.
18. Discuss the reasons for half-wave rectifiers not being commonly used although they are very simple.
19. Describe how to test a diode using a multimeter.
Power supplies 2 20. Define the forward current (IF), surge current (ISFM), forward diode voltage (VD) and
peak inverse voltage (PIV) for a rectifier diode with a simple sketch.
21. Mathematically determine the average and effective values and the ripple factor for half wave and full wave rectified voltages.
22. Discuss the determination of the peak inverse voltages in selecting rectifier diodes.
23. Discuss the necessity for smoothing and circuit modalities used for this purpose.
24. Calculate the smoothing capacitor required for a supply with output voltage 12 V, current 0.5 A, frequency of the main's supply 60 Hz and ripple factor 10%.
25. Calculate the approximate charging and discharging times at steady state for the capacitor in the previous question. Determine the approximate value of the average charging current at steady state.
26. Figure shows the equivalent circuit
of a smoothing capacitor. Define each component in the circuit and discuss how they affect the performance of the capacitor in a power supply.
27. Explain the reason for heaving a small non-electrolytic capacitor across the smoothing capacitor.
28. Discus the reason for adding a small resistance between the output of the rectifier and smoothing capacitor.
29. Discuss how to choose a smoothing capacitor for a given power supply application.
30. What is the "bleeding" resistor, where and why it is used?
31. An unregulated power supply has 2200 µF aluminum electrolytic smoothing capacitor in parallel with 0.1 µF polystyrene capacitor. The nominal value of the output voltage is 10 V for the output current of 0.5 A and main's voltage 220 V at 60 Hz.
a. Calculate the DC component of the output voltage and the ripple voltage for the load current of 0.1A.
b. Repeat (a) for the load current of 1 A.
c. Calculate the output voltage and ripple for the output current 500 mA as the main's voltage dropping to 200 V.
d. Repeat (c) for the main's voltage rising to 240 V.
Power supplies 3 e. Generate a comparison table and discuss the effect of load current and input
voltage variations on the performance of the power supply.
32. Discuss the need for a voltage regulator in power supplies that are used in electronics.
33. What is a zener diode and how it differs from an ordinary rectifier diode?
34. Design a zener diode regulated power supply assuming that:
a. The required output voltage is 5 V
b. The output current is between 0 and 100 mA c. Transformer used is 220 V / 6 V.
d. Using commercial components, select the rectifier, smoothing capacitor and limiting resistor.
35. Search for 5 IC voltage regulators from component catalogs and/or web and make a table of comparison for their characteristics.
36. Design a linear regulated dual power supply that would provide 1 A load current at ± 6 V from a mains supply of 220 V / 60 Hz. Use practical values for the components and justify your selections.
37. What is a crowbar and how it is used in protecting power supplies?
38. What is a switched regulator and how it differs from the linear regulator?
39. What are the major advantages of switching regulators over the linear ones?
40. What are the major disadvantages/limitations of switching power supplies?
41. Explain the function of the high frequency switch in switching regulators.
42. Draw the functional block diagram of a switching power supply and explain the similarities and differences between a regular transformer used in ordinary power supplies and high-frequency transformer used in switching power supplies.
43. Explain the similarities and differences between the input and output rectifiers used in switching power supplies.
44. Explain the similarities and differences between the input filter capacitors and output filter capacitors.
45. Explain the function of the pulse width modulator (PWM) in regulating the output voltage.
46. Why we have problem of RF interference in switching supplies and how it can be eliminated?
47. What element contributes most to the weight of the power supply and why the switching supply is much lighter than its linear counterparts?
48. The current in a 10 Ω resistor is 5*cos(314t) A
Power supplies 4 a. Draw the waveform of the current
b. Define and calculate the following values for the current:
i. Peak
ii. Peak to peak iii. Average
iv. Root Mean Square (RMS)
c. Calculate the value of the power dissipated by the resistor
d. How much would be the current if it would be DC to generate the same power on the resistor?
49. For a transformer in a power supply, the required average output voltage is 10 V, the ripple voltage is 1 V and the voltage drop across the rectifier is 2 V and the required output current (average) is 1 A. The efficiency (η) of the transformer is 0.8. Calculate:
a. the required output voltage of the transformer
b. the input current of the transformer if the input voltage is 220 V c. the output power delivered by the power supply
d. the power loss by the transformer.
50. A series R-L circuit has R = 0.1 kΩ and L = 10 mH. The circuit is excited by Vi = 5 + 10 sin(1000t) V
a. Draw the circuit diagram
b. Calculate the voltages across R and L.
51. The circuit shown is driven off by a 12 V DC supply. The inductor is 10 mH and the resistor is 100 Ω. The switch works at 1 kHz with 40% duty cycle (i.e. it is "on" for 0.4 ms and "off" for 0.6 ms in a 1 ms cycle). Determine and draw the waveform of the voltage across the resistor. What happens if the frequency of the switch goes to 10 kHz? What happens if the switch works at 100 kHz? (Assume that the diode is ideal, i.e. it works as an electronic switch).
