TUTORIAL 3

1. TUTORIAL 3 Draw and label a block diagram the elements of a DC power supply which receives an input from the 240 V; 50 Hz mains and produces an output of regulated 5 V DC. Describe briefly the function of each element. Illustrate your description with suitable figures. Q1

2. TUTORIAL 3 – VOLTAGE REGULATORS Solution

3. TUTORIAL 3 SOLUTION (cont’d) Function of each element Transformer reduces (steps down) the 240 VAC to a suitably low AC value (See figure)

4. TUTORIAL 3 SOLUTION (cont’d) Function of each element Rectifier converts the low VAC received from the transformer to pulsating VDC (See figure)

5. TUTORIAL 3 SOLUTION (cont’d)

6. TUTORIAL 3 SOLUTION (cont’d) Function of each element Smoothing circuit, also known as filter, minimizes the ripples appearing in the voltage waveform at the output of the rectifier. The output of this circuit is as smoothened waveform as in the following figure.

7. TUTORIAL 3 SOLUTION (cont’d)

8. TUTORIAL 3 SOLUTION (cont’d) Function of each element Regulator maintains a constant output DC voltage (within certain percentage) despite variations, within certain limit, in either the line voltage (VAC), load or both.

9. TUTORIAL 3 Explain the following terms as applied to a voltage regulator; a) line regulation b) load regulation Q2

10. TUTORIAL 3 SOLUTION a) Line regulation as applied to a voltage regulator is a measure of its ability to maintain a constant output voltage despite variation in the input voltage. It is the change of output voltage per unit change in the input voltage and is normally expressed in percentage form, thus;

11. TUTORIAL 3 SOLUTION (cont’d) b) Load regulation as applied to a voltage regulator is a measure its ability to maintain a constant output voltage despite variation in the load connected its terminal, and is normally express in percentage form, thus;

12. TUTORIAL 3 The output voltage of a regulator is 30 V when not connected to any load. The rated maximum output current of the regulator is 1 A. It was found that output voltage at the maximum output current was 29.4 V. Determine the load regulation of the regulator. Q3

13. TUTORIAL 3 SOLUTION

14. TUTORIAL 3 A voltage regulator is rated at a nominal output voltage of 12 V for the corresponding input voltage of 20 V. It is found that its output voltage decreases to 11.84 V when the input voltage decreases to 16 V. Determine the line regulation of the regulator. Q4

15. TUTORIAL 3 SOLUTION

16. TUTORIAL 3 Draw and label a block diagram showing the elements of a basic series regulator. Q5

17. TUTORIAL 3 SOLUTION

18. TUTORIAL 3 The regulator in the following figure is to produce a nominal output voltage, Vo = 18.6 V and a maximum load current IL(max) = 1 A. It is known that , the zener diode current IZ shall not drop to less than 15 mA, the current I1 = 1 mA and the input voltage Vi may vary between 22 V to 30 V. Determine the values of R1, R2 and R3. Q6

19. TUTORIAL 3 Q6 (cont’d)

20. TUTORIAL 3 SOLUTION Substituting values;

21. TUTORIAL 3 SOLUTION (cont’d) Neglecting I;

22. TUTORIAL 3 SOLUTION (cont’d) Since it follows that;

23. TUTORIAL 3 SOLUTION (cont’d) Neglecting I+; Substituting values;

24. TUTORIAL 3 For the shunt regulator shown in the following figure; a) find Vo; b) if IL and Vo are constant, find the change in IC1 if Vi changes by 1 V; c) if the maximum input voltage Vi = 25 V, find IL if the load resistance, RL is shorted Q7

25. TUTORIAL 3 Q7 (cont’d)

26. TUTORIAL 3 SOLUTION a)

27. TUTORIAL 3 SOLUTION (cont’d) If IL and Vo are constant, I3 will also be constant. Hence; b)

28. TUTORIAL 3 SOLUTION (cont’d) c) When RL is shorted, Vo = 0. Therefore;

29. TUTORIAL 3 Describe the operation of the following regulator; Q8

30. TUTORIAL 3 SOLUTION If the load resistance RL suddenly decreases, the output voltage Vo will also decrease. The decrease in Vo will be followed by a decrease in V4. The output of the op-amp VOA decreases because VOA = AO(V4 – VZ) and VZ being constant.

31. TUTORIAL 3 SOLUTION (cont’d) The drop in VOA reduces the VBE bias voltage of Q1 and hence reduces IC1. The reduction in IC1 means more of IR1 can flow through RL and thereby increases the output voltage Vo. This increase compensates for the previous drop in Vo.

32. TUTORIAL 3 SOLUTION (cont’d)

33. TUTORIAL 3 Draw and label a block diagram showing the elements of a basic shunt regulator. Q9

34. TUTORIAL 3 SOLUTION (cont’d) The block diagram of a basic shunt regulator;

35. TUTORIAL 3 Describe the operation of the following regulator; Q10

36. TUTORIAL 3 SOLUTION DESCRIPTION TO FOLLOW