1. 1.0 LINEAR DC POWER SUPPLY 1.1.1 The importance of DC Power Supply Circuit For electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific value. Dry cells is not economical because the electronic equipment that requires high voltage use a lot of dry cells. A battery is a common DC voltage source for some types of electronic equipment especially portables like cell phones and iPods. Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices. Incoming power supply from TNB is in alternating current and high voltage ( 1 phase = 240 V, 3 phase = 415V)

2. 1.1.2 BLOCK DIAGRAM OF DC POWER SUPPLY

3. Schematic Diagrams Of a Simple Power Supply Rectifier circuit

4. 1.1.3 The Function of each Block

5. TransformerTransformer circuit symbol

6. 1.1.1 TransformerTransformer Transformer convert AC electricity from one voltage to another with little loss of power. Transformers work only with AC and this is one of the reasons why mains electricity is AC. Step-up transformers increase voltage, step- down transformers reduce voltage. Most power supplies use a step-down transformer to reduce the dangerously high mains voltage (230V in UK) to a safer low voltage.

7. The input coil is called the primary and the output coil is called the secondary. There is no electrical connection between the two coils, instead they are linked by an alternating magnetic field created in the soft-iron core of the transformer. The two lines in the middle of the circuit symbol represent the core. Transformers waste very little power so the power out is (almost) equal to the power in. Note that as voltage is stepped down current is stepped up.

8. TransformerTransformer:Turn Ratio The ratio of the number of turns on each coil, called the turns ratio, determines the ratio of the voltages. A step- down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

9. Transformer only The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor.

10. Transformer + Rectifier The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for electronic circuits unless they include a smoothing capacitor.

11. Transformer + Rectifier + Smoothing The smooth DC output has a small ripple. It is suitable for most electronic circuits.

12. Transformer + Rectifier + Smoothing + Regulator The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

13. 1.2 Rectifier Most of the devices in electronic system need the dc power supply to operate. Source of the power that supplied to our home is AC. To get DC voltage, we use the rectifier circuit. Rectifier is the circuit that used 1/more than 1 diode to convert the AC volt to the pulsating voltage. There are 3 types of rectifier: 1) Half Wave Rectifier 2) Full Wave Rectifier 3) Bridge Rectifier

14. 1.21 Half Wave Rectifier The arrow on the diode indicates the direction of the current in the diode Because of the diode, the alternating current in the load resistor is reduced to the positive portion of the cycle

15. 1.2.2 Operation of Half wave- Rectifiers During the positive cycle of the input signal, the diode D forward bias. D acts as a switch is closed the current can pass through. Voltage drop on the RL is equal to the magnitude of the positive cycle if we ignore the input signal voltage drop on the diode.

16. 1.2.2 Operation of Half wave- Rectifiers During the negative cycle of input signal, the diode D reverse biased. D acts as a switch is open so current can not pass through. Voltage drop in the RL during the negative cycle is empty.

17. 1.2.3:The input & Output Waveform Plot of Input Vi & Output Vo waveform for half-wave rectifier ideal diode

18. Output Voltage Output voltage for half-wave rectifier circuit current produced only positive cycle. Since the current through the diode and the diode voltage drop is 0.7V (assuming silicon diode), the output voltage is: - Frequency The frequency output signal is equal to the input frequency. Vout = Vin – 0.7V

19. Example 1: A half-wave rectifier circuit for input voltage 20 Vp-p, 50 Hz. By making the assumption there is no voltage drop on the diode, calculate: - i) Rectifier output voltage signal Vin = 20 Vp-p Vp = 10 Vp Vout = 10 Vp ii) Frequency of the output signal Output frequency = Input frequency signal = 50 Hz

20. Example 2: Calculate the peak output voltage and peak current flowing through the diode during the forward biased state.

21. Answer: Peak output voltage, Vp (out) = 5 – 0.7 = 4.3V Peak current, Ip = Vp (out) / R = 4.3 / 47 = 91.5mA

22. 1.3 Full-Wave Rectifiers

23. 1.3.1 Full-Wave Rectifier Circuit with Center Tap Transformer

24. 1.3.2 Operation Of the Circuits During positive cycle of Vin, D1 is forward-biased and D2 is reverse-biased FIGURE 1.3.2 (a)

25. During negative cycle of the input, the network appears as shown in FIGURE 1.3.2 (b), reversing the role of the diodes, D2 is forward biased & D1 is reverse biased. The net effect is same output as the appearing in FIGURE 1.3.2 (a) FIGURE 1.3.2 (b)

26. Input & Output Waveform Of Full-Wave Rectifier Figure 1.3.2 ( C ):Waveform of Full-Wave rectifier

27. 1.4 Bridge Rectifier 1.4.1 Full-Wave Bridge rectifier Circuit When we connect the oscilloscope across the RL we will find the output waveform is the same as figure 1.4.1 (a) Figure 1.4.1 (a)

28. 1.4.2 The operation circuit of Bridge Rectifier Positive Half-cycle

29. 1.4.2 The operation circuit of Bridge Rectifier Negative Half-cycle

30. 1.4.2 The operation circuit of Bridge Rectifier-contd Output Voltage Output voltage of full wave bridge rectifier circuit resulting in two cycles. Since in a cycle of current through the two diodes and the voltage drop on the diode is 1.4V (assuming silicon diode), the output voltage is: Frequency Frequency output signal is twice frequencies input. Vo = V M-N – 1.4V

31. 1.5 & 1.6:Types of filter There are 4 types of general filter are used: 1)Capacitor filter 2)RC filter 3)LC filter 4)  filter

32. 1) The operation of capacitor filter diagram 1.5.1 The operation and filtering process of RC filter circuits using O/P wave (half-Wave)

33. The operation and filtering process of Capacitor filter circuits using O/P wave (half-Wave)-contd During the first cycle, the capacitor charges from points A to B reaching the maximum voltage, V max. Between points B and C, when the input voltage is less than V max, the capacitor starts discharging through the resistor, R L. The capacitor will discharge until point C during which the input voltage is equal to the capacitor voltage and the capacitor starts to charge again. This process is repeated for the next cycle.

34. 1.5.2 Ripple Voltage Definition: A small variation occurs in the DC because the capacitor discharges a small amount between the positive and negative pulses. Then it recharges. This variation is called ripple. The ripple can be reduced further by making the capacitor larger. The ripple appears to be a sawtooth shaped AC variation riding on the DC output. A small amount of ripple can be tolerated in some circuits but the lower the better overall.

35. Types of filter-CONTD 2) RC Filter  RC filter is added after the filter capacitor.  RC filters are produced : a resistor in series with the load (RL) and a capacitor in parallel with (RL). Effects of RC filter  Resistor (R) will decrease the voltage ripple.  C2 will filter the balance of the ripple voltage.

36. Types of filter-CONTD 3) LC Filter  LC filters have been produced by combination inductor in series and capacitor in parallel.  The inductor, L function is to reduce the ripple even more without a lot of dc resistance.  Provided the ripple reduction is sufficient and the power loss in the resistor is not excessive.

37. Types of filter-CONTD 4)  filter   filter acts to overcome the problems generated by the RC filter.  Resistor in the RC filter is replaced with the inductor (L).  The filtering action of each component:  C1 : - low reactance to AC - infinite resistance to DC - bypass AC to ground while DC moved towards L  L : - high reactance to AC - zero resistance to DC - allows DC to pass through it and blocks AC  C2 : - similar to C1 - bypass AC which could not blocked by L - only DC available at the output

38. 1.7 The operation of voltage regulator circuit 1.7 Voltage Regulation The function of a voltage regulator is to: a) Stabilize the output voltage even though there is a variation of the input current or output current. b) Reduce the ripple at the output voltage of the filter circuit. There are three types of voltage regulator circuit : - 1) Voltage regulator zener diode 2) Transistor series voltage regulator 3) Voltage regulator integrated circuit (78XX series)

39. 1.7.1 (a) Zener Diode Basic Zener diode regulator circuit is shown in figure below. This is a type of shunt regulator. In designing such a circuit, one has to know the value of the series resistor, Rs, the maximum current that the Zener diode will have to handle, the variation of the input voltage, Vin, and the variation of the load current, I L. For variations in Vin, the Zener circuit maintains constant voltage, Vo, across the load, RL, by changing the current through the series resistor Rs.

40. 1.7.1 (b) Serial Transistor Transistors connected in series with the load will control the input voltage to output. Referring to figure below, if the output voltage decreases, the increase in the VBE will cause the transistor to produce more than the current flow will increase the output voltage and maintain the output voltage. The zener diode will act as a reference voltage. The same process occurs if the output voltage increases. The transistor will reduce the current value, causing the output voltage and maintaining the output voltage decreases FIGURE 1.71.b: Simple types of series regulator using a Zener diode and a transistor

41. 1.7.1 (b) Serial Transistor-CONTD The Zener diode will keep the base voltage constant while the transistor forms a variable series resistor. Since the emitter is always 0.7 V or so different from the base voltage, a constant output voltage will result. The transistor amplifies the effect of the Zener diode, so not only will better regulation result, but higher current can be drawn with less power dissipation than the equivalent shunt regulator.

42. 1.7.1 (c )Integrated Circuits LM Series 78XX (where XX = 05, 06, 08, 10, 12, 15, 18 or 24) is a three-terminal voltage regulator. IC LM7805 will produce output voltage +5 V, the LM7806 will produce +6 V output voltage and thus LM7824 will produce output voltage +24 V. Figure 1.7.1 (c ) shows the integrated circuit voltage regulator circuit.

43. The pulsating DC input voltage from the rectifier is filtered by the capacitor, C1 which filters the undesired ripples before it is connected to terminal 1. The regulated output voltage of +5V is produced at terminal 2 filter by the capacitor C2. C2 will also filter all the high frequency distortion in the system. Terminal 3 is grounded. 1.7.1 (c) Integrated Circuits

44. 1.8 Voltage Divider Circuits In some systems of electronic equipment, especially equipment large or complex, it consists of several stages, each circuit is sometimes used voltages at different value. For example TV system, there are more than ten different levels of circuit functions and some that require voltage 100V, 48V, 12V and ect. By using DC power supply, it can be accomplished by having the voltage divider network after a high voltage value has been obtained.

45. 1.8.1 Voltage Divider Circuits- Contd

46. 1.9 Schematic Diagrams Of a Simple Power Supply Rectifier circuit

47. 1.9 Schematic Diagrams Of a Simple Power Supply Figure 1.9a shows schematic diagrams of a simple power supply. From left to right, we find the center tap transformer will reduce the ac supply voltage. The full-wave rectifier converts ac voltage to DC.  filters are used to ensure the DC voltage are pulsed and then the DC will be stabilize by zener diode.