Lecture No# 8 Prepared by: Engr. Qurban Ali Memon Incharge of Project Lab.

To construct the dc power supply by using bread board. LAB EXPERIMENT NO # 07 To construct the dc power supply by using bread board. Components: 1. Bread board 2. step down transformer 3. Zener diode(5V or 10V) 4. four diodes (IN4001) 5. two resistors (580Ω & 680Ω) 6. Few hard wires 7. Multimeter 8. Led( Light Emitting Diode). DISCUSSION: The dc power supply is an electronic system that is used to convert ac into dc. Power is used in everyday life. People need power to run their homes, cars, computers, etc. The dc power supply could be used in all electronic components. In dc power supply contain ac and dc components. Fig. 7.1(b): output (dc) Fig. 7.1(a): Input (ac)

Four circuit could be used in dc power supply. Fig. 7.2: Block diagram of d.c power supply

Fig. 7.3: symbol of capacitor 1. Transformer Step-up transformers increase voltage, step-down transformers reduce voltage. A step down transformer is a transformer which converts high voltage into low voltage. A transformer is commonly used to step down the input AC voltage into desire level. Most electronic circuits operate on lower voltage than the AC line voltage. 10: 1 220Ac Fig. 7.3: symbol of capacitor

Fig.7.4: Circuit diagram of d.c power supply For example: a transformer with a turns ratio of 10 to 1 would convert the 220 volt 50 Hz input sine wave into a 22 volt sine wave Ns/Np= Sv 1/10=220v 22v 220AC 5v Fig.7.4: Circuit diagram of d.c power supply

2. Rectifier: Another widely used rectifier is the bridge rectifier. It uses four diodes. This is called a full wave rectifier as it produces an output pulse for each half cycle of the input sine wave. On the positive half cycle of the input sine wave, diodes D1 and D2 are forward biased so act as closed switches appearing in series with the load. On the negative half cycle, diode D1 and D2 are reverse biased and diodes D3 and D4 are forward biased so current flows through the load in the same direction. Fig.7.5(b): pulsating dc output Fig.7.5(a): Input ac Fig.7.5(c): circuit diagram of bridge rectifier

Fig. 7.6(a): symbol of capacitor The capacitor has two legs. One leg has (-) signs next to it, this is the negative side. The other side is positive, sign (+) this is the positive side. A filter is used to remove the pulsations and create a constant output. The most common filter is a large capacitor . Fig.7.6(b): pulsating dc output Fig. 7.6(a): symbol of capacitor + t Fig.7.6(c): pure dc -

4. Regulator: Voltage regulator Ics or zener diodes are available with fixed (typically 5, 12 and 15V) or variable output voltages. The main function of zener diode is used to maintain constant voltage. The Zener diode has two states, namely; on and Off. On state: It means that the anode must be connected to the negative side of the voltage source and the cathode must be connected to the positive side. When reverse voltage across from a Zener diode the Vz is equal to or more than breakdown voltage Vz, the current increases very sharply. Under such condition, the Zener is said to be the “ON” State Off state: . It means that the anode must be connected to the negative side of the voltage source and the cathode must be connected to the positive side. When reverse voltage across from a Zener diode is less than Vz but greater than 0V, the Zener diode is in the “Off” state. Under such condition, the Zener diode can be represented by open switch. Suppose: If Vz=5v or Vz > 5v Suppose: If Vz<4v 0r 3v but Vz > 0v

Fig. 7.7(b): dc pulsating output 220AC 5v - Fig. 7.7(a)Circuit diagram of d.c power supply Fig. 7.7(c): pure dc Fig. 7.7(b): dc pulsating output Fig. 7.7(a): input (ac)