UNIT V - RECTIFIERS AND POWER SUPPLIES

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Clare Byrd
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1 UNIT V - RECTIFIERS AND POWER SUPPLIES OBJECTIVE On the completion of this unit the student will understand CLASSIFICATION OF POWER SUPPLY HALF WAVE, FULL WAVE, BRIDGE RECTIFER AND ITS RIPPLE FACTOR C, L, LC and CLC filters AND ITS RIPPLE FACTOR ZENER DIODE REGULATOR SMPS POWER CONTROL USING SCR RECTIFIERS A rectifier is an electrical device that converts alternating current (AC) to direct current (DC), a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid state diodes, vacuum tube diodes, mercury arc valves, and other components. A device which performs the opposite function (converting DC to AC) is known as an inverter. When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC. Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(i) oxide or selenium rectifier stacks were used.

2 Early radio receivers, called crystal radios, used a "cat's whisker" of fine wire pressing on a crystal of galena (lead sulfide) to serve as a point-contact rectifier or "crystal detector". In gas heating systems flame rectification can be used to detect a flame. Two metal electrodes in the outer layer of the flame provide a current path and rectification of an applied alternating voltage, but only while the flame is present. HALF WAVE RECTIFIER In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Halfwave rectification can be achieved with a single diode in a one phase supply, or with three diodes in a three-phase supply. FULL WAVE RECTIFIER

3 For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) form a full-wave rectifier (in this case, the voltage is half of that for the non-tapped bridge circuit above, and the diagram voltages are not to scale). BRIDGE RECTIFIER A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tapped transformer, four diodes are required instead of the one needed for half-wave rectification. Four rectifiers arranged this way are called a diode bridge or bridge rectifier. FILTERS C- FILTER The simple capacitor filter is the most basic type of power supply filter. The application of the simple capacitor filter is very limited. It is sometimes used on extremely high-voltage, low-current power supplies for cathode-ray and similar electron tubes, which require very little load current from the supply. The capacitor filter is also used where the power-supply ripple frequency is not critical; this frequency can be relatively high. The capacitor (C1) shown in figure 4-15 is a simple filter connected across the output of the rectifier in parallel with the load.

When this filter is used, the RC charge time of the filter capacitor (C1) must be short and the RC discharge time must be long to eliminate ripple action.

Better filtering also results when the input frequency is high; therefore, the full-wave rectifier output is easier to filter than that of the half-wave rectifier because of its higher

4 When this filter is used, the RC charge time of the filter capacitor (C1) must be short and the RC discharge time must be long to eliminate ripple action. In other words, the capacitor must charge up fast, preferably with no discharge at all. Better filtering also results when the input frequency is high; therefore, the full-wave rectifier output is easier to filter than that of the half-wave rectifier because of its higher frequency. CL FILTER The LC choke-input filter is used primarily in power supplies where voltage regulation is important and where the output current is relatively high and subject to varying load conditions. This filter is used in high power applications such as those found in radars and communication transmitters.

5 Notice in figure that this filter consists of an input inductor (L1), or filter choke, and an output filter capacitor (C1). Inductor L1 is placed at the input to the filter and is in series with the output of the rectifier circuit. Since the action of an inductor is to oppose any change in current flow, the inductor tends to keep a constant current flowing to the load throughout the complete cycle of the applied voltage. As a result, the output voltage never reaches the peak value of the applied voltage. Instead, the output voltage approximates the average value of the rectified input to the filter, as shown in the figure. The reactance of the inductor (X L ) reduces the amplitude of ripple voltage without reducing the dc output voltage by an appreciable amount.

6 C-L-C FILTER Capacitive and inductive filters connected in a pi-type configuration. Because of the combination of filtering devices, the ability of the pi filter to remove ripple voltage is superior to that of either the capacitance or inductance filter. SERIES VOLTAGE REGULATOR In electronics, a linear regulator is a voltage regulator based on an active device (such as a bipolar junction transistor, field effect transistor or vacuum tube) operating in its "linear region" (in contrast, a switching regulator is based on a transistor forced to act as an on/off switch) or passive devices like zener diodes operated in their breakdown region. The regulating device is made to act like a variable resistor, continuously adjusting a voltage divider network to maintain a constant output voltage. ZINER DIODE REGULATOR

7 The image shows a simple zener voltage regulator. It is a shunt regulator and operates by way of the zener diode's action of maintaining a constant voltage across itself when the current through it is sufficient to take it into the zener breakdown region. The resistor R1 supplies the zener current I Z as well as the load current I R2 (R2 is the load). R where, V Z is the zener voltage, and I R2 is the required load current. This regulator is used for very simple low power applications where the currents involved are very small and the load is permanently connected across the zener diode (such as voltage reference or voltage source circuits). Once R1 has been calculated, removing R2 will cause the full load current (plus the zener current) to flow through the diode and may exceed the diode's maximum current rating thereby damaging it. The regulation of this circuit is also not very good because the zener current (and hence the zener voltage) will vary depending on V S and inversely depending on the load current. SUMMARY Parts of DC power supply 1. Stepdown transformer 2. Rectifier 3. Filter 4. Voltage Regulator Rectifier 1. Half wave rectifier 2. Full wave rectifier 3. Bridge rectifier Performance parameters of Rectifier 1. Average load current 2. Average load voltage 3. RMS load current 4. RMS Load Voltage 5. Ripple factor 6. Voltage Regulation 7. Rectification efficiency 8. TUF

8 Ripple Factor Indicates how close the rectified output is to close to the pure ideal dc voltage. Transformer Utilization Factor (TUF) Indicates how well the input transformer is being is used. Filters 1. Capacitor input filter 2. Choke input filter 3. LC filter 4. Pi filter Voltage Regulators 1. Linear Regulator 2. Switching Regulator 3. Series Regulator 4. Shunt Regulator Performance parameters of Rectifier 1. Load Regulation 2. Line Regulation 3. Output Resistance 4. Temperature stability factor 5. Ripple Rejection

9 REVIEW QUESTIONS 2 MARK 1. Define ripple factor and rectification efficiency of a rectifier. 2. What are the disadvantages of FWR with center tapped transformer? 3. State the advantages and disadvantages of a bridge rectifier. 4. What is a filter? 5. Explain necessity of filter in the rectifier circuits. 6. Define critical inductance. 7. State the advantages and disadvantages of a choke input filter. 8. State advantages and disadvantages of an LC filter. 9. What are the advantages of a Pi type filter over the other filters? 10. What are the limitations of voltage multiplying circuits? 11. How is the voltage multiplier circuits classified? 12. What is a bleeder resistor? 13. Define transformer utilization factor and ripple factor in half wave rectifier. 14. Indicate two advantages of bleeder resistor. 15. What are the advantages of bridge rectifier over its center tapped transformer. 16. Compare the transformer utility factor of half wave rectifier and bridge rectifier and reason out the better one. 17. Why is a simple capacitor filter not suitable for heavy loads? 18. Where is SMPS used? 19. What are the basic concepts of SMPS? 20. How shunt regulator is differentiated from series regulator? 21. Mention the advantages of switched mode power supplies. 22. Define line regulation and load regulation.

10 8 Marks 1. With a capacitor filter, explain the working of a half wave rectifier and obtain its ripple factor compare it with Pi filter instead of capacitor filter. 2. What are the advantages and disadvantages of bridge rectifier circuit? 3. Deriver the expression for ripple factor FWR with Pi section filter. 4. Derive the expression of ripple factor of C, L, LC, and Pi filter. 5. Describe the working principles of full wave rectifier and derive the expression for ripple factor. 6. Design a simple zener regulator with own parameters. 7. With a neat diagram explain the operation of transistorized series type voltage regulator. 8. Explain with diagram how SCR can be used as variable full wave rectifier. 9. Explain the function of switching regulator. 10. Draw the block diagram of a SMPS and explain its operation. 11. Describe with block diagram the series and shunt voltage regulator and explain the operation of transistorized series voltage regulator.

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) (ISO/IEC Certified)

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) (ISO/IEC Certified) WINTER 16 EXAMINATION Model Answer Subject Code: 17213 Important Instructions to examiners: 1) The answers should be examined by key words and not as word-to-word as given in the model answer scheme. 2)