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### Chapter 1.

1. B.L.D.E.A’S SSM Polytechnic, Vijayapur. Department of Mechatronics Engineering CHAPTER -1. PowER SuPPliES By-N.T.BIRADAR. Lect
2. UNIT 1.Power Supplies A power supply is an electronic device that supplies electric energy to an electrical load. The primary function of a power supply is to convert one form of electrical energy to another. 1.1 Exlain the Block diagram of Linear Power supply. Most non-portable equipment uses power supplies that operate from the AC power line but produce one or more DC outputs. A presentation of eSyst.org A battery is a common DC voltage source for some types of electronic equipment especially portables like cell phones and iPods.For electronic circuits made up of transistors and/or ICs, this power source must be a DC voltage of a specific value.All electronic circuits need a power source to work.Power Supply Main circuits in most power supplies. A presentation of Components of a Power Supply is as follows Transformer: with a turns ratio of 10 to 1 would convert the 120 volt 60 Hz input sine wave into a 12 volt sine wave. A transformer is commonly used to step the input AC voltage level down or up. Most electronic circuits operate from voltages lower than the AC line voltage so the transformer normally steps the voltage down by its turns ratio to a desired lower level.Transformer The rectifier: converts the AC sine wave into a pulsating DC wave there are several forms of rectifiers used but all are made up of diodes. Filter: is a large capacitor. A filter is used to remove the pulsations and create a constant output.The rectifier produces a DC output but it is pulsating rather than a constant steady value over time like that from a battery.
3. Regulator: fixes the output voltage to the desired level then maintains that value despite any output or input variations. Most electronic circuits cannot withstand the variations since they are designed to work properly with a fixed voltage.Changes in the load or the AC line voltage will cause the output voltage to vary.The regulator is a circuit that helps maintain a fixed or constant output voltage Regulator. 1.2 Explain the operation of Half Wave Rectifier. Fig.1.2 Half Wave Rectifier A rectifier is an electronic device that converts AC voltage into DC voltage. In other words, it converts alternating current to direct current. A rectifier is used in almost all the electronic devices. Mostly it is used to convert the main voltage into DC voltage in the power supplysection. By using DC voltage supply electronic devices work. In a single-phase half-wave rectifier, either negative or positive half of the AC voltage flows, while the other half of the AC voltage is blocked. Hence the output receives only one half of the AC wave. A single diode is required for a single-phase half-wave rectification and three diodes for a three-phase supply. Half wave rectifier produces more amount of ripple content than full wave rectifiers and to eliminate the harmonics it requires much more filtering. During the positive half cycle, when the secondary winding of the upper end is positive with respect to the lower end, the diode is under forward bias condition and it conducts current. During the positive half cycles, the input voltage is applied directly to the load resistance when the forward resistance of the diode is assumed to be zero. The wave forms of output voltage and output current are same as that of the AC input voltage. During the negative half cycle, when the secondary winding of the lower end is positive with respect to the upper end, the diode is under reverse bias condition and it does not conduct
4. current. During the negative half cycle, the voltage and current across the load remains zero. The magnitude of the reverse current is very small and it is neglected. So, no power is delivered during the negative half cycle. 1.3 Explain Bridge rectifier. A bridge rectifier circuit is a common part of the electronic power supplies. Many electronic circuits require rectified DC power supply for powering the various electronic basic components from available AC mains supply. We can find this rectifier in a wide variety of electronic AC power devices like home appliances, motor controllers, modulation process, welding applications, etc Fig1.3 Bridge rectifier During the Positive half cycle of the input AC waveform diodes D1 and D2 are forward biased and D3 and D4 are reverse biased. When the voltage, more than the threshold level of the diodes D1 and D2, starts conducting – the load current starts flowing through it, as shown as red lines path in the diagram below. During the negative half cycle of the input AC waveform, the diodes D3 and D4 are forward biased, and D1 and D2 are reverse biased. Load current starts flowing through the D3 and D4 diodes when these diodes starts conducting as shown in the figure. We can observe that in both the cases, the load current direction is same, i.e., up to down as shown in the figure – so unidirectional, which means DC current. Thus, by the usage of a bridge rectifier, the input AC current is converted into a DC current. The output at the load with this bridge wave rectifier is pulsating in nature, but for producing a pure DC requires
5. additional filter like capacitor. The same operation is applicable for different bridge rectifiers, but in case of controlled rectifiers thyristors triggering is necessary to drive the current to load. 1.4 Necessity of filters. • We know that rectifiers are used to convert AC to DC, but not a pure DC. There would be considerable AC component in their output, called ‘ripple’, in addition to the desired d.c. component. • Most sophisticated electronic systems need pure DC supply to drive, or power them. To construct a good power supply which gives pure DC output, we need to remove or filter out the AC component from the output of rectifiers. 1.5 Explain L type and Pi type filters. This filter circuit employs a choke and a capacitor. The circuit of an inverted L-type filter circuit is shown in fig. below. Fig 1.4 L type filters Since the choke offers high reactance to high frequencies, it blocks them and the capacitor C short them to ground as it offers negligible reactance to the high frequencies. Thus only low frequencies below cutoff frequency fc are allowed to pass through without significant attenuation.In this filter, the output is taken across the capacitor C.These components have two elements: 1 capacitor and 1 inductance, and are second-order filters
6. The theoretical insertion loss is 40 dB per decade. For optimum performance the filter must be placed with the inductor situated on the side which has the lower impedance. These filters are ideal for circuits with imbalanced source / load impedances 1.6 Explain Pi type filters. This filter circuit employs a choke and two capacitors. The circuit of an inverted pi-type filter circuit is shown in fig. below Fig 1.5 Pi type filters Here a second capacitor C2 is added in the circuit to improve the filtering action by grounding higher frequencies.The inductor or choke is always connected in series between the input and output and the capacitors are grounded in parallel.The output voltage is taken across the capacitor C2 .The capacitor-input filter operates in three steps: • The capacitor C1 offers low reactance to the AC component of the rectifier output while it offers infinite resistance to the DC component. As a result the capacitor shunts an appreciable amount of the AC component while the DC component continues its journey to the inductor L. • The inductor L offers high reactance to the AC component but it offers almost zero resistance to the DC component. As a result the DC component flows through the inductor while the AC component is blocked. • The capacitor C2 shunts the AC component which the inductor had failed to block. As a result only the DC component appears across the load RL. 1.7 Explain the Zener diode regulator.
7. However, the Zener Diode or “Breakdown Diode”, as they are sometimes referred too, are basically the same as the standard PN junction diode but they are specially designed to have a low and specified Reverse Breakdown Voltage which takes advantage of any reverse voltage applied to it.The Zener diode behaves just like a normal general-purpose diode consisting of a silicon PN junction and when biased in the forward direction, that is Anode positive with respect to its Cathode, it behaves just like a normal signal diode passing the rated current.However, unlike a conventional diode that blocks any flow of current through itself when reverse biased, that is the Cathode becomes more positive than the Anode, as soon as the reverse voltage reaches a pre-determined value, the zener diode begins to conduct in the reverse direction. Fig.1.6 Zener diode regulator The current now flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor) and once achieved, this reverse saturation current remains fairly constant over a wide range of reverse voltages. The voltage point at which the voltage across the zener diode becomes stable is called the “zener voltage”, ( Vz ) and for zener diodes this voltage can range from less than one volt to a few hundred volts.
8. The resistor, RS is connected in series with the zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilised output voltage Vout is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected so to limit the maximum current flowing in the circuit. With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. Also a small value of the series resistor RS will result in a greater diode current when the load resistance RL is connected and large as this will increase the power dissipation requirement of the diode so care must be taken when selecting the appropriate value of series resistance so that the zener’s maximum power rating is not exceeded under this no-load or high-impedance condition. 1.8 Explain Series voltage regulator. The series voltage regulator or series pass voltage regulator uses a variable element placed in series with the load. By changing the resistance of the series element, the voltage dropped across it can be varied to ensure that the voltage across the load remains constant.The advantage of the series voltage regulator is that the amount of current drawn is effectively that used by the load, although some will be consumed by any circuitry associated with the regulator. Unlike the shunt regulator, the series regulator does not draw the full current even when the load does not require any current. As a result the series regulator is considerably more efficient.One of the simplest implementations of this concept is to use a single pass transistor in the form of an emitter follower configuration, and a single Zener diode drive by a resistor from the unregulated supply. This provides a simple form of feedback system to ensure the Zener voltage is maintained at the output, albeit with a voltage reduction equal to the base emitter junction voltage - 0.6 volts for a silicon transistor Fig1.7 Series voltage regulator The Zener diode will generally need a minimum of around 10mA for a small Zener to keep its regulated voltage. The resistor should then be calculated to provide the base drive current and the minimum Zener current from a knowledge of the unregulated voltage, Zener voltage and the current required. [ (Unregulated voltage - Zener voltage ) / current ]. A small margin should be
9. added to the current to ensure that there is sufficient room for margin when the load, and hence the transistor base is taking the full current. The power dissipation capacity for the Zener diode should be calculated for the case when the load current, and hence the base current is zero. In this case the Zener diode will need to take the full current passed by the series resistor. 1.9 Explain IC Voltage Regulators: IC 723 Voltage regulators, the 723 Voltage Regulators IC. The functional diagram of the voltage regulator is shown below. It consists of a voltage reference source (Pin 6), an error amplifier with its inverting input on pin 4 and non-inverting input on pin 5, a series pass transistor (pins 10 and 11), and a current limiting transistor on pins 2 and 3. The device can be set to work as both posistive and negaive voltage regulators with an output voltage ranging from 2 V to 37 V, and output current levels upto 150 m A. The maximum supply voltage is 40 V, and the line and load regulations are each specified as 0.01%. The figure shown below is a positive voltage regulator with an IC 723. The output voltage can be set to any desired positive voltage between (7-37) volts. 7 volts is the reference starting voltage. All these variations are brought with the change of values in resistors R1 and R2 with the help of a potentiometer. A darlington connection is made by the transistor to Q1 to handle large load current. The broken lines in the image indicate the internal connections for current limiting. Even foldback current limiting is possible in this IC. A regulator output voltage less than the 7 V reference level can be obtained by using a voltage divider across the reference source. The potentially divided reference voltage is then connected to terminal 5. Fig.1.8 Voltage Regulators: IC 723 Another importat point to note about this IC is that the supply voltage at the lowest point on the ripple waveform should be at least 3 V greater than the output of the regulator and greater than Vref. If it is not so a high-amplitude output ripple is possible to occur.
10. 1.10 Explain IC 78XX Voltage Regulator is one of the most important and commonly used electrical components. Voltage Regulators are responsible for maintaining a steady voltage across an Electronic system.Voltage fluctuations may result in undesirable effect on an electronic system, so to maintaining a steady constant voltage is necessary according to the voltage requirement of a system.Let us assume a condition when a simple light emitting diode can take a max of 3V to the max, what happens if the voltage input exceeds 3V ?, of course the diode will burn out. This is also common with all electronic components like, led’s, capacitors, diodes etc. The slightest increase in voltage may result in the failure of entire system by damaging the other components too. For avoiding Damage in such situations voltage regulator are used for regulated power supply. Fig.1.9 IC 78XX 1.11Explain IC 79XX regulator 79xx voltage regulators are very commonly used in electronic circuits. The main purpose of this IC is to supply required regulated negative voltage to the circuits. IC 79xx can supply a constant negative voltage output, in spite of any voltage fluctuations in its input voltage. It can be mainly found in the circuits in which integrated circuits that require +Vcc and – Vcc are used
11. Fig.1.10 IC 79XX IC 79xx is a three pin negative voltage controller IC. It is a small integrated circuit used in a circuit to supply a constant negative input voltage. The number 79 indicates that it is a negative voltage regulator and xx indicates the output voltage of the IC. ‘xx’ can be replaced by the controlled output voltage provided by the regulator, for example, if it is 7905, then the output voltage of the IC is -5 V. Similarly if it is 7912, then output voltage of the IC is -12 volts and so on. The name of the IC may vary based on the manufacturer as LM79xx, L79xx, MC79xx etc.IC 79xx requires heat sink for its safe operation. Heat sink boosts heat dissipation therefore the life of the device can be extended. 1.11 Explain Block diagram of UPS. In order to protect a sensitive system from power losses and blackouts, an alternative power source is required that can switch into operation immediately when disruption occurs. An interruptible power supply (UPS) is just such an alternative source.
12. Fig 1.11 Block diagram of UPS A UPS generally consists of a rectifier, battery charger, a battery bank and inverter circuit which converts the commercial ac input into dc suitable for input to the battery bank and the inverter. The rectifier should have its input protected and should be capable of supplying power to the inverter when the commercial supply is either slightly below the normal voltage or slightly above. In case of On-line UPS, the battery operated inverter works continuously whether the mains supply is present or not. Triac T1is on for all the times while Triac T2 has been provided to bypass the UPS inverter, only when a fault develops in the UPS inverter. When the mains supply fails, the UPS supplies power only until the batteries get discharged. However, once the mains power resumes, the batteries will get charged again. The switching times of these supplies is considered to be zero. Usually sealed maintenance free batteries are used and the running time of the inverter is low (approximately 10 to 30 minutes). In the case of Off-Line UPS, the inverter is off when the mains power is on and the output voltage is derived directly from the mains. The inverter turns on only when the mains supply fails. Its switching time is less than 5 ms. These UPS are generally used with PCs or computers or other appliances where a small duration (5 ms or less) interruption in power supply can be tolerated. Usually, sealed batteries or lead-acid batteries are used. The running time of these supplies is also low (about 10 to 30 minutes). 1.12 Explain Block diagram of SMPS The electric power is not normally used in the form in which it is produced or distributed.• Practically all electronic systems require some form of Power conversion.• A device that transfers electric energy from a source to a load using electronic circuits is referred to as power supply.• A typical application of a power supply is to convert utility AC voltage into regulated DC voltages required for electronic equipment. Block diagram of power supply Parts of a power supply:
13. Fig.1.12 Block diagram of SMPS Categories of Power SuppliesThere are two broad categories of power supplies:• Linear regulated power supply• switched mode power supply (SMPS) In some cases one may use a combination of switched mode and linear power supplies to gain some desired advantages of both the types. The electric power is not normally used in the form in which it is produced or distributed.• Practically all electronic systems require some form of Power conversion.• A device that transfers electric energy from a source to a load using electronic circuits is referred to as power supply.• A typical application of a power supply is to convert utility AC voltage into regulated DC voltages required for electronic equipment. Categories of Power SuppliesThere are two broad categories of power supplies:• Linear regulated power supply• switched mode power supply (SMPS) In some cases one may use a combination of switched mode and linear power supplies to gain some desired advantages of both the types. The AC voltage is connected to a transformer, which steps that ac voltage down to the level for the desired dc output.• A diode rectifier then provides a full-wave rectified voltage.• This is initially filtered by a simple capacitor filter to produce a dc voltage.• This resulting dc voltage usually has some ripple or ac voltage regulation.• A regulator circuit can use this dc input to provide a dc voltage that not only has much less ripple voltage but also remains the same dc value even if the input dc voltage varies somewhat or the load connected to the output dc voltage changes.• This voltage regulation is usually obtained using one of the voltage regulator IC units. Transformer• Transformer convert Ac electricity from one voltage to another with little loss of power.• Transformers work only with AC & this is one of the reasons why mains electricity is AC.TYPES OF TRANSFORMER• Step-up Transformer• Step-down Transformer Step-up transformers increase voltage, step-down transformers reduce voltage.• The input coil is called the primary & the output coil is called the secondary.• There is no electrical connection between the two coils, instead they are linked by the alternating
14. 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. So, as voltage is stepped down current is stepped up. RECTIFIER• In mains supplied electronic systems the AC input voltage must be converted into a DC voltage with the right value & degree of stabilization.• Rectifier does this work.• In other words a rectifier circuit is necessary to convert a signal having zero average value into one that has a nonzero average.• Two types of rectifiers : a. Half wave rectifier. b. Full wave rectifier.Figure above uses a center-tapped transformer with two rectifier diodes.• Figure below uses a simple transformer & four rectifier diodes usually known as a bridge rectifier. SMOOTHING/FILTER• We need a way to smooth out the pulsations& get a much cleaner dc power source for the load circuit.• This is done by a filter circuit.• In power supply, a filter must remove or reduce the ac variations while still making the desired dc available to the load circuitry.• Any given filter involve capacitors, inductors,&/resistors in some combination.
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