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01
Power supply
CONTENTS PAGE
INTRODUCTION……………………………………………………………………………………..
TASK 1
2.0 Linear Power Supply………………………………………………………………………………
3.0 Switched-mode power supply SMPS…………………………………………………………….
TASK 2
4.0 Project Description and Introduction…………….…………………………………………………………………
5.0 Power Supplies circuit………………………………………………………………………………
6.0 Operation of Circuit……………………………………………………………………………
Transformers 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) to a safer low voltage.
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.
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.
turns ratio = Vp
= Np
and power out = power in
Vs Ns Vs × Is = Vp × Ip
Vp = primary (input) voltageNp = number of turns on primary coilIp = primary (input) current
Vs = secondary (output) voltageNs = number of turns on secondary coilIs = secondary (output) current
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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. For this circuit, type of this transformer is step down and input for this transformer we use 230V AC and the output is 6v.
DATA SHEET OF TRANSFOMER
Specification Model : HT-6E1Input AC 115V/230VOutput 6V 0 6VCapacity 1000mANet weight 331g
021
Diode
Example: Circuit symbol:
Function
Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.
Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).
Reverse Voltage
When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.
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RectifierThere are several ways of connecting diodes to make a rectifier to convert AC to DC. The bridge rectifier is the most important and it produces full-wave varying DC. A full-wave rectifier can also be made from just two diodes if a centre-tap transformer is used, but this method is rarely used now that diodes are cheaper. A single diode can be used as a rectifier but it only uses the positive (+) parts of the AC wave to produce half-wave varying DC.
Rectifier diodes (large current)
Rectifier diodes are used in power supplies to convert alternating current (AC) to direct current (DC), a process called rectification. They are also used elsewhere in circuits where a large current must pass through the diode.
All rectifier diodes are made from silicon and therefore have a forward voltage drop of 0.7V. The table shows maximum current and maximum reverse voltage for some popular rectifier diodes. The 1N4001 is suitable for most low voltage circuits with a current of less than 1A.
Alternate pairs of diodes conduct, changing over the connections so the alternating directions of AC are converted to the one direction of DC
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Capacitors
Electrolytic Capacitors (Electrochemical type capacitors)
Aluminum is used for the electrodes by using a thin oxidization membrane.Large values of capacitance can be obtained in comparison with the size of the capacitor, because the dielectric used is very thin.The most important characteristic of electrolytic capacitors is that they have polarity. They have a positive and a negative electrode.[Polarised] This means that it is very important which way round they are connected. If the capacitor is subjected to voltage exceeding its working voltage, or if it is connected with incorrect polarity, it may burst. It is extremely dangerous, because it can quite literally explode. Make absolutely no mistakes.Generally, in the circuit diagram, the positive side is indicated by a "+" (plus) symbol.Electrolytic capacitors range in value from about 1µF to thousands of µF. mainly this type of capacitor is used as a ripple filter in a power supply circuit, or as a filter to bypass low frequency signals, etc. Because this type of capacitor is comparatively similar to the nature of a coil in construction, it isn't possible to use for high-frequency circuits. (It is said that the frequency characteristic is bad.)
The photograph on the left is an example of the different values of electrolytic capacitors in which the capacitance and voltage differ.From the left to right:220µF (25V) [diameter 8 mm, high 12 mm] 1000µF (50V) [diameter 18 mm, high 40 mm]
The size of the capacitor sometimes depends on the manufacturer. So the sizes shown here on this page are just examples.
In the photograph to the right, the mark indicating the negative lead of the component can be seen.You need to pay attention to the polarity indication so as not to make a mistake when you assemble the circuit.
Smoothing
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Smoothing is performed by a large value electrolytic capacitor connected across the DC supply to act as a reservoir, supplying current to the output when the varying DC voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC (dotted line) and the smoothed DC (solid line). The capacitor charges quickly near the peak of the varying DC, and then discharges as it supplies current to the output.
Note that smoothing significantly increases the average DC voltage to almost the peak value (1.4 × RMS value). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS (1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value giving 1.4 × 4.6 = 6.4V smooth DC.
Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small ripple voltage. For many circuits a ripple which is 10% of the supply voltage is satisfactory and the equation below gives the required value for the smoothing capacitor. A larger capacitor will reduce ripple. The capacitor value must be doubled when smoothing half-wave DC.
Smoothing capacitor for 10% ripple, C = 5 × Io
Vs × f
C = smoothing capacitance in farads (F)Io = output current from the supply in amps (A)Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DCf = frequency of the AC supply in hertz (Hz), 50Hz in Malaysia
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Ceramic CapacitorsCeramic capacitors are constructed with materials such as titanium acid barium used as the dielectric. Internally, these capacitors are not constructed as a coil, so they can be used in high frequency applications. Typically, they are used in circuits which bypass high frequency signals to ground.These capacitors have the shape of a disk. Their capacitance is comparatively small.
The capacitor on the left is a 100pF capacitor with a diameter of about 3 mm.
The capacitor on the right side is printed with 103, so 10 x 103pF becomes 0.01 µF. The diameter of the disk is about 6 mm.Ceramic capacitors have no polarity.Ceramic capacitors should not be used for analog circuits, because they can distort the signal.
Regulator
Voltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output voltages. They are also rated by the maximum current they can pass. Negative voltage regulators are available, mainly for use in dual supplies. Most regulators include some automatic protection from excessive current ('overload protection') and overheating ('thermal protection'). Many of the fixed voltage regulators ICs have 3 leads and look like power transistors, such as the 7805 +5V 1A regulator shown on the right. They include a hole for attaching a heat sink if necessary.
027
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Construction
1. Cut a piece of stripboard to.
2. Fit the five wire links (two input ac, two outputs 12V and 5V and ground).
3. Fit the four 1N4001 diodes, taking care that the polarity of each one is correct.
4. Fit the 47μF and the single 1000µf capacitors. Ensure that the polarity of the 1000µf capacitor is correct. The leads will be marked with '+' or '-'.
5. Fit the two terminal blocks.
6. Bolt the heatsink to the board.
7. Bend the leads of the LM 7805 and LM7812 and position it. Bolt it into place before soldering the leads to the board.
8. Connect up a 7V - 35V AC power supply and test the circuit by placing a volt meter across the DC Output terminals. The voltage should read approximately 5V DC.
e. Testing and troubleshoot physical circuit 12/8/08 16/08/08
f. Prepare result, discussion & conclusion 16/08/08 19/08/08
g. Creating casing 8/8/8 18/08/08
h. Solder component on stripboard 12/8/08 19/18/08
i. Complete report 12/8/08 19/18/08
j. Do some necessary change 15/08/08 26/08/08
k. Submit the assignment report 26/08/08 27/08/08
DATE TIMES(days)
031
Gantt chart
032
General Statement of the Problem
After designing any electronic equipment, the engineers or designers should know the type of the power supply to incorporate in their end product. A well designer power supply in all equipment should improve both the performance and reliability of the total system at no additional cost. The common problem for electronic designers is to identify the suitable power supply that gain competitive qualities in electronics market. One of the most important criteria for good power supply is the ability to keep the voltage constant under input voltage disturbance. Developing guideline that will facilitate engineers and designers to know the characteristics of both linear and switching mode power supply will allow them to be more competitive. It is also important to know that in most cases the average power supply purchaser doesn’t know most of these details all that much. The designer should ensure that his power supply has the unique aspects, but not far from those of other power suppliers. He should also consider the power supply’s weight, cost, power efficiency, and space of the equipment that will need this power supply.
As conclusion, we know that power supplies are essential part of all electronic system. When dealing with electronic circuits, we have to meet the basic requirement of providing electrical power for them to work.
The basic purpose of a power supply is to provide one or more fixed voltages to the working circuit, with sufficient current-handling capacity to maintain the operating conditions of the circuit.
The basic power supply is consist of major component such as transformer, rectifier, filter and regulator .All of them have their own job to make the power supply system is function. Apart from the major component, it is also included of grounded plug, fuses and switch.
As we know, generally any basic switched power supply consists of five standard components which are set up from a pulse-width modulating controller, a transistor switch, an inductor, a capacitor and a diode. A switch mode power supply is a widely used circuit nowadays and it is used in a system such as a computer, television receiver and battery charger.
033
Reference:
Brown,Marty
Practical switching Power Supply Design, Harcourt Brace Jovanovich,1990.
Gottlieb, Irving M.,
Power Supply, switching Regulators, Inverters, and Converters, 2 nd Edition .TAB Books,1994.