CONSTRUCTION OF A DEMONSTRATION VARIABLE REGULATED D.C. POWER SUPPLY UNIT

CONSTRUCTION OF A DEMONSTRATION VARIABLE REGULATED D.C. POWER SUPPLY UNIT


BY

AMADI UCHENNA

CHAPTER ONE
1.0 INTRODUCTION
With the rapid growth of highly developed technology of instant communication, computerization, industrial automation and space exploration, the feats would not have been possible without the advent of electrical / electronics engineering. In electrical and telecommunication engineering field, systems and equipments like amplifiers, satellites, microwave link systems to name but a few depend for their correct operations upon the availability of stable well regulated d.c. power supplies. In experimental laboratories, the use of well regulated d.c. power supplies becomes inevitable because the result of these experiment depends largely on the quality power supply used.

The construction of this piece of electronic equipment will find use both now and in future. The main requirement of a well regulated d.c. power supply unit are: good regulations, low ripple, low output impedance and strict output short-circuit protection.

1.1 BACKGROUND OF THE STUDY:
In recent times variable d.c power supply unit have become very useful and popular and popular to power d.c. equipments. When d.c. supplies of different values are required while accumulation of different values could be used, it is cheaper and more convenient to use a variable d.c. supply hence the important of variable d.c. power supply.

1.2 PURPOSE OF THE PROJECT:
The purpose of this work or project is to construct a demonstration how a variable d.c. power is realize and works.

1.3 SCOPE OF THE PROJECT
The project is limited to the construction and demonstration of a variable d.c. power supply and the voltage range is from 0 to 15 volts.

1.4 DEFINITION OF TERMS:
TRANSFORMER: This is an electrical device used mainly for voltage transformation to the required level.

FUSE:- This is a short piece of wire inside electrical equipment which prevent damage by melting and stopping the electricity when there is too much power.

INTEGRATED CIRCUIT: This is a very small set of electronics connection printed on a single piece of semi conductor material instead of being made from separated parts.

RECTIFICATION: This is a process of converting alternating or changing current to direct unidirectional current.

SEMI-CONDUCTOR: This is a material whose conducting ability lies between that of conducting and insulators.

BRIDGE RECTIFICATION: This is a block or group of four diodes arrange or connected such that two diodes conduct at every half ciycle to give a d.c. output.

CHAPTER TWO

2.0 LITERATURE REVIEW
2.1 TRANSFORMERS: This is an electrical device used mainly for voltage transformation to the required levels. Transformers come in various sizes, some transformers are as large as a room, and others can be as small as a cube of sugar. The basic parts of a transformer are primary winding usually connected to the input signals. The secondary winding is connected to the load. The core of the transformer is another important part of the device. The illustration in figure 2.1 shows a step down transformer. There are voltage and current transformers which could be auto transformers or separately wound transformers and many others.
Core

Primary Winding Secondary Winding

Figure 2.1 Step down transformer showing the main part of the device

2.2 RECTIFIERS: Conversion of alternating current or voltage into direct current is called rectification. A diode whether crystal or electronic is used as a rectifier because it conduct current only in one direction. Half wave rectification uses one diode, full wave rectifiers uses tow diodes whereas a bridge rectifier has four diodes.

2.3 FULL WAVE RECTIFICATION: Here the diodes conduct in both cycles of the input. Full wave rectification uses of a center tap transformers is essential when only two diodes are used. There is the need for centre tapped transformers with bridge rectifier.

2.4 FULL WAVE BRIDGE RECTIFIER: it is the most frequently used circuit for electronic d.c power supplier. It requires four diodes but the transformer used is not centre taped and has a maximum voltage across it secondary winding as show in figure 2.2

Figure 2.2 Full Wave Bridge Rectifier circuit
2.5 FILTER: The rectifier’s circuit described produces the required direct current in the load. The d.c. is however not ready or smooth. In most application it is desirable to keep this latter component small. This can be accomplished by the use of a smoothening or filter circuit. The simplest of which consist of a capacitor in parallel with the load as shown in figure 2.3.

The diodes conduct when the supply voltage V is more positive than the load voltage VR. During this conducting period, if the diode forward resistance is neglected, then the load voltage is equal to the supplied voltage.









Figure 2.3 A capacitor filter

2.6 UNREGULATED D.C. POWER SUPPLY

This consists of the transformer that steps down the a.c. supply to the desired voltage level. The output of the transformer is then rectified to give a fluctuating d.c. signal. The output of the transformer is fed into the rectifier that converts the low or stepped down a.c. into d.c. The smoothening and filter circuit smoothens the fluctuating d.c. signals and filter out the ripples. The unregulated d.c. output is now connected to the lad which could be any equipment or appliance as shown in figure 2.3.



UNREGULATED D.C. POWER SUPPLY

2.7 REGULATED VARIABLE D.C. POWER SUPPLY
This has the same circuitry from the a.c transformation through the rectifier through the smoothening and filter network. Because of the varying load current, the stabilization and regulating network are incorporated at the output of the filter. The output voltage is varied to the desired level by a potentiometer which sets the output voltage to the required level value. The variation is from zero to the maximum output voltage as shown in the figure below.

REGULATED VARIABLE D.C. POWER

2.8 ADJUSTABLE VOLTAGE REGULATOR
The LM 78xx series of three terminal regulators is available with several fixed output voltages making them useful in a wide range of application. The voltage available allows these regulators to be used in logic systems, instrumentation and other solid state electronic equipment. Although designed primarily as fixed voltage regulator. This device can be used with external components to obtain adjustable and currents.


CHAPTER THREE

3.0 CIRCUIT ANALYSIS AND DESCRIPTION

Variable D.C. Power Supply Circuit Diagram
3.1 CIRCUIT PROTECTION
The circuit is protected by a fuse. A fuse is a short piece of wire designed to melt when the current passing through it exceeds the rated value. It is placed on the live line of the circuit. The 1A fuse used in this project work is to protect the circuit from current beyond 1Amp. All domestic electrical equipment are rated specifying the maximum current, voltage or power required for safe operation.

3.2 THE TRANSFORMER UNIT
The transformer in this project steps down or reduces the mains voltage so that it can be used in the range of electronic circuits. There are two types of transformers, the step up and step down. In this project work, a step down 12v transformer takes in 240v a.c. at it’s primary voltage and reduces it to 12v and this 12v is fed to the rectifier unit.

3.3 RECTIFICATION AND SMOOTHENING NETWORK
Rectification is the process of converting a.c. to d.c. It is use in power supply units for producing low power d.c. from the mains 240v supply.

In this project work, the full wave rectification is achieved by the use of a bridge rectifier consisting of four diodes. The IN4001 Silicon diode with a maximum forward current of 1A and peak reverse voltage of 50V. after the rectification, the d.c. voltage in the circuit has ripples and has to be filtered. This smoothening is achieved by the use of an electrolytic capacitor or a resistor capacitor arrangement of 10µf. The amount of smoothening obtained depends on the size of the capacitor and the size of the load resistance. The larger the time constant, the better the smoothening and the shorter the ripple voltage.

3.4 LM 7815 REGULATOR
The LM 7815 used for this project work is available in an aluminum TO-3 package as in figure above which will allow over 1.0A load current. If adequate heat sinking is provided, current limiting is included to limit the peak output current to safe value. Internal power dissipation becomes too high for the heat sink provided the thermal shut-down circuit takes over preventing the IC from over heating. It provides output voltages ranging from 5v to 10v. Bypassing the output is not necessary though it improves transient response. Input bypassing is needed only when the regulator is located far from the filter capacitor of the supply.

TO – 3

3.5 PRINCIPLE OF OPERATION
The variable regulated d.c. power supply utilizes a 240v ac supply which is stepped down by a 12v transformer. The transformer is connected to a fuse of 1A and a switch to turn on and off the circuit. The circuit is incorporated with four diodes which forms the bridge rectifier circuit. The capacitor and resistor network does the smoothening of the voltage which is rectified by the bridge rectifier. After the smoothening, the LM 7815 voltage regulator takes in the input from it first pin while pin 2 is grounded. The regulated voltage output is on pin 3. A voltmeter is connected with a variable switch which is used to control the d.c output and the d.c. voltmeter indicates the amount of voltage at the output of the Lm 7815 voltage regulator.


CHAPTER FOUR

4.0 Constructional Description and Casing:
The variable regulated dc power supply is constructed on a veroboard measuring 7.5cm by 6.5cm. The thickness is about 2cm. The veroboard has 16 conducting strips with 19 holes by 16 holes in which the components are carefully fixed in base as on the layout diagram of figure below and soldered. The components are well laid out traceable from input to output terminals. The transformer ‘T’, neon lamp, switch, variable resistor, voltmeter as well as the fuse are not soldered on the veroboard so as to reduce the weight of parts on the veroboard, the conducting leads connect them to their respective points as per the circuit diagram particularly. The transformer is so connected not to increase the load on the veroboard which might break or crack the conducting strips. The meter is fixed on the front panel for easy and quick taking of reading. In like mannere, the switch, lamp, and variable resistor control are easily accessible for quick and easy control, the fuse on the other hand has been fixed within the casing but not soldered directly on the veroboard.

There are vent holes on the sides of the casing for cross air circulation and to lover the temperature within the enclosure as the diodes and voltage regulator are semiconductors which are temperature sensitive.

4.1 CASING
The metal casing is made with these dimensions 6cm long, 4cm wide and 7cm high to the component designed. The front view has the power supply switch (ON/OFF), lamp indicator and a variable resistor knob. Also, a voltage meter is on the front view as shown in the figure below:



Figure 4.1 Wiring / Layout Diagram

HP 6202B Regulated DC Power Supply ManualFigure 4.2 Pictorial view of a variable regulated dc power supply unit

4.2 TOOLS USED

S/NO TOOLS FUNCTION
1 Soldering iron It is used to solder components to the veroboard
2 Screw Driver It is use to screw or fasten the transformer to the casing
3 Plier It was used to cut surplus terminals of components
4 Voltmeter It measures the amount of voltage in the circuit
5 Iron brush It is use to clean the bits of the soldering iron
6 Hack saw It is use to cut material for the casing


4.3 PARTS LIST
S/N PARTS VALUES
1 D1, D2, D3, D4. Diodes 1N4001 Silicom 1A
2 Transformer 12V
3 Fixed Resistor 10 ohms
4 Integrated Circuit LM 7815
5 Fuse 1A
6 Variable Resistor B100K
7 Zener Diode 1A
8 C1, C2. Capacitor 100µf
9 Vero board 7.5cm by 6.5cm


CHAPTER FIVE

5.0 TESTING, RECOMMENDATION AND CONCLUSION
5.1 TESTING
After the construction, the system was tested and measurement indicated on the meter. The variable resistor was adjusted and meter readings noted. Some components were replaced to improve the system performance. The power supply unit was tested again for over two hours to ascertain it output level or to 10V.

5.2 RECOMMENDATION
This project has its limitations among which are time factor, technical specification and finance. These areas should be looked into for improvement on subsequent work on the same topic through the following:
1) Project construction and writing should be taught for atleast two semesters of National Diploma two (NDII).
2) Government through such corporate bodies should help fund or finance students’ project to improve the quality and possibly standardize the project for the market.
3) The polytechnic can construct more of this project for use in the laboratory / workshop instead of buying from the market thereby depleting the resources of the institution.
4) The project as a course should be weighted the same as such course as electronics, telecommunication, power system etc so that the students will take it more serious than it is obtained currently. There is still more room for improvement on the variable regulated direct current power supply.

5.3 CONCLUSION
The aim of a project is to make the student adopt the theories into the practical realization for the benefits of mankind.

The circuit was constructed and tested, the output voltage was varied from o to 10 volt. It is a huge success.