INTRODUCTION

1.1 PREAMBLE:

Most electrical devices require sources of DC power

before they can operate. Dry cells and batteries are one

form of DC source. These usually have the advantage of

being portable and ripple- free. However they have the

disadvantage of low voltage frequent replacements and

expensive when compared to conventional DC power

supplies. Since the most convenient and economical

source of power is the domestic AC supply it is

advantageous to connect this alternating mains voltage

to its DC voltage equivalent. This process of converting

AC voltage into DC voltage is called Rectification.

1.2 AIM OF THE PROJECT

The aim of the project is to design and construct a power

supply unit which would produce a dual polarity output

of -9V and +9V supply. (i.e negative and positive). It

therefore introduces the use of rectifiers filters and

regulators in order to achieve this.

1.3 PRINCIPLE OF OPERATION USING BLOCK DIAGRAM

In order to give a proper description analysis of this

system the whole system has been divided into 5

different block sections. The first sectional block (i.e.

step-down transformer) is responsible for stepping down

the ac supply voltage to its usable level (i.e. 240ac to 12v

ac). The 12v output of this section is channeled through

a rectifier circuit which simply rectifies the alternating

voltage supply to dc voltage supply after which it is

passed via the filter section in order to reduce the ripple

ac voltage. This filtered dc voltage serves as an input to

two voltage regulators ICs which would regulate the 12

volts dc to a –9volts and +9volts supply respective.

1.4 THESIS OUTLINE

This project work covers a discussion and analysis of the

different stages involved in achieving a dual DC voltage

supply source which include AC voltage transformer. AC

to DC rectification stage filtering stage and voltage

regulator stage.

1.5 OBJECTIVE OF THE PROJECT

The design objectives of this project are aimed towards

achieving the following:

(a) to design and construct a power supply unit that can

be utilized by various electric appliances (i.e a

multipurpose utility supply).

(b) to design an economical source of DC power supply

which would replace the use of dry cells and batteries

which are normally expressive

and subject to frequent replacements.

(c) to obtain DC voltage with a low level of AC ripple and

with good regulation and stability irrespective of

changes in the mains inputs voltage and even

changes in the connected load.

1.6 LIMITATIONS OF THE PROJECT

The limitation of this project work is that it can only be

used for electric devices that require not

more that -9V or + 9V at IA current flow.

However this limitation can be overcome by replacing the

ICs 7809and 7909 which gives +9v and -9v respectively

with ICs like 7824 or 7924 when higher voltage is of +24v

or -24v are required respectively. In addition for lower

dual voltage requirements (less than +_9v ) appropriate

ICs could be used.