Design Of Dual Power Supply System 5V and 12V Output

CLASS WORK PROJECT DESIGN

DESIGN OF DUAL POWER SUPPLY SYSTEM 5V AND 12V OUTPUT

Introduction

In this chapter we would be discussing the design and implementation of the dual power supply system. Electronic circuits using tubes or transistors required a source of dc. power. For example, in tube amplifiers, d.c. voltage was needed for plate, screen grid and control grid.  Similarly, the emitter and collector bias in a transistor had also to be direct current.  Batteries were rarely used for this purpose as they were costly and required frequent replacement.  In practice, d.c. power for electronic circuits was mostly conveniently obtained from commercial a.c. lines by using rectifier-filter system, called a d.c. power supply. 

The rectifier-filter combination constituted an ordinary d.c. power supply.  The d.c. voltage from an ordinary power supply remained constant so long as a.c. mains voltage or load was unaltered.  However, in many electronic applications, it was desired that d.c. voltage should remain constant irrespective of changes in a.c. mains or load.  Under such situations, voltage regulating devices were used with ordinary power supply.  This constituted regulated d.c. power supply and kept the d.c. voltage at fairly constant value.  

Background
A transformer was used to step down the AC line voltage to a smaller peak voltage Vm, which had to be somewhat larger than the ultimately desired DC output.  A diode circuit rectified the AC signal and a capacitive filter bank was then used to “smooth” or “filter” the rectified sinusoid, producing a waveform with predominant DC component.  Under normal loading conditions there was always some residual periodic variation or “ripple” in the filtered signal.  If the application required very low ripple and constant DC output over a wide range of loading conditions, then an active regulation was required to further reduce or eliminate this residual ripple.  Most active regulator circuits would require a certain minimum input-output voltage differential for proper operation.

Problem statement
To buy two different chargers become expensive to low class living individuals, therefore the team decided to designed a charger that can charge two different d.c components at the same time so such the expenses of buying two chargers is reduce by a quarter or even a half.  

Significance of the project. 

• The design was not so complex i.e. it required few components.
• It required less space unless heat sink was used. 
• Did not generate noise and heat. 
• Fast transient response meaning that the rate at which the capacitor discharged was fast. 
• Low cost.

General Objectives  

The general objective of a rectifying circuit was to convert a.c power to d.c power.

Specific Objective  

 To generate a dual power circuit that could supply two different d.c devices that used 5V and 12V at the same time.

Scope of study
To design a dual power supply of 5V and 12V dc with the following specification of both input and output. 

• A.C input of 230V
• A.C output of 14V 
• D.C input of 14V 
• D.C output of 5V and 12V.

Methodology 

Tools used. 

• Soldering lead.
• Power source. 
• Soldering gun.

Materials used  

• Step down transformer
• Diodes 
• Capacitors. 
• Voltage regulators 
• Resistor 
• LED 
• Printed board 
• USB head 
• Bread board 
• Jumpers

Bill of quantity 

S/N    Components                            Specification        Qty
1        Step-down transformer          230V/14V, 1A         1   
2        Diodes                                    1N4004                  4  
3        Capacitors                             1000µF                   2   
4        Voltage regulator                   VR7812                  1    

5        Voltage regulator                   VR7805                  1   
6        Resistor                                  1kΩ                        1   
7        LED                                        Red-5V                   1   
8        USB head                               5V                          1  
9        Printed board                         Ea                          1   
10      Jumper                                    Ea                         15   
11      Soldering lead                        Role                      1   

Circuit diagram

Safety precautions. 

• Avoid touching capacitor before it is completely discharged.
• The terminals of each components were appropriately marked for correct connection to avoid short circuit. 
• Avoid connecting components when power is on. 
• Avoid touching soldering gun when it is connected to the power source.

Procedures

•  The four diodes were connected as shown to form the rectifying circuit with its input connected to the secondary winding of the transformer while its output connected to the input of power regulators coded VR17812 and VR27805.
• Each of the input terminals of the two regulators were connected to the ground through capacitor rated 1000µF. 
• The reference terminal of each of the regulator was connected directly to the ground. 
• Connectors were connected to the output and ground of VR17812 to generate 12V d.c output. 
• 1KΩ resistor in series with the LED were connected across the output and ground of VR27805 and connectors were connected to both the output and ground terminal to generate 5V d.c output.

Mode of operation
The above circuit obtains power from a 230V AC mains supply voltage and fed into a step down transformer for transforming higher voltage supply to lower one. The transformer TR1 is of 230V primary, 14V secondary and 1A step down transformer. The stepped down voltage is fed into the bridge rectifier made of four 1N4004 diodes for the conversion of AC supply to DC one. 

The output from the bridge rectifier was filtered by the capacitor C1 and C2 to offer a steady DC level to the input pins of the regulators used in the above circuit. The DC voltage is then fed in to the IC 7805 which is a 5V regulator and also to the IC 7812 which was a 12V regulator. The output obtained from the 7805 & 7012 are 5V and 12V respectively. A LED D5 was connected through the current limiting resistor of 1KΩ to indicate the state of the device.

Schematic 

 


Conclusion
After the components were mounted on the printed board and the circuit was powered through at a step down transformer, the required voltages of 5V and 12V dc were obtained across the terminals.  

Challenges
In the process of designing and construction of the dual power supply, the following problem were encountered, 

• Most of materials used in the construction of the dual power supply were expensive.
• There was no enough time given to the students for studying and designing of project which caused a lot of expenses. 
• The project was new to the students since it was the first electronic project that they have ever studied and we had the problem of determining the terminals of the electronic components we used. 
• The circuit did not have the protective equipment’s which put all the component under used in great danger. 
• Another problem was that the circuit produced heat and this can burn the circuit components.

Improvement to above challenges
In order to have an efficient dual power supply, the following components has to be incorporated in the circuit to solve the problems notices during the design and operation of the system. 

• A.C Relay – this relay has to be connected before the rectifier circuit to monitor voltage, current, frequency, surge variation and lose connection.
• D.C Relay – It has to be connected at the output to monitor output parameters. 
• Heat sink – should be connected with the regulating circuit to dissipate heat generated by regulators.

                                                   

KIMULI ISMAIL   
LWANGA RICHARD   
ATENY GABRIEL AGUTO   
KASIIMBURA OSBERT   
SSEMWOGERERE DANIEL      
FATUMAH SULAIMAN    
KITYO MARK ANTHONY      
BUKENYA IVAN    
BOGERE INNOCENT       
DDEMBE JOSIAH