TERM PAPER ON

CONSTRUCTION OF 12 VOLTS POWER SUPPLY

WRITTEN BY
ADIE UNIMKE CHINONSO
14/ENG05/029
MECHATRONICS GROUP B2

SUBMITTED TO THE
DEPARTMENT OF ELECTRICAL &
ELECTRONICS ENGINEEERING

THEORY ON 12VOLTS POWER SUPPLY CIRCUIT

What is a power supply?
A power supply is a device which delivers an exact voltage to another
device as per its needs.
There are many power supplies available today in the market like
regulated, unregulated, variable etc, and the decision to pick the correct
one depends entirely on what device you are trying to operate with the
power supply. Power supplies, often called power adapters, or simply
adapters, are available in various voltages, with varying current
capacities, which is nothing but the maximum capacity of a power supply
to deliver current to a load (Load is the device you are trying to supply
power to).

What is a power supply circuit?

A power supply circuit is basically a circuit designed for the sole purpose
of supplying power. In this case, power is supplied from a voltage source.
This voltage is stepped down using a transformer. Of course, the input
voltage is alternating, hence the use of rectifiers is employed in this
circuit. The rectifier convert this alternating voltage to unidirectional
voltage, but this conversion is not 100%. The power supply circuit makes
use of capacitors. These capacitors smooth the output of power supplies.
The variable resistor is used to directly control significant power (more
than a watt), since the power dissipated in the variable resistor would be
comparable to the power in the controlled load. The transistor is a
semiconductor device used to amplify and switch electronic signals and
electrical power.

AC Input: Coming from the wall, the AC alternates from a minimum to a

maximum voltage at a frequency of 60Hz (in the US and other 60Hz
countries). That is what powers all of the AC appliances in your house and
shop, and it looks like the following graph. After the transformer, the
graph is similar, except the sine wave has a smaller amplitude.

AC Power Graph
Rectification: The first stage of this power supply is a rectifier. The
rectifier is an arrangement of diodes that only allows current to flow in one
direction. Think of a one way check valve for water. Because of the
arrangement of diodes in the full wave rectifier used in this design, the
positive part of the AC signal passes unimpeded and the negative part of
the AC signal is actually inverted and added back into the output signal
from the rectifier. Now our signal looks like this:

AC Power Rectified Graph

Smoothing: Now we have at least consistently positive voltage levels,
but they still dip down to zero 120 times per second. A large capacitor,
which can be thought of like a battery over very short time periods, is
installed across the circuit to even out these rapid fluctuations in power.
The capacitor charges when the voltage is high and discharges as the
voltage is low. With the help of the capacitor, the voltage curve looks like
this:

AC Power Smoothed Graph

Regulation: At this point, we use an integrated circuit to consistently
regulate the voltage to exactly the desired level. It is important in sizing
the components for all of the previous stages to drive this IC with a
voltage level sufficiently higher than the regulated voltage such that the
remaining dips 120 times per second will not drop below the required
minimum input value. However, you do not want to drive it with too high a
voltage, as that excess power will be dissipated as heat. The voltage
curve at this point is (ideally) a DC signal at the desired voltage; a
horizontal line.

DC Power Graph

ANALYSIS OF THE POWER SUPPLY CIRCUIT

Fig 1.0: 12V power supply circuit schematic diagram

The power supply circuit works under different principles of electrical
engineering. Initially, it makes use of a transformer, TX which produces the
initial AC input to the circuit. This alternating current is transferred to the
 circuit, but first, it passes through a bridge rectifier. This connection is
made via the use of jumper wires. The bridge rectifier comprises of four
diodes D1 D4, which convert the AC to DC. Now, the current entering the
remaining circuit is DC. This current is not totally DC, therefore a capacitor
is connected in parallel with the rectifier which performs the operation of
smoothening the DC waveform.
The transistor T1, amplifies and switches electronic signals and electrical
power. A voltage or current applied to one pair of the transistor's
terminals changes the current through another pair of terminals. Next up,
the variable resistor VR is used to vary resistance in the circuit, to give
required output current and voltage. The capacitor C2 makes further
smoothening of the waveform. The final DC output can be detected by the
light emitting diode

Components required

1. Jumper wires, with at least 1A current carrying capacity for AC

mains
2. 220V Step Down Transformer TX
3. Bridge rectifier D1 D4
4. 470F Capacitor C1
5. 10F Capacitor C2
6. 5k Variable Resistor VR
7. LM317 Transistor T1
8. LED (for indication)
9. 120 Resistor R1
10. 10 Resistor R2
11. General Purpose PCB
12. Adapter jack
13. Breadboard
14. Soldering iron Solder
15. Piece of veroboard

Functions of components
Jumper Wires: A jump wire, is a short electrical wire with a solid tip at each
end (or sometimes without them, simply "tinned"), which is normally used
to interconnect the components in a breadboard among others, they are
used to transfer electrical signals from anywhere on the breadboard to the
input/output pins of a microcontroller.

Jumper Wires
Transformer: A transformer is an electrical device that transfers energy
between two or more circuits through electromagnetic induction.
Commonly, transformers are used to increase or decrease the voltages of
alternating current in electric power applications.

Transformer

A varying current in the transformer's primary winding creates a

varying magnetic flux in the transformer core and a varying magnetic field
impinging on the transformer's secondary winding. This varying magnetic
field at the secondary winding induces a varying electromotive force (EMF)
or voltage in the secondary winding. Making use of Faraday's Law in
conjunction with high magnetic permeability core properties, transformers
can thus be designed to efficiently change AC voltages from one voltage
level to another within power networks.

Bridge Rectifier: A rectifier is an electrical device

that converts alternating current (AC), which periodically reverses
direction, to direct current (DC), which flows in only one direction. The
process is known as rectification.
 Bridge Rectifier
Capacitor: A capacitor (originally known as a condenser) is
a passive terminal electrical used to store energy electrostatically in
an electric field. Capacitors are widely used in electronic circuits for
blocking direct current while allowing alternating current to pass.
In analogue filter networks, they smooth the output of power supplies.
In resonant circuits they tune radios to particular frequencies. In
electric systems, they stabilize voltage and power flow.

Capacitor

Variable Resistor: A variable resistor informally is a three-

terminal resistor with a sliding or rotating contact that forms an
adjustable voltage divider. Only two terminals are used, one end and the
wiper. Variable resistors are commonly used to control electrical devices
such as volume controls on audio equipment. Variable resistors are
operated by a mechanism can be used as position transducers, for
example, in a joystick. Variable resistors are rarely used to directly control
significant power (more than a watt), since the power dissipated in the
variable resistor would be comparable to the power in the controlled load.
 Variable Resistor
Resistor: A resistor is a passive two-terminal electrical component that
implements electrical resistance as a circuit element. Resistors act to
reduce current flow, and, at the same time, act to lower voltage levels
within circuits. In electronic circuits resistors are used to limit current flow,
to adjust signal levels, bias active elements, terminate transmission
lines among other uses.

Resistor

Transistor: A transistor is a semiconductor device used

to amplify and switch electronic signals and electrical power. It is
composed of semiconductor material with at least three terminals for
connection to an external circuit. A voltage or current applied to one pair
of the transistor's terminals changes the current through another pair of
terminals. Because the controlled (output) power can be higher than the
controlling (input) power, a transistor can amplify a signal. Today, some
transistors are packaged individually, but many more are found embedded
in integrated circuits.
 Transistor

LED: A light-emitting diode (LED) is a two-lead semiconductor light

source. It is a pn junction diode, which emits light when activated. When
a suitable voltage is applied to the leads, electrons are able to recombine
with electron holes within the device, releasing energy in the form of
photons. This effect is called electroluminescence, and the colour of the
light (corresponding to the energy of the photon) is determined by the
energy band gap of the semiconductor.
An LED is often small in area (less than 1 mm2) and integrated optical
components may be used to shape its radiation pattern.

light-emitting diode (LED)

PCB: A printed circuit board (PCB) mechanically supports and

electrically connects electronic components using conductive tracks, pads
and other features etched from copper sheets laminated onto a non-
conductive substrate. PCBs can be single sided (one copper layer), double
sided (two copper layers) or multi-layer (outer and inner layers). Multi-
layer PCBs allow for much higher component density. Conductors on
different layers are connected with plated-through holes called vias.
Advanced PCBs may contain components - capacitors, resistors or active
devices - embedded in the substrate.
Printed circuit boards are used in all but the simplest electronic products.
Alternatives to PCBs include wire wrap and point-to-point construction.
PCBs require the additional design effort to lay out the circuit, but
manufacturing and assembly can be automated. Manufacturing circuits
with PCBs is cheaper and faster than with other wiring methods as
components are mounted and wired with one single part. Furthermore,
operator wiring errors are eliminated
Adapter Jack: An adapter Jack used to provide the output voltage to a
device with a particular socket
Breadboard: A breadboard is a construction base
for prototyping of electronics. Because the solderless breadboard does not
require soldering, it is reusable. This makes it easy to use for creating
temporary prototypes and experimenting with circuit design.

Breadboard

Difference between a veroboard and a PCB (Printed

Circuit Board)

1. Veroboard 2. PCB (Printed Circuit Board)

Veroboard is a widely-used type of electronics prototyping board
characterized by a 0.1 inch (2.54 mm) regular (rectangular) grid of holes,
with wide parallel strips of copper cladding running in one direction all the
way across one side of the board.
In using the board, breaks are made in the tracks, usually around holes, to
divide the strips into multiple electrical nodes. With care, it is possible to
break between holes to allow for components that have two pin rows only
one position apart such as twin row headers for IDCs.
All versions of veroboard have copper strips on one side. Some are made
using printed circuit board etching and drilling techniques, although some
have milled strips and punched holes.
Veroboard holes are drilled on 0.1 inch (2.54 mm) centers. This spacing
allows components having pins with a 0.1 inch (2.54 mm) spacing to be
inserted. Compatible parts include DIP ICs, sockets for ICs, some types of
connectors, and other devices.

PCB- A printed circuit board, or PCB, is used to mechanically support

and electrically connect electronic components using conductive
pathways, tracks or signal traces etched from copper sheets laminated
onto a non-conductive substrate. It is also referred to as printed wiring
board (PWB) or etched wiring board. A PCB populated with electronic
components is a printed circuit assembly (PCA), also known as a printed
circuit board assembly (PCBA). Printed circuit boards are used in virtually
all but the simplest commercially-produced electronic devices.

PCBs are inexpensive, and can be highly reliable. They require much more
layout effort and higher initial cost than either wire wrap or point-to-point
construction, but are much cheaper and faster for high-volume
production; the production and soldering of PCBs can be done by totally
automated equipment.

Soldering and desoldering techniques

Soldering techniques
Soldering: Soldering is a process in which two or more metal items are
joined together by melting and flowing a filler metal (solder) into the joint,
the filler metal having a lower melting point than the adjoining metal.
Soldering differs from welding in that soldering does not involve melting
the work pieces. In brazing, the filler metal melts at a higher temperature,
but the work piece metal does not melt. In the past, nearly all solders
contained lead, but environmental concerns have increasingly dictated
use of lead-free alloys for electronics and plumbing purposes.
 General soldering technique

Solder

Soldering is used in plumbing, electronics, and metalwork from flashing to

jewellery.
Soldering provides reasonably permanent but reversible connections
between copper pipes in plumbing systems as well as joints in sheet metal
objects such as food cans, roof flashing, rain gutters and automobile
radiators.
Electronic soldering connects electrical wiring and electronic components
to printed circuit boards (PCBs).
Soldering methods are broadly divided into two types:

Partial heating method

Total heating method.

Partial heating method: Heat is applied to the package leads

and/or PWB in a localized manner. Partial heating involves less heat
stress on the device and printed wiring board, but is unsuitable for
large volume production. Therefore, this method is mainly used to
correct soldering or for devices with a low heat resistance.

Total heating method: Heat is applied to the entire package

and/or PWB. Because of excellence in productivity and running cost,
 these types are widely used. However, this method can place
considerable heat stress on the semiconductor device and board.

Desoldering techniques
Desoldering: In electronics, desoldering is the removal of solder and
components from a circuit board for troubleshooting, repair, replacement,
and salvage.

Desoldering with a desoldering gun

Desoldering using a vacuum plunger (on the right) and a soldering
iron

Desoldering with a desoldering

gun.

Desoldering using a vacuum plunger (on the right) and a soldering iron.

Etching
Developing PCB using etching
Etching is usually done with ammonium persulfate or ferric chloride. For
PTH (plated-through holes), additional steps of electroless deposition are
done after the holes are drilled, then copper is electroplated to build up
the thickness, the boards are screened, and plated with tin/lead. The
tin/lead becomes the resist leaving the bare copper to be etched away.

The simplest method, used for small-scale production and often by

hobbyists, is immersion etching, in which the board is submerged in
etching solution such as ferric chloride. Compared with methods used for
mass production, the etching time is long. Heat and agitation can be
applied to the bath to speed the etching rate. In bubble etching, air is
passed through the etchant bath to agitate the solution and speed up
etching. Splash etching uses a motor-driven paddle to splash boards with
etchant; the process has become commercially obsolete since it is not as
fast as spray etching. In spray etching, the etchant solution is distributed
over the boards by nozzles, and recirculated by pumps. Adjustment of the
nozzle pattern, flow rate, temperature, and etchant composition gives
predictable control of etching rates and high production rates

As more copper is consumed from the boards, the etchant becomes

saturated and less effective; different etchants have different capacities
for copper, with some as high as 150 grams of copper per litre of solution.
In commercial use, etchants can be regenerated to restore their activity,
and the dissolved copper recovered and sold. Small-scale etching requires
attention to disposal of used etchant, which is corrosive and toxic due to
its metal content.

The etchant removes copper on all surfaces exposed by the resist.

"Undercut" occurs when etchant attacks the thin edge of copper under the
resist; this can reduce conductor widths and cause open-circuits. Careful
control of etch time is required to prevent undercut. Where metallic
plating is used as a resist, it can "overhang" which can cause short-circuits
between adjacent traces when closely spaced. Overhang can be removed
by wire-brushing the board after etching.

Chemicals used for etching

There are two major types of etching chemicals
1. Wet etching chemicals: These chemicals have High Quality (Low
containing of metal, impurity), improved permeability, reduced
particle adhesion and suppressed etching rate change. Examples
include: Aqueous Hydrofluoric Acid (HF), Ammonium Fluoride
(NH4F), Buffered Hydrogen Fluoride (BHF), Buffered Hydrogen
Fluoride with Surface-active Agent (BHF-U), Zielex Buffered
Hydrogen Fluoride (Zielex BHF).
2. Dry etching agents: In dry etching, plasmas or etchant gasses
remove the substrate material. The reaction that takes place can be
done utilizing high kinetic energy of particle beams, chemical
reaction or a combination of both. They generally have high-Purity
(More than 99.999 %). Examples include: PFC-14(CF4), PFC-
116(C2F6), PFC-C318(C4F8), HFC-23(CHF3), HFC-32(CH2F2).