power suply documentation

7/31/2019 Power Suply Documentation

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a) Hardware Discription

Power Supply Description

In electronics generally we use D.C .power . In our micro-controller

circuits we use 6volts power supply. Normally in domestic power we get

230volts A.C. Power . Using some circuits we convert this A.C. Power into

required D.C. Power .

Steps involved in power supply circuits

Step down transformer

Rectifier circuit

Filter circuit

Regulator circuit

Indicator circuit

Step down transformer:

First step involved in the power supply circuit is to reduce the

230A.C.Power into 9v A.C. for this purpose we use 9-0-9 step down

transformer. This transformer is called center tapped transformer. This

transformer reduces the 230v A.C. into 9v A.C power. This transformer

consists of two parts.

Primary

Secondary

Primary:


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Primary is the input of the transformer. Primary consists of two wires.

One is connected to phase and other is connected to neutral of the domestic

A.C. Power .

Secondary:

Secondary is the output of the transformer. Secondary consists of

three wires. The middle wire is ground terminal and the other two wires are

9v A.C terminals.

When the input is connected to the 230v A.C power, we get 9vA.C

power as the output of the transformer.

The symbol for the Transformer:

Rectifier circuit:

The output for the step down circuit is 9v A.C power. For our micro-

controller circuit we need 6v D.C power. So we introduce the rectifier circuit.

The rectifier circuit is the only circuit used to convert the A.C. Power intoD.C. Power .

Basically there are three types of rectifier circuits, half wave rectifier,

full wave rectifier and bridge rectifier. Here we use full wave rectifier circuit.

In rectifier circuits diodes are used as rectifiers. Diodes conduct only when


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the electrons flow in the forward direction. It does not conduct when the

electrons flow in the reverse direction.

Two diodes are used in the full wave rectifiers. These diodes are

connected directly to the secondary of the transformer.

Operating principle for the Full Wave Rectifier:

A full wave rectifier is a circuit, when converts an AC voltage into a

pulsating DC voltage using both half cycle of the applied AC voltage. It used

two diodes of which one conducts during one half cycle while the other

conducts during the other half cycle of the applied AC voltage.

During the positive half cycle of the input voltage, diode D1 becomes

forward biased. Hence D1 conducts and D2 remains OFF. The load current

flows through D1 and the voltage drop across Rlwill be equal to the input

voltage.

During the negative half cycle of the input voltage, drop D1 becomes

reverse biased and D2 becomes forward biased. Hence D1 remains OFF and

D2 conducts. The load current flows through d2 and the voltage drop across

R lwill be equal to the input voltage.

The symbol for the Transformer:


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Fitter Circuit:

The output of the Full wave Rectifier contains both AC and DC components.

A majority of the application, which cannot tolerate a high value ripple,

necessitates further processing of the rectifier output. The undesirable AC

components i.e. the ripple, can be minimized using filters.

The output of the rectifier is fed as input to the filter. The output of the

filter is not a perfect DC, but it also contains small AC components. Some

important filters are

Inductor Filter

Capacitor Filter

LC Filter

CLC Filter

In our circuit we use the Capacitor Filter.

Capacitor Filter

A Capacitor filter connected directly across the load is shown above.

The property of a capacitor is that it allows AC component and blocks DCcomponents. The operation of the capacitor filter is to short the ripple to

ground but leave the DC to appear at output when it is connected across the

pulsating DC voltage. During the positive half cycle, the capacitor charges up

to the peak vale of the transformer secondary voltage, Vm and will try to

maintain this value as the full wave input drops to zero. Capacitor will

discharge through Rlslowly period, which depends on the capacitor voltage.

Until the transformer secondary voltage again increases to a value greaterthan the capacitor voltage. The diode conducts when the transformer

secondary voltage becomes more than the diode voltage. This is called the

cut in voltage. The diode stops conducting when the transformer voltage

becomes less than the diode voltage. This is called cut out voltage.


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Regulator circuit:

This circuit is a small +5V power supply, which is useful when experimenting

with digital electronic. Small inexpensive wall transformer with variable

output voltage are available from any electronics shop and supermarket.

Those transformers are easily available, but usually their voltage regulation

is very poor, which makes then not very usable for digital circuit

experimenter unless a better regulation can be achieved in some way. This

circuit can give +5V out put at about 150mA current, but it can be increased

to 1Amp when good cooling is added to 7805regulator chip. The circuit has

overload and terminal protection.

If you need other voltage than +5V, you can modify the circuit by

replacing the 7805 chip with another regulator 78 chip family. The last

numbers in the chip code tells the output voltage. Remember that the input

voltage must be at least 3v greater than regulator output voltage to

otherwise the regulator does not work well.

Device control unit:

The program in VC++ language is fed to computer and it reads the

data entered through the keyboard on the computer. Through Parallel port it

sends data to Control Unit (Micro Controller Unit). The output of the

microcontroller unit is fed to the Switching unit. The switching unit is nothing but

the coils in the relay. By using this, the relay is turned ON\OFF.


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The switching unit is nothing but switch in Relay. It controls device

i.e. ON\OFF .The driver unit drives the voltage to device connected to it.

RELAYS:

A relay is best defined as a switch that is operated by an electromagnet. A relay

controller is a device that is used to control a bank of switches. A relay controller works by

turning on and off magnetic coils under logic control. A computer controlled relay driver

allows your computer to send simple commands to activate a switch or a group of switches.

Relays are ideally suited for controlling everything from lights and motors to

telecommunication, audio, and video signals. Some relays can be used for switching radio

frequency signals. Relays come in many sizes and ratings. There are literally tens of

thousands of relay varieties on the market.

NCD relay controllers allow you to switch electrical equipment from a computer via

RS232, USB, or Wireless communications. There are many advantages to using a computer

controlled relay controller. When the controlling computer is connected to the internet, relays

can be controlled from anywhere in the world.

Internet controlled relay switching allows a local computer or a remote computer to

activate a relay. Wireless relays have one additional advantage: wireless sensors can be used

to automatically activate a relay without computer intervention.

Relays typically have two or three connections: Common, Normally Open, and Normally

Closed. The Common is the part of the relay that actually makes a mechanical movement. By

default, many relays have their common (COM) lead connected to the normally closed lead

(NC). When the electromagnet is energized, the COM disconnects from the NC and


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reconnects to the Normally Open lead (NO). When the relay is deactivated, the COM

reconnects to the NC as shown fig 15.1.

Fig5.2 relay connection

relays often have two ratings: AC and DC. These rating indicate how much power can

be switched through the relays. This does not necessarily tell you what the limits of the relay

are. For instance, a 5 Amp relay rated at 125VAC can also switch 2.5 Amps at 250VAC.

Similarly, a 5 Amp relay rated at 24VDC can switch 2.5 Amps at 48VDC, or even 10 Amps at

12VDC.

Electromagnetic Relays:-

When a coil of wire is wound on a non magnetic material such as plastic, paperetc. ,it is called a air-core solenoid or simply a solenoid .if a soft iron core is inserted intothe coil, it becomes an electromagnet. this electromagnet is the basic component for
http://www.controlanything.com/wirelessintro/intro2.avi


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relay and many other electromechanical devices such as electric bell, circuit breakeretc,.

Operation

When a current flows through the coil, the resultingmagnetic field attracts an armature that is mechanically linkedto a moving contact. The movement either makes or breaks aconnection with a fixed contact. When the current to the coil isswitched off, the armature is returned by a force approximatelyhalf as strong as the magnetic force to its relaxed position.Usually this is a spring, but gravity is also used commonly inindustrial motor starters. Most relays are manufactured to operate quickly. In a lowvoltage application, this is to reduce noise. In a high voltage or high current application,this is to reduce arcing.

Pole & ThrowSince relays are switches, the terminology applied to switches is also applied to

relays. A relay will switch one or more poles, each of whose contacts can be thrown byenergizing the coil in one of three ways:

Normally-open (NO) contacts connect the circuit when the relay is activated; thecircuit is disconnected when the relay is inactive. It is also called a Form A contact or"make" contact.

Normally-closed (NC) contacts disconnect the circuit when the relay is activated;the circuit is connected when the relay is inactive. It is also called a Form B contact or"break" contact.

Change-over, or double-throw, contacts control two circuits: one normally-opencontact and one normally-closed contact with a common terminal. It is also called aForm C contact or "transfer" contact.

The following types of relays are commonly encountered:

SPST - Single Pole Single Throw. These have two terminals which can be connected ordisconnected. Including two for the coil, such a relay has four terminals in total. It isambiguous whether the pole is normally open or normally closed. The terminology"SPNO" and "SPNC" is sometimes used to resolve the ambiguity.

SPDT - Single Pole Double Throw. A common terminal connects to either of two others.Including two for the coil, such a relay has five terminals in total.


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DPST - Double Pole Single Throw. These have two pairs of terminals. Equivalent to twoSPST switches or relays actuated by a single coil. Including two for the coil, such arelay has six terminals in total. It is ambiguous whether the poles are normally open,normally closed, or one of each.

DIODE:

A pn junction diode consists of a pn junction formed either in germanium or silicon crystal.the

diode has two terminals namely anode and cathode. the anode refers to p-type region and the

cathode refers to the n-type region.

The most important characteristic of a pn juntion is its ability to conduct current in one

direction only. In other words it offers very high resistance in reverse direction. When no voltage

is applied across the diode, pn juntion will form due to the recombination of holes and electrons

as they move from one region to another. When pn juntion is forward biased the holes are

repelled by the positive terminal and electrons by negative terminal of the battery due to which

they acquire energy and penetrate through the depletion region, as a result the width of the

depletion region decreases and so does the potential barrier. Therefore, there will be large flow

of current from p to n.

When pn junction is reverse biased that is negative voltage is given to p and vice versa , then

holes in the p region are attracted to the negative terminal and electrons are attracted are attracted

to the positive region.


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Therefore depletion region widens and increases the barrier potential so only very small current

due to minority carrier flow trough the junction, which is known as the

reverse saturation current

TRANSISTORS:

On 23rd

December 1947 Walter H. Brattain (W.H. Brattain ) and john Bardeen of Beel

Telephone Laboratory in America announced the invention of a new electronic device called a

transistor or junction transistor or bipolar junction transistor (BJT). Now the transistor became

the heart of many electronic applications and replaced the vacuum tubes (not discussed in this

book). Since then, there has been a rapidly expanding effort to develop and utilize many types of

semiconductor devices such as FET, MOSFET, SCR, TRIAC, DIAC, and UJT etc. A transistor

has several advantages over vacuum tubes as mentioned below.

No heater is required. Hence no warming time and no heating power

Operates at low voltages

Physically smaller in size and lighter in weight

Have longer life

High efficiency due to low power consumption

Shock-proof since operates at low voltages

A transistor basically consists of two p-n junctions connected back-to-back.

OR

When a third doped element is added to a crystal diode, the resulting device is known as a

transistor.


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A transistor is a semiconductor device that can either amplify an electrical signal or act as

an electronic switch. Basically a transistor consists of a germanium or silicon crystal which

contains three separate regions. The three regions may consists of either two n-type regions

separated by a p-type region or two p-type regions separated by an n-type region.

The transistor is our most important example of an active component, a device that

can amplify, producing an output signal with more power in it than the input signal. The

additional power comes from an external source of power (the power supply, to be exact). Note

that voltage amplification isnt what matters, since, for example, a step -up transformer, a

passive component just like a resistor or capacitor, has voltage gain but no power gain.

Devices with power gain are distinguishable by their ability to make oscillators, by feeding some

output signal back into the input.

It is interesting to note that the property of power amplification seemed very important to

the inventors of the transistor. Almost the first thing they did to convince themselves that they

had really invented something was to power a loudspeaker from a transistor, observing that the

output signal sounded louder than the input signal.

The transistor is the essential ingredient of every electronic circuit, from the simplest

amplifier or oscillator to the most elaborate digital computer. Integrated circuits (ICs), which

have largely replaced circuits constructed from discrete transistors, are themselves merely arrays

of transistors and other components built from a single chip of semiconductor material.

A good understanding of transistors is very important, even if most of your circuits are made

from ICs, because you need to understand the input and output properties of the IC in order to


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connect it to the rest of your circuit and to the outside world. In addition, the transistor is the

single most powerful resource for interfacing, whether between ICs and other circuitry orbetween one sub circuit and another. Finally, there are frequent (some might say too frequent)

situations where the right IC just doesnt exist, and you have to rely on discrete transistor

circuitry to do the job. As you will see, transistors have an excitement all their own. Learning

how they work can be great fun

It has two p-n junctions or three semiconductor regions.

Three terminals (E,B,C) are taken from three regions.

The middle section is very thin

Three regions of the transistor are named as:

-Emitter (E)

-Base (B)

-Collector(C)

EMITTER

It form the left-hand section or region of the transistor as shown in Fig. It is heavily doped

than the other two regions. Its main function is to supply majority charge carriers* to the base.

The emitter is always forward biased w.r.t. base. The emitter of an n-p-n transistor supplies free

electrons where as a p-n-p transistor supplies holes to its junction with the base.

BASE


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It is the middle region of the transistor. It is very thin as compared to either emitter or

collector and is very lightly doped. The base-emitter junction is forward biased and the base-

collector junction is reverse biased.

COLLECTOR

It forms the right-hand section or region of the transistor as shown in fig. It is moderately

doped. Its main function is (as indicated by its name) to collect the majority charge carriers

coming from the emitter and passing through base. The collector is always reverse biased. In

most transistors, collector region is made physically larger than the emitter region because it has

to dissipate more heat.

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