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Network A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media. A computer network consists of a collection of computers, printers and other equipment that is connected together so that they can communicate with each other.  

Network application
A Network application is any application running on one host and provides a communication to another application running on a different host, the application may use an existing application layer protocols such as: HTTP(e.g. the Browser and web server), SMTP(e.g. the email-client). And may be the application does not use any existing protocols and depends on the socket programming to communicate to another application. So the web application is a type of the network applications. 
There are lots of advantages from build up a network, but the th…



In this lesson we will learn about basic electricity & about some important electrical components so that we can better understand the concept of electricity require for a pc & their let’s go ahead.

Current: Current is a flow of electrical charge carriers, usually electrons or electron-deficient atoms. The common symbol for current is the uppercase letter I. The standard unit is the ampere, symbolized by A. One ampere of current represents one coulomb of electrical charge (6.24 x 1018 charge carriers) moving past a specific point in one second. Physicists consider current to flow from relatively positive points to relatively negative points; this is called conventional current or Franklin current. Electrons, the most common charge carriers, are negatively charged. They flow from relatively negative points to relatively positive points. Electric current can be either direct or alternating. Direct current (DC) flows in the same direction at all points in time, although the instantaneous magnitude of the current might vary. In an alternating current (AC), the flow of charge carriers reverses direction periodically. The number of complete AC cycles per second is the frequency, which is measured in hertz.
Current is basically two types AC & DC. Let’s discuss about AC current & DC current in this segment. 

AC (Alternative Current) Alternating current (AC) occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement. 

DC (Direct Current) DC (direct current) is the unidirectional flow or movement of electric charge carriers (which are usually electrons). 


Amount of energy that can be carried
Safe to transfer over longer city distances and can provide more power.
Voltage of DC cannot travel very far until it begins to lose energy.
Cause of the direction of flow of electrons
Rotating magnet along the wire.
Steady magnetism along the wire.
The frequency of alternating current is 50Hz or 60Hz depending upon the country.
 The frequency of direct current is zero.
It reverses its direction while flowing in a circuit.
It flows in one direction in the circuit.
It is the current of magnitude varying with time
It is the current of constant magnitude.
Flow of Electrons
Electrons keep switching directions - forward and backward.
Electrons move steadily in one direction or 'forward'.

The basic difference between AC & DC 

SMPS Overview  
Switch Mode power Supply 

Introduced in 1960s with the Apple-II, SMPS are a light weight, economic and power efficient solution in regulating power supply to a computer. Switched mode power supplies are unanimously preferred as computer power supply units for their small size and high efficiency.  SMPS units dissipate less power by employing a transformer that regulates input voltage and provides constant current to the peripherals attached to the computer. The input voltage is chopped to different levels or is switched through different levels using a normal linear voltage supply can works at 30% efficiency, while an SMPS based power supply unit can provide 70-80% of efficiency.   

An SMPS also has the features of being light-weight and economical as compared to other power supplies. Also, the circuit topology of it can be varied depending upon the power requirements of the device to which it is attached. 

The electronic power supply integrated with the switching regulator for converting the electrical power efficiently from one form to another form with desired characteristics is called as Switch-mode power supply. It is used to obtain regulated DC output voltage from unregulated AC or DC input voltage. 

Topologies of Switch Mode Power Supply There are different types of topologies for SMPS, among those, a few are as follows 

  • DC to DC converter 
  • AC to DC converter 
  • Fly back converter 
  • Forward converter 

 DC to DC Converter SMPS Working Principle

In a DC-to-DC converter, primarily a high-voltage DC power is directly obtained from a DC power source. Then, this high-voltage DC power is switched at a very high switching speed usually in the range of 15 KHz to 50 KHz. And then it is fed to a step-down transformer which is comparable to the weight and size characteristics of a transformer unit of 50Hz. The output of the step-down transformer is further fed into the rectifier. This filtered and rectified output DC power is used as a source for loads, and a sample of this output power is used as a feedback for controlling the output voltage. With this feedback voltage, the ON time of the oscillator is controlled, and a closed-loop regulator is formed. 

DC to DC converter SMPS

The output of the switching-power supply is regulated by using PWM (Pulse Width Modulation). As shown in the circuit above, the switch is driven by the PWM oscillator, such that the power fed to the step-down transformer is controlled indirectly, and hence, the output is controlled by the PWM, as this pulse width signal and the output voltage are inversely proportional to each other. 
If the duty cycle is 50%, then the maximum amount of power is transferred through the step-down 
transformer, and, if duty cycle decreases, then the amount of power transferred will decrease by decreasing the power dissipation. 

AC to DC Converter SMPS Working Principle

The AC to DC converter SMPS has an AC input. It is converted into DC by rectification process using a rectifier and filter. This unregulated DC voltage is fed to the large-filter capacitor or PFC (Power Factor Correction) circuits for correction of power factor as it is affected. This is because around voltage peaks, the rectifier draws short current pulses having significantly high-frequency energy which affects the power factor to reduce. 

AC to DC  converter SMPS 

It is almost similar to the above discussed DC to DC converter, but instead of direct DC power supply, here AC input is used. So, the combination of the rectifier and filter, shown in the block diagram is used for converting the AC into DC and switching is done by using a power MOSFET amplifier with which very high gain can be achieved. The MOSFET transistor has low on-resistance and can withstand high currents. The switching frequency is chosen such that it must be kept inaudible to normal human beings (mostly above 20 KHz) and switching action is controlled by a feedback utilizing the PWM oscillator. This AC voltage is again fed to the output transformer shown in the figure to step down or step up the voltage levels. Then, the output of this transformer is rectified and smoothed by using the output rectifier and filter. A feedback circuit is used to control the output voltage by comparing it with the reference voltage 
  • Fly-back Converter type SMPS Working Principle 
The SMPS circuit with very low output power of less than 100W (watts) is usually of Fly-back converter type SMPS, and it is very simple and low- cost circuit compared to other SMPS circuits. Hence, it is frequently used for low-power applications. 
  • Forward Converter type SMPS Working 
Forward converter type SMPS is almost similar to the Fly-back converter type SMPS, but in the forward converter type, a control is connected for controlling the switch and at the output of the secondary winding of the transformer, and the rectification and filtering circuit is complicated  as compared to the fly-back converter. 

It can be called as a DC to DC buck converter, along with a transformer used for isolation and scaling. In addition to the diode D1 and capacitor C, a diode D2 and an inductor L are connected at the output end. If switch S gets switched ON, then the input is given to the primary winding of the transformer, and hence, a scaled voltage is generated at the secondary winding of the transformer.       

The SMPS essential Parts are: 
➣ Input rectifier 
➣ Inverter 
➣ Voltage converter 
➣ Output regulator 

Input rectifier

The AC input from mains is first rectified in the SMPS using a rectifier to convert it into DC. The rectifier consisting of a full wave diode bridge or module that produces an unregulated DC voltage to the Smoothing capacitor. The input AC passing into the rectifier has AC voltage pulses that may reduce the power factor. So control techniques are used to force the average input current to follow the sine wave. 


This stage converts the rectified DC into AC using a power oscillator. The power oscillator has a small output transformer with a few windings at the frequency 20-100 kHz. Switching is controlled by a MOSFET amplifier. The output AC voltage is usually isolated optically from the input AC by using an Opt coupler IC for safety reasons. 

Voltage converter 

This stage has a high frequency transformer and the inverted AC drives its primary windings. This creates the up and down voltage at the output. If DC is required, the output AC is converted to DC using a rectifier circuit using Silicon diodes (fast recovery and minimum loss of current and low forward voltage drop). The rectified output DC is then filtered using the filter section consisting of inductors and capacitors. Some non-isolated SMPS contains an inductor instead of the transformer and the circuit act as boost converter or buck converter. In high voltage SMPS, Capacitor-Diode multiplier is used instead of inductors or transformer. 

Output regulator 

The output stage always monitors the output voltage by comparing with a reference voltage using a feedback system. For safety reasons, the output stage is isolated by an opt isolator as seen in the SMPS of computers. In some SMPS, Open loop regulation is used without feedback circuit and constant voltage is fed to the transformer input.  

SMPS components 

Most of the components used in SWITCH MODE POWER SUPPLY (SMPS) can be easily identified. However, some components are difficult to identify. Some components are designed especially for a particular model. The different components of SWITCH MODE POWER SUPPLY (SMPS) are: 

Switch Mode (chopper) Transistors and other Semiconductors
  • Transistor: A transistor is small and fast semiconductor used for amplification and voltage stabilization. 
  • Bipolar Power Transistors: is High voltage power transistor, which is used as the main switch Mode transistor. 
  • Diodes and Rectifiers: AC line rectifier uses a combination of 2 or 4 diodes. High efficiency diodes are used to rectify output voltages. These diodes look like normal axial lead diodes. They can be combined in pairs inside TO220 type packages. To troubleshoot SWITCH MODE POWER SUPPLY (SMPS) problems, test for shorted and open junctions. 

Capacitors (filter and bypass) in an SWITCH MODE POWER SUPPLY (SMPS)
  • Filter Capacitors: Used to filter the rectified AC line input voltage and various outputs of the power supply. 
  • Bypass Capacitors: These are green colored, high quality plastic dipped or rectangular molded capacitors part of RFI filter. They are placed at output section and seldom fail. 
  • Resistors: Test for proper value with the help of millimeter. While measuring the resistor in-circuit, if the value is higher than normal value, then resistor is bad. 
  • Startup Resistors: They are high value (100K) medium wattage resistors. Test for open circuit results in dead supply without blown fuses or fusible resistors. 
  • Flameproof or Fusible Resistor: They are rectangular ceramic blocks which are blue or gray in color. They function as fuses; hence they cannot be replaced with higher wattage types. Test for open circuits. 
  • WTC Resistors (thermistors): Negative Temperature Coefficient resistors act as inrush surge limiters. There may be one or two of these in series with the AC input. These often look like fat black disk capacitors. 

Transformers and Inductors required in a SWITCH MODE POWER SUPPLY (SMPS) 

  • High Frequency Transformers: They consist of the switch Mode power transformer and feedback transformer. They provide line isolation and generate multiple output voltage. They rarely cause problems and can be tested for open circuit. Some supplies use small Transformers for feedback instead of opposite isolators. Opposite isolator is used to isolate the input line with the rest of the section. 
  • Inductors: Output inductors rarely fail and they can be tested for open circuit. Replace the faulty inductor if found. 
  • Coupled Inductors: They are used as a part of Pi type RFI filter which is placed in the AC input circuit. The windings of inductor are in series with AC line. They look like small transformer. They are very reliable and help to reduce noise and spikes. They can be tested for open circuit. 

Description & Details of SMPS Cables or Connectors: 

Here is the complete list and description of connectors or cables of a PSU that supply power to all your major hardware components of your PC

20 + 4 Pin ATX / Motherboard Connector 

This is the main ATX connector that supplies power to your motherboard and other components like RAM, Low end graphics card, PCI cards that are connected to your motherboard. Earlier motherboards have 20 pin sockets only but now modern day motherboards require 24-pin power connectors. Therefore nowadays all the SMPS comes with 24 pin detachable connector (20 + 4) that can be split into 20 pin and 4 pin cables.

CPU 4 + 4 Pin Connector

This is the 12 Volt connector that supply power to your CPU or Processor. Some motherboards have 4 pin 12V CPU socket and some have 8 pin 12V socket, therefore most of the good power supplies come with 8 pin (4 + 4) detachable connectors which can be split into two 4 pin connectors. 

SATA Power Connector

This power connector is used to power up SATA devices that mostly includes Hard Disks, DVD Writers / Drives. 

Floppy 4 Pin Connector

This 4 pin power connector is used to power floppy drives. Floppy Drives are not being used commonly, but in some cases they can prove very useful.

Peripheral 4 Pin Molex Connector

Peripheral 4 Pin Molex Connector is used to provide power to IDE devices and other peripherals like Fans, case lights etc. You can also use this Molex 4 pin connector to power other devices like SATA devices, Graphics cards etc. by the use of converters or adapters e.g. Molex to SATA power connector etc. 

PCI-e 6 Pin / PCI-e 8 Pin Connector

PCI-e 6 Pin or PCI-e 6+2 Pin are 12 Volt Connectors that are used to provide power to mid to high range graphics cards that requires extra amount of power for their working. Budget and low-end graphics card do not need them as they draw their power from the motherboard PCI-Express x16 slot only. Older mid to high end graphics card comes with PCI-e 6 Pin slot while the newer high end graphics comes with PCI-e 8 Pin slot. The new PCI Express 8 Pin cable can provide maximum of 150 Watts power which is the double that of the PCI Express 6 Pin cable i.e. 75 Watts. 

SMPS color code: 

The most common computer power supplies are built to conform to the ATX form factor. This enables different power supplies to be interchangeable with different components inside the computer. The SMPS has so many wires & all of them providing different power now let’s see the various Voltages of SMPS wires. 

SMPS Testing 

Testing a SMPS is the first thing we check when we troubleshooting a dead computer and if you having a dead computer, there will be a 90% of chance your power supply would be faulty. To check a SMPS outside from computer is a very easy task to do you doesn’t needs any technical or electrical knowledge to perform this testing thing, you’ll just need a screw driver and a paper clip that’s it.

 Step 1  Unplug power connections. 
The first step is to do is unplug all the power connection to the computer, make sure your computer is turned off completely. 

Step 2   Open computer case 

Open your computer case by removing some of screws at the left side of computer case. You’ll just need to remove only one side of case. 

Step 3  Unplug PSU connection. 

Unplug all the connection from the PSU (SMPS) to all internal hardware devices. Most of connections contain clip attachment so make sure you lose the clip before removing any connection. 

Name of some connections to remove:- 

1. 24 Pin ATX from Motherboard. 2. 4 Pin ATX from Processor. 3. SATA Power Connector from Hard Disk or DVD-R/W. 4. 4-6 Pin Power Connector from Graphic Card (if attached). 

Step 4  Make a “U” shape metal wire. 

After the SMPS connection was removed, make a “U” shape of metal wire or you can use a paper clip by bending it to a “U” shape. 

Step 5   Test the SMPS.

First find the 20 or 24 pin ATX connector from your SMPS and locate the green and black wire connection. Note that there is only one green wire. 

After finding the connection, attach the power cord to the SMPS and insert the metal wire into the green and black wire connector and short the circuit to test the SMPS is working or not. 

If the SMPS fan runs perfectly without any of lags and stoppage that mean your SMPS is working fine. If your fan doesn’t move or create movement for a sec and stops that mean you have a faulty SMPS by your side so, replace it with a new SMPS. 


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