Introduction to the Personal Computer

Cases and Power Supplies 
The computer case is a vital component of the computer system; it provides both protection and cooling for the internal components. The size, location of the I/O ports, and number of internal drive bays are a few attributes to consider when buying a case. It needs to be compatible with the form factor of the motherboard in addition to the power supply.

Computer Cases
 The computer case contains the framework to support the internal components of a computer while providing an enclosure for added protection. Computer cases are usually made of plastic, steel, or aluminum and are available in a variety of styles. In addition to providing protection and support, cases also provide an environment designed to keep the internal components cool. Case fans move air through the computer case. As the air passes warm components, it absorbs heat and then exits the case. This process keeps the computer components from overheating. Cases also help to prevent damage from static electricity. The computer’s internal components are grounded via attachment to the case. All computers need a power supply to convert alternating current (AC) power from the wall socket into direct current (DC) power. Every computer also needs a motherboard. The motherboard is the main circuit board in a computer. The size and shape of the computer case are usually determined by the motherboard, power supply, and other internal components. The size and layout of a case is called the form factor. The basic form factors for computer cases include desktop and tower. Desktop cases can be slimline or full-sized. Tower cases can be mini or full-sized.

Figure 1-1 Types of Computer Cases You can select a larger computer case to accommodate additional components that may be required in the future. Or you might select a smaller case that requires minimal space. In general, the computer case should be durable, be easy to service, and have enough room for expansion. Computer cases are referred to in a number of ways:
󲐀 Computer chassis
󲐀 Cabinet
󲐀 Tower
󲐀 Box
󲐀 Housing You need to consider several factors when choosing a case:
󲐀 Size of the motherboard
󲐀 Number of external or internal drive locations, called bays
󲐀 Available space

Power Supplies
The power supply must provide enough power for the components that are currently installed and allow for other components that may be added at a later time. If you choose a power supply that powers only the current components, you might need to replace the power supply when other components are upgraded. The power supply, converts alternating current (AC) power coming from a wall outlet into direct current (DC) power, which is a lower voltage. DC power is required for all components inside the computer. There are three main form factors for power supplies, Advanced Technology (AT), AT Extended (ATX), and ATX12V. The ATX12V is the most common form factor used in computers today

A computer can tolerate slight fluctuations in power, but a significant deviation can cause the power supply to fail. An uninterruptible power supply (UPS) can protect a computer from problems caused by changes in power. A UPS uses a power inverter. A power inverter provides AC power to the computer from a built-in battery by converting the DC current of the UPS battery into AC power. This built-in battery is continually charged via DC current that is converted from the AC supply.  Connectors Most connectors today are keyed. A keyed connector is designed to be inserted in only one direction. Each power supply connector uses a different voltage, as described in Table 1-2. Different connectors are used to connect specific components to various ports on the motherboard: 

󲐀 A Molex keyed connector connects to optical drives, hard drives, or other devices that use older technology. 

󲐀 A Berg keyed connector connects to a floppy drive. A Berg connector is smaller than a Molex connector. 

󲐀 A SATA keyed connector connects to an optical drive or a hard drive. The SATA connector is wider and thinner than a Molex connector. 

󲐀 A 20-pin or 24-pin slotted connector connects to the motherboard. The 24-pin connector has two rows of 12 pins each, and the 20-pin connector has 2 rows of 10 pins each. 

󲐀 A 4-pin to 8-pin auxiliary power connector has 2 rows of 2 to 4 pins and supplies power to all areas of the motherboard. The auxiliary power connector is the same shape as the main power connector but smaller. It can also power other devices within the computer. 

󲐀 A 6/8-pin PCIe power connector has 2 rows of 3 to 4 pins and supplies power to other internal components. Older standard power supplies used two connectors called P8 and P9 to connect to the motherboard. P8 and P9 were unkeyed connectors. They could be installed backward, potentially damaging the motherboard or power supply. The installation required that the connectors be lined up with the black wires together in the middle. Note If you have a difficult time inserting a connector, try repositioning it, or check to make sure that no bent pins or foreign objects are in the way. If it is difficult to plug in a cable or other part, something is wrong. Cables, connectors, and components are designed to fit together snugly. Never force a connector or component. If a connector is plugged in incorrectly, it can damage the plug and the connector. Take your time and make sure that you are handling the hardware correctly.  

Voltage Wire Color Use +12V Yellow Disk drive motors, fans, cooling devices, and system bus slots –12V Blue Some serial port circuits and early PROMs +3.3V Orange Modern CPU, some memory, and AGP video cards +5V Red Motherboard, early processors, many motherboard components –5V White ISA bus cards and early PROMs 0V Black Ground (the return loop for circuits)  

Electricity and Ohm’s Law (1.1.1.3) These are the four basic units of electricity: 

󲐀 Voltage (V) 

󲐀 Current (I) 

󲐀 Power (P) 

󲐀 Resistance (R) 

Voltage, current, power, and resistance are electronic terms that a computer technician must know. 

󲐀 Voltage is a measure of the force required to push electrons through a circuit. Voltage is measured in volts (V). A computer power supply usually produces several different voltages. 

󲐀 Current is a measure of the amount of electrons going through a circuit. Current is measured in amperes, or amps (A). Computer power supplies deliver different amperages for each output voltage. 󲐀 Power is a measure of the pressure required to push electrons through a circuit (voltage), multiplied by the number of electrons going through that circuit (current). The measurement is called watts (W). Computer power supplies are rated in watts. 

󲐀 Resistance is the opposition to the flow of current in a circuit, measured in ohms. Lower resistance allows more current, and therefore more power, to flow through a circuit. A good fuse has low resistance or almost 0 ohms. A basic equation, known as Ohm’s law, expresses how three of the terms relate to each other. It states that voltage is equal to the current multiplied by the resistance: V = IR In an electrical system, power is equal to the voltage multiplied by the current: P = VI In an electrical circuit, increasing the current or the voltage results in higher power.  

You can use Ohm’s triangle , shown in Figure 1-3, to calculate voltage, current, or resistance when two of the variables are known. To see the correct formula, cover up the variable that is not known and perform the resulting calculation. For example, if voltage and current are known, cover the R to reveal the formula V / I. Calculate V / I to find R. You can use the Ohm’s law chart shown in Figure 1-3 to calculate any of the four basic units of electricity using any two known units

Computers normally use power supplies ranging from 250W to 800W output capacity. However, some computers need 1200W and higher-capacity power supplies. When building a computer, select a power supply with sufficient wattage to power all components. Each component inside the computer uses a certain amount of power. Obtain the wattage information from the manufacturer’s documentation. When deciding on a power supply, choose one that has more than enough power for the current components. A power supply with a higher wattage rating has more capacity; therefore, it can handle more devices. On the back of most power supplies is a small switch called the voltage selector switch . This switch sets the input voltage to the power supply to either 110V/115V or 220V/230V. A power supply with this switch is called a dualvoltage power supply. The correct voltage setting is determined by the country where the power supply is used. Setting the voltage switch to the incorrect input voltage could damage the power 

supply and other parts of your computer. If a power supply does not have this switch, it automatically detects and sets the correct voltage. Caution Do not open a power supply. Electronic capacitors located inside of a power supply, shown in Figure 1-4, can hold a charge for extended periods of time. The power supply is considered to be a field replaceable unit (FRU), which means that if it no longer works, don’t fix it, but instead replace it. 

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