Form Factors
Motherboards adhere to design specifications called form factors. The form factor determines the physical characteristics of a motherboard, including its dimensions, number of expansion slots, and mounting hole locations, as well as the back panel dimensions, arrangement, and orientation.
ATX
The ATX (advanced technology extended) form factor is the most commonly used form factor. Because of its popularity, several variants of the ATX form factor exist. Each variant has different specifications for dimensions and number of expansion slots. However, all ATX variants share the following characteristics:
Standard ATX
The standard ATX form factor is the form factor that all other variants are modeled after. ATX motherboards:
Extended ATX
(EATX)
The EATX form factor is the largest ATX variant. EATX:
microATX
The microATX form factor is a smaller version of the ATX form factor. The microATX form factor:
ITX
The ITX form factor was designed for low-power, small form factor (SFF) computers. The most common ITX form factor is the Mini-ITX form factor. The Mini-ITX form factor:
Other ITX form factors include the following:
The Mini-ITX form factor uses the same mounting locations and back panel specifications as the ATX form factor, allowing Mini-ITX motherboards to fit in ATX cases.
BTX
The BTX (balanced technology extended) form factor was designed as a replacement for the ATX form factor. However, it did not gain widespread adoption. With BTX:
BTX was implemented mainly by computer manufacturers such as Dell
ATX Full-tower
ATX full-tower cases are the largest computer cases. Full-tower cases have a lot of space for external and internal components. ATX full-tower cases are compatible with the following form factors:
ATX Mid-tower
ATX mid-tower cases are slightly smaller than full-tower cases. Mid-tower cases have fewer external and internal bays. ATX mid-tower cases are compatible with the following form factors:
microATX Tower
microATX towers are smaller cases designed to be placed on desktops. microATX towers typically have only one drive bay and are compatible with the following form factors:
Some microATX towers have a slim design. These cases are typically half the width of a microATX tower and are designed to lie flat or upright.
Mini-ITX Tower
Mini-ITX towers are designed to house mini-ITX motherboards. They are typically smaller than microATX towers.
HTPC
Home theatre PC (HTPC) cases are designed to connect to TVs and be used as a home media computer. HTPC cases are compatible with microATX and Mini-ITX form factors.
Notebook
Notebook cases are generally proprietary and often vary among models.
When you purchase a computer case, it will usually come with the following components:
Power supplies perform the following functions:
Older ATX units use a reverse air flow that blows air directly over the CPU. This method is not as efficient
You should be aware of the following facts about power supplies
Most modern power supplies eliminate the voltage switch and instead automatically switch between voltages as necessary. These power supplies automatically adjust to accept input voltages in the range of 100 to 240 volts.
Alternatively, there are several online tools you can use to estimate a computer’s watt requirements.
Name and describe the connector

24-pin (20+4 pin) ATX connector
The 24-pin ATX power plug supplies power to the motherboard.
Older motherboards used 20-pin power plugs. With a 24-pin ATX power plug, the four extra pins supply an additional 3.3, 5, and 12 volts of DC power.
Name and describe the connector

4-pin 12 V (P4) power
Starting with the Pentium 4 (P4) processor, CPUs required more power than could be provided through the ATX power plug. The 4-pin 12 V connector:
The 4-pin 12 V CPU connector is not the same as the 20+4-pin ATX power connector.
Name and describe the connector

8-pin EPS12V CPU power
Modern processors consume even more power. The 8-pin EPS12V connector provides four lines of 12 V power.
Some power supplies have two 4-pin connectors (4+4) that are meant to be used side-by-side in the 8-pin plug.
Name and describe the connector

6+2-pin PCIe
Newer video cards require more power than can be supplied through the PCI Express bus. The 6+2-pin PCIe connector plugs directly into the video card to supply additional, dedicated power. The 6+2-pin PCIe:
Some motherboards have only a 6-pin PCIe connector. These connectors provide up to 75 watts.
Name and describe the connector

4-pin peripheral power
The 4-pin peripheral power connector (colloquially called a 4-pin Molex connector) is used by legacy components (e.g., IDE hard drives and PATA optical drives), case fans, and other accessory devices. The connector provides both 5 V (red wire) and 12 V (yellow wire).
Name and describe the connector

SATA power
The SATA power connector has 15 pins and provides 3.3, 5, and 12 volts. As its name implies, it powers SATA devices.
Name and describe the connector

4-pin mini-Molex
The 4-pin mini-Molex connector provides both 5 and 12 volts and is used by floppy drives.
Most modern power supplies do not have a 4-pin mini-Molex connector.
When troubleshooting a power supply, keep the following in mind
troubleshooting a power supply, keep the following in mind:
Some computer manufacturers, such as Dell or HP, produce proprietary power supplies. These power supplies might have a unique shape or use different wiring schematics on connectors. When replacing a power supply, identify whether a standard ATX or a proprietary power supply is required.