Standard voltages

Standard voltages

Tip Know this stuff for the test. The power supply provides the following voltages to the motherboard and drives:

l +5V: The standard voltage of motherboards with all processors below 100 MHz (early Pentium, 486, 386, and so on) and many peripheral boards.

l +12V: Used primarily for disk drive motors and similar devices. Modern motherboards also pass this voltage to ISA bus expansion slots.

l –5V and –12V: Included in most power supplies for compatibility with older systems. Most modern motherboards don't use either of these voltages. Power supplies that produce these values do so at very low (less than 1 amp) amperage. Check the label on your power supply.

l +3.3V: The standard voltage level for motherboards for 100 MHz processors and above (Pentium, Pentium Pro, and equivalent chips run at 3.3V (some use even lower voltages internally).

Upgraded motherboards must convert the 5V signal from the power supply into 3.3V for the processor, requiring a voltage regulator on the motherboard. Newer power supplies provide the 3.3V power for the CPU directly.

Time Shaver Concentrate on which voltages are used with which types of devices, especially the voltage of all Pentium-class processors, listed in Table. For example, disk drives use the +12V lines, the Pentium processor uses 3.3V, and the other lines (+/–5V and –12V lines) are primarily used for backward compatibility.

Processor voltages

Processors actually have two levels of power that they use. The first, which is called external voltage, or I/O voltage is the voltage used to power the devices mounted to motherboards compatible with a certain processor. The second level of power for a processor is its internal voltage. The internal voltage is important in the sense that the higher this number, the more heat the processor generates. As you study the voltages listed in Table 10-1 for the Pentium-class processors, you'll notice a trend of smaller internal voltage levels.

Fitting it in the box

Form factor refers to the shape and dimensions of a device. PC cases are designed to hold a particular power supply form factor. The power supply must match the designed form factor of the case as well as match its power to the motherboard. The form factor of a power supply is not often an issue except for upgrades and build-your-owns, because the power supply is usually purchased already installed in the case. The form factor of the case is usually more of an issue.

In general, the form factors of the motherboard are the same for the case and the power supply. The most common form factors used today are the Baby AT, the oldest standard that has been used for PCs

Processor Voltage Levels