The Case and Power Supply - With or Without a Power Supply?
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Many cases come with power supplies already mounted. This may sound like a bargain as well as a convenience, but as youíll see a little later in this chapter, you must take vast considerations into account when choosing a power supply. Itís not necessarily a good idea to let a case manufacturer make that choice for you.
As mentioned, cases and power supplies are often overlooked components of computers. Between the two, the power supply is the more overlooked, making it the most underappreciated piece of equipment in an entire PC. Thatís a crime, since itís so vitally important to the operation of the PC itself.
I tend to choose cases without power supplies so that I can purchase a power supply to meet my systemís individual needs. Itís probably a good bet for you to do the same, since youíre constructing a high-end, custom machine whose needs might not be fulfilled by a possibly generic and low-end power supply included with a case.
The Power Supply
Without a power supply, your computer would be nothing more than a doorstop. The power supply serves many purposes, from transforming the AC power fed into it into clean DC power for the PCís components to providing the proper current to each bit of equipment in the computer. Everythingóthe processor, the expansion bus, the BIOS, the motherboard chipset, the video and audio cards, the USB portsórely on the proper operation of the power supply.
That makes it a vital component of the computer and one not to be overlooked or taken for granted. The following sections explain the anatomy of a power supply, how power supplies are used, and how to select the right power supply for your computer (overkill is a good thing). They explain the difference between a good, high-quality power supply and a cheapie knock-off that youíll want to avoid. They also explain how to mount a power supply into your chassis.
Suffice it to say, itís worth it to spend a little more time and money selecting the best power supply possible. (Note that a power supply is often referred to as a PSU, for power supply unit.)
What a Power Supply Does
To the typical user, the power supplyís role is this: you plug your PC into the wall through it, flip the little switch (if there is one) to the ďonĒ position, and then turn on the computer. From that moment, the power supply is forgotten, unless it has a really noisy fanóin which case, the user turns up the speakers.
Despite its seemingly simple roll in this scenario, the power supply is doing far more than it would seem. Itís regulating current, transforming electricity from noisy AC to smooth DC, helping cool the system, and maybe even protecting the computer from damage caused by surges and brownouts.
The power supply furnishes the computer with the following voltages and devices powered:
Voltage Devices Powered
Motherboard chipset, PCI and AGP cards, memory, serial and parallel ports
Voltage regulators, disk-drive controllers, legacy items like ISA slots and SIMM memory, PCI and AGP cards, serial and parallel ports
Motors, voltage regulators
Legacy ISA slots
Legacy serial and parallel ports
Since youíll probably be working with current ATX equipment, you can pretty much ignore the negative voltages. Figure 1-4 shows an ATX power supply.
Figure 1-4 An ATX power supply
Figure 1-5 The rear of an ATX power supply
You can also ignore other legacies of older form factors, such as hot-toggle switches. An ATX power button doesnít do anything other than complete a logic circuit that tells the power supply to power up the system. Older form factors, such as AT, used actual switches in the front of the box that opened and closed an electrical circuit. Those switches had to be wired with a cable that stretched from the power supply to the front of the computer.
Current ATX power supplies (like the one shown in Figure 1-5) provide PS_ON (power on) and SVSB (SV Standby) current on a constant basis, unless theyíre shut off via the switch on the back or they are unplugged completely. This allows the PC to detect when the power-on circuit has been activated, and it also makes possible other functions, such as Wake on LAN, which allows remote programs on a network to wake the computer. This is often used by backup servers, maintenance programs implemented by a server, and other office stuff.
Along with supplying power to run a system, the power supplyís job is to ensure that itís providing good, clean, DC current thatís free of spikes, dips, and noise. It also makes sure that enough juice is provided to run the system. Thatís what the Power_Good signal is for. Most power supplies complete a series of internal checks and tests before powering up the system. If everything checks out, the power supply sends a signal to the motherboard called PWR_OK, or Power_Good, and that signal must be maintained continuously while the computer is on. If, for some reason, the normally 5V Power_Good signal fell outside its parameters (usually 2.5V to 6V), the power supply would cease to send the signal and the motherboard would reset the processor. Conditions that can cause the Power_Good signal to fail include brownouts, blackouts, and surges. Thus, the Power_Good signal is one way for the power supply to protect the PC from outside harm.
Beware that some low-priced, low-quality power supplies do not perform the necessary checks to maintain a proper Power_Good signal and instead simply send a 5V current through the circuit, thus ensuring that the motherboard thinks itís within normal parameters. Thatís one reason you shouldnít skimp on your purchase of a power supply.
This chapter is from Build Your Own High Performance Gamers' Mod PC, by Chen and Durham (McGraw-Hill/Osborne, 2004, ISBN: 0072229012). Check it out at your favorite bookstore today. Buy this book now.
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