Upgrading and Repairing PCs

Power-Use Calculations

When expanding or upgrading your PC, you should ensure that your power supply is capable of providing sufficient current to power all the system's internal devices. One way to see whether your system is capable of expansion is to calculate the levels of power drain in the various system components and deduct the total from the maximum power supplied by the power supply. This calculation can help you decide whether you must upgrade the power supply to a more capable unit. Unfortunately, these calculations can be difficult to make because many manufacturers do not publish power consumption data for their products.

In addition, getting power-consumption data for many +5V devices, including motherboards and adapter cards, can be difficult. Motherboards can consume different power levels, depending on numerous factors. Most motherboards consume about 5 amps or so, but try to get information on the one you are using. For adapter cards, if you can find the actual specifications for the card, use those figures. To be on the conservative side, however, I usually go by the maximum available power levels set forth in the respective bus standards.

For example, consider the power-consumption figures for components in a modern PC, such as a desktop or Slimline system with a 200-watt power supply rated for 20 amps at +5V and 8 amps at +12V. The ISA specification calls for a maximum of 2.0 amps of +5V power and 0.175 amps of +12V power for each slot in the system. Most systems have eight slots, and you can assume that four are filled for the purposes of calculating power draw. The calculation shown in Table 21.15 shows what happens when you subtract the amount of power necessary to run the various system components.

Table 21.15. Power Consumption Calculation

	Available 5V Power (Amps):	20.0A		Available 12V Power (Amps):	8.0A
Less:	Motherboard	–5.0A	Less:	4 slots filled at 0.175 each	–0.7A
	4 slots filled at 2.0 each	–8.0A		3 1/2'' hard disk drive motor	–1.0A
	3 1/2'' floppy drive logic	–0.5A		3 1/2'' floppy drive motor	–1.0A
	3 1/2'' hard disk drive logic	–0.5A		Cooling fan motor	–0.1A
	CD-ROM/DVD drive logic	–1.0A		CD-ROM/DVD drive motor	–1.0A
	Remaining Power (Amps):	5.0A		Remaining Power (Amps):	4.2A

In the preceding example, everything seems okay for now. With half the slots filled, a floppy drive, and one hard disk, the system still has room for more. Problems with the power supply could come up, however, if this system were expanded to the extreme. With every slot filled and two or more hard disks, problems with the +5V current definitely would occur. However, the +12V does seem to have room to spare. You could add a CD-ROM drive or a second hard disk without worrying too much about the +12V power, but the +5V power would be strained.

If you anticipate loading up a system to the extreme—as in a high-end multimedia system, for example—you might want to invest in the insurance of a higher-output power supply. For example, a 250-watt supply usually has 25 amps of +5V current and 10 amps of +12V current, whereas a 300-watt unit usually has 32 amps of +5V current available. These supplies enable you to fully load the system and are likely to be found in full-sized desktop or tower case configurations in which this type of expansion is expected.

Motherboards can draw anywhere from 4 to 15 amps or more of +5V power to run. Depending on which processor is installed, the figure can go higher. As an example, the AMD Athlon XP 3000+ (2.167GHz) processor draws up to 74.3 watts of 1.65V power, which is supplied by motherboard-based voltage regulators. These regulators are powered by +5V in most Socket A motherboards, so assuming an 80% (typical) efficiency in the regulator power conversion, this would equate to consuming 18.57 amps of +5V power. This, plus the power draw from the rest of the motherboard, would put the +5V power draw near the 24A–30A maximum allowed by the power supply connectors.

By comparison, the 3.06GHz version of the Pentium 4 draws up to 81.8 watts of 1.55V power. Intel wisely decided that Pentium 4 systems were power hungry enough that the motherboard-based voltage regulators should run on +12V instead of +5V. Again, assuming an 80% efficiency, the regulators would require 20.45 amps if running on +5V but only 8.52 amps if running on +12V. By moving the regulators to the 12V power, the load on the 5V is greatly eased. I assume Socket A motherboards will also make the move to 12V regulators due to the better balanced design. If your motherboard has the ATX12V connector, you can be assured that the CPU voltage regulator module is running off 12V and not 5V.

Considering that systems with two or more processors are now becoming common, you could have up to 40 amps of 5V (using 5V-powered CPU voltage regulators) or up to 16 amps of 12V power being drawn by the processors alone. A dual processor system using 5V-based regulators would almost certainly exceed the 30A maximum allowable power draw through the ATX main and auxiliary connectors! Add in the power draw for the motherboard, video card, memory, and disk drives, and you'll see that very few "no-name" power supplies can supply this kind of current. For these applications, you should consider only high-quality, high-capacity power supplies from a reputable manufacturer, such as PC Power and Cooling.

In these calculations, bus slots are allotted maximum power in amps, as shown in Table 21.16.

Table 21.16. Maximum Power Consumption in Amps per Bus Slot

Bus Type	+5V Power	+12V Power	+3.3V Power
ISA	2.0	0.175	n/a
EISA	4.5	1.5	n/a
VL-bus	2.0	n/a	n/a
16-bit MCA	1.6	0.175	n/a
32-bit MCA	2.0	0.175	n/a
PCI	5	0.5	7.6

As you can see from the table, ISA slots are allotted 2.0 amps of +5V and 0.175 amps of +12V power. Note that these are maximum figures; not all cards draw this much power. If the slot has a VL-bus extension connector, an additional 2.0 amps of +5V power are allowed for the VL-bus.

Floppy drives can vary in power consumption, but most of the newer 3 1/2'' drives have motors that run on +5V current in addition to the logic circuits. These drives usually draw 1.0 amp of +5V power and use no +12V at all. 5 1/4'' drives use standard +12V motors that draw about 1.0 amp. These drives also require about 0.5 amps of +5V for the logic circuits. Most cooling fans draw about 0.1 amps of +12V power, which is negligible.

Typical 3 1/2'' hard disks today draw about 1 amp of +12V power to run the motors and only about 0.5 amps of +5V power for the logic. 5 1/4'' hard disks, especially those that are full-height, draw much more power. A typical full-height hard drive draws 2.0 amps of +12V power and 1.0 amp of +5V power.

Another problem with hard disks is that they require much more power during the spin-up phase of operation than during normal operation. In most cases, the drive draws double the +12V power during spin-up, which can be 4.0 amps or more for the full-height drives. This tapers off to a normal level after the drive is spinning.

The figures that most manufacturers report for maximum power supply output are full duty-cycle figures. The power supply can supply these levels of power continuously. You usually can expect a unit that continuously supplies a given level of power to be capable of exceeding this level for some noncontinuous amount of time. A supply normally can offer 50% greater output than the continuous figure indicates for as long as 1 minute. Systems often use this cushion to supply the necessary power to spin up a hard disk drive. After the drive has spun to full speed, the power draw drops to a value within the system's continuous supply capabilities. Drawing anything over the rated continuous figure for any extended length of time causes the power supply to run hot and fail early, and it can create nasty symptoms in the system.

Tip If you are using internal SCSI hard drives, you can ease the startup load on your power supply. The key is to enable the SCSI drive's Remote Start option, which causes the drive to start spinning only when it receives a startup command over the SCSI bus. The effect is that the drive remains stationary (drawing very little power) until the very end of the POST and spins up right when the SCSI portion of the POST begins. If you have multiple SCSI drives, they all spin up sequentially based on their SCSI ID settings. This is designed so that only one drive is spinning up at any one time and so that no drives start spinning until the rest of the system has had time to start. This greatly eases the load on the power supply when you first power the system on. In most cases, you enable Remote Start through your SCSI host adapter's setup program. This program might be supplied with the adapter on separate media, or it might be integrated into the adapter's BIOS and activated with a specific key combination at boot time.

The biggest cause of power supply overload problems has historically been filling up the expansion slots and adding more drives. Multiple hard drives, CD-ROM drives, and floppy drives can create quite a drain on the system power supply. Be sure you have enough +12V power to run all the drives you plan to install. Tower systems can be especially problematic because they have so many drive bays. Just because the case has room for the devices doesn't mean the power supply can support them. Be sure you have enough +5V power to run all your expansion cards, especially PCI cards. It pays to be conservative, but remember that most cards draw less than the maximum allowed. Today's newest processors can have very high current requirements for the +5V or +3.3V supplies. When selecting a power supply for your system, be sure to take into account any future processor upgrades.

Many people wait until an existing component fails to replace it with an upgraded version. If you are on a tight budget, this "if it ain't broke, don't fix it" attitude might be necessary. Power supplies, however, often do not fail completely all at once; they can fail in an intermittent fashion or allow fluctuating power levels to reach the system, which results in unstable operation. You might be blaming system lockups on software bugs when the culprit is an overloaded power supply. In addition, an inadequate or failing supply causing lockups can result in file system corruption, which causes even further system instabilities (that could remain present even after replacing the power supply). If you use bus-powered USB devices, a failing power supply can also cause these devices to fail or malfunction. If you have been running your original power supply for a long time and have upgraded your system in other ways, you should expect some problems, and you might want to consider reloading the operating system and applications from scratch.

Although there is certainly an appropriate place for the exacting power-consumption calculations you've read about in this section, a great many experienced PC users prefer the "don't worry about it" power calculation method. This technique consists of buying or building a system with a good-quality 300-watt or higher power supply (or upgrading to such a supply in an existing system) and then upgrading the system freely, without concern for power consumption.

Tip My preference is the 425W supply from PC Power and Cooling, which is probably overkill for most people, but for those who keep a system for a long time and put it through several upgrades, it is an excellent choice.

Unless you plan to build a system with arrays of SCSI drives and a dozen other peripherals, you will probably not exceed the capabilities of the power supply, and this method certainly requires far less effort.