Estimating Required Capacity

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Computer power supplies aren't infinite founts of power for a computer. It's possible to overload a power supply, which can lead to erratic behavior. Each power supply also has a limited number of internal power connectors, and, if you run out of these connectors, it's necessary to add more by using special power splitter cables.

Power Supply Electrical Capacity

To understand power supply capacity, it's important to start with a fundamental relationship between three measures related to electricity: volts (v), watts (W), and amperes (amps). This relationship is

Watts = Volts x Amps

For instance, if a 5v device draws 10 amps, it consumes 50 watts. Power supply capacity is measured in watts. For instance, a power supply might be rated at 235W. To understand how this translates into volts and amps, however, it's necessary to know the voltage drawn by the devices in question. In fact, various devices attached to modern computers each draw different quantities of power in 3.3v, 5v, and 12v amounts. To understand the capacity of a power supply, therefore, it's necessary to understand how many watts or amps it can provide at any given voltage. For example, a power supply might be capable of providing 14 amps at 3.3v, 22 amps at 5v, and 8 amps at 12v, for a total of 252.2W. Many power supplies, however, include a maximum combined output for combinations of voltages. For instance, there might be a 125W maximum for 3.3v and 5v operation, resulting in an adjusted total wattage of 221W.

If you want to be meticulous about acquiring an adequate power supply, you should seek out specifications on the power requirements of every component in your computer. Unfortunately,

Chapter 4

such information is hard to come by for some components, such as most expansion cards. Table 4.1 summarizes typical power consumption required by various types of devices.

Table 4.1 Power Consumption in Amps for Various Devices

Device

+3.3v

+5v

+12v

ISA card

0

2.0

0.175

EISA card

0

4.5

1.5

16-bit MCA card

0

1.6

0.175

32-bit MCA card

0

1.6

0.175

VL-Bus card

0

2.0

0

PCI card

7.6

5

0.5

3.5-inch floppy disk

0

1.5

1.5

3.5-inch hard disk

0

0.5

1.0

CD-ROM drive

0

1.0

1.0

Fan

0

0

0.1

Expansion card figures are maximums; those for other devices are typical values.

Expansion card figures are maximums; those for other devices are typical values.

The motherboard's power draw varies with the CPU. Note that voltage regulators, which are required when using anything but one of the voltage levels provided by the power supply, result in some loss, so actual watts consumed is greater than you would expect by multiplying volts by amps.

As an example, consider a computer with one ISA card, two PCI cards, a 3.5-inch floppy disk, two 3.5-inch hard disks, a CD-ROM drive, and two extra cooling fans. Adding up these values, 4

this computer requires 50.16W from 3.3v, 77.5W from 5v, and 70.5W from 12v, for a total of -

198.16W. The motherboard consumes a few more watts, as well—perhaps 25W or so, depending on the board and CPU. S n

Of course, not all devices require their maximum amounts of power at the same time. Typically, for instance, hard disks require a great deal of power when they start up, to overcome the inertia inherent in stopped disk platters. A tape backup unit consumes little power unless it's in operation; likewise, for a floppy disk. This fact means that a power supply can be adequate most of the time but still fail at certain times, such as when the hard disks power up or when you use a tape drive and CD-ROM drive simultaneously. If you experience problems when you use a device, it's possible that an inadequate power supply is to blame.

If you have several SCSI hard disks, you can reduce the startup power drain by configuring your SCSI drives to start up only when they're told to do so by the SCSI host adapter. Most drives can be configured in this way with a jumper setting, typically called remote start or something similar. The drives then power up after the motherboard has finished its initial tests, and the SCSI host adapter signals each drive to start in sequence, so they don't all start up at once.

As a general rule of thumb, a 235W power supply is adequate for small systems. If you add more than a handful of disk devices or loads of high-drain expansion cards, you might need to boost the capacity of your power supply. 300W and 400W power supplies are available for ATX systems, and are quite desirable for tower systems filled with devices.

Power Connecting Cables

One other aspect of power supply capacity is the number and types of power connect cables provided by the power supply. Most devices, including hard disks, CD-ROM drives, and tape backup drives, use standardized four-wire power connectors that are keyed to plug into the device in only one way. 3.5-inch floppy drives use smaller connectors, and motherboards use either one or two connectors, depending upon the type of motherboard.

Caution

AT and Baby AT motherboards use a pair of power connectors that must be inserted into the motherboard with the black wires next to each other. If you insert the power connectors backward, you can destroy the motherboard!

If you add more devices than the power supply has connectors, you can use one or more power splitters (shown in Figure 4.11) to increase the number of connectors. Instead of inserting a power supply power plug into a disk drive or other device, you plug it into the splitter, which then lets you deliver power to two devices rather than just one.

While internal power splitters can be extremely useful things, you should not over-use them, for two reasons:

• If you need lots of power splitters, that might be a sign that you're exceeding the capacity of the power supply. One or two splitters probably does not constitute a problem, but if you need half a dozen, it might be time to consider a more capable power supply.

Chapter 4

Each power splitter represents a potential failure point. If the power splitter's connection is weak, both attached devices can fail, sometimes mysteriously. I once traced a strange hard disk problem to a flaky power splitter.

Figure 4.11

A power splitter lets you add devices to a computer even if the power supply doesn't have enough power leads.

Figure 4.11

A power splitter lets you add devices to a computer even if the power supply doesn't have enough power leads.

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