The power supply is a critical component in a PC, as it
supplies electrical power to every component in the system. In
my experiences, it is also one of the most failure-prone
components in any computer system. Because of its importance
to proper and reliable system operation, you should understand
both the function and limitations of a power supply, as well
as its potential problems and their solutions.
Power Supply Function and
Operation
The basic function of the power supply is to convert the
type of electrical power available at the wall socket to that
which is usable by the computer circuitry. The power supply in
a conventional desktop system is designed to convert the
120-volt, 60Hz, AC current into something the computer can
use--specifically, +5- and +12v DC current, and +3.3v as well
on some systems. Usually, the digital electronic components
and circuits in the system (motherboard, adapter cards, and
disk drive logic boards) use the 3.3v or +5v power, and the
motors (disk drive motors and any fans) use the +12v power.
The power supply must ensure a good, steady supply of DC
current so that the system can operate properly.
If you look at a specification sheet for a typical PC power
supply, you see that the supply generates not only +5v and
+12v, but also -5v and -12v. Because it would seem that the
+5v and +12v signals power everything in the system (logic and
motors), what are the negative voltages used for? The answer
is, not much! In fact, these additional negative voltages are
not used at all in many modern systems, although they are
still required for backwards compatibility.
Although -5v and -12v are supplied to the motherboard via
the power supply connectors, the motherboard itself uses only
the +5v. The -5v signal is simply routed to the ISA bus on pin
B5 and is not used in any way by the mother-board. It was
originally used by the analog data separator circuits found in
older floppy controllers, which is why it was supplied to the
bus. Because modern controllers do not need the -5v, it is no
longer used but is still required because it is part of the
ISA Bus standard.
NOTE: Power supplies in systems with a Micro
Channel Architecture (MCA) Bus do not have -5v. This power
signal was never needed in these systems, as they always
used a more modern floppy controller design.
Both the +12v and -12v signals also are not used by the
motherboard logic, and instead are simply routed to pins B9
and B7 of the ISA bus (respectively). These voltages can be
used by any adapter card on the bus, but most notably they are
used by serial port driver/receiver circuits. If the
motherboard has serial ports built in, the +12v and -12v
signals can sometimes be used for those ports.
NOTE: The load placed on these voltages by a
serial port would be very small. For example, the PS/2 Dual
Async adapter uses only 35mA of +12v and 35mA of -12v (0.035
amps each) to operate two ports.
Most newer serial port circuits no longer use 12v
driver/receiver circuits, but instead now use circuits that
run on only 5v or even 3.3v. If you have one of these modern
design ports in your system, the -12v signal from your power
supply is likely to be totally unused by anything in the
system.
The main function of the +12v power is to run disk drive
motors. Usually a large amount of current is available,
especially in systems with a large number of drive bays, such
as in a tower configuration. Besides disk drive motors, the
+12v supply is used by any cooling fans in the system, which,
of course, should always be running. A single cooling fan can
draw between 100mA to 250mA (0.1 to 0.25 amps); however, most
newer ones use the lower 100mA figure. Note that although most
fans in desktop systems run on +12v, most portable systems use
fans that run on +5v or even 3.3v instead.
In addition to supplying power to run the system, the power
supply also ensures that the system does not run unless the
power being supplied is sufficient to operate the system
properly. In other words, the power supply actually prevents
the computer from starting up or operating until all the
correct power levels are present.
Each power supply completes internal checks and tests
before allowing the system to start. The power supply sends to
the motherboard a special signal, called Power_Good. If
this signal is not present, the computer does not run. The
effect of this setup is that when the AC voltage dips and the
power supply becomes over-stressed or overheated, the
Power_Good signal goes down and forces a system reset or
complete shutdown. If your system has ever seemed dead when
the power switch is on and the fan and hard disks are running,
you know the effects of losing the Power_Good signal.
IBM originally used this conservative design with the view
that if the power goes low or the supply is overheated or
over-stressed, causing output power to falter, the computer
should not be allowed to operate. You even can use the
Power_Good feature as a method of designing and implementing a
reset switch for the PC. The Power_Good line is wired to the
clock generator circuit (an 8284 or 82284 chip in the original
PC/XT and AT systems), which controls the clock and reset
lines to the microprocessor. When you ground the Power_Good
line with a switch, the chip and related circuitry stop the
processor by killing the clock signal and then reset the
processor when the Power_Good signal appears after you release
the switch. The result is a full hardware reset of the system.
Instructions for installing such a switch in a system not
already equipped can be found later in this chapter.
Newer systems with ATX or LPX form factor motherboards
include a special signal called PS_ON which can be used
to turn the power supply (and thus the system) off via
software; this is sometimes called the soft-off
feature. This is most evident in Windows 95 when you
select the Shut Down the Computer option. If the power supply
soft-offs, Windows will automatically shut down the computer
rather than display a message that it's safe to shut down the
computer.
Power Supply Form Factors
The shape and general physical layout of a component is
called the form factor, and items that share form
factors are generally interchangeable. When a system is
designed, the designers can choose to use one of the popular
standard form factors, or they can "roll their own." Choosing
the former means that a virtually inexhaustible supply of
inexpensive replacements is available in a variety of quality
and power output levels. Going the custom route means that the
supply will be unique to the system and available only from
the original manufacturer in only the model(s) they produce.
If you cannot tell already, I am a fan of the
industry-standard form factors!
The form factor of the power supply that a particular
system uses is based on the case design. Six popular case and
power supply types can be called industry standard. The
different types are: PC/XT style Baby AT style
AT/Desk style Slim style
AT/Tower style ATX style Each of these supplies are
available in numerous different configurations and power
output levels. Of these standard types, the Slim style and ATX
style are found in most modern systems, while the others are
largely obsolete.
PC/XT Style
When IBM introduced the XT, it used the same basic power
supply shape as the original PC, except that the new XT supply
had more than double the power output capability (see Figure
8.1). Because they were identical in both external appearance
and the type of connectors used, you could easily install the
better XT supply as an upgrade for a PC system. Because of the
tremendous popularity of the original PC and XT design, a
number of manufacturers began building systems that mimicked
their shape and layout. These clones, as they have been
called, could interchange virtually all components with the
IBM systems, including the power supply. Numerous
manufacturers have since begun producing these components, and
nearly all follow the form factor of one or more IBM
systems.
FIG.
8.1 PC/XT-form factor power
supply.
AT/Desk Style
When IBM later introduced the AT desktop system, it created
a larger power supply that had a form factor different from
the original PC/XT. This system was rapidly cloned as well,
and to this day still represents the basis for most IBM-
compatible designs. The power supply used in these systems is
called the AT/Desktop style power supply (see Figure
8.2). Hundreds of manufacturers now make motherboards, power
supplies, cases, and so on that are physically interchangeable
with the original IBM AT. If you are buying a compatible
system, I recommend those that have form factors that are
compatible with the IBM AT, because you will have numerous
motherboards and power supplies from which to choose.
AT/Tower Style
The compatible market has come up with a couple of other
variations on the AT theme that are popular today. Besides the
standard AT/Desktop type power supply, we also have the
AT/Tower configuration, which is basically a full-sized
AT-style desktop system running on its side. The power supply
and motherboard form factors are basically the same in the
Tower system as in the Desktop. The tower configuration is not
new; in fact, even IBM's original AT had a specially mounted
logo that could be rotated when you ran the system on its side
in the tower configuration. The type of power supply used in a
tower system is identical to that used in a desktop system,
except for the power switch location. Most AT/Desktop systems
required that the power switch be located right on the power
supply itself, while most AT/Tower systems use an external
switch attached to the power supply through a short 4-wire
cable. A full sized AT power supply with a remote switch is
now called an AT/Tower form-factor supply (see Figure
8.3).
FIG.
8.2 AT/Desktop form factor power
supply.
FIG.
8.3 AT/Tower form-factor power
supply.
Baby-AT Style
Another type of AT-based form factor that has been
developed is the so called Baby-AT, which is simply a
shortened version of the full-sized AT system. The power
supply in these systems is shortened on one dimension;
however, it matches the AT design in all other respects. These
Baby-AT style power supplies can be used in both Baby-AT
chassis and the larger AT-style chassis; however, the full
size AT/Tower power supply does not fit in the Baby-AT chassis
(see Figure 8.4).
FIG.
8.4 Baby-AT form factor power
supply.
Slim Style
The fifth type of form factor that has developed is the
Slimline (see Figure 8.5). These systems use a
different motherboard configuration that mounts the slots on a
"riser" card that plugs into the motherboard. The expansion
cards plug into this riser and are mounted sideways in the
system. These types of systems are very low in height, hence
the name Slimline. A new power supply was specifically
developed for these systems and allows interchangeability
between different manufacturers' systems. Some problems with
motherboard interchanges occur because of the riser cards, but
the Slimline power supply has become a standard in its own
right. The slimline power supply is by far the most popular
power supply design in use today. Despite how it might sound,
even most full-sized AT Desktop and Tower cases today are
designed to accept the slimline form factor power supply.
FIG.
8.5 Slimline/Low Profile form factor
power supply.
ATX Style
The newest standard on the market today is the ATX form
factor (see Figure 8.6). This describes a new motherboard
shape, as well as a new case and power supply form factor. The
ATX supply is based on the slimline or low-profile design, but
has several differences worth noting. One difference is that
the fan is now mounted along the inner side of the supply,
blowing air across the motherboard and drawing it in from the
outside at the rear. This flow is the opposite of most
standard supplies, which blow air out the back of the supply
and also have the fan positioned at the back. The reverse flow
cooling used in the ATX supply forces air over the hottest
components of the board, such as the CPU, SIMMs, and expansion
slots. This eliminates the need for the notoriously unreliable
CPU fans that have unfortunately become common today.
Another benefit of the reverse flow cooling is that the
system will remain cleaner and free from dust and dirt. The
case is essentially pressurized, so air will push out of the
cracks in the case, the opposite of what happens in non-ATX
systems. For example, if you held a lit cigarette in front of
your floppy drive on a normal system, the smoke would be
inhaled through the front of the drive and contaminate the
heads! On an ATX system with reverse flow cooling, the smoke
would be blown away from the drive because the only air intake
is the single fan vent on the power supply at the rear. Those
who use systems that operate in extremely harsh environments
could add a filter to the fan intake vent, which would ensure
even further that all air entering the system is clean and
dust free.
FIG.
8.6 ATX form factor power
supply.
The ATX system format was designed by Intel in 1995, but
became popular in the new Pentium Pro-based PCs in 1996. The
ATX form factor takes care of several problems with the
Baby-AT or Slimline form factors. Where the power supply is
concerned, this covers two main problems. One problem is that
the traditional PC power supply has two connectors that plug
into the motherboard. The problem is that if you insert these
connectors backwards or out of their normal sequence, you will
fry the motherboard! Most responsible system manufacturers
will have the motherboard and power supply connectors keyed so
they cannot be installed backwards or out of sequence, but
many of the cheaper system vendors do not feature this keying
on the boards or supplies they use.
To solve the potential for disaster that awaits those who
might plug in their power supply connectors incorrectly, the
ATX form factor includes a new power plug for the motherboard.
This new connector features 20 pins, and is a single-keyed
connector. It is virtually impossible to plug it in backwards,
and because there is only one connector instead of two nearly
identical ones, it is impossible to plug them in out of
sequence. The new connector also can optionally supply 3.3v,
eliminating the need for voltage regulators on the motherboard
to power the CPU and other 3.3v circuits. Although the 3.3v
signals are labeled as optional in the ATX specification, they
should be considered mandatory in any ATX form factor power
supply you purchase. Many systems will require this in the
future.
Besides the new 3.3v signals, there is one other set of
signals that will be found on the ATX supply not normally seen
on standard supplies. They are the Power_On and
5v_Standby signals, which are also called Soft
Power. Power_On is a motherboard signal that can be used
with operating systems like Windows 95 or Windows NT, which
support the ability to power the system down with software.
This will also allow the optional use of the keyboard to power
the system back on, exactly like the Apple Macintosh systems.
The 5v_Standby signal is always active, giving the motherboard
a limited source of power even when off.
The other problem solved by the ATX form factor power
supply is that of system cooling. Most of the high-end Pentium
and Pentium Pro systems have active heat sinks on the
processor, which means there is a small fan on the CPU
designed to cool it. These small fans are notoriously
unreliable, not to mention expensive when compared to standard
passive heat sinks. In the ATX design, the CPU fan is
eliminated, and the CPU is mounted in a socket right next to
the ATX power supply, which has a reverse flow fan blowing
onto the CPU.
You will find it easy to locate supplies that fit these
industry-standard form factors. Several vendors who
manufacture PC power supplies in all these form factors are
listed later in this chapter. For proprietary units, you will
likely have to go back to the manufacturer.
Power Supply Connectors
Table 8.1 shows the pinouts for most standard AT or
PC/XT-compatible systems. Some systems may have more or fewer
drive connectors. For example, IBM's AT system power supplies
have only three disk drive power connectors, although most of
the currently available AT/Tower type power supplies have four
drive connectors. If you are adding drives and need additional
disk drive power connectors, "Y" splitter cables are available
from many electronics supply houses (including Radio Shack)
that can adapt a single power connector to serve two drives.
As a precaution, make sure that your total power supply output
is capable of supplying the additional power.
Table 8.1 Typical PC/XT and AT Power Supply
Connections
Connector |
AT Type |
PC/XT Type |
P8-1 |
Power_Good (+5v) |
Power_Good (+5v) |
P8-2 |
+5v |
Key (No connect) |
P8-3 |
+12v |
+12v |
P8-4 |
-12v |
-12v |
P8-5 |
Ground (0) |
Ground (0) |
P8-6 |
Ground (0) |
Ground (0) |
P9-1 |
Ground (0) |
Ground (0) |
P9-2 |
Ground (0) |
Ground (0) |
P9-3 |
-5v |
-5v |
P9-4 |
+5v |
+5v |
P9-5 |
+5v |
+5v |
P9-6 |
+5v |
+5v |
P10-1 |
+12v |
+12v |
P10-2 |
Ground (0) |
Ground (0) |
P10-3 |
Ground (0) |
Ground (0) |
P10-4 |
+5v |
+5v |
P11-1 |
+12v |
+12v |
P11-2 |
Ground (0) |
Ground (0) |
P11-3 |
Ground (0) |
Ground (0) |
P11-4 |
+5v |
+5v |
P12-1 |
+12v |
-- |
P12-2 |
Ground (0) |
-- |
P12-3 |
Ground (0) |
-- |
P12-4 |
+5v |
-- |
P13-1 |
+12v |
-- |
P13-2 |
Ground (0) |
-- |
P13-3 |
Ground (0) |
-- |
P13-4 |
+5v |
-- |
Notice that the Baby-AT and Slimline power supplies also
use the AT/Desktop or Tower pin configuration. The only other
type of industry standard power supply connector is found on
the new ATX form factor power supply. This is a 20-pin keyed
connector with pins configured as shown in Table 8.2.
Table 8.2 ATX Power Supply Connections
Signal |
Pin |
Pin |
Signal |
3.3v* |
11 |
1 |
3.3v* |
-12v |
12 |
2 |
3.3v* |
GND |
13 |
3 |
GND |
Pwr_On |
14 |
4 |
5v |
GND |
15 |
5 |
GND |
GND |
16 |
6 |
5v |
GND |
17 |
7 |
GND |
-5v |
18 |
8 |
Power_Good |
5v |
19 |
9 |
5v_Standby |
5v |
20 |
10 |
12v |
* = Optional signal
NOTE: The ATX supply features several signals
not seen before, such as the 3.3v, Power_On, and 5v_Standby
signals. Because of this, it will be difficult to adapt a
standard slimline or low-profile form factor supply to work
properly in an ATX system, although the shapes are virtually
identical.
Although the PC/XT power supplies do not have any signal on
pin P8-2, you can still use them on AT-type motherboards, or
vice versa. The presence or absence of the +5v signal on that
pin has little or no effect on system operation. If you are
measuring voltages for testing purposes, anything within 10
percent is considered acceptable, although most manufacturers
of high-quality power supplies specify a tighter 5 percent
tolerance. I prefer to go by the 5 percent tolerance, which is
a tougher test to pass.
Desired Voltage |
Loose Tolerance Min. (-10%) |
Tight Tolerance Max. (+8%) |
Min. (-5%) |
Max. (+5%) |
+/-5.0v |
4.5v |
5.4v |
4.75 |
5.25 |
+/-12.0v |
10.8v |
12.9v |
11.4 |
12.6 |
The Power_Good signal has tolerances different from the
other signals, although it is nominally a +5v signal in most
systems. The trigger point for Power_Good is about +2.5v, but
most systems require the signal voltage to be within about 3v
to 6v.
A power supply should be replaced if the voltages are out
of these ranges.
Power Switch Connectors.
The AT/Tower and Slimline power supplies use a remote power
switch. This switch is mounted in the front of the system case
and is connected to the power supply through a standard type
of 4-wire cable. The ends of the cable are fitted with spade
connector lugs, which plug into the spade connectors on the
power switch itself. The switch is usually a part of the case,
so the power supply comes with the cable and no switch. The
cable from the power supply to the switch in the case contains
four color-coded wires. There may also be a fifth wire
supplying a ground connection to the case as well.
CAUTION: The remote power switch leads carry
110v AC current at all times. You could be electrocuted if
you touch the ends of these wires with the power supply
plugged in! Always make sure the power supply is unplugged
before connecting or disconnecting the remote power switch.
The four or five wires are color-coded as follows:
- The brown and blue wires are the live and neutral feed
wires from the 110v power cord to the power supply itself.
These wires are always hot when the power supply is plugged
in.
- The black and white wires carry the AC feed from the
switch back to the power supply itself. These leads should
only be hot when the power supply is plugged in and the
switch is turned on.
- A green wire or a green wire with a yellow stripe is the
ground lead. It should be connected somewhere to the PC
case, and helps to ground the power supply to the case.
On the switch itself, the tabs for the leads are usually
color-coded; if not, they can still be easily connected. If
there is no color coding on the switch, then plug the blue and
brown wires onto the tabs that are parallel to each other, and
the black and white wires to the tabs that are angled away
from each other. See Figure 8.7 for a guide.
FIG.
8.7 Power supply remote switch
connections.
As long as the blue and brown wires are on the one set of
tabs, and the black and white leads are on the other, the
switch and supply will work properly. If you incorrectly mix
the leads, you can create a direct short circuit, and you will
likely blow the circuit breaker for the wall socket.
Disk Drive Power Connectors.
The disk drive connectors are fairly universal with regard
to pin configuration and even wire color. Table 8.3 shows the
standard disk drive power connector pinout and wire colors.
Table 8.3 Disk Drive Power Connector
Pinout
Pin |
Wire Color |
Signal |
1 |
Yellow |
+12v |
2 |
Black |
Gnd |
3 |
Black |
Gnd |
4 |
Red |
+5v |
This information applies whether the drive connector is the
larger Molex version or the smaller mini-version used on most
3 1/2-inch floppy drives. In each case, the pinouts and wire
colors are the same. To determine the location of pin 1, look
at the connector carefully. It is usually embossed in the
plastic connector body; however, it is often tiny and
difficult to read. Fortunately, these connectors are keyed and
therefore are difficult to insert incorrectly. Figure 8.8
shows the keying with respect to pin numbers on the larger
drive power connector.
FIG.
8.8 A disk drive female power supply
cable connector.
Notice that some drive connectors may supply only two
wires--usually the +5v and a single ground (pins 3 and
4)--because the floppy drives in most newer systems run on
only +5v and do not use the +12v at all.
Physical Connector Part Numbers.
The physical connectors used in industry-standard PC power
supplies were originally specified by IBM for the supplies
used in the original PC/XT/AT systems. They used a specific
type of connector between the power supply and the motherboard
(the P8 and P9 connectors), as well as specific connectors for
the disk drives. The motherboard connectors used in all the
industry-standard power supplies have not changed since 1981
when the IBM PC appeared. With the advent of 3 1/2-inch floppy
drives in 1986, however, a new smaller type of drive power
connector appeared on the scene for these drives. Table 8.4
lists the standard connectors used for mother-board and disk
drive power.
Table 8.4 Physical Power Connectors
Connector Description |
Female (on Power Cable) |
Male (on Component) |
Motherboard P8/P9 |
Burndy GTC6P-1 |
Burndy GTC 6RI |
Disk Drive (large style) |
AMP 1-480424-0 |
AMP 1-480426-0 |
Disk Drive (small style) |
AMP 171822-4 |
AMP 171826-4 |
You can get these raw connectors through electronics supply
houses (Allied, Newark, Digi-Key, and so on) found in the
vendor list. You also can get complete cable assemblies
including drive adapters from the large to small connectors,
disk drive "Y" splitter cables, and motherboard power
extension cables from a number of the cable and miscellaneous
supply houses such as Cables To Go, the Cable Connection, Ci
Design, and Key Power.
The Power_Good Signal
The Power_Good signal is a +5v signal (+3.0 through +6.0 is
generally considered acceptable) generated in the power supply
when it has passed its internal self tests and the outputs
have stabilized. This normally takes anywhere from 0.1 to 0.5
seconds after you turn on the power supply switch. This signal
is sent to the motherboard, where it is received by the
processor timer chip, which controls the reset line to the
processor.
In the absence of Power_Good, the timer chip continuously
resets the processor, which prevents the system from running
under bad or unstable power conditions. When the timer chip
sees Power_Good, it stops resetting the processor and the
processor begins executing whatever code is at address
FFFF:0000 (usually the ROM BIOS).
If the power supply cannot maintain proper outputs (such as
when a brownout occurs), the Power_Good signal is withdrawn,
and the processor is automatically reset. When proper output
is restored, the Power_Good signal is regenerated and the
system again begins operation (as if you just powered on). By
withdrawing Power_Good, the system never "sees" the bad power
because it is "stopped" quickly (reset) rather than allowed to
operate on unstable or improper power levels, which can cause
parity errors and other problems.
In most systems, the Power_Good connection is made via
connector P8-1 (P8 Pin 1) from the power supply to the
motherboard.
A well-designed power supply delays the arrival of the
Power_Good signal until all voltages stabilize after you turn
the system on. Badly designed power supplies, which are found
in many low-cost compatibles, often do not delay the
Power_Good signal properly and enable the processor to start
too soon. The normal Power_Good delay is from 0.1 to 0.5
seconds. Improper Power_Good timing also causes CMOS memory
corruption in some systems. If you find that a system does not
boot up properly the first time you turn on the switch but
subsequently boots up if you press the reset or
Ctrl+Alt+Delete warm boot command, you likely have a problem
with Power_Good. Thi is happens because the Power_Good signal
is tied to the timer chip that generates the reset signal to
the processor. What you must do in these cases is find a new
high-quality power supply and see whether it solves the
problem.
Many cheaper power supplies do not have proper Power_Good
circuitry and often just tie any +5v line to that signal. Some
motherboards are more sensitive to an improperly designed or
improperly functioning Power_Good signal than others.
Intermittent startup problems are often caused by improper
Power_Good signal timing. A common example occurs when
somebody replaces a motherboard in a system and then finds
that the system intermittently fails to start properly when
the power is turned on. This ends up being very difficult to
diagnose, especially for the inexperienced technician, because
the problem appears to be caused by the new motherboard.
Although it seems that the new motherboard might be defective,
it usually turns out to be that the original power supply is
poorly designed and either cannot produce stable enough power
to properly operate the new board, or more likely has an
improperly wired or timed Power_Good signal. In these
situations, replacing the supply with a high-quality unit is
the proper solution.
Power Supply Loading
PC power supplies are of a switching rather than a linear
design. The switching type of design uses a high speed
oscillator circuit to generate different output voltages, and
is very efficient in size, weight, and energy compared to the
standard linear design, which uses a large internal
transformer to generate different outputs.
One characteristic of all switching type power supplies is
that they do not run without a load. This means that you must
have the supply plugged into something drawing +5v and +12v or
the supply does not work. If you simply have the supply on a
bench with nothing plugged into it, the supply burns up or
protection circuitry shuts it down. Most power supplies are
protected from no-load operation and will shut down. Some of
the cheap clone supplies, however, lack the protection circuit
and relay and are destroyed after a few seconds of no-load
operation. A few power supplies have their own built-in load
resistors, so that they can run even though no normal load is
plugged in.
According to IBM specifications for the standard 192-watt
power supply used in the original AT, a minimum load of 7.0
amps was required at +5v and a minimum load of 2.5 amps was
required at +12v for the supply to work properly. Because
floppy drives present no +12v load unless they are spinning,
systems without a hard disk drive often do not operate
properly. Most power supplies have a minimum load requirement
for both the +5v and +12v sides, and if you fail to meet this
minimum load, the supply shuts down.
Because of this characteristic, when IBM used to ship AT
systems without a hard disk, they had the hard disk drive
power cable plugged into a large 5-ohm 50-watt sandbar
resistor mounted in a little metal cage assembly where the
drive would have been. The AT case had screw holes on top of
where the hard disk would go, specifically designed to mount
this resistor cage. Several computer stores I knew in the
mid-1980s would order the diskless AT and install their own
20M or 30M drives, which they could get more cheaply from
sources other than IBM. They were throwing away the load
resistors by the hundreds! I managed to grab a couple at the
time, which is how I know the type of resistor they used.
This resistor would be connected between pin 1 (+12v) and
pin 2 (Ground) on the hard disk power connector. This placed a
2.4-amp load on the supply's 12-volt output, drawing 28.8
watts of power--it would get hot!--thus enabling the supply to
operate normally. Note that the cooling fan in most power
supplies draws approximately 0.1 to 0.25 amps, bringing the
total load to 2.5 amps or more. If the load resistor was
missing, the system would intermittently fail to start up or
operate properly. The motherboard draws +5v at all times, but
+12v is normally used only by motors, and the floppy drive
motors are off most of the time.
Most of the 200-watt power supplies in use today do not
require as much of a load as the original IBM AT power supply.
In most cases, a minimum load of 2.0 to 4.0 amps at +5v and a
minimum load of 0.5 to 1.0 amps at +12v are considered
acceptable. Most motherboards will easily draw the minimum +5v
current by themselves. The standard power supply cooling fan
draws only 0.1 to 0.25 amps, so the +12v minimum load may
still be a problem for a diskless workstation. Generally the
higher the rating on the supply, the more minimum load is
required; however, there are exceptions, so this is a
specification you want to check into.
Some high-quality switching power supplies, like the Astec
units used by IBM in all the PS/2 systems, have built-in load
resistors and can run under a no-load situation because the
supply loads itself. Most of the cheaper clone supplies do not
have built-in load resistors, so they must have both +5v and
+12v loads to work.
If you want to bench test a power supply, make sure that
loads are placed on both the +5v and +12v outputs. This is one
reason why it is best to test the supply while it is installed
in the system instead of separately on the bench. For
impromptu bench testing, you can use a spare motherboard and
hard disk drive to load the +5v and +12v outputs,
respectively.
Power-Supply Ratings
Most system manufacturers will provide you with the
technical specifications of each of their system-unit power
supplies. This type of information is usually found in the
system's technical-reference manual and also on stickers
attached directly to the power supply. Power supply
manufacturers can supply this data, which is preferable if you
can identify the manufacturer and contact them directly.
Tables 8.5 and 8.6 list power-supply specifications for
several of IBM's units, from which most of the compatibles are
derived. The PC-system power supplies are the original units
that most compatible power supplies have duplicated. The input
specifications are listed as voltages, and the output
specifications are listed as amps at several voltage levels.
IBM reports output wattage level as "specified output
wattage." If your manufacturer does not list the total
wattage, you can convert amperage to wattage by using the
following simple formula: Wattage = Voltage x Amperage By
multiplying the voltage by the amperage available at each
output and then adding them up, you can calculate the total
capable output wattage of the supply.
Table 8.5 Power Supply Output Ratings for IBM
"Classic" Systems
|
PC |
Port-PC |
XT |
XT-286 |
AT |
Minimum Input Voltage |
104 |
90 |
90 |
90 |
90 |
Maximum Input Voltage |
127 |
137 |
137 |
137 |
137 |
110/220v Switching |
No |
Yes |
No |
Auto |
Yes |
Output Current (amps):+5v |
7.0 |
11.2 |
15.0 |
20.0 |
19.8 |
-5v |
0.3 |
0.3 |
0.3 |
0.3 |
0.3 |
+12v |
2.0 |
4.4 |
4.2 |
4.2 |
7.3 |
-12v |
0.25 |
0.25 |
0.25 |
0.25 |
0.3 |
Calculated output wattage |
63.5 |
113.3 |
129.9 |
154.9 |
191.7 |
Specified output wattage |
63.5 |
114.0 |
130.0 |
157.0 |
192.0 |
Table 8.6 shows the standard power supply output levels
available in industry-standard form factors. Most
manufacturers that offer power have supplies with different
ratings. Supplies are available with ratings from 100 watts to
450 watts or more. Table 8.6 shows the rated outputs at each
of the voltage levels for supplies with different
manufacturer-specified output ratings. To compile the table, I
referred to the specification sheets for supplies from Astec
Standard Power and PC Power and Cooling. As you can see,
although most of the ratings are accurate, they are somewhat
misleading for the higher wattage units.
Table 8.6 Typical Compatible Power Supply
Output Ratings
Specified Output Wattage |
100W |
150W |
200W |
250W |
300W |
375W |
450W |
Output Current (amps):+5v |
10.0 |
15.0 |
20.0 |
25.0 |
32.0 |
35.0 |
45.0 |
-5v |
0.3 |
0.3 |
0.3 |
0.5 |
1.0 |
0.5 |
0.5 |
+12v |
3.5 |
5.5 |
8.0 |
10.0 |
10.0 |
13.0 |
15.0 |
-12v |
0.3 |
0.3 |
0.3 |
0.5 |
1.0 |
0.5 |
1.0 |
Calculated output wattage |
97.1 |
146.1 |
201.1 |
253.5 |
297.0 |
339.5 |
419.5 |
Most compatible power supplies have ratings between 150 to
250 watts output. Although lesser ratings are not usually
desirable, it is possible to purchase heavy-duty power
supplies for most compatibles that have outputs as high as 500
watts.
The 300-watt and larger units are excellent for enthusiasts
who are building a fully optioned desktop or tower system.
These supplies run any combination of motherboard and
expansion card, as well as a large number of disk drives. In
most cases, you cannot exceed the ratings on these power
supplies--the system will be out of room for additional items
first!
Most power supplies are considered to be universal,
or worldwide. That is, they run on the 220v, 50-cycle
current used in Europe and many other parts of the world. Most
power supplies that can switch to 220v input are automatic,
but a few require that you set a switch on the back of the
power supply to indicate which type of power you will access.
(The automatic units sense the current and switch
automatically.)
If your supply does not autoswitch, make sure the voltage
setting is correct. If you plug the power supply into a 110v
outlet while set in the 220v setting, there will be no damage,
but it will certainly not operate properly until you correct
the setting. On the other hand, if you are in a foreign
country with a 220v outlet and have the switch set for 110v,
you may cause some damage.
Power Supply Specifications
In addition to power output, many other specifications and
features go into making a high-quality power supply. I have
had many systems over the years. My experience has been that
if a brownout occurs in a room with several systems running,
the systems with higher-quality power supplies and higher
output ratings always make it over power disturbances, whereas
others choke. I would not give $5 for many of the cheap, junky
power supplies that come in some of the low-end clone
systems.
High-quality power supplies also help to protect your
systems. A power supply from a vendor like Astec or PC Power
and Cooling will not be damaged if any of the following
conditions occur:
- A 100 percent power outage of any duration
- A brownout of any kind
- A spike of up to 2,500v applied directly to the AC input
(for example, a lightning strike or a lightning simulation
test)
Decent power supplies have an extremely low current leakage
to ground of less than 500 microamps. This safety feature is
important if your outlet has a missing or improperly wired
ground line.
As you can see, these specifications are fairly tough and
are certainly representative of a high-quality power supply.
Make sure that your supply can meet these specifications. The
vendors recommended in this chapter produce supplies that meet
or exceed these specifications.
Power-Use Calculations
One way to see whether your system is capable of expansion
is to calculate the levels of power drain in the different
system components and deduct the total from the maximum power
supplied. This calculation might help you decide when to
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.
It is difficult to get power consumption data for most +5v
devices, including mother-boards and adapter cards.
Motherboards can consume different power levels, depending on
numerous factors. Most 486DX2 motherboards consume about 5
amps or so, but if you can get data on the one you are using,
so much the better. 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 as set forth in the respective bus
standards.
For example, consider the typical power consumption figures
for components in a modern PC system. Most standard desktop or
slimline PC systems today come 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 and 0.175
amps of +12v power for each slot in the system. Most systems
have eight slots, and you can assume that four of them are
filled for the purposes of calculating power draw. The
following calculation shows what happens when you subtract the
amount of power necessary to run the different system
components:
5v Power: |
20.0 Amps |
Less: |
Motherboard -5.0 4 slots filled at
| |