Technical The Internal Components The Power Supply Part 2

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The Power Supply - part 2

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 colour-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 240v 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 colour-coded as follows:

• The brown and blue wires are the live and neutral feed wires from the 240v 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 colour-coded; if not, they can still be easily connected. If there is no colour 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.

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 colour. Table 3 shows the standard disk drive power connector pinout and wire colours.

Table 3 Disk Drive Power Connector Pinout

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 colours 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.

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 4 lists the standard connectors used for motherboard and disk drive power.

Table 4 Physical Power Connectors

You can get these raw connectors through electronics supply houses. 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.

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 stabilised. 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 stabilise 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>+<DEL> warm boot command, you likely have a problem with Power_Good. This 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 start-up 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 plus 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 5 and 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 5 Power Supply Output Ratings for IBM "Classic" Systems

Table 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 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 6 Typical Compatible Power Supply Output Ratings

Most compatible power supplies have ratings between 200 to 350 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 240v, 50-cycle current used in Europe and many other parts of the world. Most power supplies that can switch to 240v 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 are in a foreign country with a 110v outlet and have the switch set for 240v, there will be no damage, but it will certainly not operate properly until you correct the setting. On the other hand, if you plug the power supply into a 240v outlet while set in the 110v setting, 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.

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 motherboards 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 230-watt power supply rated for 23 amps at +5v and 9 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:

In the preceding example, everything seems alright for now. With half the slots filled, two floppy drives, and one hard disk, the system still has room for more. There might be trouble if this system were expanded to the extreme. With every slot filled and two or more hard disks, there definitely will be problems with the +5v. 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 will be strained.

If you anticipate loading up a system to the extreme, as in a high-end multimedia system, for example, you may want to invest in the insurance of a higher output supply. For example, a 250-watt supply usually has 25-amps of +5v and 10-amps of +5v current, whereas a 300-watt unit has 32-amps of +5v power. These supplies would permit a fully expanded system and are likely to be found in full-sized desktop or tower case configurations in which this capability can be fully used.

Motherboards can draw anywhere from 4 to 15 amps or more of +5v power to run. In fact, a single Pentium 66MHz CPU draws up to 3.2 amps of +5v power all by itself. Considering that dual Pentium processor systems are available, you could have 6.4 amps or more drawn by the processors alone. A motherboard like this with 64M of RAM might draw 15 amps or more all by itself. Most 486DX2 motherboards draw approximately 5 to 7 amps of +5v. Bus slots are allotted maximum power in amps, as shown in Table 7.

Table 7 Maximum Power Consumption in Amps per Bus Slot

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 and not all cards draw this much power. If the slot has a Local-Bus extension connector, an additional 2.0 amps of +5v power is allowed for the Local-Bus.

Floppy drives can vary in power consumption, but most of the newer 3 1/2-inch drives have motors that run off +5v in addition to the logic circuits. These drives usually draw 1.0 amp of +5v power and use no +12v at all. Most 5 1/4-inch 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-inch 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. The 5 1/4-inch 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 amps of +5v power.

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

The figures most manufacturers report for maximum power supply output are full duty-cycle figures, which means that these levels of power can be supplied continuously. You usually can expect a unit that continuously supplies some level of power to supply more power for some noncontinuous amount of time. A supply usually can offer 50 percent greater output than the continuous figure indicates for as long as one minute. This cushion is often used to supply the necessary power to start spinning a hard disk. 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 long length of time causes the power supply to run hot and fail early, and it can prompt several nasty symptoms in the system.

TIP: If you are using internal SCSI hard drives, you can ease the start-up load on your power supply. The key is to enable the Remote Start option on the SCSI drive, which causes the drive to start spinning only when it receives a start-up command over the SCSI bus. The effect is such 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 is begun.
If you have multiple SCSI drives, they all spin up sequentially based on their SCSI ID setting. This is designed so that only one drive is spinning up at any one time, and 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.
This tip is essential when dealing with portable type systems in which power is at a premium. I burned up a supply in one of my portable systems before resetting the internal drive to Remote Start.

The biggest causes of overload problems are filling up the slots and adding more drives. Multiple hard drives, CD-ROM drives, floppy drives, and so on can place quite a drain on the system power supply. Make sure that you have enough +12v power to run all the drives you are going to install. Tower systems can be a problem here because they have so many drive bays. Make sure that you have enough +5v power to run all your expansion cards, especially if you are using Local-Bus or EISA cards. It pays to be conservative, but remember that most cards draw less than the maximum allowed.

Many people wait until an existing unit fails before they replace it with an upgraded version. If you are on a tight budget, this "if it ain't broke, don't fix it" attitude works. Power supplies, however, often do not just fail; they can fail in an intermittent fashion or allow fluctuating power levels to reach the system, which results in an unstable operation. You might be blaming system lockups on software bugs when the culprit is an overloaded power supply. If you have been running with your original power supply for a long time, you should expect some problems.

Disks - The Basics

Other Co-processors