Upgrading and Repairing PCs (17th Edition)

Power supplies have several specifications that define their input and output capabilities as well as their operational characteristics. The following sections define and examine most of the common specifications related to power supplies.

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 convert the higher wall-socket AC voltage to the much lower DC voltage used to power the PC and PC components. Switching-type power supplies are noted for being very efficient in size, weight, and energy in comparison to the linear design, which uses a large internal transformer to generate various outputs. This type of transformer-based design is inefficient in at least three ways. First, the output voltage of the transformer linearly follows the input voltage (hence the name linear), so any fluctuations in the AC power going into the system can cause problems with the output. Second, the high current-level (power) requirements of a PC system require the use of heavy wiring in the transformer. Third, the 60Hz frequency of the AC power supplied from your building is difficult to filter out inside the power supply, requiring large and expensive filter capacitors and rectifiers.

The switching supply, on the other hand, uses a switching circuit that chops up the incoming power at a relatively high frequency. This enables the use of high-frequency transformers that are much smaller and lighter. Also, the higher frequency is much easier and cheaper to filter out at the output, and the input voltage can vary widely. Input ranging from 90V to 135V still produces the proper output levels, and many switching supplies can automatically adjust to 240V input.

One characteristic of all switching-type power supplies is that they do not run without a load. Therefore, you must have something such as a motherboard and hard drive plugged in and drawing power for the supply to work. If you simply have the power supply on a bench with nothing plugged in to it, either the supply burns up or its protection circuitry shuts it down. Most power supplies are protected from no-load operation and shut down automatically. Some of the cheapest supplies, however, lack the protection circuit and relay and can be destroyed after a few seconds of no-load operation. A few power supplies have their own built-in load resistors, so they can run even though there isn't a normal load (such as a motherboard or hard disk) plugged in.

Some power supplies have minimum load requirements for both the +5V and +12V sides. According to IBM specifications for the 192-watt power supply used in the original AT, a minimum load of 7.0 amps was required at +5V and a minimum of 2.5 amps was required at +12V for the supply to work properly. As long as a motherboard was plugged in to the power supply, the motherboard would draw sufficient +5V at all times to keep those circuits in the supply happy. However, +12V is typically used only by motors (and not motherboards), and the floppy or CD/DVD drive motors are off most of the time. Because floppy or optical (CD/DVD) drives don't present any +12V load unless they are spinning, systems without a hard disk drive could have problems because there wouldn't be enough load on the +12V circuit in the supply.

To alleviate problems, when IBM used to ship the original AT systems without a hard disk, it plugged the hard disk drive power cable into a large 5-ohm, 50-watt sandbar resistor that was mounted in a small, 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.

Note

Several computer stores I knew of in the mid-1980s ordered the diskless AT and installed their own 20MB or 30MB 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 +12V output, drawing 28.8 watts of power (it would get hot!) and thus enabling the supply to operate normally. Note that the cooling fan in most power supplies draws approximately 0.10.25 amps, bringing the total load to 2.5 amps or more. If the load resistor were missing, the system would intermittently fail to start up.

Most of the 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 00.3 amps at +3.3V, 2.04.0 amps at +5V, and 0.51.0 amps at +12V is considered acceptable. Most motherboards easily draw the minimum +5V current by themselves. The standard power supply cooling fan draws only 0.10.25 amps, so the +12V minimum load might still be a problem for a diskless workstation. Generally, the higher the rating on the supply, the more minimum load required. However, exceptions do exist, so this is a specification you should check when evaluating power supplies.

Some switching power supplies have built-in load resistors and can run in a no-load situation. Most power supplies don't have internal load resistors but might require only a small load on the +5V line to operate properly. Some supplies, however, might require +3.3V, +5V, and +12V loads to workthe only way to know is by checking the documentation for the particular supply in question.

No matter what, if you want to properly and accurately bench test a power supply, be sure you place a load on at least one (or preferably all) of the positive voltage outputs. This is one reason it is best to test a supply while it is installed in the system, instead of testing it separately on the bench. For impromptu bench testing, you can use a spare motherboard and one or more hard disk drives to load the outputs.

Power Supply Ratings

A system manufacturer should be able to provide you the technical specifications of the power supplies it uses in its systems. This type of information can be found in the system's technical reference manual, as well as on stickers attached directly to the power supply. Power supply manufacturers can also supply this data, which is preferable if you can identify the manufacturer and contact them directly or via the Web.

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:

watts = volts x amps

For example, if a motherboard is listed as drawing 6 amps of +5V current, that equals 30 watts of power according to the formula.

By multiplying the voltage by the amperage available at each main output and then adding the results, you can calculate the total capable output wattage of the supply. Note that only positive voltage outputs are normally used in calculating outputs; the negative outputs, Standby, Power_Good, or other signals that are not used to power components are exempt.

Table 19.19 shows the standard power supply output levels available in industry-standard form factors. Most manufacturers offer supplies with ratings from 100 watts to 450 watts or more. Table 19.20 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 Power (note that they no longer make PC power supplies) and PC Power and Cooling. Although most of the ratings are fairly accurate, there appears to be overly optimistic rounding going on in some of the ratings.

Adding a +3.3V output to the power supply modifies the equation significantly. Table 19.20 contains data for various ATX/ATX12V power supplies from PC Power and Cooling.

^[*] Note the calculated maximum output is theoretical, assuming the maximum draw from the +3.3V, +5V, and +12V simultaneously. Virtually all power supplies place limits on the maximum combined draw for the +3.3V and +5V combined. This would make the true maximum rating somewhat less than the calculated maximum shown here.

If you compute the total output using the formula described earlier, these power supplies seem to produce an output that is much higher than their ratings. The 300W model, for example, comes out at 340 watts. However, notice that the supply also has a maximum combined output for the +3.3V and +5V of 150 watts. This means you can't draw the maximum rating on both the +5V and +3.3V circuits simultaneously but must keep the total combined draw between them at 150W or less. This brings the total output to a more logical 294 watts.

Most PC power supplies have ratings between 150 and 300 watts. Although lesser ratings are not usually desirable, you can purchase heavy-duty power supplies for most systems that have outputs as high as 600 watts or more.

The 300-watt and larger units are recommended for fully optioned desktops or tower systems. These supplies run any combination of motherboard and expansion card, as well as a large number of disk drives and other peripherals. In most cases, you can't exceed the ratings on these power suppliesthe system will run out of room for additional items first!

Most power supplies are considered to be universal, or worldwide. That is, they also can run on the 240V, 50-cycle current used in Europe and many other parts of the world. Many power supplies that can switch from 120V to 240V input do so automatically, but a few require you to set a switch on the back of the power supply to indicate which type of power you will access.

Note

In North America, power companies are required to supply split-phase 240V (plus or minus 5%) AC, which equals two 120V legs. Resistive voltage drops in the building wiring can cause the 240V to drop to 220V or the 120V to drop to 110V by the time the power reaches an outlet at the end of a long circuit run. For this reason, the input voltage for an AC-powered device might be listed as anything between 220V and 240V, or 110V and 120V. I use the 240/120V numbers throughout this chapter because that is the intended standard figure.

Caution

If your supply does not switch input voltages automatically, make sure the voltage setting is correct. If you plug the power supply into a 120V outlet while it's set in the 240V setting, no damage will result, but the supply won't operate properly until you correct the setting. On the other hand, if you plug in to a 240V outlet and have the switch set for 120V, you can cause damage.

Other 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 are far more likely to make it through the power disturbances unscathed, whereas others choke.

High-quality power supplies also help protect your systems. A high-quality power supply from a vendor such as PC Power and Cooling will not be damaged if any of the following conditions occur:

• A 100% 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 an 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.

You can also use many other criteria to evaluate a power supply. The power supply is a component many users ignore when shopping for a PC, and it is therefore one that some system vendors choose to skimp on. After all, a dealer is far more likely to be able to increase the price of a computer by spending money on additional memory or a larger hard drive than by installing a better power supply.

When buying a computer (or a replacement power supply), you always should learn as much as possible about the power supply. However, many consumers are intimidated by the vocabulary and statistics found in a typical power supply specification. Here are some of the most common parameters found on power supply specification sheets, along with their meanings:

• Mean Time Between Failures (MTBF) or Mean Time To Failure (MTTF). The (calculated) average interval, in hours, that the power supply is expected to operate before failing. Power supplies typically have MTBF ratings (such as 100,000 hours or more) that are clearly not the result of real-time empirical testing. In fact, manufacturers use published standards to calculate the results based on the failure rates of the power supply's individual components. MTBF figures for power supplies often include the load to which the power supply was subjected (in the form of a percentage) and the temperature of the environment in which the tests were performed.

• Input Range (or Operating Range). The range of voltages that the power supply is prepared to accept from the AC power source. For 120V AC power, an input range of 90V135V is common; for 240V power, a 180V270V range is typical.

• Peak Inrush Current. The greatest amount of current drawn by the power supply at a given moment immediately after it is turned on, expressed in terms of amps at a particular voltage. The lower the current, the less thermal shock the system experiences.

• Hold-up Time. The amount of time (in milliseconds) that a power supply can maintain output within the specified voltage ranges after a loss of input power. This enables your PC to continue running without resetting or rebooting if a brief interruption in AC power occurs. Values of 1530 milliseconds are common for today's power supplies, and the higher (longer), the better. The ATX12V specification calls for a minimum of 17ms hold-up time.

• Transient Response. The amount of time (in microseconds) a power supply takes to bring its output back to the specified voltage ranges after a steep change in the output current. In other words, the amount of time it takes for the output power levels to stabilize after a device in the system starts or stops drawing power. Power supplies sample the current being used by the computer at regular intervals. When a device stops drawing power during one of these intervals (such as when a floppy drive stops spinning), the power supply might supply too high a voltage to the output for a brief time. This excess voltage is called overshoot, and the transient response is the time that it takes for the voltage to return to the specified level. This is seen as a spike in voltage by the system and can cause glitches and lockups. Once a major problem that came with switching power supplies, overshoot has been greatly reduced in recent years. Transient response values are sometimes expressed in time intervals, and at other times they are expressed in terms of a particular output change, such as "power output levels stay within regulation during output changes of up to 20%."

• Overvoltage Protection. Defines the trip points for each output at which the power supply shuts down or squelches that output. Values can be expressed as a percentage (for example, 120% for +3.3 and +5V) or as raw voltages (for example, +4.6V for the +3.3V output and +7.0V for the +5V output).

• Maximum Load Current. The largest amount of current (in amps) that safely can be delivered through a particular output. Values are expressed as individual amperages for each output voltage. With these figures, you can calculate not only the total amount of power the power supply can supply, but also how many devices using those various voltages it can support.

• Minimum Load Current. The smallest amount of current (in amps) that must be drawn from a particular output for that output to function. If the current drawn from an output falls below the minimum, the power supply could be damaged or automatically shut down.

• Load Regulation (or Voltage Load Regulation). When the current drawn from a particular output increases or decreases, the voltage changes slightly as wellusually increasing as the current rises. Load regulation is the change in the voltage for a particular output as it transitions from its minimum load to its maximum load (or vice versa). Values, expressed in terms of a +/percentage, typically range from +/1% to +/5% for the +3.3V, +5V, and +12V outputs.

• Line Regulation. The change in output voltage as the AC input voltage transitions from the lowest to the highest value of the input range. A power supply should be capable of handling any AC voltage in its input range with a change in its output of 1% or less.

• Efficiency. The ratio of power input to power output, expressed in terms of a percentage. Values of 65%85% are common for power supplies today. The remaining 15%35% of the power input is converted to heat during the AC/DC conversion process. Although greater efficiency means less heat inside the computer (always a good thing) and lower electric bills, it should not be emphasized at the expense of precision, stability, and durability, as evidenced in the supply's load regulation and other parameters.

• Ripple (or Ripple and Noise, or AC Ripple, or PARD [Periodic and Random Deviation]). The average voltage of all AC effects on the power supply outputs, usually measured in millivolts peak-to-peak or as a percentage of the nominal output voltage. The lower this figure, the better. Higher-quality units are typically rated at 1% ripple (or less), which if expressed in volts would be 1% of the output. Consequently, for +5V that would be 0.05V or 50mV (millivolts). Ripple can be caused by internal switching transients, feed through of the rectified line frequency, and other random noise.

Power Factor Correction

Recently, the power line efficiency and harmonic distortion generation of PC power supplies has come under examination. This generally falls under the topic of the power factor of the supply. Interest in power factor is not only due to an improvement in power efficiency, but also because of a reduction in the generation of harmonics back on the power line. In particular, new standards are now mandatory in many European Union (EU) countries that require harmonics be reduced below a specific amount. The circuitry required to do this is called power factor correction (PFC).

The power factor measures how effectively electrical power is being used and is expressed as a number between 0 and 1. A high power factor means that electrical power is being used effectively, whereas a low power factor indicates poor utilization of electrical power. To understand the power factor, you must understand how power is used.

Generally, two types of loads are placed on AC power lines:

• Resistive. Power converted into heat, light, motion, or work

• Inductive. Sustains an electromagnetic field, such as in a transformer or motor

A resistive load is often called working power and is measured in kilowatts (KW). An inductive load, on the other hand, is often called reactive power and is measured in kilovolt-amperes-reactive (KVAR). Working power and reactive power together make up apparent power, which is measured in kilovolt-amperes (KVA). The power factor is measured as the ratio of working power to apparent power, or working power/apparent power (KW/KVA). The ideal power factor is 1, where the working power and apparent power are the same.

The concept of a resistive load or working power is fairly easy to understand. For example, a light bulb that consumes 100W of power generates 100W worth of heat and light. This is a pure resistive load. An inductive load, on the other hand, is a little harder to understand. Think about a transformer, which has coil windings to generate an electromagnetic field and then induce current in another set of windings. A certain amount of power is required to saturate the windings and generate the magnetic field, even though no work is being done. A power transformer that is not connected to anything is a perfect example of a pure inductive load. An apparent power draw exists to generate the fields, but no working power exists because no actual work is being done.

When the transformer is connected to a load, it uses both working power and reactive power. In other words, power is consumed to do work (for example, if the transformer is powering a light bulb), and apparent power is used to maintain the electromagnetic field in the transformer windings. In an AC circuit, these loads can become out of sync or phase, meaning they don't peak at the same time, which can generate harmonic distortions back down the power line. I've seen examples where electric motors have caused distortions in television sets plugged in to the same power circuit.

PFC usually involves adding capacitance to the circuit to maintain the inductive load without drawing additional power from the line. This makes the working power and apparent power the same, which results in a power factor of 1. It usually isn't just as simple as adding some capacitors to a circuit, although that can be done and is called passive power factor correction. Active power factor correction involves a more intelligent circuit designed to match the resistive and inductive loads so they are seen as the same by the electrical outlet.

A power supply with active power factor correction draws low distortion current from the AC source and has a power factor rating of 0.9 or greater. A nonpower factor corrected supply draws highly distorted current and is sometimes referred to as a nonlinear load. The power factor of a noncorrected supply is typically 0.60.8. Therefore, only 60% of the apparent power consumed is actually doing real work!

Having a power supply with active PFC might or might not lower your electric bill (it depends on how your power is measured), but it will definitely reduce the load on the building wiring. With PFC, all the power going into the supply is converted into actual work and the wiring is less overworked. For example, if you ran a number of computers on a single breaker-controlled circuit and found that you were blowing the breaker periodically, you could switch to systems with active PFC power supplies and reduce the load on the wiring by up to 40%, meaning you would be less likely to blow the breaker.

The International Electrical Committee (IEC) has released standards dealing with the low-frequency public supply system. The initial standards were 555.2 (Harmonics) and 555.3 (Flicker), but they have since been refined and are now available as IEC 1000-3-2 and IEC 1000-3-3, respectively. As governed by the EMC directive, most electrical devices sold within the member countries of the EU must meet the IEC standards. The IEC1000-3-2/3 standards became mandatory in 1997 and 1998.

Even if you don't live in a country where PFC is required, I highly recommend specifying PC power supplies with active PFC. The main benefits of PFC supplies is that they do not overheat building wiring or distort the AC source waveform, which causes less interference on the line for other devices.

Power Supply Safety Certifications

Many agencies around the world certify electric and electronic components for safety and quality. The most commonly known agency in the United States is Underwriters Laboratories, Inc. (UL). UL standard #60950Safety of Information Technology Equipment, Third Editioncovers power supplies and other PC components. You should always purchase power supplies and other devices that are UL-certified. It has often been said that, although not every good product is UL-certified, no bad products are.

In Canada, electric and electronic products are certified by the Canadian Standards Agency (CSA). The German equivalents are TüV Rheinland and VDE, and NEMKO operates in Norway. These agencies are responsible for certification of products throughout Europe. Power supply manufacturers that sell to an international market should have products that are certified at least by UL, the CSA, and TüVif not by all the agencies listed, and more.

Apart from UL-type certifications, many power supply manufacturers, even the most reputable ones, claim that their products have a Class B certification from the Federal Communications Commission, meaning that they meet FCC standards for electromagnetic and radio frequency interference (EMI/RFI). This is a contentious point, however, because the FCC does not certify power supplies as individual components. Title 47 of the Code of Federal Regulations, Part 15, Section 15.101(c) states as follows:

The FCC does NOT currently authorize motherboards, cases, and internal power supplies. Vendor claims that they are selling 'FCC-certified cases,' 'FCC-certified motherboards,' or 'FCC-certified internal power supplies' are false.

In fact, an FCC certification can be issued collectively only to a base unit consisting of a computer case, motherboard, and power supply. Thus, a power supply purported to be FCC-certified was actually certified along with a particular case and motherboardnot necessarily the same case and motherboard you are using in your system. This does not mean, however, that the manufacturer is being deceitful or that the power supply is inferior. If anything, this means that when evaluating power supplies, you should place less weight on the FCC certification than on other factors, such as UL certification.