In a personal computer (PC), the power supply is the metal box usually found in a corner of the case. The power supply is visible from the back of many systems because it contains the power-cord receptacle and the cooling fan.

The power cord from the wall plugs into it. It has a fan in it. A bundle of wires come out of it and plug onto your motherboard. The original PCs had TWO connectors that plug onto the motherboard. The black wires of each go adjacent to each other. The orange wire goes toward the back of the computer. Some other plugs are available for connecting power to the floppy and hard drives. That is the power supply. While the rest of the PC uses low voltages (+5, +12, -5, -12), DANGEROUS HIGH

Power supplies can be fried by bizarre electrical line problems (lightning strikes are the classic example); that's why it's a good idea to plug a surge protector into your wall outlet and your PC into the surge protector. If a big blast of electricity heads down the line toward your PC, the surge protector will stop it -- actually, it will burn out, sacrificing itself for the greater good of your computer.

Without the Power Supply your computer would not run, it would just be a useless metal box full of wires, plastic and metal; would make a great doorstop. The power supply converts the alternating current (AC) line from your home to the direct current (DC) needed by the personal computer. VOLTAGES are inside the power supply, so you should never remove the cover unless you are a qualified power supply repair technician.

The small, red switch at right, above the power-cord connector, is for changing line voltages in various countries.

Connection to Motherboard

The switching power supplies used today are much smaller and lighter. They convert the 60-Hertz (Hz, or cycles per second) current to a much higher frequency, meaning more cycles per second. This conversion enables a small, lightweight transformer in the power supply to do the actual voltage step-down from 110 volts (or 220 in certain countries) to the voltage needed by the particular computer component. The higher-frequency AC current provided by a switcher supply is also easier to rectify and filter compared to the original 60-Hz AC line voltage, reducing the variances in voltage for the sensitive electronic components in the computer.

PC power-supply cables use standardized, keyed connectors that make it difficult to connect the wrong ones. Also, fan manufacturers often use the same connectors as the power cables for disk drives, allowing a fan to easily obtain the 12 volts it needs. Color-coded wires and industry standard connectors make it possible for the consumer to have many choices for a replacement power supply.

There are basically two different types of PC Power Supplies

ATX systems are almost the standard now. Intel's ATX specification was created to allow more convenient access to built-in I/O devices on the motherboard, and to accommodate power supplies that generate +3 VDC for use with modern low-voltage CPUs and chipset's. An ATX-compatible power supply offers a 20-pin motherboard connector and provides +3 VDC.

AT the original ISA-compatible supply offers a 12-pin connector (normally two 6-pin connectors side by side) and NO +3 VDC output. All non-ATX systems that require +3V have an on-board power supply and regulator (on the motherboard) that convert +5V into +3V for the new CPUs. The +3V CPUs consume less energy and generate less heat than
the +5V parts.

ATX power supply is different from AT power supply. It depends a logic circuit on the motherboard to turn it on. Once you flipped the power switch on the ATX power supply to on (some ATX power supplies do not even have such a switch), the ATX power supply sends a 5V 720MA current to the motherboard through pin 9 on the power connector. That current is for WOL (Wake-up On Lan) and power on circuits. There is a power-on jumper on the motherboard that connects to the pushbutton located in front of ATX case. When the pushbutton is pressed, it sends a signal to the mothboard, which in turn notifies the ATX power supply to turn on the full power. The location of the power-on jumper on the motherboard is manufacture dependent. You will need to read your motherboard manual to locate that jumper. At any event, do not try to manually jump-start the power supply without attaching motherboard. Since the power supply is expecting certain sensing circuit feedback to regulate the output voltage, munaully starting it without attaching it to the motherboard could cause damage to the power supply.

Advanced Power Management (APM) offers a set of five different states that your system can be in. It was developed by Microsoft and Intel for PC users who wish to conserve power. Each system component, including the operating system, basic input/output system (BIOS),motherboard and attached devices all need to be APM-compliant to be able to use this feature. Should you wish to disable APM because you suspect it is using up system resources or causing a conflict, the best way to do this is in the BIOS. That way, the operating system won't try to reinstall it, which could happen if it were disabled only in the software.

A 400-watt switching power supply will not necessarily use more power than a 250-watt supply. A larger supply may be needed if you use every available slot on the motherboard or every available drive bay in the personal computer case. It is not a good idea to have a 250-watt supply if you have 250 watts total in devices, since the supply should not be loaded to 100 percent of its capacity.

There are many tables out there that show what each device uses for wattage. AS I look at the different tables it is interesting the wattages are sometimes different for the same device. I have found, what I think, is a great page that you can figure your wattage required.

http://journeysystems.com/power_supply_calculator.php

The ATX motherboard is designed to operate with at least a 145 Watt power supply for typical system configurations. A higher-wattage supply may be required for heavily-loaded configurations. The power supply must provide +5VSB (voltage stand by) with 720mA of current (see the ATX2.01 or later specification). If the power supply does not supply sufficient current, some system configurations with the motherboard may not power up. Additionally, if the power supply can not supply sufficient current, and does not have overload protection, the power supply may be damaged. That is why we pay close attention to the manufacture process of the power supply in our case so that it would not be a problem of the end users. If you are using AMD CPU chip, it is recommended to use AMD Certified power supply.

What power supply requirements does the ATX motherboard have if a WOL (Wake on
LAN*) capable Network Interface Card (NIC) is installed?

A power supply that provides at least 720mA on 5VSB must be used when building a WOL capable system. If your power supply is not capable of meeting the 720 mA current requirement, your system may not power up. Also you could experience damage to your power supply if it does not have any over-current protection. If you are using a WOL capable NIC but are not using the WOL capability you can remove the cable from the NIC to the WOL header on the motherboard. This will prevent the NIC from drawing additional current from the 5VSB.

This model has the typical characteristics of an ATX12V 2.0 power supply (this industry standard doesn't describe models with an output power of over 400W, so you have to extrapolate). The +12V rail is “virtually” divided in two as is done in the absolute majority of new PC power supplies. In other words, there is only one power rail inside the power supply and its load current is up to 32A, but there are two separate current limiters, 18A each, set up on the PSU's output. I already wrote in my reviews about the reasons why the sum of the max currents on the 12V1 and 12V2 rails is bigger than the total current on the +12V rail: this power rail is split in two only to comply with the EN-60950 safety regulation which states that the maximum output power on user-accessible contacts must not exceed 240VA (that is, the current must not be higher than 20A at 12V voltage; the protection is set at 18A for a small safety margin). Of course, it is logical to set the current limit higher for each of the 12V rails, i.e. at 18A, to provide more flexibility in load distribution across the PSU's outputs.

This model has the typical single +12 rail

It is said that the AMD Athlon XP+ processor requires a minimum of 15A on the 12v rail. This is due to the surge of power to start it. This is taken from the AMD site:

Power Supply Failures and Symptoms

Bad Regulation

Regulation is the act of keeping the power supply's output voltages constantly at a particular DC level. To do this properly, each output voltage must have its own regulator. Most power supplies today only regulate +5V, but some are available that regulate all of them. Current drain from the power supply is known as "load". When the hard drive heads move to a new position, the load increases momentarily because of the current to the head positioner motor. Because of power supply design, the increase in load causes the output voltage to drop momentarily. It is the job of the regulator circuit inside the power supply to push the voltage back up instantly, so the drive motor does not "see" a fluctuating voltage. If the regulator is operating properly, the output will be a constant DC level, regardless of the changes in load. One major problem with modern cheap power supplies is that +12 VDC and +5 DC circuits share the same output transformer, but the only the +5 output has a regulator. Thus, if the load on +5 is low, as is the case in most systems, the +5 output tends to rise, the regulator pulls it back down by reducing current in the transformer, and as a result, the +12 VDC output is pulled down right along with it. This causes the +12 VDC output to be too low. In case you are wondering,, low +12 VDC will affect drive motors.

Power Good

A side issue to regulation is the Power Good signal. It is supposed to be present when the DC outputs are good and not present when they are not good. Obviously, if the +12 VDC output drops too low, the power is not good. However, many power supply manufacturers only cause Power Good to go away when the outputs are so catastrophically bad that the system will not run at all. I have seen power supplies with the Power Good signal completely removed, and the adjacent +5V signal jumper over to the Power Good pin in the motherboard connector.

Brown-Out

Brown-out is a dramatically reduced AC source voltage. It is caused by excessive load on the power company, such as during hot summer days when everyone in town is running the air conditioner. In a good quality power supply, brown-out will not affect the DC outputs at all until it drops to a certain level. Exactly at that level, the power supply cleanly shuts off all DC outputs, immediately shutting down the system. In a bad quality power supply, brown-out makes the DC outputs drop too low, but the supply does not cleanly shut down. When the outputs drop, especially +5V, some of the circuits in the system begin to malfunction, causing data loss and data corruption, especially when writing to disk drives. It also makes the drive motors overheat, thereby reducing their expected life or burning them up (yes, LOW voltage causes overheating and premature failure). The system can continue operating and unbeknownst to you, the data is being corrupted and parts are being damaged.

Transients / Spikes

A transient is a voltage surge of short duration that exceeds the nominal voltage by a large amount, perhaps 10% to 20% or more. A transient is commonly referred to as a "spike" or a "glitch". There are many kinds of transients, some dangerous to your computer, and some dangerous only to your data. Transients are bad and should not occur on the DC outputs of a power supply. Transients on the DC outputs are usually caused by surges in the AC power feeding the power supply. A defective power supply can cause transients too, but it is rare compared to AC transients. AC transients are caused when the power company switches power off and on, or by a motorized appliance (air conditioner, refrigerator, air compressor, etc) near the computer switching on or off. Lightning strikes near or on the power lines also cause them. The power supply should prevent transients from passing through to the DC outputs. A transient that passes through the supply can damage the motherboard or drive motors. This happens all the time with lightning, and it is forgivable for a power supply to pass THAT through (lightning burns out modems attached to phone lines, and through the modem it burns out the computer, as well). But the more common circumstance is for the transient to interrupt operation of the system in some way.

Noise

Noise is unwanted AC signal riding on the DC level. It is commonly referred to as power supply ripple. The input section of the power supply has to filter out the AC line frequency of 50 Hz to 60 Hz. Inside the power supply a pair of power transistors that switch on and off at a relatively high frequency, usually around 30 thousand times a second (30 kHz). The output filter is supposed to block the 30kHz AC signal. Sometimes the filtering does not work properly, thereby allowing AC power line frequency or internal switching frequency to be mixed with the DC outputs. This can create severe problems in operation of the system. If you look on your motherboard you will see tiny components adjacent to each chip.

These are filter capacitors, and their job is to filter out high frequency switching transients produced inside the chips, to prevent transients from being put on the DC power lines and thereby affecting adjacent chips. The switching transients are rather like the water hammer you hear in some houses when you turn the water on and off quickly. The filter capacitors absorb the shock wave of the "electronic hammer". These filters are NOT intended to filter out power supply ripple. When you have power supply ripple entering the motherboard on the +5V or +3V power lines, that ripple can so alter the DC voltages that it causes the transistor circuits to malfunction. The result is lost or corrupted data.

Feedback into AC Power Lines

The power supply not only has to filter noise out of the DC outputs, but it also has to prevent its own switching noise AND any high frequency or radio frequency motherboard or disk drive noise from feeding back into the AC power line. Such feedback will not affect the computer, but it can affect nearby radios, televisions, cellular phones, and portable phones. Such emissions are a violation of FCC regulations and similar regulations outside the USA.

Radiation

Radiation is the radio frequency electromagnetic energy in the air. If the power supply and computer are not properly designed, the radio frequency energy generated by computer components can leak out into the air and affect radio and television type devices nearby. This also is a violation of the governments' emissions regulations.

Faulty or Old Components

When components inside the power supply start getting old or deteriorating from excessive heat, they break down and fail to function properly. Often the power supply will keep on working, but not correctly. One indication of an aging supply is when you hear noise other than fan noise coming from inside the supply. The most typical kind of such noise is a repeated chirping sound. A less typical one is a low hiss or hum. Such a supply should be replaced. Note: the power supply's internal switcher circuit can issue a high-frequency sound, but it is normally beyond the range that humans can hear.

Bad Fans

This important device mounted inside the back of the power supply pulls air through the supply from the ducts inside the computer. If the fan stops working, the air flow will stop, the power supply and the computer circuits will overheat, and the computer will die.The power supply may die too, and the damage may be permanent. Before it dies, it may operate erratically, causing mysterious hang ups, data corruption, and data loss.

Dirt, Dust, Debris

Computer cases normally are not outfitted with air flow filters, so any dust and dirt in the air adjacent to to the computer will flow into the system. Normally, the particles catch onto the edges of the air flow holes on the case and on the power supply, building up as time goes by. It also collects on PC boards and circuit components. The thicker it gets, the more heat is trapped inside the components. As they overheat, they begin to malfunction, and they fail prematurely. The system must be blown and vacuumed out occasionally to prevent this.

Shorted Outputs

While it is not a problem with the supply itself, it may be that some device in the system is shorting a DC output to ground. Good power supplies simply shut off that voltage till the short is removed. Cheap power supplies may blow an internal fuse (DANGER: don't try to replace one unless you are a qualified maintenance technician - the voltages inside the supply can kill you), or become permanently damaged.

How to detect a Bad Power Supply

Most people assume the power supply is okay if the system is running. That is a wrong assumption much of the time. Some power supply failures are immediately obvious, but others cause all kinds of problems and ineffective remedial actions before being discovered. Among the many symptoms of a bad supply are: noisy fan (grinding, screeching), NO fan sound at all, chirping sound, erratic system or component operation, intermittent system operation, one part works but another does not, burning smell, and dead system. The problem is: almost anything in the system can cause MOST of these problems.

Aside from the obvious problem of a completely dead system, there are two ways to detect a bad : the HARD way and the EASY way.

The Hard Way: Fancy Equipment
You can buy a voltmeter and a storage oscilloscope, learn how to use them, read up on the specifications and testing procedures , then test the supply, replace it if defective, and let that equipment sit around till next time you use it. Cost: $500 to $2000+, depending on how fancy the equipment is.

The Easy Way: PC Power Check

You can buy a PC Power Check test card. You plug the card into a slot, or plug the power supply directly onto the card, and switch power on. You look at the lights. They tell you what is out of specification. Then you use it to help check out your friends' power supplies (you will get more friends this way), and you use it to make more service money because it is WORTH the price. The lights tell you the status of the system Reset signal and the Power Good signal.

They show you if each of the 4 voltages is Too high Too low Noisy Affected by a Transient Four summary lights on the rear edge of the card show if any errors occurred. A switch on the rear edge lets you reset the detector circuits to clear any error lights.

The circuits on the card are pre-adjusted to IBM's original over- and under-voltage limits for power supply outputs. They are set to detect 2% or more noise (IBM had no spec for that) because too many supplies fail at 1%. They will also detect a transient that is .75 VDC over nominal and lasts for 200 nanoseconds or longer.

Jumpers on the card let the high and low detectors operate in trap or continuous monitor mode. The alarm lights operate as follows: High and Low detectors in trap mode latch on when an error occurs and stay on till you cycle power or the card's reset switch. This lets you "trap" an error that occurs while you are not looking. High and Low detectors in continuous mode turn the lights on and off as errors come and go.

This lets you see the effect of load changes on regulation. If the lights blink when the drive motor is accessing, the regulators are defective or missing. Noise detectors always operate in continuous mode. Transient detectors always operate in trap mode. PC Power Check has small load resistors built onto the card, and an ISA motherboard connector so you can plug the supply directly onto the card and load it down enough to test it. This allows you to bench test several power supplies without risking damage to your computer by plugging an unknown supply into it for testing.

There are many stores out there you can purchase a Power Supply. Hundreds listed on the Internet, a couple favorites places that I shop are: Newegg.com and 3btech.net