This is an article from one of the hardware sites that I frequently visit:
"Computer power supplies are generally an afterthought for most individuals buying computer components. In past years, most personal computer cases on the market came with a factory installed power supply but over time this has changed. In fact, many of the cases that do still have power supplies in them tend to have inferior or low quality units installed in order to reduce costs for the manufacturers. Most high end computer cases will not even come supplied with a power supply so that users can get a unit that bests matches their components.
Power Supply Basics
While a computer gets plugged into a wall outlet that typically has 115 volts of electricity coming from it, the power needed by the computer components needs a lower voltage. Electricity running at such high voltage levels would burn out the transistors within the computer chips rendering everything in the computer useless. This function of converting the higher voltage power from the utilities to the lower voltages required by the computer components is the primary function of the power supply unit.
There are typically three voltage levels that the power supply unit needs to step down the high voltage to: 3.3v, 5v and 12v. The power supply delivers the lower voltage electricity to the various components through the myriad of power connectors that run out of the power supply. Most of these connectors are either the 4 pin Molex connectors that are plugged into the hard and optical drives inside of the computer. A smaller 4 pin floppy connector also exists but is increasingly being used for other devices instead of floppies. Finally there is the power block which connectors into the motherboard.
Wattage: Is More Better?
All power supplies are generally sold based on their wattage rating. What people don't know is that this is the combined total wattage rating of the power supply at full load across each of the individual voltage lines. Since the power supply has multiple voltage outputs, each voltage rail will pull its own current from the power supply. Thus, two different power supplies both rated at 450 watts may have completely different power characteristics when it comes to the amount of power delivered to each of the voltage lines.
Below is a chart of the power ratings that might be found for a power supply unit:
DC Ouput +5v +12v +3.3v -5v -12v +5v SB
Max 35A 18A 28A .5A 1.0A 2.0A
Min 0A .8A 0A 0A 0A 0A
Now if you are really into math and know all the equations for wattage based on currents and voltages you can figure out the maximum wattage for the power supply, but that isn't really necessary. The key to most power supplies is going to be the +12v rail and its maximum current.
Today's desktop processors use a large amount of power to run at their fast speeds. The primary power delivered to the processor is based on the +12v rail. So to make sure the power supply has sufficient power for the computer, you want to make sure that the +12v rail has a maximum current that is more than enough for your processor. The maximum current rating for the +12v rail should be at least 15 amps or higher. Anything less that this will have an unstable processor voltage.
The total wattage required to run the system is really going to be dependent upon all the other components that are in the system. Many small form factor systems that can only support at most 2 hard drives and one optical drive have power supplies as small as 200W. A full tower system on the other hand may require a large 550 watt power supply if it is running large raid drive arrays, a number of high speed cooling fans and several optical drives. That full tower system won't necessarily be running the full 550 watts all the time, but when everything is running it could potentially come close.
Below is a chart of various PC case sizes and the recommended ratings of a power supply for that size case with the number of devices that could potentially be installed in the case:
Case Size Recommended Wattage Min. +12v Max Load
Mini-Tower 330-350W 15A
Mid-Tower 350-380W 15A
Full Tower 380-450W 18A
Server Tower 450-550W 20A
ATX versus ATX12V
The ATX form factor built a set of standards that included specific formats and functions for computer power supplies. The major development was the ATX power connector and controls. What this did was take two separate power connectors and pulled them together into a single power block that was easy to install and also had a few power control functions between the motherboard and the power supply.
What this did was changed the system from being fully powered on when the switch was powered on at the power supply to a constant low power stream being fed to the motherboard. The motherboard in turn controlled when to enable the ATX power supply from being turned on fully through a soft power switch that was attached to the motherboard. This also allowed for a wide range of functions such as Wake-On-LAN, Suspend and Hibernate to be added into the operating systems and motherboards.
The problem came about with the development of the Pentium 4. This new processor required more power than the standard ATX platform could deliver to the motherboard. To help compensate for this, Intel developed a new ATX12V connector and power supply standard. This allowed an ATX12V compliant power supply to provide additional current on the +12v rail to the motherboard through an additional 4-pin block so it could properly power the processor. This same standard is now also used by AMD for its Athlon based processors. It is important to remember this when purchasing a power supply for an Athlon or Pentium 4 based processor as they require a ATX12V unit. Older processors are compatible with ATX12V units as they still have the standard ATX power connector. Really old systems may still use the old AT power supplies, but it would be very rare to have a system that uses that style anymore.
Fans and Noise
Heat is dangerous to all forms of electronics and it is particularly true for power supplies. Power supplies generate a large amount of heat in the process of converting the high voltage current into the lower voltage currents. As the load increases on the power supply, so does the heat that the power supply will generate. This is why active cooling fans have always been placed inside of computer power supplies. Many of the newer power supplies will often have 2 fans, one to pull air into the unit and one to expel the hot air out of the unit.
Of course, computer cooling fans are the biggest cause for noise generated by a computer system. With the introduction of additional fans into the units due to increasing temperatures, power supplies are now one of the key noise generators from a computer. It also does not help that the primary fans inside of these units also resides directly to the back of the computer case.
Manufacturers are starting to recognize this problem and are adding features to help reduce the noise. The easiest solution is to use ball bearing based fans instead of sleeve bearings. The ball bearings generate far less noise and also tend to have a longer life span than sleeve bearing based fans. Another major introduction has been temperature controlled fans. These units will monitor their internal temperature and increase the speed of the fan as temperatures rise. This keeps the units running within their acceptable levels while reducing the amount of noise generated.
How to Recognize a Dying Power Supply
When a power supply is dying, it has the unfortunate result of causing additional damage to the components inside of the computer system. The most common cause of power supply failures is overheating. Once power supplies get to very high temperatures, the components inside of them fail resulting in unacceptable or no voltage being supplied to the computer components.
Power supplies generally overheat due to improper airflow through the unit. This typically is the result of either a failing or failed cooling fan or a blocked air intake. One is fairly easy to detect, but the other is more difficult. As fans bearings tend to wear out (a precursor to their failure), they produce a greater amount of noise. Often this will be a scraping or high pitch tone. If your power supply is showing these audible signs, its best to take the time and money to replace the power supply. If the fan fails completely, the power supply will likely overheat which can result in greater damage to your components.
Power supplies also tend to be a collection area for dust. If the dust buildup is great enough, it could impede the flow of air through the unit that will increase the temperatures inside of the system. An easy way to prevent this is to clean the power supply. This is done by using compressed air to blow out the dust from the inside of the power supply. The important thing to do is to blow the air into the unit from the intake fan inside the computer and not the external fan. If you use the air from the external fan, you will end up blowing the dust into the interior of your case rather than cleaning it out. It is best to do this type of cleaning about once a year."
Although I agree that a well-built PSU like Antec and Fortron and Enermax is recommended to keep a stable running system, I've never had the problem of a bad PSU. Even cheap PSUs have a fuse integrated into its system that will blow before over-voltage will reach a system.
All I'm saying is even cheap no-name PSUs are built to within the limits of todays standards.
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