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Home > Guides > Power Supply Guide

  Power Supply Guide  

The power supply

Power supply guide

          The power supply (PSU) is the single most important part of your computer system, and is also the most often overlooked. If your power supply is faulty or isn’t working, then nothing in your system is working. A broken power supply can also damage all of your other expensive components, leaving you with a very large paperweight. As PC’s become exponentially more powerful, the importance of a reliable power supply is parallel. With the growing needs of the PC enthusiast, power supply companies have come up with many solutions to satisfy those demands. With so many new features available and factors to decide upon, it is extremely easy to become lost in all the abbreviations and complex definitions. This guide is designed to help you determine the right power supply for your specific application.

Here is a list of the many factors that must be considered when choosing a new power supply; Please click on one of the below links to receive information about the topic.

1. Watts
2. Cables
3. Modular
4. Form Factor Version
5. Efficiency
6. PFC (Power Factor Correction)
7. Noise

            Tells you how powerful a power supply is in Watts. Determine what your average power consumption will be considering the components that are installed on your system. (I.E. CPU, Video card, Hard drives/SSDs, fans, etc…) and choose a power supply that is going to be able to reliably deliver the required amount of power for your specific system/setup. There are many power supply calculators on the internet that allow you to put in your systems configuration and will estimate how much power you will need.
            A power supply’s voltage is separated in to different sections called “rails”. It’s voltage is sectioned into seperate rails to make sure a sufficient amount of power is supplied your components at all times. The wattage rating on some power supplies can be misleading because the large number that is usually advertised is the combined wattage of all the rails; the 5V, 12V, 3.3V, -12V, -5V and 5VSB. You may or may not even be using all or one of the rails, depending on your setup, so learning how to read the power supply labels is important to determine which power supply will provide you with the most usable power.

How to read a power supply label:

AC Input: The first row will show your ac input (your wall voltage).
DC Output: The second row will show the PSU's peak amperage on each of the rails.
Peak: The third row will show the max wattage for all of the rails.
Continuity: The fourth row will show the PSU's continuous amperage (what the power supply can do over a period an extended period of time.)
Max Power: The fifth row shows the maximum wattage for the entire power supply.

Please note that your power supply cannot feed both the 3.3V and the 5V rails maximum amperage at once. Meaning on this particular power supply you can load the 3.3V rail to 30 amps, and you can load the 5V rail to 35 amps, but you cannot load both to their maximum at the same time. They are not additive.

Notice how this power supply has two 12V rails? This is becoming a more common feature amongst the newer power supplies. With the higher demand on the +12V rail common in today's high speed systems Intel decided it would be safer to split the duty of supplying the +12V across two rails. It is safer because inexpensive transistors capable of supplying more amperage at any kind of decent efficiency are subject to being destroyed. By splitting the duty of the +12V rail, you can use cooler running, cheaper transistors to supply the power. Furthermore this isolates components on one rail from the other. Electromagnetic interference (EMI) is also is also reduced by light inverters and drive motors, by isolating the CPU and video card components.

Just as the 3.3V and the 5V rails are not additive, neither are the two +12V rails. Peak power is for one or the other, but not both at the same time.

Because of the non-additive properties of power supplies rails, it's easy to just look at the big number in maximum watts, and think that is sufficient for picking the right power supply for you, but as a more informed consumer you could find that just because a different power supply might have more "max" power, it might not have have more usable power given your specific needs. What good is power without the ability to use it?

It should be also be noted that dual 12V rails doesn't always mean better. Some problems people have are when the CPU is on it's own rail, and everything else on the other rail. The the CPU has too much overhead, while the other components (PCI cards, Video cards, drives, etc...) don't have nearly enough power. Some power supply manufacturers are getting creative and are splitting up the duties of the +12V rails, so that the distribution of power is balanced and all your components are getting enough power equally. This is why it is always good to check the label, to see the power distribution through the rails.

As important as it is; power output is just one factor in choosing the right power supply for you.
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Your power supply is responsible for feeding all of your components power, without the correct amount or type, you cannot successfully hook up your system. There are many different cable configurations available, and most newer power supplies contain more than enough connectors for all of your needs. It's still good to check and see if the particular power supply you are looking at has the ability to connect to all of your components and peripherals.
Here is a list of the cables that most newer power supplies support:

1. ATX power
(20pin + 4pin)
The ATX 20/24 pin connector connects your power supply to your motherboard. Older ATX form factor boards required the 20pin connector, while a lot of the newer boards require the 24pin connector. A lot of the newer power supplies offer a 24pin connector that can be converted to 20pin by sliding off the last 4pins on the connector.
2. 12V
4pin connector
Intel Pentium 4, 4pin connector. Supplies an extra 12V to the Pentium 4 CPU, now also utilized in many newer AMD motherboards.
3. 4pin
4pin "berg" connector. For attaching small form factor devices such as 3.5" floppy drives, or external audio cards.
4. 4pin Molex This is the standard 4pin connector used to connect all of your peripherals. Hard drives, optical drives, fan controllers, light tubes, the list goes on.
5. 6pin PCI-E This is a newer connector used to power high end video cards using the PCI Express slot. Many new power supplies will come with two of these for use in dual card, high end Crossfire/SLI setups.
6. SATA Power This is a newer power connector used to provide power to SATA (serial ATA) hard drives.

 After you decide if your power supply has all the connectors you will need (and perhaps seeing a bit into the future for further expansion possibilities). It's time to go on to the next factor in choosing the right power supply to you.
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More recently modular power supplies have expanded all over the market. No more cluttered wires, or overflowing corners of your case with tons of molex connectors that aren't being used. Behold the solution in the modular power supply; giving you the ability to use only the cables you need and none of the ones you don't. Although there might be a slight increase in initial price, modular power supplies are vastly becoming the norm in terms of newer power supplies. Not only does the modular power supply allow you to use less cabling, the cables are more often than not sleeved, which allows you to tuck them away giving your case a more aesthetically pleasing appearance, and providing you with improved airflow.

Example of a modular cable:

Note that both ends have a connector, which allows you to unplug all unnecessary cables from your power supply, streamlining the internal flow of air in your case and cleaning up internal layout.
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ATX Form Factors:
ATX12V 1.3, ATX12V 2.2, ATX12V 1.0... sounds very important, but what do all those numbers mean? Form Factors are just a standard form used by manufacturers to ensure compatibility within a given set of requirements. Meaning if something (power supply, motherboard, case, etc...) has a form factor, it must comply to a certain set of rules or requirements so that it can be utilized with any number of products and peripherals that adhere to that specific form factor. It should be duly noted that form factors are more relevant to actual computer manufacturers then computer consumers. The form factor revisions have to do with how the power supply is built and to which specifications are to be adhered to. The most common form factor for modern pc computers is the ATX form factor, and you see this ATX designation on power supplies, cases, motherboards, etc. Supplied here is a brief version history of the ATX12V (12V designating 12 volts) power supply form factors and their most relevant changes.

Changes made from ATX12V 1.1
to ATX12V 2.0
Changed the optional mounting hole to a required hole in chassis implementations. Changed
a previously required mounting hole to “not needed.”

Added optional power connector for fan control, fan monitor, IEEE-1394 voltages, and
remote 3.3 volt sensing.
Changes made from ATX12V 2.01 to ATX12V 2.2 Voltage Tolerances: -5VDC and -12VDC should comply
to ± 10%, not ±5%
Updated 3.3 V tolerance.
Main Power Connector changed from 20 pin to 24 pin to support PCI-Express requirements.
Changes made from ATX12V 2.2
to ATX12V 2.3
Recommended efficiency was increased to 80% (with at least 70% required), and the 12 V minimum load requirement was lowered.
Over-current limit of 240VA per rail was removed, allowing 12V lines to provide more than 20A per rail.

The most notable differences in the power supply form factor versions is the change from 20 pin to the 24 pin main power connector. almost all new power supplies come with a 24 pin main power connector with a removable 4 pin module, allowing it backwards compatibility with 20 pin motherboards. If you decide to purchase an older power supply which does not have a 24 pin main power connector and you are using a newer motherboard which requires a 24 pin main power connector, there are plenty of adapters that allow you to change your older 20 pin main power connector into the newer 24 pin connector that are also conveniently very inexpensive.
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Another parameter to consider when choosing a power supply is how efficient it is. When a power supply is more efficient, it will use less power from the wall than one that is less efficient even if it produces the same amount of DC power. The obvious benefits of a more efficient power supply is a lower power bill, also, the difference in wattage is dissipated in heat, so a more efficient power supply runs cooler. This can be used to an advantage one of two ways. One: A power supply will last longer if it’s not exposed to prolonged temperatures. Two: A quiet fan can be installed because not as much air flow is required to cool a more efficient power supply. In regards to overclocking, having an efficient power supply helps immensely. Not only are efficient power supplies keeping temperatures down, they also supply voltages with typiclaly less noise. Most manufacturer's will put their power supplies efficiency in their specifications. One way power supply manufacturer's are creating more efficient power supplies are with the addition of PFC, or Power Factor Correction. Read on to learn about PFC and how it makes your power supply more efficient.
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PFC (Power Factor Correction):
Power Factor Correction's main job is to turn complex variable loads (switching power supplies) to more constant loads (simple AC appliances like toasters). This is done through PFC and can be implemented in two different ways. Active PFC and passive PFC. Active PFC uses a circuit to correct power factor, it is capable of a full range of voltages and diminishes total harmonics. While active PFC is a more complex way to correct power factor, it is definitely more expensive to produce an active PFC power supply.
Passive PFC utilizes a capacitive filter at the AC input to correct poor power factor. This method, while being more efficient for your utility company may actually be less efficient for your power supply. Such capacitive filtering may cause a significant amount of heat generated from within your power supply which can transfer to other components within your power supply causing it to run hotter, which translates to less efficient operation.
In some parts of the world customers of utility companies are actually charged more for poor power factor. In Europe you are simply not allowed to use electronic device with a complex AC load without power factor correction. So certain inexepensive power supplies simply aren't available in Europe.
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There has been an increasing rise in the market for quiet pc components, and for good reason. most modern computers are very loud. All those high speed fans, rotating CD-ROM spindles, and higher RPM hard drives create enough noise to drive anyone to distraction. It isn't myth anymore that repeated low-level noise pollution – such as the subtle but ultimately insidious background noise in most offices, schools, and homes – can significantly impact your health. Once you have experienced a truly quiet pc, going back to the world of overbearing fans simply is not an option. There are many ways to quiet your pc, and the power supply is one of the most significant ways to do so. There are many manufacturers that advertise quiet solutions for your power supply, many use larger fans (120mm), some even use a fanless design, or near silent fanless operation which have onboard sensors to moderate fan speed to power load ratios. Most quiet power supplies feature their power supply operation noise to prove just how quiet they are. (Noise information is shown in decibels (dBA)).
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