PC Power Supply: A buyers guide to picking the right power supply for your computer

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Captain, I need more power!

It’s here, the package containing your new video card and promised hours of gaming goodness. You open your computer, install the new card, careful to follow every direction, and then, full of anticipation, hit your power-button. And with a sickening beep your computer boots and…shuts down. What could it be? You check the hardware requirements for the video card and go down the list, checking everything off until you hit “power supply” and see a bunch of numbers that make little sense to anyone but an electrician.

The sad fact of PC computing is that power supplies were until recently an afterthought. They came in little white boxes and unless you were a geek who builds computers for fun (raises hand) you never even saw one. They just sat in your box, powering everything just fine, and would often outlast everything else in the machine. I used to build a new box and just cannibalize old power supplies with little care, confident that everything would work. Often a single power supply would work across several builds. This is no longer the case.

Symptoms of a Weak Power Supply

Many power supplies simply don’t cut it, leading to loss of performance, crashes, and even causing the PC to fail to boot. This might be due to poor manufacturing or due to the power supply not being able to supply enough electricity to everything in the machine, but in any case it can be hard to track down because it will often look like other problems.

The following issues are signs your power supply might not be up to the task:

  1. During boot, your machine takes a very long time to get past the POST screen. Post is the Power On System Test, and sends a burst of power through every system critical component, testing them in a certain combination to make sure the PC can boot without damaging the hardware. In a normal machine, the POST screen will burst past quickly, but if you have a poor power supply, it can take a long time for everything to get done. Especially slow will be recognition of hard drives.
  2. During heavy graphics use, you start getting hard drive errors. The hard drives and the video card both live on the 12V rail (more on what this means later), and thus if the video card starts pulling too much power it can cause hard drive errors.
  3. Memory errors in programs, but when you test the memory it tests clean. This is a similar issue to the second one. The electricity in a computer has to maintain a certain flow and that flow is used to run the memory. If that flow starts spiking around, it will cause memory errors.
  4. Random shutdowns and reboots during CPU intensive activity. Like the memory, the CPU has to maintain a certain frequency across its calculations. That frequency is directly related to the voltage the CPU receives, and if this voltage fluctuates too much, the CPU will fail just like memory will. When memory fails, a program will crash, but when the CPU fails, the whole machine crashes.
  5. Hangs during boot where POST never even comes up that can then be fixed by resetting the computer, sometimes multiple times. This is the same as the POST problem, where the hardware isn’t getting checked properly and hangs while waiting.
  6. Your computer starts to boot, but then just shuts down, or gets locked in a loop of constantly trying to boot.

If you are having any of these issues, you need to look at the power supply first rather than later. I have replaced everything else, only to find it was the power supply and only the power supply. Also, do it now. A problem power supply can burn out your whole motherboard.

What is a Power Supply?

A power supply is a small box, usually located in the back of the machine, with cables coming out of it to your motherboard, your drives, and most likely your video card. It supplies power to each component of the computer. If that was all it did, then things would not be so complicated, but the power supply has another job.

The power supply converts different levels of electrical current and maintains them at a certain voltage. This voltage is consistent and runs a particular kind of device in the computer. When you look on your power supply’s hardware specifications, it will have certain numbers: +12V, -12V, +3.33V, etc. These are the different “rails”, and different hardware pulls current from different rails to function properly. Since several devices share rails, the current has to be steady and clean at all times.

A poor power supply will either fail to provide enough wattage or amperage. What these mean doesn’t matter, but they are the two numbers that important.

How to Know What You Need

There are some basic issues to consider. Wattage must be high enough to run everything, and for a modern machine that often means 500 watts or higher. However, people are buying 500-650 watt power supplies and still having problems, and the reason is that not all wattage is created equal. Amperage is just as important, and arguable more important on modern machines. Your video card especially will have an amp requirement. It should be right on the hardware specs where it will say “Requires a minimum of 28A on the 12V rail” or something similar.

Now, here is the tricky part: many modern power supplies share their amperage across multiple 12V rails. This was originally meant to distribute the current equally, making sure nothing failed. However, no one checked with the video card companies, who had figured out a way to dynamically alter the voltage required by the video card based on use. This gave us all those neat features like cards that can overclock when you start a game or where the fan changes speed based on the heat of the card. However, it also means those shared rails get all confused and try to balance the load, often causing graphical slowdown and hard drive errors.

This means a power supply may say “30A” and then have 4 12V rails sharing that 30A and balancing the load, which will not be enough for a video card that says “minimum 30A on the 12V rail”. You will get tons of errors. This gives you two choices: shoot for about 5 amps higher than your card says, which may be pretty hard to find, or find a power supply that offers the amperage you need in a single rail.

Final Word

If you want to get very fine tuned about what level power supply you need, there is a good calculator available here. Most users won’t need to get that detailed, as the wattage and amperage issues are the most important and can be estimated. The other key issue is that you get what you pay for, and that means that you need to find a good brand and avoid the “sale” white-box power supplies that you often see in computer shops. Antec, Zalman, and Thermaltake are all good brands. Remember, just because the box says the wattage and amperage is there, it doesn’t tell you how clean the signal actually is. This is one place you don’t want to scrimp.

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