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Inner Workings of PC Power Supplies

by Mystic Overclocker
2004/11/02

Inner workings of PC power supplies

The power supply is a very important part of your computer, and also the most likely to fail. Also, if you are planning a big upgrade, a weak power supply can keep your new upgrade from working.

How do you know your power supply is bad? Well, beyond the obvious problem of not turning on at all, there are some things you can look for. These can include overheating, periodic boot-up failures or errors, parity errors, lots of noise, and even electrical shock when the case or connections are touched.

If your power supply is showing any of these signs, it probably needs to be replaced. Never attempt to repair one yourself. Always take it to a technician. Or just buy a new one. Replacement is probably the most economical choice.

Power supplies are rated by the wattage output. An old computer will have a 70 watt power supply. Today, they go as high as 500 watts. The average computer today uses 230 watt or 250 watt. If you are buying a new power supply, buy one with this rating, and you will be set to go with numerous attachments.

The basics about how a power supply works.

Your power supply converts the standard 120 volt power from your wall into a form usable by your computer. In converts the AC current from the wall into a DC current or 5 volts and 12 volts. It then distributes this power to all the devices in your system. Usually, the motherboard, adapter cards, and diskette drives get the 5 volt power, while the disk motors, and cooling fans get the 12 volt leads. The newest Pentium 4 motherboards also require a 12 volt adapter in addition to the typical 5 volt lead.

The power supply also gives the system the final go-ahead before it boots. It sends an electric signal to the motherboard assuring that the system has enough power to operate correctly.

The main fan of your power supply is very important. It cools the power supply and keeps a constant and bearable temperature inside the rest of the computer's case. Obviously, the fan depends on a nice supply of air to operate. It pulls air through the vent at the front of the case, circulates it, and blows it out of the hole at the back of the power supply. Therefore, it is best to keep the case on most of the time while the PC is turned on to ensure a nice flow of air. Also, make sure you have dust plates covering each hole on the back of the computer that is not being used by an expansion card.

At vs. ATX form factors

Baby AT power supplies are the most common. They are plain and simple power supplies. The ATX form factor requires a special power supply. This supply is much more advanced. It contains special circuitry which allows software control of the power on signal. It supplies a steady 5 volt power to the motherboard even when the rest of the system is off, thus giving the system the ability to boot itself through software. The fan on the ATX power supply is reversed. This blows air into the case instead of out. The air blows right over the CPU, eliminating the need to a CPU fan. It also pressurizes the inside of the case, keeping it clean.

Connectors and Leads

When working with the power supply, you need to know about some of the connectors. Most importantly if you are using a new case, you might have to connect the supply to the power switch on the front of the case. Some people have a hard time doing this, since there are four wires, and four tabs, and finding which wires goes to what tab can be difficult. The instructions are straight forward.

You will usually have four color-coded wires. Some cases provide a fifth that is simply attached to the case for grounding. The wires are color-coded as follows:

  • The brown and blue wires are the feed wires from the 110V supply. They bring the full 110V to the switch. They are always hot when the power supply is plugged in, meaning ALWAYS HAVE THE POWER SUPPLY UNPLUGGED WHEN ATTACHING ITS CONNECTORS!
  • The black and white wires carry the AC current from the switch to the power supply. These wires are only hot when the system is on, because the switch actually closes the circuit.
  • If you have a green wire, sometimes with a stripe, it is a ground wire, and is attached to the case somewhere.

When connecting, place the blue and brown wires on the tabs that are parallel and next to each other. Place the black and white wires on the tabs that are angled and next to each other. I've heard you can actually mix these up a bit without problems, but ALWAYS make sure the black and brown are right next to each other, and the same with the others.

The drive connectors are all about the same, except for size. In response to some requests for a pin-out, here it is.

1 Yellow +12 V
2 Black Ground
3 Black Ground
4 Red +5 V

What you can do to make your power supply last longer.

  • House the computer in a friendly environment. Since the fan in the thing is working to keep things cool, it needs to have cool air to blow. The fan is ineffective in hot air. Therefore keep the things in a air conditioned room.
  • Try to minimize the amount of dust and pollution in the air around your computer. The fan draws air in, so you don't want it to blow in smoke or something that will just make your computer cough and hack. You may notice that the fan hole in your power supply is covered with dust. The little furry kind. You may want to clean this off, but make sure you don't stick anything into the power supply.
  • Use a surge protector. This is a device that will keep electric surges from entering your computer through the cord and frying your computer. Make sure the surge protector has phone line protection too so that you don't get a surge through your phone cords into the modem.
  • You might want to use a UPS, or Uninterruptible Power Supply. This is a little box with a built in surge protector plus a battery capable of running your computer for 10-15 minutes if the power shuts off. This will give you enough time to save your work.

 

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