Understanding and Troubleshooting the PC Power Supply

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Electrical Considerations

The unit of voltage in the International System of Units (SI) is a volt. It represents the potential difference between two points of a conducting wire carrying a constant current of one ampere when the power dissipated between these two points is equal to one watt. By understanding the relationship between voltage, current, and power, we can easily calculate one value if the other two are known. We simply use the formula, Watts = Volts x Amps.

By using the label on the power supply, it is possible to calculate the output rating if you do not have the specification from the manufacturer. Typically, the label shows four voltages with corresponding current levels. To calculate power supply’s output rating, sum the power present on each of the four voltages; calculate the power (Watts) by multiplying voltage times current (Watts = Volts x Amps) and then add these four values together to get the total power of the unit.

Not only is the total output important, each voltage signal is important. 3.3v and 5v signals needed to be within 5 percent, while 12v should be within 10 percent. Typically, problems arise when the power supply is outside of these specifications. The electronic components use the positive (+) 3.3-volt signal and 5-volt signal while fans use the 12-volt signal, which is usually supplied to disk drives and other storage devices.

Negative (-) 5-volt signal is vital for ISA operation and therefore usually abandoned by newer systems. In fact, the motherboard does not use the -5v signal, rather it is routed to the ISA slot on pin B5. The +5Vsb is important in power management as well as stability of the system.

Early ATX systems required a minimum of 720 mA of current. However, the AMD Athon systems are stable when this value is above 1A. Intel’s new D815EEA motherboard requires 2A. Therefore, Do-It-Yourself assemblers need to make sure that they know this value.

200, 250, 300 or more

It isn’t uncommon to read a post from someone in a newsgroup asking which is the correct power supply to use with a particular motherboard. Invariably someone posts that a minimum power supply is 300 watts or 400 watts, etc.

How did they come up with this figure? Most likely they read someone else’s post and thought it was a good answer. The best answer to this type of general question is, ‘It depends.’ So the next time you see this post – feel free to help the person by suggesting the following rules of thumb:

  1. Sum the power requirements of each component. Then purchase a power supply that is roughly double this requirement – thereby leaving room for growth.
  2. Higher clock speed processors draw more power than lower speed grades. Therefore, the faster processor systems should use power supplies with higher output ratings.
  3. The newer hard drives require less power than older models. However, if you are using more than one hard drive, one DVD-ROM, and one floppy, you should consider using a power supply greater enough to ‘spin’ the drives up at the same time. Some SCSI drives have jumpers for delaying spin up, removing this requirement.
  4. The wattage of the power supply can be greater than the system requires. It never hurts to spend a little more for the higher rated output. It always hurts to under spend.
  5. Always consider airflow. Gateway and CoolerMaster use an air duct that attaches to the power supply.

Because these rules of thumb do not give a direct and final answer, it is sometimes easier to just answer newsgroup posts, ‘Get a 400-watt power supply’ and be done with it. In fact, we usually recommend a minimum of a 300-watt power supply with a standby voltage of 2A for entry level systems. This helps ensure the assembler is covered in a wide range of configurations. We also suggest 400-watt power supplies for systems that have dual processors, while those with RAID, or multiple SCSI or IDE drives should consider 600-watt power supplies (possibly redundant too).

Pre-assembled versus Do-It-Yourself assembly

To contrast between the Do-It-Yourself and Pre-assembled markets, we had a discussion with a very helpful technician from Gateway named David. According to David, Gateway has a team of engineers who pre-qualify components. This is the role that Do-It-Yourself assemblers need to undertake while choosing the right components.

The power supply used by Gateway is the ASTEC model ATX 202-3545. This 200-watt power supply provides the 1.7A on the +5Vsb signal. It also has the connector between the power supply and the motherboard for power management. Because power management is immature, David suggests that some systems operate better without the power management enabled. We recommend this same action for most Do-It-Yourself assemblies.


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