by Mikael Strand Introduction The aim of this guide is: • To explain the basic functions of a power supply • To explain the difference between the ATX 1.3 and the ATX 2.01/2.2 PSU standards. • To explain different PSU parameters and how these will make a difference to your system. • To help you choose the right size of PSU for your system and what to consider when purchasing a power supply. What does a PSU do? The main purpose of a power supply is to convert the 230V AC (110V in the US) electric supply that comes out of the wall socket into 3.3V, 5V and 12V DC that the computer uses. A secondary purpose is to provide air flow in the case. The heat output of computer components is increasing. To cope with this case fans have become the primary exhaust in most systems. This has lessened the importance of the PSU as a provider of airflow in the case. What are the ATX 1.3 and the ATX 2.01/2.2 PSU standards? The ATX PSU standard is a standard set by Intel. The ATX 1.3 standard was released in April 2003. This standard was the first PSU standard to include SATA connectors. At the moment this is the most common standard for PSUs. ATX 2.01 was released in June 2004. This was a major update of the ATX PSU standard, making the PSUs more suited to the modern PC with its increasing load on the 12V line of the PSU. Graphics cards and the CPU(s) are the main consumers on the 12V line. The latest version of this standard, ATX 2.2, was released in March 2005. What are the main differences between the ATX 1.3 and ATX 2.01/2.2 standard? The ATX connector The ATX connector has changed from having 20 connectors to 24 connectors. The added connectors are 12V1, 5V, 3.3V and COM. Second 12V line added The purpose of the addition of a second 12V line on the power supply was to eliminate power surges on the 12V line when the CPU suddenly changes from idle to full load. SATA connectors The SATA connectors have changed from being optional in the 1.3 standard to being compulsory. The standard does not state how many SATA connectors the PSU should have. Efficiency Efficiency has been changed from just being recommendations to being a requirement. The required minimum efficiency is 70% for full load and typical load, and 60% for low load. There is still recommended efficiencies in ATX 2.01/2.2, these are 78% for full load, 80% for typical load and 75% for low loads. Full load is 100%, typical load is stated as 50% of overall output and low load as 20% of overall output.