What is the difference between AT and ATX power supplies?

AT power scheme is the original power scheme that most of the older computers used. AT-style computer cases had a power button that is directly connected to the system computer power supply. Pushing the power button kills all the power instantly (whether the Operating System is ready for it or not!)


An ATX power supply is typically controlled by an electronic switch. Instead of a hard switch at the main power input, the power button on an ATX system is a sensor input monitored by the computer. ATX systems allow the Operating System to control the final “off” signal to the power supply; this gives the OS time to save all information and complete important tasks before turning off the power supply using a dedicated output signal (PS_ON#). ATX power supplies also support lower power modes. They have an additional “Standby” power output (5VSB) that stays on to power standby devices whenever the system goes into low-power mode.


The ATX specification is the newer and more power efficient design. It is superior to the AT power supply scheme, and is used on almost all modern desktop and laptop computers. ADL Embedded Solutions offers PC/104 power supplies that support the ATX power scheme with 5V, 12V and 5VSB outputs only. Although the full ATX specification has additional voltages (12V, -5V and 3.3V), these extra power inputs are intended for Desktop systems; most embedded systems have no use for these extra voltage inputs and so they are omitted to save power and space.

 For power rating of each connection, please see below.